Nitrification Inhibitors for Agriculture, Health, and the Environment

NITRIFICATION INHIBITORS FOR AGRICULTURE, HEALTH,

AND THE ENVIRONMENT

Rajendra Prasad and J. F. Power2

I Division of Agronomy Indian Agricultural Research Institute

New Delhi, India *United States Department of Agriculme

Agricultural Research Service University of Nebraska

Lincoln, Nebraska 68583

I. Introduction II. Nitrification Inhibitors (NIs)

A. Relative Effectiveness B. Soil Factors Affecting Effectiveness of NIs C. NIs and Nitrogen Losses and Immobilization

m. NIs, NH,+/NO,- Ratios, and Plant Growth IV. NIs and Crop Yields

A. Rice B. Corn C. Grain Sorghum D. Wheat E. Sugarcane F. Potato G. Cotton

V. Phytotoxicity of NIs VI. Health and Nitrates

A. Nitrates and Human Health B. Nitrates and Animal Health C. Nitrate Content of Drinking Water D. NIs and Nitrate Content in Vegetables

A. NIs and Nitrate in Groundwater B. Ozone Layer Depletion C. Global Warming References

VII. NIs and Environment

233 Aduances in Agranamny. Volume H

Copyright Q 1995 by Academic Press, Inc. All righu of reproduction in any form reserved.

234 RAJENDRA PRASAD AND J. F. POWER

I. INTRODUCTION

Nitrification inhibitors (NIs) emerged as a group of agrichemicals with the development of N-Serve [2-chloro-6(trichloromethyl)pyridine] (Dow Elanco trade name for nitrapyrin) by Goring ( 1962ab), although inhibition of nitrification by a number of herbicides, insecticides, nematicides, and fungicides were known long before. Except for a few field experiments, research on nitrification inhibitors during the 1960s was mostly restricted to laboratory studies (Prasad el al., 1971). Intensive field investigations were carried out in the late 1960s and 197Os, and the American Society of Agronomy, the Crop Science Society of America, and the Soil Science Society of America jointly sponsored a symposium on December 6, 1978, at Chicago, Illinois, the proceedings of which were published in 1980 (Meisinger et al., 1980). Three years later a technical workshop on the nitrification inhibitor dicyandiamide (DCD) was held on December 4-5, 1981, in Muscle Shoals, Alabama; this workshop was jointly sponsored by the National Fertilizer Development Center, the Tennessee Valley Authority at Muscle Shoals, Alabama; the International Fertilizer Development Center, Muscle Shoals, Alabama; and SKW Trostberg AG, West Germany (Hauck and Behnke, 1981). A second workshop on DCD was held on December 4-5, 1987, at Atlanta, Georgia, and the proceedings were published as a special issue in Communications in Soil Science and Plant Analysis (Vol. 20, Nos 18 and 19, 1989) (Hauck et al., 1989).

In addition to specific chemicals such as nitrapyrin or DCD, natural products like those from neem (Azadirachta indica Juss) are reported to have nitrification- inhibiting properties (Reddy and Prasad, 1975; Sahrawat and Parmar, 1975) and have been widely evaluated in India. Prasad et al. (1993) addressed the N use efficiency aspects of urea coated with neem cake and other neem products at the Neem World Conference held at Bangalore, India (February 24-28, 1993).

An ideal nitrification inhibitor should be mobile, persistent, and, above all, eco- nomic in use (Hauck, 1972). It should also be nontoxic to other soil organisms, animals, and humans and should move with the fertilizer or nutrient solution. Compounds with high vapor pressure may move fast and compounds easily absorbed may not be very effective. An ideal NI should stay effective in soil for an adequate time period; at least for the growth period of a crop. Above all, the real testing ground is in the economics of use; most studies indicate that about a 0.3 to 0.5 mg ha - ' yield increase will pay for the cost. This one factor alone has stopped many nitrification inhibitors from reaching the farm level. The major goal in using a NI is to increase the efficiency of fertilizer N applied to agricultural/horticultural crops by reducing nitrate leaching losses as well as nitrification losses as N,O or N,. Thus ideal situations where NIs are likely to be the most effective are those where such losses predominate, such as rice paddies, areas receiving heavy precipitation, irrigated areas (especially furrow) because of leaching, and crops receiving high rates of N fertilization or manures.

NITRIFICATION INHIBITORS 2 3 5

During the 1980s there was considerable effort by ecologists, environmental- ists, and some agriculturists to reduce fertilizer N use on the farm, mainly due to its likely role in increasing nitrate concentrations in groundwater and because N fertilizers are manufactured from a nonrenewable natural resource (natural gas). However, on a global scale this will neither be possible nor desirable if we are to feed the increasing world population. The available estimates indicate that 2422 Tg of cereals will be required in 2000 AD (Prasad, 1986) compared to the 1991 estimated production of 1884 Tg of cereals (FAO, 1991). Thus an additional 28.5% of cereals will have to be produced in the next decade, most of it in the developing countries.

While this increase in cereal production can be achieved in most African and South American countries by bringing more land under cultivation, Asia has done it by increasing productivity per unit land per unit time. This calls for a sizable increase in the consumption of fertilizer, especially nitrogen. It is estimated by 2000 AD that 145.4 Tg of fertilizer N will be consumed annually (UNIDO, 1978), which is nearly double the 1990-1991 consumption of 77 Tg of fertilizer N (FAO, 1991). Furthermore, a large number of the developing countries, especially those in south and southeast Asia, grow rice as a principal crop, the crop for which fertilizer N losses are greatest (Prasad and De Datta, 1979; Fillery and Vlek, 1986; Reddy and Patrick, 1986; De Datta, 1986). In addition to the large amounts of fertilizer N needed in the developing countries, high costs involved in their production or purchase also need to be considered. Also the sustainability of synthetic fertilizer production from natural gas at some time in the future is a concern. Thus, efficient use of fertilizer N is necessary, suggesting that nitrification inhibitors have a role to play. This chapter provides an overview of the literature available on the use of nitrification inhibitors in relation to production and quality of agricultural and horticultural crops, human and animal health, and the environment.

11. NITRIFICATION INHIBITORS

A fairly large number of chemicals have been reported as nitrification inhibitors: Nitrapyrin (abbreviated as NP in this chapter) or N-Serve [2-chloro-6-(trichloromethyl)pyridine] (Goring 1962ab); AM (2-amino-4-chloro-6-methylpyri- midine) (Toyo Koatsu Industries, 1965); DCD (Amberger and Guster, 1978); terrazole or Dwell or etridiazole (5-ethoxy-3-trichloromethyl- 1,2,bthiadiazole) (Olin Corp., I976ab); DCS [N-(2,5-dichlorophenyl)succinamic acid] (Namioka and Komaki, 1975ab); KN? (potassium azide) (Hughes and Welch, 1970); ATC (4-amino- 1,2,4-triazole) (Guthrie and Bomke, 1980); TU (thiourea); MBT (2- mercaptobenzothiazole); 2-ethynyl pyridine (McCarty and Bremner, 1986); MPC (3-methyl-pyrazole- I-carboxamide) (McCarty and Bremner, 1990); ST (2-sul- fanil-amido thiazole) (Mitsui Toatsu, 1968); CS, (Ashworth et ul., 1977); 2-mer-


capto- 1,2,4-triazole, sodium diethylthiocarbamate; 2,5-dichloroaniline; 4-amino- 1,2,4-triazole (Bundy and Bremner, 1973); C2H, (acetylene) (Hynes and Knowles, 1981; Berg et al., 1982); gaseous hydrocarbons such as C,H, (ethane), C,H, (ethylene), and CH, (methane) (McCarty and Bremner, 1991); ammonium thiosulfate (Goos, 1985); and thiophosphoryl triamide (Radel er al., 1992). Of these, only eight (NP, AM, DCD, ST, TU, Dwell, MBT, and C2H,) have been widely tested.

In addition to specific chemicals, allelochemicals also have nitrification-inhibiting properties. For example, Rice ( 1984) postulated that because inhibition of nitrification results in conservation of both energy and nitrogen, vegetation in late succession or climax ecosystems contains plants that release allelochemicals that inhibit nitrification in soil. However, a critical appraisal of the available informa- tion does not lend support to such a hypothesis (Bremner and McCarty, 1993). As an example, terpenoids thought to be released by a ponderosa pine (Pinus ponde- rosu Dougl.) and supposed to inhibit nitrification in soil had no such effects (Bremner and McCarty, 1988). However, some natural products are reported as nitrification inhibitors. These include “neem” (A. indica Juss.) cake or an ace- tone/alcohol extract of seed (Reddy and Prasad, 1975; Sahrawat and Parmar, 1975) and “karanj” (Pongarnia glabra Vent.) seed, bark, and leaves (Sahrawat et a!., 1974).

A. RELATIVE EFFECTIVENESS OF M S

Rajale and Prasad (1970) found AM as effective as NP, while Bundy and Brem- ner (1973, 1974) found that AM was less effective than NP and DCD. Sommer (1970) compared a number of NIs and ranked them in the following order: Terrazole > NP > DCS > guanylthiourea > AM > MAST (2-amino-4-methyl- 6-trichloromethyltraizine) > ST. McCarty and Bremner (1989) compared 12 compounds and found 6 of them to be effective NIs: 2-ethynylpyridine > Dwell > NP > MPC > ATC > DCD (Table I).

In a number of U.S. studies NP and DCD were found to be equally effective. In their studies in Illinois, Malzer et al. (1989) at Urbana, Monmouth (Typic Hapla- quolls), and Dekalb (Aquic Arguidoll), showed that the disappearance of ammonium was similar between DCD (5% DCD-N) and NP (0.5 kg ha ~ ’). At Brown- ston (Mollic Albaqualf), however, ammonium disappearance was slower with DCD than with NP. Bronson et al. (1989) from Alabama reported that DCD in Norfolk loamy sand (Typic Paleudult) was equal to NP for up to 42 days, but was less effective than NP in Decatur silt loam (Rhodic Paleudult).

Etridiazole and NP are equally effective in reducing nitrification of ammonium N in soils up to 160 days after application on silty loam soils (Typic Ochraqualfs and Aquic Hapludalfs) in Illinois (Shyilon et al., 1984).

Blending of urea with neem cake inhibited nitrification by 70, 40, and 5% at

NITRIFICATION INHIBITORS 237

Table I

Effects of 5 mg kg-' Soil with Different Compounds on Nitrification of Ammonium in Soils"

Compound

Soil

Harps Webster Storden

2-Ethynylpyridine

Etridiazole (Dwell)

Nitrapyrin (N-Serve)

3-Methylpyrazole- I -carboxamide

4-Amino- I ,2,4-triazole

Dicyandiamide

Potassium azide

N-(2,5-Dichlorophenyl)succinamide

Sodium thiocarbonate

Thiourea

2-Mercaptobenzothiazole

Ammonium thiosulfate

g%, inhibition of nitrification

79 80 I00

61 70 97 45 56 94

43 53 93

41 52 92

8 20 41

0 3 5

0 2 5

0 0 0

0 0 0

0 0 0

0 0 0

"Samples of soil (20 g) were incubated at 25°C for 25 days after treatment with 6 ml water containing 4 mg N as ammonium sulfate and 0 or 100 y g of the compound specified. Adapted from McCarty and Bremner (1989).

the end of 1,2, and 3 weeks of incubation, respectively; the corresponding figures for NP at 1% of N were 85,93, and 90% (Reddy and Prasad, 1975). Thomas and Prasad (1982) evaluated neem cake-coated urea on a number of soils (Entisols, Vertisols, Ultisols) and found it to be 50% as effective as NP. The active compounds in neem responsible for retardation of nitrification are thought to be meliacins (epinimbin, nimbin, desacetyl nimbin, salanin, desacetylsalanin, and azadi- rachtin) (Devkumar, 1986). Nitrification retardation after 2 weeks was 73.6,44.6, and 12.5% for NP, epinimbin, and desacetylnimbin, respectively (Devkumar and Goswami, 1992).

Neem cake and DCD were evaluated for their efficiency in inhibiting nitrification of prilled urea-derived NH,+-N in a wheat field (Joseph and Prasad, 1993a,b). Prilled urea was blended with 10 and 20% DCD-N or with 10 and 20% neem cake and incorporated into the soil just before the wheat was sown. Both DCD and neem cake partially inhibited the nitrification of prilled urea-derived NH,; DCD was better than neem cake. The nitrification-inhibiting effects of DCD lasted for 45 days, while that of neem cake lasted for only 30 days.

Most NIs inhibit nitrification by retarding the oxidation of NH,+-N to NO,--N by Nitrosomonas sp. Research with different strains of Nirrosomonas


4 0

30 - 1 I

0- 0 3 6 9 1 2 0 3 6 9 1 2 0 3 6 9 1 2

D A Y S

Figure 1. Effect of dicyandiamide (DCD), nitrapyrin (NP), and thiourea (TU) on the activity of Nitrosomonas eurupoeo i n pure culture. Adapted from Zacheri and Ainberger ( 1990).

sp. showed remarkable differences in sensitivity to nitrapyrin (Belser and Schmidt, 1981), and it was concluded that NP does not retard the activity of the entire population of Nitrosomonas sp. Results of a study done by Zacheri and Amberger (1990) on the effect of three NIs are shown in Fig. 1. Growth of a pure culture N . europaea was completely suppressed by 10 ppm NP or 0.5 ppm TU; inhibition by 300 ppm DCD was 83%. Ammonium oxidation and respiration of Nitrosomonas cell suspensions were reduced by 93% with 10 ppm NP, 95% with 0.5 ppm TU, and 73% with 300 ppm DCD. When used at 1000 ppm, DCD had bacteriostatic effects. Enzymatic investigations revealed that hydroxylamine oxi- doreductase was not affected by high concentrations of inhibitors (200 ppm DCD, 100 ppm TU). Cytochrome oxidase activity was increased 10% with 200 ppm DCD, was not affected by 100 ppm TU, and was inhibited by 52% with 100 ppm NP. These results suggest that different NIs probably have different modes of action.

B. SOIL FACTORS AFFECTING EFFECTIVENESS OF N I S

A number of studies have investigated the effect of different soil factors on the effectiveness of NIs, and this subject has been well reviewed by Slangen and Kerk- hoff (1984). The main findings are summarized below.

1. Organic Matter

Hendrickson and Keeney ( 1979b) found complete inhibition of nitrification with NP at 0.5 mg kg - I in a soil with 1% organic matter and none in the same soil when organic matter was raised to 5% by adding active carbon. Similar results were obtained by McClung and Wolf (1980) with NP and terrazole when they


added compost to the soil. The influence of organic matter is probably due to its effect on sorption and rate of decomposition of the chemical.

2. Temperature

Most reports suggest that nitrification inhibitors are more effective at relatively low temperatures, i.e., below 20°C (Goring, 1962a; Bundy and Bremner, 1973). This is mainly due to the effect of temperature on degradation of a NI and the consequent persistence. Herlihy and Quirke (1975) found that the half-life of NP was 43 to 77 days at 10" C and 9 to 16 days at 20°C. Touchton et al. (1979) found the half-life of NP to be 22 days at 4" C and less than 13 days at 2 1 " C for a loamy soil with pH 6.8 and an organic matter content of 2%. In a soil with pH 5.5 and an organic matter content of 5%, the half-life of NP was 92,44, and 22 days at 4, 13, and 2 I " C, respectively. Touchton ef a/. ( 1979) reported that the half-life of NP in a Cisne silt loam was 7 days at 2 1" C and 22 days at 8" C.

DCD is highly sensitive to temperature. Vilsmeier (1980) reported that after 60 days, 0.67 mg DCD-N 100 g ~ I soil degraded to 0.60 mg at 8" C, 0.4 at 14" C, and 0.1 mg at 20" C in a sandy silt loam soil of Germany with a pH of 6.2. Bronson et a/. ( 1989) found that the half-life of DCD decreased from 52.2 days at 8" C to 22 days at 22°C in Norfolk loamy sand and from 25.8 days at 8°C to 7.4 days at 22" C in Decatur soils. Data of McCarty and Bremner (1989) for Iowa soils showed that in 28 days inhibition of nitrification decreased from 72% at 15" C to 19% at 30°C in Harps silty clay soil when DCD was added at 10 mg kg - I soil (Table 11). At 30°C the inhibitory effect at 10 mg kg - ' of soil with etridiazole exceeded that at 100 mg kg ~ soil with DCD and the inhibitory effect of 10 mg

Table I1

Influence of Soil Temperature on Effectiveness of Dicyandiamide (DCD) for Inhibition of Nitrification of Ammonium in Soils"

Amount of Soil temperature ("C) DCD added

Soil (mg kg ~ ' soil) 15 20 25 30

% inhibition of nitrification

Harps 10 72 60 48 19 50 83 82 72 49

Webster 10 78 65 51 25 50 85 84 13 62

Storden 10 90 75 53 23 50 97 94 89 81

"Samples of soil (20 g) were incubated at 15, 20, or 30°C for 28 days after treatment with 6 ml water containing 4 mg N as ammonium sulfate and 0.0.2, or I .O mg of DCD. Adapted from McCarty and Bremner (1989).

2 40 RAJENDRA PRASAD AND J. F. POWER

kg- ' soil with NP exceeded that at 50 mg kg - I soil with DCD (McCarty and Bremner, 1989). In Illinois, DCD and NP were equally effective on Drummer silty clay loam at 7.2"C. but DCD was more effective than NP at 15.5"C (Sawyer, 1985).

3. pH

The influence of soil pH on the persistence of NP is reported to be minimal (Hendrickson and Keeney, 1979a). This can be expected since a number of genes of nitrifying organisms are involved in nitrification, each with different pH optima (Bhuija and Walker, 1977).

4. Soil Water

Hydrolysis of NP is enhanced in water-saturated soils (Hendrickson and Kee- ney, 1979a) as compared to aerobic conditions in soils at field capacity (0.01 to 0.033 M Pa). Volatilization of NP is more pronounced in wet than in dry soils (McCall and Swann, 1978).

In addition to these factors, method and time of fertilizer application and source of N used can affect the effectiveness of NIs under field conditions (Singh and Prasad, 1985; Sudhakara and Prasad, 1986a; Thomas and Prasad, 1987).

c. N S AND NITROGENLOSSES AND IMMOBILIZATION

1. Urea Hydrolysis

Most of the NIs, such as NP, AM, ST, ATC, KN3, CS,, and DCD, have little effect on urea hydrolysis. However, TU, ammonium, and potassium ethylxanthate and thiosulfate retard urea hydrolysis (Mahli and Nyborg, 1979; Goos, 1985; Ash- worth ef al., 1980).

2. Ammonia Volatilization

Since NIs retard nitrification, ammonium-N can accumulate and result in a higher soil pH (Bundy and Bremner, 1974), which is conducive to NH3 volatilization. Enhanced NH3 volatilization losses due to application of NI have been reported (Bundy and Bremner, 1974; Smith and Chalk, 1978; Prakasa Rao and Put- tanna, 1987). While Bundy and Bremner (1974) reported a 28-34% N loss from volatilization of added urea N with a NI (NP, ATC, CL- 1850) and 9% without, Smith and Chalk (1978) found NH3-N losses of 86 and 92 mg kg ~ I soil without and with NP. High NH3-N losses reported by Bundy and Bremner (1974) could


be due to high rates of N applied (400 mg N kg - I soil). Volatilization losses of NH, with or without NI can be reduced by incorporation of the fertilizer N. Clay et al. (1990) reported that NH, volatilization from bare soil was lower with urea and DCD than with untreated urea. However, when the soil surface was covered with residue, NH, volatilization was similar with or without DCD. Sudhakara and Prasad (1 986a) reported that when 120 kg N ha - ' was applied 20 days after sowing rice, the NH, volatilization loss was 8.37% of the applied N from urea compared to 3.89% with neem cake-coated urea. Thus at rates of N generally applied in field crops, an increase in NH, volatilization due to NIs can be considerably reduced by the incorporation of fertilizer N and NIs in soil. Another possibility for reducing NH, volatilization is the use of dual purpose (NIIurease inhibitor) compounds such as thiophosphoryl triamide (Radel et ai., 1992).

3. Denitrification

By retarding nitrification, NIs slow down and reduce the potential for N loss by denitrification as N,O or NZ; this, however, should not be confused with the reduction of denitrification per se. For example, some workers reported that NIs (NP, DCD, NaN,, Dwell, KN3, ST, PM, ATC) directly retard denitrification (Mitsui et ul., 1964; Henninger and Bollag, 1976; McElhannon and Mills, 1981), especially when added at the rate of 50 or 100 mg kg ~I soil (Bremner and Yeo- mans, 1986). Such rates are too high for general applications to field crops. Brem- ner and Yeomans (1986) evaluated the effect of 28 NIs and found that only KN, and 2,4-diamino-6-trichloromethyl-S- triazine, when added at the rate of 50 mg kg ~I soil, inhibited denitrification. The other NIs had no appreciable effect on denitrification.

4. Nitrogen Losses from Plants

Plants also lose some amount of N from the foliage (Wetselaar and Farquhar, 1980; Patron et al.. 1988; Francis et al., 1993). A high loss of N was observed by Daiger e ta / . (1976) from winter wheat at different locations in western Nebraska, following different rates of N application. In general, dry matter and N content of tops and roots reached a maximum at anthesis. Thereafter, dry matter declined by about lo%, while losses of N from the tops plus roots ranged from about 20 to over 60% depending on the fertilizer N rate. Tanaka and Navasero (1964) reported a loss of 47 kg ha - I in the N content of rice tops in the 3 weeks before flowering and maturity at high N rates. Patron et ai. (1988) reported a NH, loss of 60- 120 ng N m - > sec - I from spring wheat plants during the presenescence time period (before milk stage) and 200 to 300 ng m - ? sec ~I during final plant senes- cence. They found that NH, loss rates on a leaf area basis were similar for the low and high N plants despite significantly higher N concentrations in high N plants.


Twice the leaf area was attained by the high N plants, resulting in similar NH, volatilization rates per plant which translates into nearly twice as high on a plant N basis for the low N plants. Farquhar et al. (1979) reported an evolution of 0.6 nmol m -, sec - I (36 g N ha ~ I day ~ I at LA1 5) from senescing leaves of corn. In a study at Lincoln, Nebraska, postanthesis fertilizer N losses as NH, from the aboveground biomass of corn plants ranged from 10 to 25% of the fertilizer applied (Francis et al., 1993); the apparent total N losses from the aboveground plant material ranged from 49 to 81 kg N ha- ' . Francis er al. (1993) observed that postanthesis N losses from aboveground plant biomass in corn accounted for 52 to 73% of the total unaccounted for fertilizer N and suggested that failure to include such losses can lead to overestimation of N losses from soil by denitrification and leaching.

Mosier er al. (1990a,b) reported a N, + N,O gas flux of 270 g N ha - I day ~I 15 days after transplanting rice where plants were included in the measur- ing chamber as compared to only 240 g N ha - I day - I when the plants were not included in the chamber. They concluded that young rice plants facilitated the efflux of N, and N,O from the soil to the atmosphere. Effects of NI on such losses of N from the plants have so far not been reported.

5. Immobilization

Immobilization of fertilizer N by soil microorganisms is significantly enhanced in the presence of a nitrification inhibitor (Osiname et al., 1983; Juma and Paul, 1983). This has been attributed to the NIs maintaining more of the applied fertilizer N as NH,+ for a longer period of time (Prasad et al., 1983; Shyilon et al., 1984; Norman and Wells, 1989) and preferential utilization of NH,+-N by heterotrophic microorganisms (Broadbent and Tyler, 1962; Alexander, 1977). Bjarnason (1987) reported that not only is NH,' preferentially immobilized, but its remineralization is at a slower rate.

Norman and Wells (1 989) found that immobilization of fertilizer N by soil microorganisms in a Crowley silt loam (Typic Albaquelf) in Arkansas was approxi- mately the same in urea and urea + DCD-amended soils during the 4-week period when the soils were not flooded (Fig. 2) . Immobilization appeared to level off after 2 weeks and stayed relatively constant for the remaining 2 weeks. After flooding, immobilization of fertilizer N was much greater in the urea + DCD-amended soil than in the urea-amended soil, and by the end of 8 weeks soil with urea + DCD had nearly 1.5 times more NH,+ than that treated with urea only. Osiname et al. (1983) reported more N immobilization with NP than with DCD. Preferential immobilization of NH,+-N rather than NO,- -N has been suggested by a number of researchers (Wickramsingha et al., 1985; Rice and Tiedje, 1989). This would support higher immobilization of fertilizer N with NIs.

Budot and Chone (1985) suggested an interesting pathway of nitrite incorpo-


NONFLOODED FLOODED

LSD 0.05=4.83 mglkg

0 1 2 3 4 5 6 7 8 9 1 0 1 1

INCUBATION TIME (weeks)

Figure 2. Immobilization of fertilizer N under nonflooded and flooded conditions. Adapted from Norman and Wells (1989). (0) Urea; (A) urea + DCD.

ration into the organic N fraction via nitrite self decomposition and fixation on organic matter in a humic-rich acid forest soil (pH 4.5; organic matter 46%). Azhar et al. (1986a,b,c) also supported this pathway. NP not only reduced the loss of nitrite via chemodinitrification, but Nelson ( 1982) also discussed the incorporation of nitrite into the organic N fraction.

Thus it appears that NIs increase immobilization of N by increasing the persistence of ammonium-N. Also, NIs retard nitrite accumulation in soils and thus reduce fixation of nitrites into organic matter.

III. NIs, NH,+/NO,- RATIOS, AND PLANT GROWTH

Because NIs maintain a higher concentration of NH,' in the soil/solution for a longer time by retarding nitrification, these chemicals have a role in determining amounts of NH,+ and NO,- and their ratios to crop plants during different stages of crop growth.

Ammonium can be more efficiently metabolized than NO, ~ -N because it does not need to be reduced when incorporated into amino acids or other organic materials. However, NH,', or rather NH;', is toxic to all plants at certain concentrations (Magalhaes and Wilcox, 1984) and this toxicity is related to the pH of the growing media (Pill and Lambeth, 1977). Magalhaes and Huber (1989) reported that NH,' toxicity was more severe at lower (3.5) than at higher pH (5.7). There is also a difference between crops with respect to tolerance to NH,'. Prasad et al.


M A I Z E I

1 2 3 1 2 3 b

WEEKS AFTER FERTILIZER APPLICATION

Figure 3. Ammonium-N and nitrate-N concentration in maize and rice soils. 0, without nitrapyrin; 0, with nitrapyrin; -, ammonium-N; -.-, nitrate-N. Adapted from Prasad ef al. (1983).

(1983) suggested the term "ammoniphilic plants" for species growing better with NH,'. They maintained high concentrations (40-60 mg kg ~ I NH,+-N soil) using NP (Fig. 3) and found that while maize plants suffered in growth, rice plants did not (Table 111). Rice absorbed more N with NH4+, while maize absorbed less N in the presence of higher concentrations of NH,'. They identified rice as an ammoniphilic plant. Other species of ammoniphilic plants are known (Gigon and

Table 111

Plant Height and Dry Matter Accumulation in Rice and Maize Plants Affected by N-Serve (NP) Treatment"

~~~ ~

Dry matter (g per plant) Plant height (cm)

Treatment Rice Maize Rice Maize

Without N-serve 17.1 32.9 0.24 1.45 With N-serve 18.2 18.6 0.26 0.90

LSD (P = 0.05) 0.66 3.47 0.023 0.13

"Adapted from Prasad eta/ . (1983).


Rorison, 1972; Ingestad, 1976). In view of the growing concern over nitrate pollution of groundwater, there is a need for research on high-yielding ammoniphilic cultivars of upland crop species.

Leaving aside the case of very high NH,+ concentrations resulting in toxicity, Olsen (1986) cited several studies where the addition of NH,' to an all NO,- system resulted in increased corn yields. Hageman ( 1 980, 1984) reviewed the effects of NH,' and NO,- nitrogen nutrition on plant growth and cited several experiments indicating that higher crop yields were obtained with a mixture of NO, - and NH,' than with either source alone. Ganmore-Neumann and Kafkafi (1983), working with nutrient solutions varying in NO,- and NH,' concentrations, obtained optimal growth for strawberries with an equal ratio of NO,- to NH,'. Bock (1987) observed a 19 to 47% increase in wheat yield with basal NO,--N + urea + nitrapyrin compared to NO,--N alone. In two greenhouse studies, Cam- berato and Bock (1989) reported a 15- 18% increase in the grain yield of sorghum when a higher NH,' concentration was obtained using urea and NP. Under field conditions Israeli et ul. (1985) obtained a maximum yield of bananas when equal ratios of NH,' to NO,- were present in the soil extracts. Bock (1986) found that nutrient solution culture studies differed from those obtained under field conditions. Also, crop variety and stage of growth should be taken into account for optimal utilization of the NH,+/NO,- ratio. Cosgrove et al. (1985), working with snapbeans, found that the NO,- to NH,' ratio is critical for maximum yields. Teyker and Hobbs (1992) reported that with coarse-textured soils and slightly alkaline pH, an enhanced NH,' regime may be advantageous for the growth of corn. They also observed that the differences in pH regimes between the hydroponic and soil-based experiments may account for the contrasting results. In a study at Illinois (Gentry and Below, 1992), a continuous supply of mixed NO,--N and NH,+-N increased corn yield by an average of 12% compared to NO,-N alone.

Shaviv er al. (1987) reported on the basis of pot culture experiments that wheat and millet (Setaria italica) exposed to NH,' only with DCD produced lower yields than those exposed to a mixture of NH,' and NO,-- with DCD. In wheat, NH,' to NO,- ratios of 50/50 and 75/25 seem to be optimal. A 25/75 NH,+/ NO,- ratio produced the highest yield at maturity. Calcium and Mg2' uptake by wheat and Mg2+ uptake by millet were reduced as the proportions of NH,+ in soil were increased. In the studies of Diest (1976) and Gashaw and Mugwira (1981). maximum growth of wheat was obtained with a solution culture of 50: 50 propor- tion of NH,+ and NO,-. Based on data from a field study using DCD, Joseph (1992) and Joseph and Prasad ( 1993a,b) reported that the optimum concentration of NH,+-N in soil for maximum grain yield of wheat gradually decreased with the age of the crop from 54.6 to 63.6 mg kg - I soil at 15 days after sowing (DAS) to 22.7 to 26 mg kg- - ' soil at 30 DAS. In the case of NO,--N, its optimum

2 46 RAJENDRA PRASAD AND J. I;. POWER

concentration for maximum grain yield increased with the age of the crop from 25.1 to 30 mg kg-I soil at 15 DAS to 31.6 to 34 mg kg-I soil at 45 DAS and decreased thereafter.

Tsai et a/. ( 1 978) found that a greater amount of sucrose in corn (as measured by I4C) was translocated from leaves to grain under NH,+-rich conditions, resulting in higher grain yield. Warren et al. (1975) found a reduction in “stalk rot” incidence and increased yield of corn when N was kept as NH,’ for a longer period with the help of NP.

As compared to NO3-, the assimilation of NH,’ in plants is not as well under- stood. According to Ivanko and Inguerson (1971) and Raven and Smith (1976), NH,’ is almost completely converted to organic N in roots prior to transloca- tion. Ammonium can be assimilated either through reductive amination of a- ketoglutarate with the glutamine dehydrogenase enzyme (GDH) system or by incorporation into glutamate with glutamine synthetase (GS) and subsequent transfer of the amide amino group of glutamine to a-ketoglutamate with glutamate synthetase (GOGAT) (Givan, 1979; Milfin and Lea, 1976; Srivastava and Singh, 1987). Although increased activity of these enzymes does not necessarily indicate their role in assimilation, increased GDH in the presence of NH,+ has been reported in roots of pea, pumpkin, soybean, sunflower, and corn (Weisman, 1972; Magalhaes and Huber, 1989). GS activity in roots and shoots of rice is reported to be higher than in the tissues of tomato and corn; in rice it increased sharply in the presence of NH,+ (Magalhaes and Huber, 1989).

The NH,+/NO,- ratios and plant growth studies lead to the following conclu- sions: (1) The growth of most upland crop plants is best when both NH,’ and NO3- forms of N are available for absorption; their relative amounts and ratios vary with species, cultivars and age of plant; and (2) NIs can help in maintaining NH,’ in soil in larger amounts and for longer periods of plant growth.

IV. NIs AND CROP YIELDS

Experiments with NIs have been conducted with a fairly large number of crops, including rice (Oryza sativa L.), corn (Zea mays L.), wheat (Triticurn aestivum L.), grain sorghum (Sorghum bicolor L. Moench), sweet corn (Zea mays L. Ri- gosa), sugarcane (Saccharum oficinarum L.), bell pepper (Capsicum annum L.), potato (Solanurn tuberosum L.), tomato (Lycopersicon esculentum Mill.), cotton (Gossypium hirsutum L.), barley (Hordeum vulgare L.), oat (Avena sativa L.), sugarbeet (Beta vulgaris L.), spinach (Spinacia oleracea L.), lettuce (Latuca saliva L. var. Capitata L.), radish (Raphanus sativus L. var. radicula Pers.), cucum- ber (Cucumis safivus L.), cabbage [Brassica oleracea convar. Capitata (L) Alef var. Alba DC], endive (Cichorium endivia L), turnip (Brassica rapa L.), and sev-


era1 grasses, including Lolium prenne L., Dactylis glomerata L., and Kentucky bluegrass (Poa pmtensis L.) (Slangen and Kerkhoff, 1984; Waddington et al., 1989). This chapter is restricted to major food and fiber crops of the world: rice, corn, wheat, grain sorghum, potato, sugarcane, and cotton.

A. RICE

The wet conditions that exist during rice production and the preference of rice for NH,+-N over NO,-N suggest that the application of NIs with NH4+- or NH,+-producing fertilizers, such as urea, would be a sound N management prac- tice. Prasad et al. (1986) suggested the use of NP for increasing N efficiency in rice. Field experiments were conducted with NP, AM, and ST (Lakhdive and Pra- sad, 1970; Reddy and Prasad, 1977) and these clearly showed that on rice soils with high percolation rates, nitrification inhibitors can be usefully employed for increased rice yields and N efficiency. Nitrification inhibitors were specifically effective in reducing N losses under alternate wetting and drying conditions frequently encountered in rice fields (Rajale and Prasad, 1972). Thomas and Prasad (1987) reported that for direct-seeded rice, NP-blended urea produced 4.7 mg grain ha ~ ' compared to 3.7 mg ha ~ I with urea. However, under similar conditions, DCD showed no advantage (Sudhakara and Prasad, 1986b).

Results from experiments conducted at different centers in Japan showed that ammonium sulfate treated with NP increased rice yield by 15-20% over untreated ammonium sulfate (Nishihara and Tsunyoshi, 1968). Similarly, in field tests carried out with AM in Japan, they showed that yields of transplanted as well as direct-seeded rice were increased by the use of 5-6 kg ha- ' AM along with ammoniacal fertilizers.

In the United States, Wells (1976) reported rice grain yield increases from the addition of 1.12 and 2.24 kg ha- ' of NP applied with 67 to 178 kg ha- ' of preplant-applied urea-N. In another study in Arkansas and Louisiana, no increase in yield due to NP was recorded in 1977, but in 1978 there was a positive grain yield response to NP (Touchton and Boswell, 1980). In Louisiana, Patrick et al. (1968) reported no advantage with NP for rice. Wells ef al. (1989) summarized results with DCD from Arkansas, California, Louisiana, Mississippi, and Texas. DCD delayed nitrification and tended to result in rice grain yield increases compared to urea-applied preplant without DCD in drill seeding. In water-seeded con- tinuously flooded rice, using DCD was advantageous only if the flood was delayed for more than 14 days after urea application. At the International Rice Research Institute, application of prilled urea with 10 or 15% DCD during the final harrow- ing produced lowland rice yields comparable to those with split applied prilled urea without DCD (De Datta, 1986).

Bains et al. (1971) reported the effectiveness of neem (A. indica Juss) seed


extract-treated urea for increasing rice yields and N efficiency. Reddy and Prasad (1975) showed that the coating of urea with neem cake controlled nitrification for a period of about 2 weeks and resulted in a significant increase in rice grain yield over prilled urea. Prasad and Prasad (1980) reported increased rice yields and N efficiency with neem cake-coated urea. These results have been confirmed by a large number of workers in India (Budhar et al., 1987, 1991; Govindaswamy and Kaliyappa, 1986; John et al., 1989; Joseph et al., 1990; Latha and Subramanian, 1986; Mishra et al., 1991; Prasad et ul., 1989; Singh et al., 1984, 1990a,b; Singh and Singh, 1991; Velu et al., 1987). NIs have therefore a definite place in rice culture, especially in conditions where N losses due to leaching and denitrification are high.

B. CORN

The results of field experiments with corn in the eastern corn belt of the United States (Nelson and Huber, 1980) illustrated that 70% of the trials in Indiana showed increased yields with NIs (NP and Terrazole); the average corn yield increase from NI was 24 and 5.2% for fall-applied anhydrous ammonia and urea liquid solution fertilizers, respectively. Yield increases were also obtained in Ken- tucky, Michigan, and southern Illinois but not in Wisconsin and northern Illinois. Hergert and Wiese (1980), summarizing the results of experiments with NP in the western corn belt of the United States, observed that the data obtained from Min- nesota, Kansas, and Nebraska indicated the largest impact of NIs on irrigated sandy soils, particularly where rainfallhrrigation provides excess water; the response of NIs on fine-textured soils was rather limited. From the results of later experiments with DCD and NP in the north central states, Malzer et al. (1989) also concluded that the greatest benefit for NI use was obtained on coarse-textured soils; their results are shown in Table IV. The data in Table IV also suggest that DCD was superior to NP when used with urea ammonium nitrate or urea. This was further confirmed in a later study conducted on installed lysimeters at the Herman Rosholt Bonanza Valley irrigation farm located in west central Minnesota (Walters and Malzer, 1990a). The soil on the experimental site was an Estherville sandy loam (Typic Hapludoll). The N1 treatment increased fertilizer use efficiency only at the 90 kg N ha ~ I rate when the leaching load was high. It was concluded that incorporation of NI with moderate N rates coupled with conservative irrigation management should reduce the risk of yield loss and minimize nitrate movement to groundwater.

Results from experiments with NIs in the southeastern United States (Touchton and Boswell, 1980; Frye et al., 1989) suggest limited benefits to corn from NIs due to relatively high soil temperatures, which permit nitrification of fall-applied ammonium-N during winter months, highly permeable coarse-textured soils, and nitrate leaching from excessive winter and spring rainfall. No yield advantage


Table IV

Relative Effectiveness of Dicyandiamide (DCD) and Nitrapyrin (NP) with Several Applications and N Sources on the Corn Yield from Coarse-Textured Soils in the Midwest"."

No. of positive No. of significant Average relative

comparisons responses' response (%) Application

Time" N sourced DCD NP DCD NP DCD NP

Fall Urea 6 - 4.9 - 3

Spr. PP Urea 20 20 9 10 27. I 16.1

Spr. PP UAN 6 6 4 2 28.9 11.4

-

Spr. PP AA 12 6 8 3 20.6 8.2

SD/split Urea 15 15 4 5 5. I 4.1

SD/split UAN 6 6 2 2 1.5 I .o

"Adapted from Malzer et al. (1989). "Data include all N rates at or below the optimum rate fertilization within each experiment (13

'Spr. PP. spring preplant: SD/split, side dress or split N application. "UAN, urea ammonium nitrate (28% N solution); AA, anhydrous ammonia. 'Significant at the 90% probability level.

experimental site years).

with DCD was obtained in 22 comparisons in the mid-Atlantic region of the United States (Fox and Bandel, 1989). In five comparisons there was a lowering of corn yield with DCD only. The reduced yield in three of these was attributed to increased NH, volatilization losses in the presence of DCD. Townsend and McRae (1980), from Nova Scotia, Canada, also observed that except on light sandy soils, no yield advantage was gained with NP.

Thus soil characteristics and the amount of precipitation received during the crop-growing season may affect the response of NI. For example, Kapusta and Varsa (1972), working on clay pan soils in Illinois known for losses due to denitrification, found a positive response in the first year which was characterized by good precipitation, but not in the next year, which was drier. Similar results were obtained in a 2-year study at New Delhi, India (Prasad and Turkhede, 1971). Benefits of the NIs in corn production are therefore limited to coarse-textured soils and in situations where excessive soil water leads to heavy N leaching.

C. GRA~NSORGHUM

NP or Dwell applied with urea, anhydrous ammonia, or urea ammonium nitrate solution did not increase yield nor improve efficiency of N applied to grain sorghum during a period of 4 years (1976- 1979), even with supplementary irrigation


to promote leaching and/or denitrification (Westerman et al., 198 1). Two tests on grain sorghum were conducted in the coastal plain of Alabama on Dothan and Norfolk sandy loam soils. In the first test (Touchton and Reeves, 1985), DCD increased grain yields when applied at the 90-kg N ha- ' rate in both 1982 and 1983. Yields with 90 kg N ha- ' and DCD were equal to yields with 134 kg N ha - without DCD. In both years, conditions were favorable for N losses via leaching and denitrification. In the second test (Frye et al., 1989), no increase in yield was obtained with DCD. Mascagni and Helms (1989) also failed to obtain an increase in the yield of grain sorghum with DCD or NP on a poorly drained Sharkey sandy clay (Vertic Haplaquepts) or on a well-drained Herbert sandy loam (Mesic Ochraqualfs) soil of Arkansas. Success with NIs on sorghum has been limited.

D. WHEAT

Extensive studies in the United States with NP and DCD showed increased yields of winter wheat due to NP in the Pacific Northwest (Washington, Idaho) (Harrison et al., 1977; Papendick and Engibous, 1980). Greater yields of wheat were obtafned with DCD in three of eight experiments in the mid-Atlantic region (Maryland and Pennsylvania) (Fox and Bandel, 1989) and in one of four experiments in the north central states (Illinois and Indiana) (Harms, 1987). In the eastern part of the Midwest (Illinois, Kentucky, Michigan, Ohio, and Wisconsin) a yield increase was on the order of 9.9 to 24% (Nelson and Huber, 1980; Shyilon et al., 1984). Nitrification inhibitors were more effective in the southern part of the Midwest due to higher rainfall and the associated nitrate leaching. In the western part of the Midwest (Nebraska, Kansas, Colorado, Minnesota), NIs were not effective in increasing wheat yields due to the virtual absence of leaching of N below the root zone (Hergert and Wiese, 1980). Little advantage with NP (Nelson et al., 1977; Boswell et al., 1976) or with DCD (Frye et al., 1989) was obtained in the southeastern states of the United States (Alabama, Virginia, Georgia, and Tennessee).

Increased wheat yields with NP were obtained in Alberta, Canada (Mahli and Nyborg, 1978). Sommer and Rossig (1978) from Germany reported that injection of NH,+-N and NP gave similar yields as obtained with a split application of N. Lewis and Stefanson (1975) obtained no yield advantage with NP under field conditions in Australia. In a field experiment on a sandy loam soil at New Delhi (Singh and Prasad, 1992). wheat yield with 80 kg N ha- ' + DCD was greater than that obtained with 120 kg N ha - I without DCD (Fig. 4). Application of DCD beyond 15% of N as DCD reduced wheat yield.

An increase of 4- 12% in grain yield of wheat due to neem cake-coated urea compared to urea was obtained in India at Kanpur (Agarwal et al., 1980), Hissar


'I 5

0-0 80-0 80-5 80-10 80-15 80-20 80-25 120-0

K g N ha1-*/. D C D - N

Figure 4. Effect of DCD on wheat grain yield. Total N applied (fertilizer and DCD). Adapted from Singh and Prasad (1992).

(Bhatia et al., 1985), and Pusa (Prasad et al., 1986; Mishra et al., 1991). Success with NIs in wheat in the United States has been mixed. Nitrification inhibitors are effective in increasing wheat yields in the Pacific Northwest and the southern Mid- west but not in the southeastern states and the western Midwest. The data from other parts of the world are too limited.

E. SUGARCANE

In a 2-year study at New Delhi, India, Parashar ef al. (1980) found a significant increase in cane yield with neem cake-coated or mixed urea at 75 kg N ha - I and with NP applied with 150 kg N ha ~ I . Furthermore, there was a significant residual effect on a ratoon crop and 75 kg N ha ~ I as NP-treated urea or neem cake-coated urea produced almost the same yield as 150 kg N ha - applied as prilled urea (Sharma et al., 1981). Singh er al. (1987) also found an increased cane yield with neem cake-coated urea. Nitrification inhibitors could have a place in sugarcane culture, but more field data are needed before a definite conclusion can be drawn.

F. POTATO

On sandy loam soils of Michigan, no yield advantage was obtained with NP, while the yield and number of marketable tubers increased with NP on Idaho soils (Potter etal., 1971). Broadcast application of N, as urea, with spraying of inhibitor

2 5 2 RAJENDRA PRASAD AND J. F. POWER

(NP on terazole) followed by thoroughly mixing the compounds with the soil gave some potato yield increases on Washington soils, whereas no effect was found with band (row) application (Roberts, 1979). Hendrickson et al. (1978) found a yield reduction and a decreased quality of tubers with up to 4.4 kg ha-l NP applied with ammonium sulfate and diammonium phosphate.

On a Plummer fine sand (Grossarenic Paleudults) at Hastings, Florida, application of 5-6 kg ha ~ I DCD significantly increased the tuber yield in 1983 but not in 1984 (Frye et al., 1989). Also, no increase in tuber yield was recorded due to DCD at Gainsville, Florida.

On alluvial soils in Ludhiana (India), ammonium sulfate and calcium ammonium nitrate are superior to urea in the absence of NP, but urea treated with NP is comparable to ammonium sulfate and is better than calcium ammonium nitrate (Sahota and Singh, 1984). Treatment with NP increased N uptake and N recovery by potato and decreased the optimum dose by 1 1-40 kg N ha - I. Increased potato yields with neem cake-coated urea were found at Simla (Sharma etal., 1980) and Palampur (Sharma et al., 1986).

G. COTTON

Reeves and Touchton (1989) in pot culture studies found that cotton was sensitive to DCD. Although significant reductions in plant growth did not occur un- less DCD exceeded rates normally applied, their results suggest a need for caution when applying DCD to cotton. On a Norfolk sandy loam (Rhodic Paleudult) in Alabama there was a tendency for yields to decline with DCD, while on a Decatur silt loam in the same state and on a BeulahBosket very fine sandy loam (Typic Dystrochrepthlollic Hapludalf) in Mississippi there was no significant increase in cotton yield (Frye et al., 1989). However, in India, an increased yield of cotton due to neem cake coating of urea was reported by several workers (Seshadri and Prasad, 1979; Jain et al., 1982). Cotton seems to be sensitive to DCD and therefore this NI should not be used for cotton.

V. PHYTOTOXtCITY OF NIs

Some of the results obtained in field experiments could be due to phytotoxicity of NIs, although obvious symptoms may not have appeared under field conditions at dosages used. Joseph (1992) reported that wheat benefited when DCD was applied at a 10% N level, while yield was reduced when the level of DCD-N was raised to 20%. Reeves and Touchton (1989) applied DCD at 0, 2.5,5, 10, 15, and 20 mg DCD N kg ~I soil along with urea or sodium nitrate at 50 mg N kg ~ I soil


in a pot culture study with Norfolk sandy loam (Typic Paleudult). Six days after application of DCD at 15 or 20 mg kg ’ soil, cotton leaves developed mottled chlorosis. After 20 days, mottled chlorosis developed on leaves of all plants treated with DCD. The chlorosis intensified with DCD rates and progressed to necrosis with DCD-N rates of 20 mg kg ~ I soil. Symptoms were similar for cotton treated with both N sources. Reductions in leaf dry weight and foliar toxicity symptoms suggested that the primary site of phytotoxicity of DCD was in leaf tissue and not in root tissue. DCD linearly increased the leaf tissue concentrations of N, P, and K and lowered concentrations of Ca” and Mg”. Lack of DCD x N source interaction suggested that reduced Ca’ ’- and Mg + uptake resulted from direct effects of DCD and not from indirect effects caused by the inhibition of nitrification and an increased NH,+ uptake. It was suggested that when banded N applications are made or root growth is restricted due to compaction, phytotoxicity from DCD-N concentrations at 5 mg kg ~ I in the root zone of cotton might diminish any potential benefits derived from increased N efficiency gained through the inhibition of nitrification.

In a greenhouse study with Cherry Belle radish, Feng and Barker (1989) found that as the concentration of NP or Captan in the medium with NH,+-N increased, growth of roots and shoots in radish was restricted and leaves were stunted, show- ing interveinal chlorosis, marginal necrosis, and upward cupping. The roots were stunted and twisted and failed to expand properly. Ca” and Mg2‘ contents in shoots, 4 weeks after seeding, were considerably lowered when NP was applied with ammonium sulfate or urea; on the other hand, K + contents were increased. Many reports (Kirkby, 1968; Wilcox ef a/., 1973) using various plants have shown that acidity of the medium and deficiencies of K + , Ca”, and Mg” are major reasons for toxic effects of NH,’. However, Goyal et a/. ( 1982) observed that even though the pH of the nutrient solution was regulated at or near neutrality, toxicity persisted in radish plants; large amounts of K + and Ca2+ in the solution did not correct the toxicity.

Plants grown with ammonium fertilizer and NI usually contain lower concentrations of Ca” and Mg2+ (English et al., 1980; Mathers et al., 1982). This tendency is attributed to competitive absorption between NH,’ and other cations. English et al. (1980) suggested that chemical inhibition affects the permeability of plant cell membranes by altering their integrity or activity. Ca2’ and Mg” concentrations are correlated negatively to the residual NH,+ in the medium but are correlated positively to residual nitrate. Plant weight is also negatively correlated with the residual NH,’.

Yield reductions and phytotoxicity from use of DCD have been reported by a number of researchers (Cowie, 191 8; Maftoun and Sheibany, 1979). Symptoms of DCD phytotoxicity developed in the greenhouse within 3 to 20 days after application of DCD, depending on the crop and DCD rate (Reeves and Touchton, 1986). Symptoms expressed on corn and sorghum were chlorosis and necrosis that


began at the leaf tips and progressed down the leaf margin. Symptoms on other crops were mottled interveinal chlorosis and leaf margin chlorosis and necrosis. Based on visual symptoms, sorghum and cotton are more sensitive to DCD than corn (Reeves and Touchton, 1986).

Concentrations as low as 2.5 mg DCD-N kg - I increased the stomatal conductance of water in cotton plants grown in the greenhouse (Reeves et al., 1988). This effect was noted under conditions of high transpirational demand in the afternoon. Concentrations of 5 - 10 mg DCD-N kg I increased responsiveness of stomata to decreasing soil water content over the entire range of available soil water. The effect of DCD on stomatal conductance was believed to be a direct effect of the compound and not directly due to soil water availability. When soil water is limited, DCD might increase water stress and decrease yield (Frye et al., 1989). Use of NI had a deleterious effect on the tuber grade in potato (Hendrickson er al., 1978). Although total tuber yield increased, the percentage of grade A-USDA tubers was reduced 2.4% with NP and 5. I % with DCD (Malzer et al., 1989).

The studies referred to earlier indicate the following: ( I ) Some field crops such as cotton are sensitive to some NIs; and (2) phytotoxicity symptoms observed could be due to direct or indirect effects of NIs; the indirect effects being the result of higher than normal NH,+-N concentrations.

VI. HEALTH AND NITRATES

NIs may possibly play a role in human and animal health by reducing the NO, content in drinking water, food, feed, and forage.

A. NITRATES AND HUMAN HEALTH

The well-known problem associated with NO3- /NO,- toxicity in humans is methemoglobinemia or “blue-baby ” syndrome. It generally occurs when infants under the age of 4 months consume too much nitrate (Rosenfield and Huston, 1950). Microbes in the stomach reduce nitrate to nitrite. When nitrites reach the bloodstream, they convert ferrous ions in the hemoglobin to the ferric form and produce methemoglobin (MHb), which has no oxygen-carrying capacity. Very young children are susceptible because their hemoglobin has a greater affinity for nitrite than hemoglobin of older children and adults. Methemoglobinemia resulting from high nitrate concentrations in drinking water was first recognized by Comly ( I 945) at the University of Iowa. Associated symptoms are diarrhea and vomiting, and the child’s complexion becomes slate blue (Ewing and Mayon- White, 1951). In addition to drinking water, the incidence of methemoglobi-


nemia has occurred in young children fed unrefrigerated spinach or high nitrate- containing fruitjuices (WHO, 1978; Keating etal., 1973). In a survey of Nebraska physicians, doctors reported 15 infants with suspected nitrate-induced methemoglobinemia (Grant, 1981). In addition to water and vegetable products, infant methemoglobinemia can occur when infant foods are prepared with nitrate- contaminated water (Johnson et al., 1987). This may also happen in older individuals who have genetically impaired enzyme systems for the reduction of methemoglobin. The largest outbreak was reported in Hungary (Deak, 1985) where 1353 cases occurred between 1976 and 1982.

Nitrite produced from NO,- could react in the stomach with secondary amines resulting from the breakdown of meat and fish forming N-nitroso compounds, which can cause stomach cancer (Fritsch and de Saint Blanquat, 1985; Saul et al., 1981). However, it should be mentioned that nitrites which are a potential health hazard are widely used as a preservative in salted meat and sausages (Davis, 1990) where they prevent the growth of Clostridium botulinum, the organism that causes botulism (WHO, 1978). Thus the risk of stomach cancer may not be closely linked with the nitrate content in drinking water. In addition to methemoglobinemia and stomach cancer, other health disorders reported due to the large ingestion of NO, - in drinking water are hypertension (Malberg et al., 1978), increased infant mor- tality (Super et al., 1981), central nervous system birth defects (Dorsch et al., 1984), and non-Hodgkins lymphoma (Weisenburger, 199 1); nevertheless, none of these have been conclusively proved to be due to NO,- ingestion (Spalding and Exner, 1993). Normally, in humans only about 20% of their NO,- intake comes from liquids and drinking water (Table V) (Isermann, 1983). In addition, overfer- tilization, heavy manuring, or irrigation with high NO3- water can also result in

Table V

Nitrate Uptake through Food and Drinks"

Product mg nitrate Percentage of

person - day ~ total daily intake

Milk and dairy products

Meat and meat products

Cereals

Oils and fats

Sugar

Fruits

Vegetable\ (155 g day - I )

Drinks and water (2.75 liter day ~ ' )

0.23

5.7

1 .s - -

0.9

63.5

19.0

0.2

6.2

1.6

-

I .o 70.0

2 I .o

"Adapted from Iserrnann (1983).


large NO,- accumulations in many vegetables, which increases the human nitrate load.

B. NITRATES AND ANIMAL HEALTH

Nitrate or nitrite poisoning is also reported in animals and is again due to MHb formation in blood with consequent asphyxiation. The conversion of nitrate to nitrite is carried out by bacteria in the rumen and ruminants are therefore especially vulnerable to nitrate poisoning. Goats, especially Angora, may be more susceptible to NO,- poisoning than either sheep or cattle (Schneider et ul., 1990). Mature single-stomach animals (except horses) are more resistant to nitrate toxi- cosis. Other than lack of oxygen, dilation of blood vessels is another secondary effect of nitrate poisoning. Abdominal pain and diarrhea are also reported. Other effects of nitratehitrite poisoning in animals include poor growth rates, reduced milk production, increased susceptibility to infections, and even abortions late in pregnancy (Schneider et al., 1990).

Nitratehitrite poisoning symptoms appear when MHb concentrations reach 20-30% of total hemoglobin, and death due to asphyxia may occur when the MHb level exceeds 75% of total hemoglobin. Blood containing MHb usually has a chocolate brown color.

Feed/forage with nitrate concentrations exceeding 2.25 g kg - I NO, - -N (1 0 g kg - NO,-) have a high risk of causing acute nitrate poisoning in ruminants; about half of this concentration should not be exceeded in the diets of pregnant beef cows. Drinking water for young livestock should contain less than 35 mg liter - I NO,--N. Nitratehitrite poisoning in adult animals is likely when the N03- -N concentration in water is more than 100 mg liter-' (Schneider et ul., 1990).

c. NITRATE CONTENT OF DRINKING WATER

There is growing concern regarding NO,- content in drinking water and the World Health Organization (WHO) has set a maximum limit of 100 mg NO,- liter-' (22.6 mg NO,--N liter-!) and a recommended limit of 50 mg N03- -N liter - I ( 1 1.3 mg N liter - I); the latter limit is also fixed by the Council of European Communities (1980).

Groundwater is the source of domestic water for almost 90% of the rural population of the United States and for about 50% of the total population (Power and Schepers, 1989). In Denmark, West Germany, The Netherlands, and Great Britain the use of groundwater accounts for 99,73,70, and 30%, respectively, of the total water consumption (Strebel et ul., 1989). Groundwater forms a substantial part


of the drinking water in other parts of the world also. In addition, groundwater contributes substantially toward irrigation; estimates for the United States are 75-80% of the total water used for irrigation (Power and Schepers, 1989). Main- tenance of groundwater quality is thus of major concern.

Nitrates in groundwater can originate from geological sources, precipitation, cultivation, animal waste, niineralization of organic N, and fertilization. Data from the U.S. Geological Survey and the Texas Department of Natural Resources over a period of 25 years showed that states where 9% or more of groundwater samples contained 10 mg N03--N liter-' (45 mg NO,- liter ~ I ) or more were Arizona, California, Delaware, Kansas, Minnesota, Nebraska, New York, Oklahoma, Rhode Island, and Texas (Madison and Brunett, 1985). After a careful examination of the U.S. Environmental Protection Agency's National Pesticide Survey (NPS), the Monsanto Company's National Alachlor Well Water Survey, and state-wide surveys in Iowa, Kansas, Nebraska, North Carolina, Ohio, Texas, Arkansas, Califor- nia, Delaware, Pennsylvania, Washington, Minnesota, and South Dakota, Spald- ing and Exner (1993) concluded that the highest incidence of contamination occurs in groundwater in the middle of the contiguous United States where NO, - -N levels in ~ 2 0 % or more of sampled wells in Iowa, Nebraska, and Kan- sas exceeded 10 mg liter ~ I; in contrast, the contamination was lower in Texas, North Carolina, and Ohio (Fig. 5). Power and Schepers (1989) observed that use of high rates of fertilizer N may be a major source of nitrates in wells in the potato- producing area of northern Maine. The high density of septic tanks, along with application of fertilizers and manures on agricultural lands, probably contributed to high NO3- on Long Island. Intensive dairy operations with associated problems of manure disposal may be a primary source of nitrates in wells in southeast Penn-

I Statstrca~~y ; Statewide i WlnerabC : other Randomized : Surveys : Surveys

Figure 5. Incidence of' NO,--N contamination in large selected surveys (number of counties surveyed is in parentheses). IA, Iowa; KS, Kansas: NE. Nebraska: NC, North Carolina: OH, Ohio; TX, Texas: AR, Arkansas: CA, California; DE. Delaware; PA. Pennsylvania; WA, Washington; MN. Minnesota; SD, South Dakota. From Spalding and Exnrr (1993).


Table VI

Correlations between Groundwater Nitrate-N Concentrations and Site Characteristics in

Nebraska"

Correlation of groundwater NO,-N concentration with r value

Irrigation well density Total fertilizer used N fertilizer use Irrigation well depth Water pH Livestock density Percentage land cultivated Human population Percentage land with legume

0.425* 0.283* 0.202*

- 0.275* - 0.233*

0.184* - 0.068 - 0.064 - 0.042

"Adapted from Muir er al. (1973). *Significant correlations.

sylvania and northern Maryland. High NO,- concentration in Delaware and parts of North Carolina may arise from intensive poultry operations plus septic tanks. A long extended belt of high NO,- wells extends from central Minnesota and Wisconsin to west Texas. Much of this area is irrigated, often for potato, corn, and sugarbeet production. Extensive irrigated areas in Colorado, Arizona, California, and Washington also have NO,- problems in the groundwater. The NO,- found in the waters of Yellowstone Park in northwest Wyoming is probably of geological origin.

From a study done in Nebraska, Muir et al. (1973) found that groundwater NO,- -N concentrations were positively correlated to total fertilizer used and irrigation well and livestock densities (Table VI). Kilmer et al. (1974) reported in North Carolina that NO, --N in groundwater under steeply sloping, moderately grazed grassed watersheds exceeded 10 mg liter - I when 1 12 kg N ha - I was applied each year. In the small pastured watersheds on well-drained residual silt loams (Typic Dystrochepts and Hapludults) in eastern Ohio, subsurface NO, - -N ranged from 3 to 5 mg liter - I with applications of 56 kg N ha ~ I year and 8.18 mg liter - I year - I with applications of 224 kg N ha - ' year - I (Owens et al., 1983). On some pastures, application of 56 kg N ha - I year - I in the first 5 years and 168 kg N ha - I year - I in the next 10 years resulted in NO,--N concentrations of 10 to 16 mg liter - I (Fig. 6) in the 9th and 10th years of high N application (Owens et al., 1992).

In a long-term regional study, seven creeks draining agricultural watersheds and representing agriculturally important physiographic regions of Kentucky were sampled in 1971 - 1972 (Thomas and Crutchfield, 1973). These creeks were re-

NITRIFICATION INHIBITORS

1 8 '

1L -

m l o - E z -

1 0 0 z 6 -

2 -

0 I " ' " ' 1 ' ' 1 ' ' 1 '

YEAR

1974 76 78 80 82 8 L 86 88 19

YEAR

259

Figure 6. Average flow-weighted seasonal (growing and dormant) concentration of NO,--N for subsurface flow throughout the 5-year prestudy periods (1974- 1979) during which 56 kg N ha - ' was annually applied and the 10-year study period (1979- 1989) during which 168 kg N ha - I was annually applied as ammonium nitrate. Adapted from Owens rt d. ( 1992).

sampled in 1989- 1990 (Thomas er al., 1991) and analyzed for nitrate concentration. The data obtained (Table VII) showed no increase in nitrate concentration in creek waters despite the fact that fertilizer N consumption in Kentucky nearly doubled during that period.

Based on analysis of water samples over a 20-year period in North Carolina, Gilliam (1991) observed that drainage conditions prevailing in soil profiles affected nitrate concentrations in soil water. In the lower coastal plain region, soils

Table VII

Calculated Total Flux of NO, --N ( f SD) in Seven Kentucky Streams"

Creek I972 1979

Cave Fiat McG i I I s Perry ( A ) Perry (B) Plum Rose West Bays

kg h a - '

15.29 t 2.33 1.93 t 1.93 0.67 t 0.67 2.07 ?z 1.29 2.33 t 1.12 3.1 1 t 3.00 3.74 t 2.63 - 12.63 t 9.42

4.36 t 3.22 2.54 t 1.94 9.70 t 7.08 4.25 -+ 3.1 1 3.08 t 2.16 3.94 f 0.58

17.74 2 2.97

"Adapted from Thomas cf ul. ( I99 I ).


are poorly drained and have high organic matter content and high water tables. In these soils there is sufficient organic matter to provide an energy source for microorganisms so that denitrification occurs and reduces nitrate concentration (Gam- brell et al., 1975). Trudell er al. (1986) and Gillham (1991 ) have confirmed anoxic conditions in shallow groundwater, which promotes denitrification. Increasing N03--N concentrations in drinking water in Europe has been a matter of great concern and by 1995, 20% of the French population will be drinking water exceeding the European Community's (EC) limit of 11.3 mg N liter - [ (Fried, 1991). Similarly, 8% of the public waterworks in Denmark and 5% of those in the former Federal Republic of Germany have groundwater that exceeds EC limits for N03--N (Fried, 1991). Handa (1987), in India, reported that numerous wells, especially those in the drier region, contain water with high nitrate contents. For example, in the state of Haryana, a well water contained as high as 296 mg NO,--N liter ~ I . Handa (1987) supports the hypothesis that the nitrate content in groundwater originates from anthropogenic activities. The effects of land use and N fertilization on NO,- concentrations in groundwater based on the experience in Western Europe are summarized in Table VIII. Arable lands, which are sub- jected to heavily fertilized vegetable cropping, had the highest NO,- concentration in the groundwater. Similarly, intensively grazed grassland with heavy fertil-

Table VIII

Measured Site and Land Use-Specific Nitrate N Input into Groundwater (Mean Concentration of the Annual Groundwater Recharge)"

Mean nitrate Land use concentration

Soil (crop rotation, N fertilizer) (mg NO,--Nliter-')

Sand Arable land (cereal-sugarbeet/potatoes-

Arable land (cereal-winter catch crops- cereal, = 120 kg N ha-' year-') 25-30

sugarbeet/potatoes-cereal, = 120 kg N h a - ' ) 14- I6

Grassland (meadow, - 250 kg N ha - year - I ) 3-7 Grassland (intensively grazed pasture, 250 kg

N ha- ' year-I; = 2 livestock units ha-I, = 180 grazing days) 14-20

Field cropping of vegetables, including special crops such as asparagus, tobacco (= 300-600kgNha- 'year- ' ) 34-70

Woodland (coniferous tree stands) 2.5 Woodland (alder tree stands) 10

Loess Arable land (cereal-sugarbeet-cereal = I50 kgN ha- ' year-') 7- I4

'Adapted from Strebel et al. (1989).


ization gave high concentrations of nitrates in groundwater. In England (Haigh and White, 1986; Roberts, 1987), application of 100- 1 1 1 kg N ha - I frequently resulted in groundwater concentrations above the EC limit. With intensively man- aged grassland systems on sandy soils in the Netherlands, a strong correlation was observed between the level of N fertilization and NO,--N leaching losses to groundwater (Steenvoorden er al., 1986). In Sweden (Bergstrom, 1987), NO,- fluxes from grass and lucerne lays were mostly below 5 kg NO, - -N ha - I year - I, while that from barley receiving 120 kg N ha - I as Ca(N0,)2 was 36 kg NO,- -N ha ~ I year - I.

D. NIs AND NITRATE CONTENT IN VEGETABLES

About 70 to 81% of the NO,- intake in the human diet is from vegetables (White, 1975). Nitrate concentrations in vegetables may be extremely high when vegetables are grown with high levels of N and under reduced light or moisture conditions (Brown and Smith, 1966; Jackson et al., 1967). Furthermore, effects of higher N rates on NO,- concentration are more likely on fast maturing vegetables such as radish, spinach, and lettuce (Huber et al., 1977). The effects of NIs and N fertilization on NO,- concentrations in vegetables were reviewed by Slangen and Kerkhoff (1984). They concluded that a relatively high amount of NH,+-N in the growth medium achieved by the use of NIs does not assure increased yield in vegetable crops as seen in cereals, nor did it always reduce nitrate concentration in vegetables. Sommer and Mertz ( 1 974), in Germany, reported that an application of NP with ammonium sulfate reduced NO,- concentrations in several vegetable crops as compared to Ca(N0, ~ ) 2 (Table IX). Moore (1973) reported a reduction of 34 and 79% in nitrate concentration in lettuce and spinach, respectively, following application of NP. With 20 ml m - I of NP, a 40% reduction

Table Ix

Nitrate-N Content of Some Vegetable Crops Affected by NP"

NO,-N (5% of DM) Total N (% of DM)

Crop Without NI With NP Without N1 With N P

Chinese cabbage I .oo 0.13 4.83 5.5 1 Mustard 0.13 0. 14 2.62 3.5 1 Black radish 0.39 0.11 3.50 3.99 Savoy cahbage 0. I9 0.12 2.91 3.96

Carrots 0.39 0.16 3.33 4.18 Lettuce 0.4 I 0.18 3.53 5.10

Spinach 0.46 0.28 4.69 5.80

"Adapted from Soinrner and Mertz ( 1974).

262 RAJENDRA PRASAD AND J. F. POW,R

Table X

Nitrate-N Accumulation (mg kg - ' dry wt) in Radish Shoots Affected by N Sources and NP"

Ammonium NI sulfate Urea

Without NP 4000a" 8000a With NP I ooc 200b

"Adapted from Feng and Barker (1989). "Means followed by the same letter do not differ sig-

nificantly (P = 0.05).

in nitrate concentration in Chinese cabbage [ Brassica pekinensis (Lour.) Repr.] was reported by Roorda van Eysinga and Van der Meijs (1980). Feng and Barker (1989) reported that the NO,- content in radish shoots 4 weeks after seeding was 1.3 I , 0.4, and 0.8% with potassium nitrate, ammonium sulfate, and urea, respectively; the values with NP (averaged over 20, 40, and 60 mg kg - I ) were I . 19, 0.01, and 0.02% (Table X). Thus, reduction i n nitrate concentration in vegetables can be achieved with the help of NIs. This is one way NIs can help in preventing health hazards.

VII. NIs AND ENVIRONMENT

Nitrification inhibitors can possibly play some, if not a great, role in environmental conservation. The two areas where NIs can contribute are: (1) reducing NO,- content in groundwater, and (2) reducing the evolution of N,O. Nitrification inhibitors may also help in reducing global warming but the data on this are lacking.

A. NIs AND NITRATE CONTENT IN GROUNDWATER

Studies on the effect of NIs on NO,- leaching are rather limited. In soil column studies, Rudert and Locascio (1979) found that NP reduced nitrate leaching losses from a Kanapha fine sand during the first 2 weeks of a 5-week leaching period. Owens (198 1 ) found that 42 and 53% of the applied urea N (672 kg N ha - I ) had leached from NP-treated and untreated sandy loam columns, respectively, after 144 days. When leaching conditions occurred on sandy Coastal Plain soils, the addition of NP to urea significantly reduced N losses (Chancy and Kamprath,


1982). From pot culture studies simulating lowland rice conditions, Prakasa Rao and Prasad (1980) reported a leaching loss of 1 1.5% applied N with prilled urea and 9.2% when prilled urea was blended with NP.

Based on data with nonweighing field lysimeters, Timmons (1984) reported that NP reduced leaching losses from sandy loam soil columns and reduced annual NO, ~ leaching losses from urea by about 7% during a 3-year period. In long-term studies on a Rayne silt loam (mesic Typic Hapludult) using monolith lysimeters at Coshcoton, Ohio, Owens (1987) found that an average N loss from the central lysimeter (no NP) during the 6-year study was 160 kg ha- ' or 48% of the N applied. Lysimeters treated with NP had an average N loss of 1 17 kg ha - I or 35% of the N applied. Based on the studies made with ISN-enriched urea on field lysimeters on an Estherville sandy loam (mesic Typic Hapludoll) growing corn, Walters and Malzer ( 1990b) reported that the leaching losses of fertilizer-derived N were delayed 25 to 50 days when urea + NP were incorporated; the total N loss was, however, not affected.

In addition to lysimeter percolates, Owens (1 987) also collected water samples from a spring receiving groundwater from a watershed cropped similarly to the lysimeters that received 168 kg N ha ~ I and 1.12 kg ha - ' NP. The nitrate concentration in the groundwater was reduced when NP was used (Fig. 7).

Cattle manure can substantially contribute toward the occurrence of NO,- in groundwater. NIs have been usefully employed in reduced nitrate leaching from cattle manure. From a pot culture study, Amberger and Vilsmeier (1979) showed that 15.30 kg ha - I of DCD applied with 150 m 3 cattle manure h a - ' resulted in

Conventioni I Meadow No-Ill1 Corn Wheat - We Corn I I (received ureo w i t h nitropyrtn) ,

I I I I I I

I I

I

01. . I " ' * ' . * " . ' . * . . ' " " ' . . ' I .... ' * * ' . ' ' w s s u A w s s u A w s S U A W S S U A W S S U A W S S U A W S S U A w s s u A w S S u A wssuAWSsuAWSSU A W

1973 7L 7 5 7 6 7 7 70 7 9 8 0 81 82 83 1981. Y E A R

Figure 7. Seasonal NO,--N concentration in groundwater from an adjacent watershed (1973- 1984) cropped with no-till corn fertilized with 168 kg N ha- ' of nitrapyrin-treated urea (1977- 1982). w, winter; s, spring; su, summer; and a, autumn. Adapted from Owens (1987).


inhibition of nitrification over 60 days at 8"C, over 40 days at 14°C and over 20-40 days at 20°C. Gorlitz and Hecht (1980) found effective inhibition of nitrification at 20" C with 2% NP or DCD (2% of total N of the manure). For DCD the effect lasted up to 3 weeks and for NP up to 9 weeks. In field experiments, NP at 1-2% (of the total N content of the slurry) proved to be effective until March after applying cattle manure in September of the previous year. DCD was somewhat less effective. Cooper (1980) found that NP and ATC were effective inhibitors of nitrification with pig slurry, while NaNO, was ineffective.

B. OZONE LAYER DEPLETION

In recent years concern about air pollution has extended from the obvious effects at ground level to the depletion of the ozone layer in the stratosphere (14-32 km above the ground surface), resulting in increased penetration of ultraviolet light with a wavelength between 209 and 330 mm (UV-B). The ozone layer in the stratosphere works as a shield against ultraviolet radiation, which with pro- longed exposure is associated with skin cancer in humans, particularly in fair skinned persons (NAS, 1975). Estimates (Shea, 1988) suggest a 4.6% increase in cases of skin cancer with each 1 % drop in ozone. Crutzen and Enhalt ( I 977) suggest that a doubling of the atmospheric N 2 0 could cause a decrease in the ozone layer which would increase the ultraviolet radiation reaching the earth surface by 20%.

The ozone hole over Antarctica at a height of 14-22 km, first identified in 1985 (Farman et al., 1985; Hofman et al., 1986), develops each southern spring and has become increasingly worse (Thompson, 1991) than when it was first detected.

Recent measurements of ozone by the TOMS (Total Ozone Mapping Spec- trometer) on the satellite NIMBUS-7 show much smaller ozone losses over the North Pole than over the South Pole (perhaps 10 to 60%). but the losses at middle latitudes in each hemisphere are comparable (Pyle, 1991). One consequence of the ozone hole is ozone loss beyond Antarctica during the austral spring and summer when air masses with chemically induced stratospheric ozone loss penetrate toward mid latitudes (Thompson, 1991). Stolarski et al. (1991) have shown significant total ozone loss in both northern and southern hemispheres to within 35" C of the equator. At 40" N, the stratospheric ozone has dropped by about 8% in the past decade during the late winter and early spring (Pyle, 1991).

During 1972- 1975, the Climatic Impact Assessment Program (CIAP) admin- istered by the U.S. Department of Transportation carried out extensive measurements of stratospheric nitrogen oxides and other species and concluded that an increase in stratospheric NO, would decrease the stratospheric ozone layer; the relationship suggested is shown in Fig. 8 (Grobecker et al., 1975). This proposed relationship indicates that doubling the NO, concentration would reduce ozone by

NITRIFICATION INHIBITORS

z z 3 810.0

R W z

0 6 W m Q w 5 ’ 0 -

W 0 c

265

-

/

0

o/ /

I 1 J 10 0 100 0 1000 0

‘1. INCREASE IN NOx COLUMN

Figure 8. The percentage decrease of ozone as a function of the percentage increase in stratosphere nitrogen oxides, as determined by the CIAP study. The line represents the equation A(O,)/ (0,) = (1/5)A(NO~)/(NO~). The asterisk represents the ozone depletion following an explicitly as- sumed doubling ofNzO. All calculations are based on an injection of NO, at a 20-km altitude. Adapted from Crobecker CI a/. ( 1975).

about 20%. The NO,-ozone decomposition catalytic cycle is shown in Fig. 9, and a report by a National Academy of Sciences Panel (1 978) suggested that the NO, catalytic cycle could be responsible for up to 50-70% of the total ozone destina- tion rate (Table XI).

N 2 0 in soils is mostly produced during biological denitrification (Payne, 1981; Reddy and Patrick 1986), but could also be produced during nitrification (Brem- ner and Blackmer, 1978; Freney et al., 1978, 1979; Aulakh et al., 1984). Some N,O is also produced by chemodenitrification (Chalk and Smith, 1983).

A global inventory of N in the biosphere shows that it is distributed in terrestrial, oceanic, and atmospheric components in the ratio 1 : 70 : 1 1,8 I8 (Winter- ingham, 1980). A number of estimates of emission of N,O and N, are available. Winteringham (1 980) estimated terrestrial and oceanic denitrification as N,O at 16-69 and 20-80 Tg (million metric tons) y e a r 1 , respectively; the values for terrestrial and oceanic denitrification as N, were estimated at 91 -92 and 5-99 Tg year ~ I. Tiedje ( 1988) estimated N loss due to denitrification at 105- 185 Tg year I for land and 25-250 Tg year - I for the sea. Bouwman (1990a) estimated global N,O emission at 9.7 to 12 Tg N year - I for natural ecosystems and 2.3 to 3.7 Tg N year I for cultivated lands; he also expressed the opinion that there is an annual increase of 0.25% in the concentration of N,O (Bouwman, 1990b). Seiler and Conrad (1987) estimated global N 2 0 emission from fertilized soils at


hv

Ozone formaton hu Ozone P hotolysl s

Ozone Photolysis

N20+0 - 2 N O

nitrous oxlde photolysis

NOx- Ozone Decomposition Catalytic Cycle

To Troposphere From Troposphere

Y

Figure 9. Main cycle of nitrogen in the stratosphere emphasizing catalytic destruction of ozone. From NAS ( 1978).

1.5 Tg N,O-N year - I . Eichner (1990) summarized N,O emission data from 104 field experiments and estimated that global release of N,O from fertilized soils to the atmosphere ranged from 0.2 to 2.1 Tg N20-N year - I. He also suggested that the fertilizer-derived emission of N,O in the year 2000 will account for 0.1 to 1.5% of the global source and will probably not exceed 3% N20-N in the atmo-

Table XI

Mechanisms for the Decomposition of Stratospheric Ozone"

Percentage of total Os Mechanism destruction rate

Photolysis (Chapman reactions) 20 '

photolysis of water I 0'

Transport to troposphere 0.5 Hydroxyl and hydroperoxyl radicals from

NO. catalytic cycle 50-70' Chlorine from natural and man-made

sources (present effect) 10-40'

OFrom NAS (1978). bThese loss processes represent an average over the ozone for-

mation region between about a 25- and 40-km altitude.


sphere. The amount of N,O emitted from a corn production system ranges from 1.3 (Mosier and Hutchinson, 1981) to 2% of applied N (Duxbury and Mc- Connaughey, 1986). Colbourn and Dowdall (1 984) observed that denitrification losses of inorganic N range from 0 to 20% of the fertilizer applied in arable soils and from 0 to 1 % on grassland soils.

NIs, which retard oxidation of ammonium to nitrate, also retard N,O emission. Bremner and Blackmer (1978) reported that application of 8 mg NP kg-I soil reduced N20-N evolved in 20 days of incubation from 148 to 10 mg kg - I soil when ammonium sulfate was the source of N; the values with urea were 122 mg kg - I soil (without NP) and 4 mg kg ~ I soil (with NP). Smith and Chalk (1980) reported that application of 10 kg NP kg - I soil with ammonia completely pre- vented N 2 0 emission for 28 days; NzO emission without NP during the same period was 57 mg N kg - I soil.

Some data are also available from field studies. Bremner et al. (198 1 ) showed that application of 0.56 kg ha - I of NP along with anhydrous ammonia ( 1 80 kg N ha ~ I ) reduced N 2 0 emissions from 1.37 kg N ha ~ I (without NP) to 0.55 kg N ha ~ I (with NP) during a period of 167 days. Magalhaes er al. (1984) reported from Australia that, under fallow conditions, NP significantly reduced the anhydrous ammonia-induced loss of N,O from a calcareous soil (pH 8.5; organic C 1.3%) but not from a noncalcareous soil (pH 7.5; organic C 2%). Bronson et al. (1992), from field studies with corn on a Nunn clay loam (mesic Aridic Argiustoll) at Fort Collins, Colorado, found losses of 1.5 to 3. I kg N,O ha - I from urea alone as compared to 0.87 to 1 .O kg N02-N ha - I from urea plus NP in the 2 years of the study (Table XII).

Acetylene, which inhibits nitrification in soils at partial pressures of 0.1 to 10 MPa, is often introduced into soil in field studies to measure the total denitrification by the acetylene block method (blockage of the reduction of N,O to N2) (Reyden et a/.. 1979; Rolston et a/. , 1982). Encapsulated CaCz (ECC) (Banerjee and Mosier, 1989) has been used as a slow-release source of C2H, to inhibit and reduce N 2 0 and N, emission in flooded rice paddies (Mohanty and Mosier, 1990; Bronson and Mosier, 1991; Bronson et a/., 1989). Bronson et al. (1992), from their studies in corn fields, reported a loss of 2.1 kg N02-N ha - I with urea + 20 or 40 kg ECC ha ~ I as compared to 3.2 kg N02-N ha - I with urea alone in 1989; the corresponding values for 1990 were 0.33-0.38 kg ha - I with most ECC and 1.5 kg ha - I with urea alone (Table XII). They also pointed out that high levels of C2H2 produced in ECC-treated plots apparently blocked the N20-N2 reduction step of denitrification (Yoshinari et al., 1977), resulting in high rates of NzO emission.

From a field study on a clay soil (Andaqueptic Haplaquoll) in the Philippines, John et al. (1989) reported that (N2 + N2O)-ISN flux during the 19 days following an application of 29 kg N ha - I as 98 atom % IsN-labeled urea never exceeded 28 g N ha - I day ~ I ; the total recovery of (N2/N,0)-N evolved from the field

268 RAJENDRA PRASAD AND J. F. P O W E R

Table XI1

Cumulative Losses of N,O-N in Irrigated Corn Affected by Urea with and without Escapsulated Calcium Carbide (ECC) or Nitrapyrin during

0-97 Days after Fertilization"

1989 1990

Treatment Mean SDb Mean SD

kg N ha- '

Urea 3.3362a' 0.973 I .65 la 0.6 18 Urea + nitrapyrin 1.174bc 0.193 0.980b 0.593 Urea + 2OkgECCha-I 2.2555ab 1.223 0.483bc 0.128 Urea + 40 kg ECC ha - 2.2270ab 0.483 0.434bc 0.192 Blank 0. I 15c 0.043 0.108~ 0.029

aAdapted from Bronson et al. (1992). 'Standard deviation. 'Means followed by the same letter are not significantly different (P = 0.05)

by Duncan's mean range test.

study was only 0.51% of the applied N, whereas total gaseous I5N loss estimated from unrecovered I5N in the I5N balance was 41% of applied N. In several other studies (Mosier et al., 1990a,b; Buresh and De Datta, 1990; Mohanty and Mosier, 1990), N gas flux measurements represented only a small fraction (1 to 10%) of the total gaseous N losses as measured using the I5N balance approach. Aulakh et al. (1992) suggested that entrapment of N gases in soil pore water and flood water in rice paddies, transmission of N gases through rice plants, and nitrogen losses through ammonia volatilization could be the cause for the low recovery of N-labeled gases. Furthermore, constancy of the N2/N20 ratio is important in es- timating denitrification losses from field soils where only N2 emissions into the atmosphere are measured (Reyden et al., 1979). Weier et al. (1993) found that the N,/N,O ratio was affected by the amount of available C, soil water content, and the amount of nitrates. The presence of high nitrate concentrations apparently inhibited the conversion of N,O to N2 and lowered the N2/N20 ratio. They did not recommend an average N2/N20 ratio for estimation of denitrification from N 2 0 field measurements. Thus there is considerable scope and need for research on more precise direct measurements of N,O losses from agricultural fields, other lands. and water masses.

c. GLOBALWARMING

The impacts of global climatic patterns of increases in greenhouse gases ab- sorbing infrared radiation have been the focal point for many studies investigating the indirect effects of atmospheric alteration (Liverman, 1986; Smit ef al., 1988).


Increases in greenhouse gases are predicted to contribute to warmer and drier climates in midlatitude regions such as the midwestern United States, southern Europe, and Asia, whereas it is predicted that higher latitudes will in all likelihood be characterized by warmer but somewhat wetter climates (Smit et al., 1988).

Recent models suggest that N,O also contributes to global warming (Yung et al., 1976); an increase of 0.2 to 0.3% N,O in concentration in the atmosphere would contribute about 5% to the supposed greenhouse warming (Enquette Ko- mission, 1989). Rodhe (1990) indicated that N,O is 300 times more radiatively active than CO, and it is estimated that nitrous oxide fluxes from climates contribute to the greenhouse warming by about 4% (UNEP, 1992).

ACKNOWLEDGMENTS

The senior author is grateful to the Director General, Indian Council of Agricultural Research (ICAR) and Director, Indian Agricultural Research Institute, New Delhi, for deputation to the Univer- sity of Nebraska, Lincoln, which enabled the preparation of this review. The authors are grateful to Drs. James S. Schepers, Gary W. Hergert, and Merle F. Vigil for their constructive criticisms.

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