Soil Science and Plant Nutrition (2009) 55, 150–159 doi: 10.1111/j.1747-0765.2008.00338.x

© 2009 Japanese Society of Soil Science and Plant Nutrition

Blackwell Publishing LtdAerobic rice under various nitrogen sourcesL. Nie et al.ORIGINAL ARTICLE

Alleviating soil sickness caused by aerobic monocropping: Responses of aerobic rice to various nitrogen sources

Lixiao NIE1, Shaobing PENG2, Bas A.M. BOUMAN2, Jianliang HUANG1, Kehui CUI1, Romeo M. VISPERAS2 and Jing XIANG1

1Crop Physiology and Production Center (CPPC), MOA Key Laboratory of Huazhong Crop Physiology, Ecology and Production, Huazhong Agricultural University, Wuhan, Hubei 430070, China and 2Crop and Environmental Sciences Division, International Rice Research Institute, Manila, Philippines

Abstract

Yield decline resulting from continuous cropping of aerobic rice is a constraint to the widespread adoptionof aerobic rice technology. Shifts in water management from flooded to aerobic conditions are known toinfluence the availability and form of N present in the soil and might require a different approach to Nmanagement in aerobic rice. The present study was conducted to determine the effects of different Nsources on the plant growth and grain yield of aerobic rice. Four pot experiments were conducted in whichrice was aerobically grown in soil that was taken from fields where aerobic rice has been cultivated for 11consecutive seasons and an adjacent field where flooded rice has been grown continuously. Nitrogen wasapplied as ammonium sulfate, urea, ammonium chloride, ammonium nitrate and potassium nitrate at fourN rates of 0.3, 0.6, 0.9 and 1.2 g N pot−1. Two unfertilized controls consisting of soil that was eitheruntreated or oven heated at 120°C for 12 h were also included. Plants were sampled during the vegetativestage or at maturity to measure plant growth, N uptake, grain yield and the yield components. Growth ofaerobic rice in aerobic soil was generally better with the application of ammonium-N than nitrate-N.Potassium nitrate decreased plant growth and caused plant death at the high N rate. Ammonium sulfatewas more effective in improving the vegetative plant growth, N nutrition and grain yield of aerobic ricethan urea at the high N rates. The application of ammonium sulfate achieved the same and even betterplant growth than the soil oven-heating treatment. These results suggest that there is a possibility of reversingthe yield decline observed in the continuous aerobic rice system by using the right source of N fertilizer atthe optimal rate.

Key words: aerobic rice, ammonium sulfate, nitrogen source, soil sickness, urea.

INTRODUCTION

Increasing scarcity of freshwater resources for agricul-ture has prevented farmers from growing floodedrice in many areas (Tuong and Bouman 2003). Variouswater-saving technologies, such as aerobic rice, havebeen developed to help farmers cope with waterscarcity in irrigated environments (Bouman et al. 2007).Aerobic rice is input-responsive and high-yieldingrice that is grown under dryland conditions, much likeother cereal crops (Bouman and Tuong 2001). Depend-

ing on the environment, aerobic rice can be grown withor without supplemental irrigation and aims to savewater by limiting losses resulting from seepage, perco-lation and evaporation. Relative to flooded rice,aerobic rice generally requires 30–50% less water, butalso tends to result in a yield penalty of 20–30% (Yanget al. 2005). Using currently available germplasm, grainyields of 5–6 t ha−1 can be achieved with aerobic rice(George et al. 2002).

At present, aerobic rice is grown commercially on80,000 ha in northern China (Wang et al. 2002);however, to improve the yield potential and the yieldstability of aerobic rice, high-yielding aerobic varietiesand sustainable crop management systems must bedeveloped. These objectives are particularly relevant inthe tropics, where several studies have reported thatcontinuous monocropping of aerobic rice can result in

Correspondence: S. PENG, Crop and Environmental SciencesDivision, International Rice Research Institute, DAPO Box7777, Metro Manila, Philippines. Email: s.peng@cgiar.orgReceived 13 June 2008.Accepted for publication 21 October 2008.

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rapid declines in the yield (George et al. 2002; Guima-raes and Stone 2000; Nishizawa et al. 1971; Peng et al.2006; Ventura and Watanabe 1978). The yield declineof continuous aerobic rice is generally believed to becaused by soil sickness, which includes biotic factors,such as nematodes and soil pathogens, and abioticfactors, such as nutrient deficiency and toxicity (Linet al. 2002; Nishio and Kusano 1975; Nishizawa et al.1971; Ventura et al. 1981).

Shifts in water management from flooded to aerobicconditions are known to influence the availabilityand form of N present in the soil and might require adifferent approach to N management for aerobicrice (Savant and De Datta 1982). Nie et al. (2008)conducted a series of pot experiments studying theindividual effects of nutrients and reported that Napplication improved plant growth under continuousaerobic rice cropping, whereas P, K and micronutrientshad little or no effect. These results suggested that Ndeficiency as a result of poor N availability and reducedplant N uptake might be important factors contributingto the yield decline of continuously cropped aerobicrice.

A number of researchers have reported differentresponses in plant growth to various N forms underupland or aerobic conditions. In general, is thedominant N form in paddy fields (Savant and DeDatta 1982) and is only stable in the oxidizedrhizosphere (Shen 1969). Lin et al. (2005) reported thataerobic rice produced higher shoot dry matter under asole supply than a sole supply during theearly growth stages and that co-provision of ammo-nium and nitrate could have a synergistic effect on thetotal N fluxes compared with the supply of eitherammonium or nitrate alone. Qian et al. (2004) foundthat aerobic rice did not show a significant preferencefor and alone and that provision of both Nforms improved the growth and N use efficiency.

The effectiveness of different N sources on alleviatingthe soil sickness caused by the monocropping of aerobic

rice is not clear. In the present study, a series of potexperiments were conducted using soils collected fromtwo adjacent fields at the International Rice ResearchInstitute (IRRI) farm: an aerobic field where aerobicrice has been grown continuously for 11 seasons and aflooded field where flooded rice has been growncontinuously. The objectives of the present study were:(1) to determine the effects of different N sources onthe plant growth and grain yield of aerobic rice grownin the continuous aerobic rice soil, (2) to compare theresponse of plants in aerobic and flooded soils to theapplication of ammonium sulfate and urea, (3) toexamine differences among rice varieties in theirgrowth responses to ammonium sulfate application incontinuous aerobic rice soil.

MATERIALS AND METHODS

Four pot experiments were conducted in the green-house at the IRRI using soil taken from the top 25 cmof the aerobic and flooded rice fields at the researchfarm of IRRI (Table 1). The aerobic soil for all fourexperiments was collected from a field where aerobicrice has been grown for 11 consecutive seasons since2001 and where a gradual decline in yield has beenobserved (Peng et al. 2006). The flooded soil forexperiment 3 was collected from an adjacent fieldwhere only flooded rice has been grown. The aerobicrice soil was Aquandic Epiaquoll with pH 7.1,16.4 g kg−1 organic C, 1.74 g kg−1 total N, 29.7 mg kg−1

Olsen P, 393 mg kg−1 available K, 40.5 meq 100 g−1

cation exchange capacity, 58% clay, 33% silt and 9%sand. The flooded rice soil was Aquandic Epiaquollwith pH 6.6, 17.6 g kg−1 organic C, 1.89 g kg−1 totalN, 13.0 mg kg−1 Olsen P, 455 mg kg−1 available K,40.3 meq 100 g−1 cation exchange capacity, 57% clay,33% silt and 10% sand.

The soil was air-dried, crushed into small pieces andmixed well for the experiments. Porcelain pots (4 L)filled with 3.0 kg of air-dried soil were used in all four

Table 1 Nitrogen sources, N rates, soil, varieties and sampling dates in the four pot experiments

Experiment Nitrogen source Nitrogen rate (g pot−1) Soil Variety Sampling stage

1 Ammonium sulfate, urea, ammonium chloride, ammonium nitrate and potassium nitrate

0.3, 0.6, 0.9 and 1.2 Aerobic soil† Apo Vegetative

2 Ammonium sulfate and urea 0.3, 0.6, 0.9 and 1.2 Aerobic soil† Apo Maturity3 Ammonium sulfate and urea 1.2 Aerobic soil†

and flooded soil‡

Apo Vegetative

4 Ammonium sulfate 1.2 Aerobic soil† Apo, IR78877-208-B-1-2 and IR80508-B-57-3-B

Vegetative

†Soil from a field where aerobic rice has been grown continuously for 11 seasons since 2001 in both dry and wet seasons. ‡Soil from a field where flooded rice has been grown continuously.

NH4+

NO3−

NO3− NH4

+

NH4+ NO3

−

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pot experiments. One day before sowing, chemicalfertilizers were applied to the pots and mixed well withthe soil and then the soil was soaked with tap water.An improved upland rice variety, Apo, was used in allexperiments because of its good performance underaerobic conditions (George et al. 2002; Lafitte et al.2002).

In experiment 1, the treatments consisted of five Nsources, ammonium sulfate, urea, ammonium chloride,ammonium nitrate and potassium nitrate, at four Nrates, 0.3, 0.6, 0.9 and 1.2 g N pot−1. In experiment 2,two N sources (ammonium sulfate and urea) were usedat the same N rates as in experiment 1. In experiment3, both aerobic and flooded soils received ammoniumsulfate and urea at a N rate of 1.2 g pot−1. In exper-iment 4, two newly developed rice varieties werecompared with Apo under the application of ammoniumsulfate at a rate of 1.2 g N pot−1. The two new varietieswere IR80508-B-57-3-B and IR78877-208-B-1-2,which were developed from crossing Apo with Aus257and IR72, respectively. Two common treatments wereincluded in all experiments: an untreated controland a soil oven-heating treatment. For the soil-heatingtreatment, pots with soil were placed inside an ovenand heated at 120°C for 12 h. Both treatments did notreceive nutrient inputs.

Each treatment was replicated five times with onepot per replicate. The distance between the pots waskept at 30 cm to avoid shading. Six pre-germinatedseeds were sown into each pot on 28 May 2006,8 February 2007, 25 July 2006 and 22 March 2007for experiments 1–4, respectively. Seedlings were thinned1 week after sowing to three uniform seedlings per pot.The pots were kept saturated for 1 week after sowingto promote good crop establishment after which thepots were kept under aerobic conditions. All pots werewatered once every 1–3 days whenever drying of thesoil surface was observed, which corresponded to asoil moisture tension at a depth of 15 cm of −15 to−25 kPa. The soil water content in the pots was notcontrolled rigorously, but frequent irrigation ensuredthat the plants did not experience drought stress andno standing water was kept in the pots throughoutthe experiment. Pesticides were sprayed on three tofour occasions to control insect damage. Weeds wereremoved manually.

The plants were sampled at 37, 40 and 40 days aftersowing for experiments 1, 3 and 4, respectively. Beforeplant sampling, the number of stems per pot wascounted and the plant height from the plant base to thetallest leaf tip in each pot was measured. Plants wereseparated into leaves and stems including sheath.The leaf area was measured with a leaf area meter(LI-3100; Li-cor, Lincoln, NB, USA). Plants were

harvested at maturity in experiment 2. Panicle numberper pot was counted. Plants were separated into leaves,stems including sheath, rachis and filled and unfilledspikelets. The dry weights of the plant organs weredetermined after oven-drying at 70°C to a constantweight in all four experiments. The above-groundbiomass was the sum of the leaf and stem dry weightsin experiments 1, 3 and 4. In experiment 2, the above-ground biomass included the dry weights of the leaves,stems, rachis and filled and unfilled grains. The spike-lets per panicle, grain-filling percentage (100 × filledspikelet number/total spikelet number) and harvestindex (filled spikelet weight/above-ground total biomass)were calculated for experiment 2. Three chlorophyllmeter (SPAD) readings were taken from one of thetopmost fully expanded leaves per pot at 39 days aftersowing in experiments 1 to 4.

The tissue N concentration was determined usingthe micro Kjeldahl digestion, distillation and titration(Bremner and Mulvaney 1982) to calculate theabove-ground N uptake. The data were analyzed usingan anova (SAS Institute 2003) and a least significantdifference (LSD) test was used to compare the meansbetween the treatments (P < 0.05).

RESULTS

In experiment 1, the application of ammonium sulfateand urea improved all measures of plant growth and Nnutrition compared with the untreated control (Fig. 1).The application of ammonium chloride and ammo-nium nitrate increased the plant height and SPAD valuecompared with the control. For stem number, leaf area,above-ground biomass and above-ground N uptake,ammonium chloride was effective only at the highestN rate, whereas ammonium nitrate had little or noeffect on these parameters compared with the control.The application of potassium nitrate decreased plantgrowth and caused plant death at the rate of1.2 g N pot−1. For the application of ammonium sulfateand urea, increasing N rates from 0.3 to 1.2 g N pot−1

generally improved plant growth and N nutrition andthe N response was greater with ammonium sulfatethan with urea. Plants responded to the applicationof ammonium chloride only up to 0.9 g N pot−1. Theapplication of ammonium sulfate and ammoniumchloride at higher rates resulted in similar SPAD valuesto the oven-heating treatment. For the other fiveparameters, only the application of ammonium sulfateat the highest rate was close to or greater than theoven-heating treatment.

In experiment 2, the application of ammoniumsulfate at all N rates improved the yield traits and Nnutrition compared with the untreated control (Fig. 2).

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However, the application of urea at 1.2 g N pot−1 didnot increase the grain filling percentage and reducedthe harvest index compared with the control. Increasingthe rate of ammonium sulfate from 0.3 to 1.2 g N pot−1

increased the panicle number, spikelets per panicle,above-ground biomass, grain yield, SPAD value andabove-ground N uptake. Spikelets per panicle andgrain yield were highest at a rate of 0.9 g N pot−1 whenN was applied as urea. Plants did not respond to N

rates of ammonium sulfate and urea consistently ingrain filling percentage and harvest index. At rates of0.3 and 0.6 g N pot−1, ammonium sulfate and ureahad similar effects on all parameters. At higher N rates,plant growth was better with ammonium sulfate thanwith urea and the differences were greatest in the1.2 g N pot−1 treatment. At a rate of 1.2 g N pot−1, thegrain yield with ammonium sulfate application was41.9 g pot−1 compared with 12.9 g pot−1 with the

Figure 1 (a) Plant height, (b) stem number, (c) leaf area, (d) above-ground biomass, (e) SPAD value and (f) above-ground Nuptake of Apo grown aerobically in soil under five N sources (ammonium sulfate, urea, ammonium chloride, ammonium nitrateand potassium nitrate) at four N rates (N1–N4 = 0.3, 0.6, 0.9 and 1.2 g N pot−1, respectively) and in an untreated control (CK)and oven-heated soil treatment in pot experiment 1. The soil was from an aerobic field where aerobic rice has been growncontinuously for 11 seasons. Oven heating of the soil was done at 120°C for 12 h. Error bars represent the standard error.

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Figure 2 (a) Panicle number per pot, (b) spikelets per panicle, (c) grain filling, (d) above-ground biomass, (e) harvest index, (f)grain yield, (g) SPAD value and (h) above-ground N uptake of Apo grown aerobically in soil with four rates of ammonium sulfateor urea application (N1–N4 = 0.3, 0.6, 0.9 and 1.2 g N pot−1, respectively) and in an untreated control (CK) and oven-heated soiltreatment in pot experiment 2. The soil was from an aerobic field where aerobic rice has been grown continuously for 11seasons. Oven heating of the soil was done at 120°C for 12 h. Error bars represent the standard error.

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application of urea. The above-ground N uptake withammonium sulfate and urea was 783 and 543 mg pot−1,respectively. Applications of ammonium sulfate at0.6–1.2 g N pot−1 and of urea at 0.9 g N pot−1 resultedin higher grain yields than the oven-heating treatment.This was mainly because of the differences in above-ground biomass, N uptake, panicle number and spikeletsper panicle between the treatments.

In experiment 3, plants were aerobically grownin aerobic or flooded soils under the application ofammonium sulfate, urea and oven heating of the soil incomparison with the untreated control. The applicationof ammonium sulfate at 1.2 g N pot−1 improved plantgrowth and N nutrition compared with the untreatedcontrol in both aerobic and flooded soils (Table 2). Theapplication of urea at 1.2 g N pot−1 improved plantgrowth and N nutrition compared with the control inthe aerobic soil. In the flooded soil, urea applicationincreased the plant height, SPAD value and above-ground N uptake of aerobic rice compared with thecontrol. In the control treatment, plant growth wasmuch poorer in the aerobic soil than in the floodedsoil. For example, the above-ground biomass of thecontrol plants grown in the flooded soil was 4.7-foldgreater than that of the control plants grown in theaerobic soil at 40 days after sowing. The application ofammonium sulfate resulted in similar plant growthbetween the aerobic and flooded soils, but this was notthe case with urea. Overall, plants grown aerobically inboth aerobic and flooded soils responded more to theapplication of ammonium sulfate than to urea. The

response of aerobic rice to N application was greaterin the aerobic soil than in the flooded soil. Plantsresponded to oven heating of the soil more in theaerobic soil than in the flooded soil. In the aerobic soil,oven heating resulted in greater stem number, leaf area,above-ground biomass and N uptake than the applica-tion of ammonium sulfate. In the flooded soil, therewas no difference between the oven-heating treatmentand ammonium sulfate application in plant growthand N nutrition. Plants produced more leaf area andabove-ground biomass in the aerobic soil than in theflooded soil when the soils were oven heated.

In experiment 4, the application of ammoniumsulfate at 1.2 g N pot−1 and oven heating of the soilconsistently improved plant growth and N nutritioncompared with the untreated control across all threevarieties (Fig. 3). The effect of ammonium sulfateapplication on plant growth and N nutrition wasgreater than that of oven heating of the soil in all threevarieties. IR80508-B-57-3-B had higher plant heightand leaf area than IR78877-208-B-1-2 and Apo in allthree treatments. The stem numbers of IR80508-B-57-3-B and IR78877-208-B-1-2 were greater than that ofApo in the control and ammonium sulfate application,whereas there was no difference in stem number amongthe three varieties in the soil oven-heating treatment.IR80508-B-57-3-B and IR78877-208-B-1-2 producedgreater above-ground biomass than Apo in all threetreatments. There were inconsistent differences amongthe three varieties with regard to the SPAD value acrossthe treatments. In general, IR80508-B-57-3-B had the

Table 2 Plant growth of Apo grown aerobically under the application of ammonium sulfate and urea and oven heating of the soilcompared with the untreated control in pot experiment 3

Parameter Control Ammonium sulfate† Urea† Oven‡

Aerobic soilPlant height (cm) 45.2 c 77.5 a 58.7 b 80.8 aStem number per pot 4.7 d 19.7 b 8.8 c 22.2 aLeaf area (cm2 pot−1) 105 d 951 b 300 c 1,423 aAbove-ground biomass (g pot−1) 0.96 d 8.51 b 2.70 c 13.98 aSPAD value 27.7 c 37.4 a 34.4 b 33.7 bAbove-ground N uptake (mg pot−1) 21 c 266 a 81 b 249 a

Flooded soilPlant height (cm) 68.8 c 75.7 ab 74.3 b 79.2 aStem number per pot 11.5 b 21.3 a 14.3 b 20.7 aLeaf area (cm2 pot−1) 589 b 923 a 690 b 1,106 aAbove-ground biomass (g pot−1) 5.44 c 8.09 ab 6.28 bc 9.21 aSPAD value 34.6 b 37.9 a 37.3 a 37.7 aAbove-ground N uptake (mg pot−1) 121 c 260 a 196 b 262 a

†The rate of ammonium sulfate and urea was 1.2 g N per pot. ‡Oven heating of the soil was done at 120°C for 12 h. Within a row, means followed by different letters are significantly different at P < 0.05 according to a least significant difference test. The aerobic soil was from a field where aerobic rice has been grown continuously for 11 seasons. The flooded soil was from an adjacent field where flooded rice has been grown continuously.

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highest above-ground N uptake, followed by IR78877-208-B-1-2 and Apo.

Increasing the rates of N as ammonium sulfatesignificantly increased the apparent N recovery rate(ANR) in both the vegetative and mature stages,whereas plants did not show any increase in ANR asN rates increased when N was applied as urea(Table 3). At higher N rates, ANR was significantly

higher with ammonium sulfate application than withurea at the same N rate. In general, root dry weightincreased as the rates of N application increased,except for the treatment of urea at the vegetative stage.At the vegetative stage, root dry weight was generallyhigher with ammonium sulfate application than withurea at higher N rates, but the reverse was true atmaturity.

Figure 3 (a) Plant height, (b) stem number, (c) leaf area, (d) aboveground biomass, (e) SPAD value and (f) above-groundN uptake of three rice varieties grown aerobically under untreated, ammonium-sulfate fertilized soil and oven-heated soil in potexperiment 4. The soil was from an aerobic field where aerobic rice has been grown continuously for 11 seasons. The threevarieties were Apo, IR78877-208-B-1-2 and IR80508-B-57-3-B. The rate of ammonium sulfate application was 1.2 g N pot−1.Oven heating of the soil was done at 120°C for 12 h. Error bars represent the standard error.

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DISCUSSION

The greater effect of oven heating on plant growth inthe aerobic soil than in the flooded soil (Table 2)suggests that the aerobic soil used in the present studywas sick compared with the flooded soil. This result isconsistent with findings from our previous studies (Nieet al. 2007, 2008). Nie et al. (2008) reported that Napplication alleviated the soil sickness of aerobic soilwhere aerobic rice has been grown continuously,suggesting that N deficiency as a result of poor soil Navailability or reduced plant N uptake was associatedwith the yield decline of monocropped aerobic rice.This is supported by the findings of Belder et al.(2005), who showed that plant 15N recoveries werelower in aerobic rice than in flooded rice at all times ofurea-N application within the same field experiment.The present study demonstrated that the aerobic riceplants grown in “sick” soil responded differently tovarious N sources.

Among the five N sources, both ammonium sulfateand urea enhanced vegetative plant growth at all Nrates in the aerobic soil compared with the control(Fig. 1). Ammonium chloride had a positive effect onplant growth only at high N rates, whereas ammoniumnitrate had little or no effect on plant growth regardlessof the N rate. We observed that potassium nitratesignificantly reduced plant growth and caused plantdeath at a rate of 1.2 g N pot−1. These results suggestthat ammonium-N was more effective than nitrate-N inalleviating the soil sickness caused by the monocroppingof aerobic rice. However, Lin et al. (2005) documentedthat aerobic rice produced significantly lower above-ground biomass under sole -N supply than undersole –N and mixed N supply.

In general, ammonium sulfate was much moreeffective in improving vegetative plant growth and Nnutrition than urea in the aerobic soil (Fig. 1). Further-more, ammonium sulfate had a greater positive effecton reproductive plant growth, reflected by grain yieldand yield components, than urea in the aerobic soil(Fig. 2). The difference between ammonium sulfate andurea became greater as the N rates increased. Theseresults were supported by the fact that ANR wassignificantly higher with ammonium sulfate than withurea at 0.6–1.2 g N pot−1 in the vegetative stage andat 0.9–1.2 g N pot−1 at maturity (Table 3). Stephen andWaid (1963) reported that the application of ammo-nium sulfate and urea at low rates gave similar yields invarious upland crops, whereas at intermediate and highrates of N application, yields with urea were considerablylower than the yields with ammonium sulfate. Thereason why ammonium sulfate was more effective thanurea in alleviating the soil sickness caused by themonocropping of aerobic rice is not clear. The soil pHvalues of the control, urea, ammonium sulfate andoven-heating treatments measured at 2 weeks after theapplication of 1.2 g N per 3.0 kg air-dried soil and wetincubation without plant growth were 6.93 (± 0.01standard deviation), 6.65 (± 0.02), 6.29 (± 0.04) and7.29 (± 0.04), respectively. Therefore, the applicationof ammonium sulfate and urea to the aerobic soilreduced the soil pH and the reduction was greater forammonium sulfate than for urea. However, ovenheating of the soil increased the soil pH. van Astenet al. (2005) reported that rice N uptake and ANRwere significantly higher on a pH-neutral soil than onan alkaline soil. Changes in the nutrient availabilityand microbial community caused by the acidificationof soil as a result of the application of ammonium

Table 3 Apparent N recovery rate and root dry weight of Apo grown aerobically under the application of ammonium sulfate andurea in pot experiment 1 (vegetative stage) and 2 (maturity)

Nitrogen rate (g N pot−1)

Vegetative stage Maturity

Ammonium sulfate Urea Ammonium sulfate Urea

Apparent N recovery rate0.3 0.11 a 0.11 a 0.45 a 0.42 a0.6 0.22 a 0.11 b 0.43 a 0.41 a0.9 0.18 a 0.07 b 0.53 a 0.42 b1.2 0.24 a 0.07 b 0.56 a 0.36 bRoot dry weight (g pot−1)0.3 0.92 a 0.95 a 4.1 a 3.5 a0.6 1.39 a 0.74 b 5.2 b 7.2 a0.9 1.30 a 0.81 a 6.8 a 6.6 a1.2 2.00 a 0.76 b 6.5 b 9.8 a

Within a row under each sampling stage, means followed by different letters are significantly different at at P < 0.05 according to a least significant difference test. The aerobic soil was from a field where aerobic rice has been grown continuously for 11 seasons.

NH4+

NO3−

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sulfate could be associated with its greater effect onplant growth and this hypothesis should be tested infuture research. Both soil acidification and urea-inducedammonia toxicity (Bremner and Krogmeier 1989)might explain the large difference in ANR in the veg-etative stage between the application of ammoniumsulfate and urea at high rates. Our study suggests thatammonium sulfate is a better N source than urea forthe basal N application of aerobic rice established withdirect seeding.

The positive effect of ammonium sulfate applicationon vegetative plant growth was observed not only inApo, but also in two other newly developed aerobicrice varieties (Fig. 3). These two new varieties producedsignificantly more above-ground biomass than Apo withthe application of ammonium sulfate at 1.2 g N pot−1.This was true even in the control without N input.We observed that the two new varieties had muchlarger root systems than Apo (data not shown).

The difference in vegetative plant biomass betweenthe application of ammonium sulfate at 1.2 g N pot−1

and the oven-heating treatment was inconsistent acrossthe experiments (Figs 1,3; Table 2). When the plantswere grown until maturity, the application of ammo-nium sulfate as low as 0.6 g N pot−1 resulted in greaterabove-ground biomass and grain yield compared withthe oven-heating treatment (Fig. 2). This suggests thatthe N released from the aerobic soil after oven heatingwas not enough for the whole growth duration. In bothexperiments 1 and 2, plant growth was significantlybetter with the ammonium sulfate application at1.2 g N pot−1 than at 0.9 g N pot−1 in the aerobic soil(Figs 1,2). Therefore, we cannot rule out the possibilitythat plant growth in the aerobic soil can be furtherimproved at a higher rate of ammonium sulfate input.The above-ground N uptake of Apo under the sametreatment was quite different among the experiments.These differences might be associated with differencesin radiation and temperature across the experiments.

Both biotic factors (pathogenic nematodes, fungi andbacteria) and abiotic factors (nutrient deficiency andtoxicity) could cause soil sickness under continuouscropping of aerobic rice (Lin et al. 2002; Nishio andKusano 1975; Nishizawa et al. 1971; Ventura et al.1981). Oven heating of the soil removed some or all ofthe factors causing soil sickness. We observed that ovenheating of continuous aerobic rice soil at 120°C for12 h increased the release of by 62% withoutincubation compared with untreated soil (data notshown). However, the increase in release wouldnot explain entirely the enhancement in plant growthand N uptake resulting from oven heating of the soil.The soil heating treatment might have also changedother soil physical and chemical properties that have an

influence on rice plant growth or removed the bioticfactors that limited the plant growth under the continuousmonocropping of aerobic rice. The present studyindicates that the application of ammonium sulfatecould achieve the same and even better plant growththan the soil oven-heating treatment, suggesting thatabiotic factors are more likely to cause the soil sicknessassociated with the continuous cropping of aerobic rice.

ConclusionsA series of pot experiments demonstrated that theplant growth of aerobic rice in the sick soil was generallybetter with the application of ammonium-N thannitrate-N. Furthermore, ammonium sulfate was moreeffective in improving the vegetative plant growth, Nnutrition and grain yield of aerobic rice than urea atthe high application rates in the aerobic soil. This wasalso true in the flooded soil, but the difference betweenammonium sulfate and urea was smaller in the floodedsoil than in the aerobic soil. In addition to Apo, a pos-itive growth response of aerobic rice to the applicationof ammonium sulfate was observed in two other newlydeveloped varieties. The application of ammoniumsulfate could achieve the same and even better plantgrowth than the soil oven-heating treatment. Theseresults suggest that there is a possibility of reversingthe yield decline observed in continuous aerobic ricesystems by using the right source of N fertilizer at theoptimal rate.

ACKNOWLEDGMENTS

This work is part of the Consultative Group on Inter-national Agricultural Research Challenge Program onWater and Food through the project “Developing aSystem of Temperate and Tropical Aerobic Rice inAsia (STAR)”. The study was also supported by theNational Natural Science Foundation of China (ProjectNo. 30528005) and the 973 Project of the Ministryof Science and Technology in China (Project No.2005CB120900).

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