Applicability of phosphate buffer extractable organic nitrogen as an indicator of available nitrogen...

Soil Science and Plant Nutrition (2008) 54, 449–458 doi: 10.1111/j.1747-0765.2008.00248.x

© 2008 Japanese Society of Soil Science and Plant Nutrition

Blackwell Publishing LtdORIGINAL ARTICLENitrogen availability in Sahelian soilsORIGINAL ARTICLE

Applicability of phosphate buffer extractable organic nitrogen as an indicator of available nitrogen in the sandy soils of the Sahel zone of Niger, West Africa

Kanako SUZUKI1, Ryoichi MATSUNAGA2, Keiichi HAYASHI3, Naruo MATSUMOTO3, Satoshi TOBITA3 and Kensuke OKADA1,4

1Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, 2National Institute of Crop Science, Ibaraki 305-8518, 3Japan International Research Center for Agricultural Sciences, Ibaraki 305-8686, and 4National Agricultural Research Center, Ibaraki 305-8666, Japan

Abstract

As phosphate buffer extractable organic nitrogen (PEON) has been successfully used to evaluate the availablenitrogen in Japanese soils, we tested whether it could also be used with sandy soils of the Sahel zone ofNiger. Soils were collected from 33 farm fields in the Fakara region of Niger. Pearl millet (Pennisetum glaucumL. cv. Haini–Kirei) was grown in these soils for 4 weeks with all nutrients except nitrogen supplied, andplant nitrogen uptake was measured. We analyzed PEON, the inorganic and total nitrogen present at thestart of the experiment, and the nitrogen mineralized during incubation in the same soils. Both PEON andmineralizable nitrogen were significantly correlated with nitrogen uptake by pearl millet. The PEON + inorganicnitrogen and mineralizable nitrogen + inorganic nitrogen were also significantly correlated with nitrogenuptake. Unlike in Japanese soils, the correlation between PEON and mineralizable nitrogen was not high inthe sandy Sahelian soils. The absorbance at 280 nm and the content of Coomassie Blue reactive substancesdetected using the Bradford method for soil extracts treated with 0.067 mol L–1 phosphate buffer were highlycorrelated with nitrogen uptake by pearl millet. These results suggest that extraction with 0.067 mol L–1

phosphate buffer is a convenient method for evaluating the available nitrogen in sandy Sahelian soils.

Key words: nitrogen uptake, pearl millet, phosphate buffer extractable organic nitrogen, Sahel, spectrophotometry.

INTRODUCTION

The zone of West Africa in which annual rainfall rangesfrom 200 to 600 mm (Heinrigs and Perret 2006), knownas the Sahel, is located in the northernmost agriculturalzone of sub-Saharan West Africa. In the Sahel, pearlmillet (Pennisetum glaucum L.) is the crop that is mosttolerant of the dry environment, and it is usually culti-vated during the rainy season (June–October). Pearlmillet is the staple crop of the Sahel, but its productivityis very low (Food and Agriculture Organization 2007).Most Sahelian soils are composed mainly of sand(approximately 90%) and are low in clay (< 10%), with anextremely low cation exchange capacity of approximately

1.0–1.6 cmolc kg–1 (Hiernaux and Ayantunde 2004);thus, they are inherently poor in nitrogen and phosphorus.

Farmers in the Sahel have traditionally depended onfallow systems to restore soil fertility in their fields(Schlecht and Buerkert 2004; Wezel and Haigis 2002).However, recent population pressures have led to adecrease in the lengths of the fallow periods, and as a result,soil fertility lacks enough time to recover sufficiently tosupport crop growth (Hiernaux and Ayantunde 2004;Schlecht and Buerkert 2004; Wezel and Haigis 2002).Chemical fertilizers are, however, too expensive forthe Sahelian farmers to use in sufficient quantities foracceptable crop growth.

The main soil fertility management practices commonlyfound among the farmers of the Sahel are manuring bycorralling livestock in a farm field or the application ofdry farmyard manure and household wastes to the field.In corralling, cows, sheep and goats are enclosed in apart of the field at night so they can leave their dung onthe field. Farmers with few livestock, however, are unable

Correspondence: K. SUZUKI, Graduate School of Agriculturaland Life Sciences, The University of Tokyo, 1-1-1, Yayoi,Bunkyo-ku, Tokyo 113-8657, Japan. Email: [email protected] 14 April 2007. Accepted for publication 6 December 2007.

450 K. Suzuki et al.


to provide manure by corralling livestock in their fieldand must instead transport household wastes and farm-yard manure to their fields by hand or in carts. How-ever, the availability of this organic matter in the regionis limited, and the farmers need to develop practicesthat will let them use the limited organic matter moreeffectively.

Nitrogen is one of the most limiting nutrients in theSahel. Thus, assessing its availability is crucial to guidingthe systematic use of organic matter so as to improvethe productivity of pearl millet. The two nitrogen frac-tions in soils that can be used by plants are inorganicnitrogen (mainly nitrate and ammonium) and decom-posable organic nitrogen. The latter consists mainly ofamino acids, amino sugars and proteins, and is easilymineralized in a relatively short time (Mengel 1996;Schulten and Schnizer 1998; Sowden et al. 1977). Theamount of decomposable organic nitrogen can be usedas an indicator of a soil’s capacity to supply nitrogen toplants.

Both microbiological and chemical methods have beenused to estimate the amount of available nitrogen insoils. The microbiological (incubation) method evaluatesthe amount of inorganic nitrogen released during incu-bation of a soil under constant temperature for severalweeks (Keeney 1982). In the various chemical methods,some component of the soil nitrogen is extracted usingchemical reagents, such as boiling water and K2SO4

(Gasser and Kalembasa 1976; Keeney and Bremner 1966),dilute H2SO4 (Fujii et al. 1990), Ba(OH)2 (Jenkinson1968), or 0.01 mol L–1 CaCl2 (Appel and Mengel 1990,1993; Houba et al. 1986); the nitrogen may also beextracted after physicochemical treatment, such asautoclaving (Stanford and Demar 1970). The chemicalextraction methods are simpler and more rapid thanthe incubation method.

Organic nitrogen compounds can be easily extractedwith 0.067 mol L–1 phosphate buffer (pH 7.0) (Higuchi1981, 1982a,b). Phosphate buffer extractable organicnitrogen (PEON) has a strong positive correlation withthe amount of mineralizable nitrogen determined usingthe incubation method (Higuchi 1982a; Ogawa et al.1989; Yanai et al. 1998a); thus, the PEON approachhas gradually become recognized as an appropriateindicator of levels of available nitrogen in soils. A numberof previous studies have reported that PEON from Jap-anese soils mainly consists of proteinaceous nitrogenwith a molecular weight of 8,000–9,000 Da and is derivedfrom bacteria rather than fungi (Higuchi 1981, 1982a;Marumoto et al. 1974; Matsumoto et al. 2000a,b).In recent studies, however, PEON has been found toconsist mainly of humic substances (Aoyama 2006) andis not uniform in its molecular weight (Miyazawa andMurayama 2007).

When the soil is treated with 0.067 mol L–1 phosphatebuffer, the soil extract becomes yellow or yellowish-brown, with the depth of the color indicating theamount of nitrogen (Ogawa et al. 1989). Several studieshave reported that the amount of PEON shows astrongly positive correlation with absorbance at a wave-length of 420 nm; this absorbance is used to determinethe amount of humic acid in the water (Ogawa et al.1989, 1992; Yanai et al. 1998b). The ultraviolet absorbanceat 280 nm (Dunn 1989) and the content of CoomassieBlue reactive substances (CBRS) detected using theBradford method (Bradford 1976) are also stronglycorrelated with PEON (Hioki et al. 2003; Saito 1988).

However, it has not yet been determined whetherPEON can be used to estimate the available nitrogen insandy Sahelian soils. The purpose of the present studywas to determine whether the organic nitrogen contentand the light absorbance of phosphate buffer extractwere useful indicators of available nitrogen in these soils.

MATERIALS AND METHODS

Collection site and soil fertility managementThe soil materials were collected from 33 farm fields inthree villages in the Fakara region of Niger (Fig. 1):Banizoumbou (13°31′N, 2°39′E), Kodey (13°22′N, 2°50′E)and Tchigo Tégui (13°30′N, 2°46′E). Surveys were con-ducted to select suitable fields for our study, which tookplace from 21 to 23 June and from 18 to 20 July 2005.During these surveys, the nature of the organic matterinput to the fields and the growth of pearl millet(P. glaucum) were investigated.

The fertility management generally practiced by farmersin the Fakara region involves a fallow period, the manualapplication of farmyard manure and household wastes,and manuring of fields by corralling livestock in thefield.

In fallow fields, the soil surface is covered with shortgrasses and scattered bushes. These fields are usuallyused for the pasturage of cows, goats and sheep, so thegrasses do not grow thickly. Using the database of theInternational Livestock Research Institute (ILRI; Hiernauxand Ayantunde 2004), we selected fields that had beenleft fallow for 1, 5 and more than 10 years near eachvillage. At Banizoumbou, we also selected three “reserved”fallows that had been fenced for 10 years to excludelivestock; as a result, the ground cover and trees hadbecome relatively thick.

When farmers manually apply farmyard manure andhousehold wastes to their fields they either uniformlyspread the organic matter over the fields or sporadicallyspread organic matter in certain parts of the fields. Weselected pearl millet fields in which manure had been

Nitrogen availability in Sahelian soils 451


applied continuously for 5 years or for more than 10 yearsbased on the ILRI database (Hiernaux and Ayantunde2004). Because organic matter is applied to differentparts of the field each year, for consistency we selectedthose points where organic matter had been appliedduring the previous dry season. Additional fields adja-cent to houses were selected where a mixture of house-hold wastes and chicken, goat and sheep manure wasapplied almost daily and the frequency and intensity oforganic matter application was much higher than in fieldsmanaged using the other manual application practices.

For corralling of livestock, we selected pearl milletfields either under the simultaneous corralling of mixedlivestock (cows, goats and sheep) or under the cor-ralling of cows only. Because the corralled parts of thefields are rotated every year, we collected soil materialsfrom the parts that had been corralled during the lastdry season. For this management practice, we did notconsider the duration of the practice.

Finally, as a control, we selected pearl millet fields whereneither chemical fertilizers nor organic matter had beenapplied for several decades.

Soil collectionSoil was collected from 27 to 30 July 2005 from theselected fields. Each field was divided into four parts. Ineach part, soils were collected to a depth of 15 cm fromfive points and mixed. The soil mixtures from the fourparts of each field were then combined and thoroughlymixed to produce a composite soil collection (approxi-mately 9 kg) from each field.

Soils were collected using an aluminum tube measuring5 cm in diameter at each sampling point. After air-drying

for 1 week, soils were passed through a 2-mm sieve foruse in the pot experiment described below. From eachcollection, a soil sample (approximately 100 g) wascollected and divided into three replicate parts for analysis.

Classification and characteristics of the collected soilsThe position of each soil collection site was recordedwith a GPS unit, and the soils were then classified basedon this location using a soil map for the Fakara region(ILRI, unpubl. data, 1996). Most of the soil sampleswere classified as Arenic Lixisols (Food and AgricultureOrganization 2006), with the exception of four soilsthat were Ferralic Arenosols and one sample that was aGleyic Arenosol (Table 1). The texture of all soil sampleswas sandy (ILRI, unpubl. data, 1996).

The soils were slightly acidic to neutral, with pH valuesranging from 5.0 to 7.3. Total nitrogen content rangedfrom 71.5 to 606.5 mg kg–1 and total carbon contentranged from 0.85 to 5.67 g kg–1 (Table 1). The otherphysicochemical characteristics of the soil samples werequite similar.

Pot experimentTo estimate the nitrogen available for crop growth, weconducted a pot experiment in the greenhouse of theSahelian Center of the International Crops ResearchInstitute for the Semi-Arid Tropics (ICRISAT; 13°15′N,2°18′E) using the 33 soils. The experiment used a com-pletely randomized block design with four replicates.Each pot had a sealed bottom and contained 1 kg ofair-dried and sieved soil. The pots were weighed and thewater content was adjusted to 70% of the maximum

Figure 1 Locations of the three villagesin the Fakara region of Niger where thesoil samples were collected and of theInternational Crops Research Institutefor the Semi-Arid Tropics (ICRISAT)Sahelian Center. BZ, Banizoumbou; KD,Kodey; TT, Tchigo Tégui.



water-holding capacity by adding deionized water daily.The average maximum and minimum temperatures inthe greenhouse were 43°C and 25°C, respectively. Sothat we could estimate the available nitrogen in the soilsamples, we applied no nitrogen fertilizer. However, wesupplied other nutrients at the following rates per pot(mg kg–1 soil): 75 P (Na2HPO4), 125 K (KCl), 50 Ca(CaCl2), 20 Mg (MgSO4) and 5 Cu (CuSO4).

After the nutrients were applied, the pots werewatered and incubated for 1 week and then 6–10 seedsof pearl millet (cv. Haini–Kirei) were sown per pot.Plants were thinned to two plants per pot 7 days aftersowing, and the plants were then grown for 3 weeks.The positions of the pots were randomized every thirdday.

The above-ground plant parts were harvested 28 daysafter sowing and immediately dried at 65°C in an ovenfor 2 days. Dry weight was then measured.

Analyses of soil and plant samplesSoil analyses were conducted using samples of the 33soils. Total nitrogen and total carbon were determinedwith an automatic NC analyzer (Sumigraph NC22F;Sumika, Tokyo, Japan). Nitrate nitrogen (NO3-N) andammonium nitrogen (NH4-N) were extracted with50 mL of 2 mol L–1 KCl solution using 5 g of air-driedsoil (Keeney and Nelson 1982) and their concentrationswere determined using colorimetry with an automaticanalyzer (TRAACS, BRAN + LUEBBE, Norderstedt,Germany). The PEON was analyzed according to the

Table 1 Properties of the soil materials and the management system used at each site

Field no. Type of practices Village Soil type pH (H2O)Total nitrogen

(mg kg–1)Total carbon

(g kg–1)

1 Fallowed for 10 years (F10) BZ Ferralic arenosol 5.45 (0.17) 162.5 (17.7) 1.62 (0.08)2 KD Arenic lixisol 5.34 (0.08) 81.3 (9.3) 1.12 (0.08)3 TT Arenic lixisol 5.39 (0.02) 152.5 (10.6) 1.66 (0.00)4 Fallowed for 5 years (F5) BZ Arenic lixisol 5.47 (0.04) 99.3 (11.0) 1.28 (0.09)5 KD Ferralic arenosol 5.73 (0.04) 96.7 (3.3) 1.15 (0.01)6 TT Arenic lixisol 5.51 (0.10) 78.0 (0.9) 1.06 (0.03)7 Fallowed for 1 year after BZ Arenic lixisol 5.08 (0.08) 84.8 (2.8) 1.15 (0.10)8 continuous cultivation (F1) KD Arenic lixisol 5.26 (0.04) 81.0 (7.1) 0.97 (0.02)9 TT Arenic gleysol 5.37 (0.04) 71.5 (6.2) 0.85 (0.08)10 Reserved fallow (with livestock BZ Arenic lixisol 5.44 (0.07) 259.0 (5.7) 3.00 (0.02)11 exclusion) for 10 years (RF) BZ Ferralic arenosol 4.95 (0.06) 186.5 (24.7) 1.87 (0.23)12 BZ Ferralic arenosol 5.13 (0.06) 163.0 (8.5) 1.61 (0.06)13 FYM and HHW applied for BZ Gleyic arenosol 5.75 (0.04) 202.5 (3.5) 2.03 (0.03)14 more than 10 years (FYM10) KD Arenic lixisol 5.73 (0.02) 229.5 (27.6) 2.20 (0.23)15 TT Arenic lixisol 6.22 (0.04) 202.5 (6.4) 2.16 (0.03)16 FYM and HHW applied for BZ Arenic lixisol 6.21 (0.06) 284.5 (13.4) 2.91 (0.00)17 more than 5 years (FYM5) KD Arenic lixisol 6.15 (0.03) 180.5 (9.2) 1.71 (0.02)18 TT Arenic lixisol 5.69 (0.06) 139.0 (15.6) 1.58 (0.21)19 Household area, FYM and BZ Arenic lixisol 6.19 (0.05) 606.5 (5.0) 5.67 (0.44)20 HHW applied daily (H) KD Arenic lixisol 6.57 (0.06) 281.0 (22.6) 2.58 (0.25)21 TT Arenic lixisol 6.73 (0.10) 276.0 (29.7) 2.50 (0.21)22 Corralled part of cows in BZ Arenic lixisol 7.30 (0.08) 254.0 (16.9) 2.35 (0.13)23 mixed corralling (C) KD Arenic lixisol 6.88 (0.05) 278.0 (29.7) 2.57 (0.12)24 TT Arenic lixisol 6.32 (0.04) 190.0 (0.0) 1.75 (0.05)25 Corralled part of small ruminates BZ Arenic lixisol 6.70 (0.11) 225.0 (4.2) 1.92 (0.02)26 in mixed corralling (CSG) KD Arenic lixisol 6.52 (0.13) 239.5 (7.8) 1.88 (0.17)27 TT Arenic lixisol 6.42 (0.03) 198.0 (12.7) 2.08 (0.21)28 Corralling with cows only (CC) BZ Arenic lixisol 5.85 (0.04) 142.0 (2.8) 1.59 (0.06)29 KD Arenic lixisol 6.22 (0.05) 196.5 (20.5) 1.73 (0.11)30 TT Arenic lixisol 6.07 (0.03) 182.1 (12.7) 1.79 (0.20)31 Control (non-treated) (NT) BZ Arenic lixisol 5.19 (0.13) 101.4 (2.3) 1.24 (0.02)32 KD Arenic lixisol 5.17 (0.07) 122.0 (11.3) 1.44 (0.04)33 TT Arenic lixisol 4.98 (0.07) 97.7 (4.7) 1.22 (0.04)

Values in parentheses indicate standard deviation. BZ, Banizoumbou; FYM, farmyard manure; HHW, household wastes; KD, Kodey; TT, Tchigo Tégui.



method of Ogawa et al. (1989). We also mixed 5 g ofair-dried soil sample with 25 mL of 0.067 mol L–1

phosphate buffer, shook the solution for 1 h, filteredit through a No. 6 filter paper (Advantec, Toyo RoshiKaisha, Osaka, Japan), determined the NH4-N content,and then digested the extract using the Kjeldahl methodbefore repeating the NH4-N analysis. The NH4-N con-centrations in the extract before and after Kjeldahl diges-tion were determined colorimetrically with an automaticanalyzer (TRAACS, BRAN + LUBBE), and the differencewas defined as the organic nitrogen concentration.

To measure mineralizable nitrogen, we placed a 10-gair-dried soil sample into a plastic bottle and adjustedthe soil water content to 60% of the maximum water-holding capacity (previously determined before begin-ning the pot experiment) by adding distilled water. Thebottles were then incubated at 30°C for 4 weeks. Afterthe incubation, 40 mL of 2 mol L–1 KCl solution wasadded to each bottle and the bottles were shaken for30 min. After filtration of the extracts, the NO3-N andNH4-N concentrations were determined colorimetri-cally with an automatic analyzer. The mineralizablenitrogen was defined as the difference between the totalinorganic nitrogen before and after the incubation.

Soil pH was measured with a glass electrode (ECOS-CAN pH5; Eutech, Nijkerk, Netherlands) in a 1:2.5 w/w mixture of soil to distilled water.

The light absorbance of the 0.067 mol L–1 phosphate-buffer extract was determined at wavelengths of 420 nmand 280 nm with a flow-cell spectrophotometer equippedwith a quartz cell with a 10-mm-long light path (UV-2500PC; Shimadzu, Kyoto, Japan). In addition, theCBRS content in the 0.067 mol L–1 phosphate-buffer

extract was measured according to the Bradford method(Bradford 1976). After reacting the phosphate-bufferextract with Coomassie Brilliant Blue G-250 using theBradford Protein Assay Kit (Bio-Rad Laboratories, Rich-mond, CA, USA) we measured the absorbance at 595 nm.The CBRS content in the phosphate-buffer extract wasexpressed by dividing the results by the correspondingabsorbance of a known quality of bovine gamma globu-lin (IgE), which was used as the basis for comparison.

Oven-dried plant samples were ground and passedthrough a 0.5-mm sieve, and the total nitrogen and totalcarbon were determined using an automatic NC analyzer(Sumigraph NC22F).

RESULTS AND DISCUSSION

Plant growth and nitrogen uptakeThe leaf color of pearl millet was pale green (indicatinga nitrogen deficiency) from 1 week after sowing. Plantheight continued to increase noticeably until day 14 aftersowing, and elongated slowly thereafter (data not shown).

The growth and nitrogen uptake of the pearl milletvaried significantly as a function of the form of soilmanagement in the field where the soil sample was col-lected (Fig. 2). The shoot dry weight ranged from 147to 946 mg pot–1, and nitrogen uptake ranged from 2.0to 16.5 mg pot–1. Because nutrients other than nitrogen(P, K, Ca, Mg and Cu) were supplied in sufficientamounts, the wide range of shoot dry weights amongthe soil samples should result primarily from differencesin available nitrogen resulting from the different soilmanagement practices.

Figure 2 Shoot dry weight and nitrogenuptake of pearl millet cultivated usingthe soils produced by different forms ofsoil fertility management. Error barsrepresent the standard deviation of themean. See Table 1 for details of thesoils for each field number.



Shoot dry weight was significantly correlated withnitrogen uptake (r = 0.981, P < 0.001; Fig. 3A). Plantheight at day 28 was also significantly correlated withnitrogen uptake (r = 0.945, P < 0.01; Fig. 3B).

Correlations between nitrogen uptake and soil nitrogen levelsInorganic nitrogen can be directly absorbed by plants;thus, its concentration should be directly correlatedwith nitrogen uptake. The concentrations of NH4-N inthe soil samples ranged from 1.39 to 12.83 mg kg–1 andwere higher than those of NO3-N, which ranged from0.14 to 7.44 mg kg–1 (Fig. 4A). The concentration ofNO3-N had a slightly higher correlation than that of

NH4-N with nitrogen uptake by pearl millet, but bothcorrelations were significant.

The correlation of total inorganic nitrogen (NO3-N + NH4-N) with nitrogen uptake by pearl millet(r = 0.707, P < 0.001; Fig. 4B) was higher than that ofeither NO3-N or NH4-N. These three inorganic nitrogenindices, however, may not be useful indicators of soilnitrogen availability because their correlations withnitrogen uptake by pearl millet were relatively weak.This may be because the NO3-N and NH4-N nitrogenpools are transient and are affected by the decomposi-tion and nitrification rates in the soil; these rates, in turn,depend on soil conditions such as moisture and temper-ature. Under field conditions in the Sahel, the inorganic

Figure 3 Correlations between nitrogen (N) uptake by pearl millet and (A) shoot dry weight and (B) shoot height. **P < 0.01;***P < 0.001.

Figure 4 Correlations between nitrogen (N) uptake by pearl millet and (A) inorganic nitrogen (NO3-N [�] and NH4-N [�]measured separately) extracted using 2 mol L–1 KCl and (B) total inorganic nitrogen (NO3-N + NH4-N). *P < 0.05; ***P < 0.001.



nitrogen pool is likely to be highly variable because ofleaching caused by the sporadic but concentrated rain-fall that occurs in this area, combined with the sandytexture of the soils.

Mineralizable nitrogen was strongly correlated withnitrogen uptake by pearl millet during the early growthstage (i.e. measured at day 28) (r = 0.732, P < 0.001;Fig. 5A). A high correlation between mineralizable nitrogenin soils and nitrogen uptake by plants has been reportedpreviously for the temperate zone (Gallagher and Bar-tholomew 1964; Ryan et al. 1971; Thomas et al. 1974)as well as for the humid tropics (Lathwell et al. 1972),but our result appears to be the first such report forthe Sahel.

The amount of organic nitrogen extracted with0.067 mol L–1 phosphate buffer (i.e. PEON) also had astrong positive correlation with nitrogen uptake bypearl millet (r = 0.753, P < 0.001; Fig. 5B). This appearsto be the first report demonstrating that nitrogen uptakefrom Sahelian soils by pearl millet can be evaluatedusing PEON measurements, which permit a more rapidanalysis than is possible using the incubation method.

In the pot experiment, the inorganic nitrogen couldnot be leached out; thus, both the inorganic nitrogenand the available organic nitrogen were considered toremain available to the plants. Therefore, we comparedplant nitrogen uptake with the sum of the availableorganic nitrogen and inorganic nitrogen in each soilsample. Plant nitrogen uptake was strongly correlatedwith both mineralizable nitrogen + inorganic nitrogen(r = 0.873, P < 0.001; Fig. 6A) and with PEON + inorganicnitrogen (r = 0.833, P < 0.001; Fig. 6B).

Although significant, the correlation between PEONand mineralizable nitrogen was not strong (r = 0.534,

P < 0.01; Fig. 7). For the soil samples with a pH below6.0, PEON tended to be higher than the amount ofmineralizable nitrogen.

Previous studies have indicated that PEON extractedfrom Japanese soils is strongly and significantly corre-lated with mineralizable nitrogen (Higuchi 1982b; Ogawaet al. 1989; Yanai et al. 1998a). In the sandy Saheliansoils examined in the present study, however, the corre-lation between PEON and mineralizable nitrogen waslow (Fig. 7). The amount of mineralizable nitrogen isdetermined by the rate of microbial decompositionduring incubation under natural soil pH. For PEON,however, the neutral phosphate buffer changed the pHof the Sahelian soil samples and PEON was extractedunder more alkaline pH conditions than the natural pHbecause most samples were from acidic soils. Previousstudies have suggested that the strengths of the sorptionof humic acids (Kretzschmar et al. 1997) and bovineserum albumin (Alkan et al. 2006) to kaolinite decreasedas the pH increased from acidic to neutral. Thus, whenPEON is extracted with a neutral phosphate buffer, thehigh molecular weight organic nitrogen compounds canbe more easily extracted as the pH increases. In addi-tion, low pH decreases the rate of microbial decomposi-tion of soil organic matter and delays the mineralizationof organic nitrogen (Alexander 1961).

Spectrophotometric analysis for estimating available organic nitrogenWe examined the use of spectrophotometric measure-ments of phosphate-buffer extract as a simple way tomeasure available nitrogen in the sandy Sahelian soils.The absorbance of phosphate-buffer extract at 420 nmwas too low to measure (data not shown), even though

Figure 5 Correlations between nitrogen (N) uptake by pearl millet and (A) mineralizable nitrogen and (B) phosphate bufferextractable organic nitrogen (PEON). ***P < 0.001.



this is the wavelength generally used to determine thehumic acid content in water, indicating that the contentof high molecular weight organic nitrogen compounds,such as humic acids, was low in the extract from thesandy Sahelian soils.

In contrast, the light absorbance at 280 nm wasstrongly correlated with the nitrogen uptake by pearlmillet (r = 0.844, P < 0.001; Fig. 8A). The CBRS con-tent in the phosphate-buffer extract, measured using theBradford method, was also strongly correlated with thenitrogen uptake by pearl millet (r = 0.868, P < 0.001; Fig. 8B).

Although the light absorbance at 280 nm had aslightly lower correlation than the CBRS content, it

provides a more convenient and rapid method for eval-uating the amount of available nitrogen in the sandySahelian soils: after extraction of the samples with0.067 mol L–1 phosphate buffer the amount of availablenitrogen can be immediately evaluated with a spectro-photometer without requiring any special reagents.

ConclusionsThe amount of PEON, which is used as one of theindicators of available nitrogen in Japanese soils, wasstrongly correlated (r = 0.753, P < 0.001) with nitrogenuptake by pearl millet grown under a range of differentforms of soil fertility management. Thus, PEON appearsto be a useful and less time-consuming alternative to theincubation method for estimating the nitrogen availa-bility in these soils. The sum of inorganic nitrogen andPEON showed an even stronger correlation than PEONalone (r = 0.833, P < 0.001) with nitrogen uptake bypearl millet. In addition, the ultraviolet absorbance ofthe phosphate-buffer extract at 280 nm was stronglycorrelated (r = 0.844, P < 0.001) with nitrogen uptakeby pearl millet, and is a convenient indicator of nitrogenavailability in the sandy Sahelian soils.

ACKNOWLEDGMENTS

Part of this study was supported by the “Improvementof Fertility of the Sandy Soils in the Semi-arid Zone ofWest Africa through Organic Matter Management”collaborative project funded by the Japanese Interna-tional Research Center for Agricultural Sciences incollaboration with the International Crops ResearchInstitute for the Semi-Arid Tropics. We thank our

Figure 6 Correlations between nitrogen (N) uptake by pearl millet and (A) the sum of mineralizable nitrogen plus inorganicnitrogen and (B) phosphate buffer extractable organic nitrogen (PEON) plus inorganic nitrogen. ***P < 0.001.

Figure 7 Correlation between phosphate buffer extractableorganic nitrogen (PEON) and mineralizable nitrogen (N). �,soil samples with pH < 6.0; �, soil samples with pH ≥6.0.**P < 0.01.



technical assistants for their help with the experimentalequipment.

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Applicability of phosphate buffer extractable organic nitrogen as an indicator of available nitrogen...

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