legume productivity and soil nitrogen dynamics in lowland rice-based cropping systems

Legume Productivity and Soil Nitrogen Dynamics in Lowland Rice-BasedCropping Systems

J. K. Ladha,* D. K. Kundu, M. G. Angelo-Van Coppenolle, M. B. Peoples, V. R. Carangal,and P. J. Dart

ABSTRACTRice (Oryza sativa L.) in wet season (WS) preceded by a dry season

(DS) fallow, commonly practiced in rainfed lowlands, causes largelosses of N through NO3 leaching and denitrification. The green-manurelegumes as NOj catch crops is economically unattractive to farmers.In a 2-yr study, we (i) assessed productivity of one grain and fourforage legumes (pigeonpea [Cajanus cajan (L.) Millsp.], crotalaria(Crotalariajuncea L.), clitoria (Clitoria ternatea L.), desmanthus [Des-manthus virgatus (L.) Willd.], and siratro [Macroptilium atropurpureum(Mocino & Sessg ex DC.) Urban]) grown in the DS, (ii) examinedNO3-N and NH4-N dynamics in soil (a Typic Tropaquept), and (iii)evaluated legume residues as a N source for succeeding rice. Nitrate-N was dominant in the 30-cm topsoil and was higher under legumescompared with weedy fallow. The legumes produced 4.9 to 9.1 taboveground biomass ha~', accumulated 132 to 306 kg N ha'1 ofwhich 67 to 81% was derived from N2 fixation. After harvests, 2.9 to5.2 t ha"1 of residues containing 81 to 162 kg N ha*1 were returnedto soil. By 3 to 4 wk after flooding, legume-treated plots had as muchas 33 to 40 kg mineral N ha'1 in topsoil compared with 10 to 13 kgN ha'1 in weedy fallowed plots. Residues significantly increased riceyield and N uptake. Rice recovered 15 to 31% of the residue N. Fallowplots required 25 to 50 kg fertilizer N ha'1 to produce comparableplant growth responses to that obtained after the legumes. Be-low ground residues of the legumes apparently contributed 13 to 37kg N ha'1 to rice. Such DS legumes that improve farm productivityas well as increase soil fertility might ensure sus tainability of productionin rainfed lowlands.

RMNFED CROPPING SYSTEMS predominate in the humidtropics, with lowland rice the principal crop. During

the rice crop, soils are usually saturated and anaerobic,but they dry and become aerobic in the DS. In the DS,rice cultivation is often not possible due to inadequatewater supply and the lands are commonly fallowed orsometimes planted to dryland crops depending on soilmoisture availability. Under aerobic conditions, the NH4form of soil mineral N is oxidized to NOs, which mayaccumulate in the soil or be utilized by crops grownthere (Buresh and De Datta, 1991; George et al., 1992).Most of the NO3 that is not utilized by plants may belost through leaching and denitrification when the soilsare subsequently flooded for rice cultivation. Buresh etal. (1993) showed that weeds and a 45- to 60-d sesbania(Sesbania rostrata Bremek. & Oberm.) crop raised be-

J.K. Ladha, O.K. Kundu, M.G. Angelo-Van Coppenolle and V.R. Caran-gal, International Rice Research Inst. (IRRI), P.O. Box 933, 1099 Manila,Philippines; M.B. Peoples, CSIRO Division of Plant Industry, Canberra,ACT 2601, Australia; and P.J. Dart, Dep. of Agriculture, University ofQueensland, Qld 4072, Australia. This study is part of a collaborativeprogram between IRRI and the Australian Centre for International Agricul-tural Research (ACIAR) under the project no. 8800 and 8731. The workwas partially supported by the Australian International Development Assis-tance Bureau (AIDAB). Received 23 Aug. 1994. *Corresponding author([email protected]).

Published in Soil Sci. Soc. Am. J. 60:183-192 (1996).

fore the rice growing season could reduce soil N lossby assimilating NO3 and recycling it through incorpora-tion into the legume green manure. Likewise, Georgeet al. (1993) reported beneficial roles of weeds, sesbaniaand mung bean [Vigna radiata (L.) R. Wilczek] cropsgrown during the short, dry-to-wet season transitionperiod. Leguminous green manures played a role inconserving soil NO3 in addition to fixing atmosphericNi. Rice farmers, however, are often reluctant to devoteland and resources to grow legumes solely for greenmanure because this provides no immediate income(Ladha et al., 1992). Considerable opportunity exists forgrowing grain and forage legumes in the postmonsoonalperiod following rice in such environments (Blair et al.,1986; Carangal et al., 1994). These legumes producefood grains and animal feeds, and their residues may beused as a N source for the following rice crops (Kulkarniand Pandey, 1988). Productivity of such dual-purposeDS legumes and their effects on soil N dynamics andtheir contributions to the yield and N uptake of thefollowing rice crop have not been investigated.

In a 2-yr study, we examined the roles of one grainand four forage legumes in enhancing farm productivityand in utilization and conservation of soil N mineralizedthrough the DS and recycling the N to WS rice crops.

MATERIALS AND METHODSExperimental Plan

A field experiment was conducted in 1991 and 1992 at theInternational Rice Research Inst., Los Baiios, Philippines, ona Typic Tropaquept. The top 20-cm soil layer had an air-driedpH (1:1 w/v water) of 6.2, cation-exchange capacity of 29cmoU kg"1, and contained 12.2 g organic C kg"', 1.08 g totalN kg~', 0.008 g Olsen extractable P kg'1, 400 g clay kg"1,and 150 g sand kg"1. In 1990, the site had been cropped tofive legumes with simultaneous maintenance of a fallow portionduring the DS (January-June) and lowland rice on the entire

N field during the WS (July-November).In the 1991 DS, five dual purpose (food, fodder, or grain)

legumes, viz. pigeonpea, crotalaria, clitoria, desmanthus, andsiratro, and a weedy fallow treatment were randomly assignedto plots, each of 5 by 7 m size, in a RGB design with threereplications. In the WS, three rates of fertilizer N (0, 25, and50 kg N ha~') were superimposed onto the fallow plots, andeach of the eight main plots was divided into two equal subplotsto impose (+) and (-) residue incorporation treatments. Afterimposing the N rate and residue treatments, the field layouthad a split plot design.

For the 1992 DS experiment, the original field layout wasused with a modification of the weedy fallow treatment intoweedy and weed-free fallows. For the following WS experi-ment, weed biomass remaining in the fallow plots was incorpo-

Abbreviations: DS, dry season; WS, wet season; BNF, biological Nifixation; %Ndfa, percentage N derived from N2 fixation; DAS, days afterseeding; WFPS, water-filled pore space; RCB, randomized complete block.

183

184 SOIL SCI. SOC. AM. J., VOL. 60, JANUARY-FEBRUARY 1996

rated into the soil while preparing land for rice. Each of theseven main plots was divided into two equal subplots to imposethe residue incorporation treatment in the legume plots andfertilizer N treatments in the fallow plots. The field layout inthe 1992 WS was also a split plot design.

Cultivation of Dry Season CropsThe DS leguminous crops were established on residual soil

moisture after harvest of WS rice, without providing any tillageoperation. In early December 1990, a drag stick was used tomake 10 furrows at 50-cm spacing in each plot and legumeseeds (at 40 kg ha"1 for pigeonpea, 25 kg ha"1 for crotalaria,20 kg ha"1 for desmanthus, 15 kg ha~' for clitoria, and 10kg ha"1 for siratro) were drilled in the furrows. Ten to fifteendays after emergence, plants were thinned to maintain 15 plantsper linear meter in each furrow. No N fertilizer was suppliedto the crops. Weeds were allowed to grow in weedy fallowbut were removed from all weed-free fallow and legume plots.

Forage legumes were periodically harvested by clipping at20 to 30 cm above ground level. The first clipping was doneat 40 DAS in crotalaria and 90 DAS in clitoria, siratro, anddesmanthus. In 1991, there was a total of five clippings forcrotalaria (on 6 Feb., 1 Mar., 21 Mar., 15 Apr., and 13 May),three for clitoria and desmanthus (on 5 Mar., 2 Apr., and 13May), and two for siratro (on 5 Mar. and 2 Apr.). In 1992,five clippings were taken from crotalaria (on 18 Feb., 16 Mar.,31 Mar., 27 Apr., and 17 May), three from clitoria (on 12Mar., 7 Apr., and 13 May), and two from desmanthus andsiratro (on 31 Mar. and 13 May). In pigeonpea, grain-podswere harvested on 25 Apr. and topping of the leftover crop(clipping of plant tops at 50 cm above the ground level topromote ratooning) was undertaken on 2 May in both years.Shoot biomass and grain yields at each harvest were estimatedfrom a 5 by 2.5 m area. Subsamples were collected from thesesamples for determining N content at each harvest.

Estimation of Biological Nitrogen Fixation by the LegumesThe contributions of BNF to total N accumulation in legume

were estimated at each sampling date by the N differencemethod and the 15N natural abundance technique with setaria(Setaria splendida Stapf) and paragrass [Brachiaria mutica(Forsskal) Stapf] as nonlegume reference plants (Shearer andKohl, 1986; Peoples and Herridge, 1990). An area of 5.0 by1.5 m on one side of each legume plot was used for the BNFestimation. Three rows each of nonlegume reference and fourrows of legume plants were grown at uniform populationdensity (with 22 plants per row) in the area. Paired samplingof legume and two nonlegume reference plants was performedin an area of 1.2 by 0.5 m (18 plants per crop) at each clipping-harvesting date. Plant material was dried at 70°C for at least48 h, weighed, and ground. Subsamples were analyzed fortotal N by digestion, distillation, and titration, adopting specialprecautions such as the use of lower digestion temperaturesand collection of larger volumes of distillates than the standardKjeldahl procedure to ensure there was complete recovery ofsample N and no isotopic fractionation at any stage duringsample preparation or analysis (Peoples et al., 1989). Noadditional safeguards were necessary to avoid cross contamina-tion of samples.

Estimates of the proportion of plant N derived from N2fixation (%Ndfa) with the N difference procedure were calcu-lated by comparing N accumulated in the legume with thenonlegume reference as follows:

_ 100 [(Legume N) - Reference N)]— —————————————————(Legume N)

The natural abundance of 15N in titrated, acidified, and concen-trated distillates was determined by isotope ratio mass spec-trometry (VG Micromass Model 903, VG Isogas, Middlewich,England) and expressed as d'5N (parts per thousand, %o) withreference to atmospheric N2 (Shearer and Kohl, 1986). Preci-sion in replicate analyses of d'5N was 0. l%o or better. Estimatesof %Ndfa were calculated from the d'5N of legume and refer-ence plants according to the following equation (adapted fromShearer and Kohl, 1986):

%Ndfa =100 [(d'5N reference) - (d15N legume)]

[(d15N reference) - (d15N fixed N)]where (d15N fixed N) represents a measure of isotopic discrimi-nation during N2 fixation determined by growing fully symbi-otic, effectively nodulated legumes in N-free culture. Valuesused for (d'5N fixed N) were -0.73%0 for crotalaria, -1.07%ofor clitoria, — 1.45%o for pigeonpea, and — 1.26%o for siratroand desmanthus. The d'5N values for setaria and paragrass(i.e., measures of the d15N of the plant-available soil N pool)ranged from 3 to 10%o in individual plots, but estimates of%Ndfa with paired reference and legume d15N measures re-sulted in standard errors for mean determinations, which rangedfrom 3 to 11 %. Since %Ndfa did not vary significantly withsetaria and paragrass, the average %Ndfa values were usedto estimate the amount of N2 fixation.

Legume Residues and Fertilizer ManagementAbout 21 d before planting rice (on 20 June 1991), each

main plot was divided into two equal subplots: the abovegroundlegume residues were removed from one but residues in theother subplot were cut, chopped into 10- to 12-cm pieces,uniformly spread onto the plots following flooding of the field,and incorporated into soil with a rotovator while the soil waspuddled for planting rice. Residue yields and their N contentswere estimated for each plot by taking Subsamples beforeincorporating the residues into soil. In both years, 13 kg Pha"' through single superphosphate was applied to all plots ina single broadcast application before planting rice.

In 1991, three fertilizer N levels, 0, 25, and 50 kg N ha'1,were assigned to the fallow plots where urea was applied intwo splits: two-thirds at transplanting and one-third at 5 to7 d before panicle initiation of rice. While preparing land forthe 1992 WS, weed biomass remaining in fallow plots wasincorporated into the soil and each of these two fallow plotswas divided into two equal subplots to apply 0 and 50 kg ha"1

of fertilizer N. The urea in mis instance was applied in asingle basal dose, just before transplanting rice.

Cultivation of Wet Season RiceThree-week-old seedlings of IR 74 were transplanted at 20

by 20 cm spacing on 7 July in 1991 and on 17 July in 1992;they were harvested in the first and second week of November,respectively. Recommended cultural practices for wetland ricewere followed. Grain and straw yields were measured froma 5.6-m2 sample area of each plot. Grain yields were reportedat 140 g kg"1 moisture content. Rice grain, straw, and thelegume and weed residues were dried to constant weight at70°C and analyzed for total N by the micro-Kjeldahl method.

LADHA ET AL.: PRODUCTIVITY AND N DYNAMICS IN LOWLAND RICE SYSTEM 185

Soil Sampling and AnalysisDuring the DS, periodic samplings for NH4 and NO3 analysis

in soil at two different depths (0-30 and 30-60 cm) were doneunder five selected treatments in 1991 (pigeonpea, crotalaria,clitoria, siratro, and weedy fallow) and under six selectedtreatments in 1992 (pigeonpea, crotalaria, clitoria, siratro,weedy fallow, and weedfree fallow). During the first monthfollowing incorporation of DS crop residues and soil submer-gence, soil was sampled to a 30-cm depth five times in 1991and four times in 1992 to determine available N under fourselected treatments (weedy fallow, crotalaria, clitoria, andsiratro). Soil samples were collected with a 4-cm-diam. auger,and each sample represented a mixed composite from fourcores taken in each plot. A 40-g wet sample was extractedwith 200 mL of 2 M KC1 solution.

For samples collected in the DS, the KC1 extracts wereanalyzed for NH4-N by steam distillation (Bremner andKeeney, 1965) and for NO3 plus NO2-N by cadmium reduction(Dorich and Nelson, 1984) with subsequent colorimetric deter-mination of NO2 (Hilsheimer and Harwig, 1976). Since NO2is likely to be small relative to NO3, the values were consideredas NO3-N for simplicity. The KC1 extracts of soil samples

collected in the WS were analyzed for NH4 plus NO3-N bysteam distillation with Devarda's alloy. All mineral N valueswere converted to kilograms N per hectares, with soil bulkdensity periodically determined with soil cores at each depth.

The WFPS in soil was calculated as described by Doran etal. (1990) with gravimetric water content for each soil sample,experimentally determined bulk densities, and an assumed soilparticle density of 2.65 t m~3.

In nearly all NO3 and NH4 data sets, the mean correlatedpositively with the variance, as frequently observed by otherresearchers (White et al., 1987; Buresh et al., 1989). There-fore, NO3 and NH4 data were transformed to log (X + 1)before analysis of variance. All reported means were calculatedwith untransformed data.

RESULTS AND DISCUSSIONDry Season Nitrogen Dynamics in Soil

Mean NO3-N and NH4-N in the 0- to 30- and 30- to60-cm soil layers under various management treatmentsat different sampling dates are presented in Tables 1

Table 1. The NO,-N and NH4-N availability in the 0- to 30-cm (LI) and 30- to 60-cm (L2) soil layers as influenced by fallow or legumecropping and rainfall during the dry season in a Philippine rice lowland, 1991.______________________________

Sampling sequence

ManagementSoil

layer24 Jan.

(23.6 mm)t5 Mar.

(42.4 mm)25 Mar.

(68.6 mm)16 Apr.

(0.4 mm)15 May

(38.7 mm)Sequence

meansProfiletotal

Weedy

Clitoria

Crotalaria

Pigeonpea

Siratro

LI meansL2 means

Weedy

Clitoria

Crotalaria

Pigeonpea

Siratro

LI meansL2 means

•kg ha-'NO3-N

LIL2LIL2LIL2LIL2LIL2

LIL2LIL2LIL2LIL2LIL2

6.93.89.8

11.410.65.2

12.35.0

13.57.2

10.66.5

6.54.26.57.7

10.97.1

10.98.37.07.28.46.9

3.84.3

16.44.7

19.04.3

24.55.9

21.76.6

17.15.2

6.53.7

10.14.18.54.9

14.13.27.25.49.34.3

12.95.7

22.38.2

24.68.4

32.56.4

33.110.125.17.8

NH..-N6.04.19.75.5

10.05.6

11.58.94.88.88.46.6

12.112.228.915.719.012.522.112.728.811.622.212.9

9.12.8

13.24.0

14.85.4

18.52.0

11.82.1

13.53.3

Mean squares:):

6.35.6

23.813.318.29.4

20.56.1

24.614.918.79.9

6.45.0

16.59.09.78.0

21.66.5

17.66.3

14.47.0

8.46.3

20.210.718.38.0

22.47.2

24.310.118.78.4

6.94.0

11.26.1

10.86.2

15.35.89.76.0

10.85.6

14.7

30.9

26.2

29.6

34.4

10.9

17.3

17.0

21.1

15.6

Sources of variationTreatment (T)

Among legumes (Tl)Weedy x Tl

Soil depth (D)T x DSampling sequence (S)T x SD x ST x D x S

df§431144

164

16

NO3-N0.465**0.071*1.647**3.549**0.139**0.457**0.025**0.108**0.024**

NH4-N0.205**0.042ns0.693**2.400**0.061**0.113**0.021**0.266**0.033**

*, ** Significant at P < 0.05 and 0.01, respectively; ns = nonsignificant.t Cumulative rainfall since the previous sampling date.I Analysis of variance conducted on data transformed to log (x + 1). Means presented are based on untransformed values.§ df = degrees of freedom.


and 2. The analysis of variance indicated significantinfluences of cropping treatment, soil depth, samplingtime, and their interactions on mineral N (NOa and NH4)in both years. Levels of soil NOs-N were substantiallylarger than that of NH4-N, the surface soil containedmuch greater levels of mineral N than the 30- to 60-cmlayer. At the beginning of the experiment, total soil Ncontent was =0.90 g kg"1 at 0- to 30-cm depth and0.60 g kg"1 at the 30- to 60-cm depth.

In the 1991 DS, availability of both NO3-N andNH4-N was significantly greater in legume-cropped thanin weedy fallow soil. Legume species differed in theirinfluence on NOa-N but supported comparable levels ofsoil NH4-N. This increased level of mineral N in legumecompared with that in weedy fallow plots could be dueto (i) enhanced mineralization of soil N caused by soildisturbance during sowing of legume seeds (Sylvester-

Bradley et al., 1988), (ii) increased availability of soilN resulting from reduced immobilization or enhancedremineralization of immobilized N in the legume rhizo-spheres (Ismaili and Weaver, 1986; Jensen and Sorensen,1988), (iii) N contributions by leaf litter or from nodu-lated roots (Watson et al., 1964), (iv) decreased denitri-fication losses resulting from improved soil structure andsmaller WFPS, promoted by the legumes, (v) reducedleaching loss of mineral N under legume covers (Agamu-thu and Broughton, 1981), or (vi) reduced utilization ofsoil mineral N by legumes (Peoples et al., 1995).

In the 1992 DS, levels of soil NO3-N and NH4-N inweed-free fallow and the legume plots were significantlylarger than in weedy fallow plots, presumably reflectingNOa-N uptake and assimilation of soil mineral N byweeds. However, the amount of N accumulated inaboveground weed biomass (35 kg N ha"1) was much

Table 2. The NO3-N and NH4-N availability in the 0- to 30-cm (LI) and 30- to 60-cm (L2) soil layers as influenced by fallow or legumecropping and rainfall during dry season in a Philippine rice lowland, 1992.______ __

Sampling sequence

SoilManagement layer

Weed free LIL2

Weedy LIL2

Clitoria LIL2

Crotalaria LIL2

Pigeonpea LIL2

Siratro LIL2

LI meansL2 means

Weed free LIL2

Weedy LIL2

Clitoria LIL2

Crotalaria LIL2

Pigeonpea LIL2

Siratro LIL2

LI meansL2 means

Sources of variationTreatment (T)Among legumes (Tl)Among weedy fallow (T2)Tl x T2Soil depth (D)T x DSampling sequence (S)T x SD x ST x D x S

13 Jan.(6.9 mm)t

10.74.0

10.74.2

13.62.7

10.42.0

14.03.5

13.84.0

12.23.4

1.91.51.91.41.62.43.53.42.01.71.31.02.01.9

df§53111S8

408

40

17 Feb.(15.3 mm)

10.94.76.73.9

17.84.6

11.75.0

17.13.0

12.82.0

12.83.9

4.63.77.23.65.24.03.13.94.63.04.94.04.93.7Mean s

NO3-N0.931*«0.075**2.737**1.695**

20.550**0.126**1.077**0.134**0.047**0.049**

25 Feb.(10.0 mm)

15.84.07.93.3

18.94.0

12.36.2

17.54.0

17.03.3

14.94.1

6.37.94.59.95.14.65.88.15.14.15.1

10.25.37.5

.quaresi

NH4-N0.121*0.112*0.173*0.095*0.967*0.054*1.214*0.083*0.172*0.031*

16 Mar.(1.6 mm)

13.73.09.18.5

22.83.7

10.89.1

14.72.8

14.52.9

14.35.0

10.711.12.41.57.79.26.13.46.08.24.78.66.37.0

23 Mar.(3.9 mm)

NO3-N20.16.4

11.51.6

24.37.8

21.85.2

20.113.325.44.4

20.56.5

NH4-N16.12.98.23.5

10.89.4

15.410.36.72.67.25.0

10.75.6

6 Apr.(1.3 mm)

kg ha-'

20.57.7

10.74.3

27.46.1

20.78.2

23.46.1

23.011.321.07.3

18.57.59.17.7

22.87.4

15.07.4

13.67.0

17.77.8

16.17.5

13 May(223.3 mm)

56.56.5

14.74.8

38.612.939.516.245.111.940.817.539.211.6

19.06.37.96.09.05.4

16.010.59.95.08.85.8

11.86.5

25 May(20.6 mm)

61.513.57.12.1

50.613.068.115.656.211.627.86.9

45.210.5

8.69.17.67.38.35.0

16.612.914.814.310.06.4

11.09.2

9 June(81.0 mm)

56.52.91.50.0

14.33.0

34.68.6

16.01.9

11.43.7

22.43.4

6.81.37.56.75.85.96.16.46.65.86.04.86.55.2

Sequencemeans

29.65.98.93.6

25.46.4

25.58.5

24.96.520.76.2

22.56.2

10.35.76.35.38.55.99.77.47.75.77.36.08.36.0

Profiletotal

35.4

12.5

31.8

34.0

31.4

26.9

16.0

11.5

14.4

17.1

13.4

13.3

*, ** Significant at P < 0.05 and 0.01, respectively; ns = nonsignificant.t Cumulative rainfall since the previous sampling date.f Analysis of variance conducted on data transformed to log (x + 1). Means presented are based on untransformed values.§ df = degrees of freedom.


less than the maximum N difference (69 kg N ha"1)measured (25 May; Table 2) between the weedy (24 kgN ha"1) and weed-free fallows (93 kg N ha"1). Similarobservations have been made by Buresh et al. (1993)and George etal. (1993). Nitrogen accumulated in under-ground weed biomass in the form of roots and nuts andin the senesced or fallen aboveground parts (not estimatedin the current study) might account for part of the ob-served difference. However, George et al. (1993) offeredseveral other possible reasons for lower NOs-N levelsin weedy fallow plots, such as depressed N mineralizationat relatively lower soil water contents in weedy plots,increased denitrification in the proximity of weed roots,and microbial immobilization of mineral N due to contin-uous weed residue turnover. Since yield and N uptakeof WS rice were significantly larger in weedy than inweed-free plots (see below), larger N losses seem un-likely in this investigation.

Average mineral N level in the 0- to 60-cm soil depthduring 1992 was larger under weed-free fallow thanunder legume crops, the difference being more markedduring later growth stages of the forage crops (Table2), which was attributed to uptake of soil N by thelegumes. The large levels of soil NO3-N formed inweed-free plots during DS were likely to be lost whenthe field was flooded and puddled before transplantingrice. Mineral N dynamics in soil and crop N removalunder crotalaria are elaborated in Fig. la and Ib. Thesharp decline in soil NOs-N and NH4-N recorded afterthe middle of May was obviously due to accelerated Nremoval by the legume. Nitrogen accumulation in finalregrowth of crotalaria following the mid-May clippingwas much larger than that in the previous clippings.Larger N removal during the late growth stage wasassociated with vigorous shoot growth of legumes follow-ing onset of monsoon rains. Legume species, however,differed significantly in their influence on soil NOa-Nand NH4-N, but the differences were not consistentbetween the two years of observation. The larger amountof mineral N in the soil under siratro and pigeonpeain 1991 DS was strongly associated with smaller Naccumulation in their biomass. In the 1992 DS, however,factors other than biomass N accumulation apparentlyhad a greater impact on mineral N level in the soil underlegumes.

In 1991, the largest concentration of soil NOs-N of25.1 kg N ha"1 (mean across all five treatments) in thesurface 0- to 30-cm soil layer was measured in the 25Mar. samples, and 12.9 kg N ha"1 at 30- to 60-cm depthwas measured in the 16 Apr. sample. In 1992, the largestNO3-N levels of 45.2 kg N ha"1 were observed in thesurface soil on 25 May, and 11.6 kg N ha"1 at 30- to60-cm depth was observed on 13 May. MaximumNOs-N availability in soil corresponded with a WFPSof 0.6 to 0.7 mL mL"1 in 1991 and 0.5 to 0.6 mL mL"1

in 1992 (Fig. la and Ib). Linn and Doran (1984) andDoran et al. (1990) reported that ammonification andnitrification proceeded rapidly in soils at WFPS valuesnear 0.6 mL mL"1. However, the year-to-year variationin peak NO3-N concentration could hardly be explainedby the WFPS variation alone. The much larger NO3-N

200

0.0

Final

JAN FEB MAR APR MAY JUN

0.0

Final

^co 20 • +

JAN FEB MAR APR MAY JUN

Fig. 1. NOj-N and NH4-N availabilty in soil (0-60 cm) and periodicN removal by a forage legume crotalaria as influenced by rainfalland soil water-filled pore space during the dry season in a Philippinerice lowland, (a) 1991 and (b) 1992. Crotalaria N derived from soiland biological N2 fixation (BNF) in different cuttings are shown bythe bars.

peak in 1992 was perhaps associated with additionalfactors. For example, higher temperatures were recordedin May 1992 (maximum 34.3 and minimum 24.3°C) than


Table 3. Aboveground biomass yields and N accumulations ofone grain and four forage legumes grown during dry seasonsin a Philippine rice lowland.

No. of cuttingsincluding residue Total aboveground

Legumes harvest biomass production

PigeonpeaCrotalariaClitoriaDesmanthusSiratro

Mean


Mean

46443

46433

t ha-'1991

6.5 bt7.6 be6.9 be8.0 c4.9 a6.8

19929.0 c7.8 b7.7 b9.1 c5.5 a7.8

Total Naccumulation

kg ha-'

154 a277 b256 b251 b132 a214

235 b279 c306 c283 c178 a256

t Under each year, values in a column followed by a common letter arenot significantly different (P < 0.05) by Duncan's multiple-range test.

in March 1991 (maximum 31.4 and minimum 22.1°C), andthis might have favored increased nitrification.

Biomass Production and Nitrogen Accumulationby Dry Season Crops

Mean shoot biomass yield of weedy fallow plots was2.2 t ha"1 with corresponding N accumulation of 24 kgha"1 in 1991 and 2.4 t ha"1 with N accumulation of 35kg ha"1 in 1992 (not shown in the tables and figures).Nitrogen accumulated by weeds was within the rangeof 22 to 42 kg N ha"1 previously reported by Buresh etal. (1993) from weedy fallow plots at nearby experimen-tal sites.

In 1991, legume aboveground biomass yields rangedfrom 4.9 to 8.0 t ha"1 with corresponding N accumula-tions of 132 to 277 kg ha"1, whereas in 1992, the biomassyields ranged from 5.5 to 9.1 t ha"1 containing 178 to306 kg N ha"1 (Table 3). The larger biomass productionin the 1992 DS resulted largely from more favorablerainfall received during the growing season (262 mm in

Table 4. Estimates of the proportion of plant N derived from N2fixation of one grain and four forage legumes determined byN difference and I5N natural abundance techniques.

Species



PlantN

difference

7380777767

6876818074

N derived from N2 fixation15N naturalabundance

19917181797971

1992746776ndtnd

Average

72.080.578.078.069.0

71.071.578.580.074.0

1992 compared with 173 mm in 1991). Among the fivelegumes studied, desmanthus produced consistently morebiomass with larger N accumulation while siratro pro-duced the least (Table 3). Although biomass productionof pigeonpea was either comparable or larger than thatof crotalaria and clitoria, its N accumulation was signifi-cantly smaller (Table 3).

Biological Nitrogen Fixation by the LegumesNitrogen accumulated in weed biomass was considered

to have come exclusively from the soil because legumeswere not a component of the weed flora. Legumes,however, derive N from both soil and atmosphere (BNF).In this study, the average plant N derived from N2 fixation(% Ndfa) in the legumes at different sampling dates rangedfrom 67 to 81% in both years of the test (Table 4).There were no significant differences between estimatesof %Ndfa determined with N difference or 15N naturalabundance. Contributions of BNF to the total abovegroundN accumulation ranged from 91 to 221 kg ha"1 in 1991and from 131 to 240 kg ha'1 in 1992 (see below).Estimates of %Ndfa for other forage legumes and pi-geonpea were within the range of 44 to 95 % (Peoplesand Herridge, 1990). Environmental conditions, how-ever, are known to have a strong influence on BNF,and sensitivity of legumes to various climatic stressesdetermines the amount of N2 fixed. The larger amountsof N2 fixed in the 1992 DS resulted primarily from bettergrowth by the legumes (Table 3). The BNF contributionswere again much larger in crotalaria, clitoria, and des-manthus (193-240 kg N ha"1) compared with pigeonpeaand siratro (91-169 kg N ha"1) in both years of the test.A large number of plant characteristics contributed toBNF, including biomass yield under clipping, legumeN demand (Table 3), capacity to fix N2 (Table 4), andadaptibility to specific environments.

Recycling of Dry Season Crop Residuesto the Field before Wet Season

Since dual-purpose legumes were used in this study,not all the accumulated N was recycled to the field.Amounts of legume residues and quantity of N returnedto the soil after harvesting of forage and food grain arepresented in Table 5. Through the legume residues, 81to 137 kg N ha"1 was available for return to the soil in

Table 5. Aboveground residues of five dry season legumes (onegrain and four forage legumes) recycled to soil as N sourcesfor wet season rice in a Philippine rice lowland.

Abovegroundresidues recycled

Legumes


Mean

1991

tha- 1 -4.7 ct3.5 ab3.0 a4.4 be3.2 a3.8

1992

5.2 b4.1 b4.6 b5.2 b2.9a4.4

Total N contentsin residues

1991

113 ab118 b114 ab137 b81 a

112

1992

143 be117 ab162 c161 c99a

136

t nd = not determined.t Values in a column followed by a common letter are not significantly

different (P s 0.05) by Duncan's multiple-range test.


1991 and 99 to 162 kg N ha'1 in 1992. Desmanthus T~returned the largest amount (137 and 161 kg N ha"1 in 51991 and 1992, respectively) and siratro the least (81 &.and 99 kg N ha"1) amount of N through their residues \at the end of the DS in both years of the study. Nitrogen |recycled to soil through residues of these dual-purpose *„legumes compared well with contributions of common, igreen-manure legumes. In different legumes grown for s60 d at nearby experimental sites, other researchers frecorded the following N accumulations in kilograms ?per hectare: crotalaria 144, cowpea 80, mung bean 136, glablab 76 (IRRI, 1986), Sesbania cannabina (Retz.) Poir. 5131 to 171, Sesbania rostrata 176 to 219 (Morris et al.,1989), common indigo (Indigofera tinctoria L.) 110,and pigeonpea 76 (Meelu and Morris, 1988). ^~

Wet Season Nitrogen Dynamics in SoilAt the beginning of the flooding and puddling opera-

tions, mineral N in the surface 0- to 30-cm soil layerranged from 6 to 10 kg ha"1 in 1991 and from 13 to19 kg ha"1 in 1992. The larger initial level of mineral Nmeasured in 1992 could partly be due to some carryovercontribution of plant residues incorporated into the soilin the previous year, since mineralization rates of theresidue N were apparently low and only 18 to 29% ofthe total residue N was recovered in the following ricecrop (see below). Vallis et al. (1983) also noted slowrelease of N from plant litter under pasture legumes,with <30% being made available in the first year.

Figure 2 shows that in both WS, mineral N in thesoil increased with time after flooding only to decline 2to 3 wk after rice transplanting. The increase in N wasattributed to mineralization of soil organic matter andplant residues. A larger mineral N observed in legumethan in weedy fallow plots was attributed to the largeramounts of plant N returned in the legume plots. Thesubsequent decline in mineral N presumably reflecteduptake and assimilation by growing rice plants as wellas losses through various transformation processes suchas leaching, volatilization, and denitrification.

In the 1991 WS, maximum levels of mineral N weremeasured at 16 d after flooding under all the treatments.The maximum available soil N ranged from 40 kg ha"1

following clitoria to 11 kg ha"1 in the fallow plots (Fig.2). In the 1992 WS, maximum levels of mineral Nmeasured were 33 kg ha"1 following clitoria, 21 kg ha"1

in crotalaria, and 14 kg ha~! in weedy fallow plots 25d after flooding and 21 kg ha"1 in siratro plots 35 dafter flooding. Higher mineral N in clitoria-treated plotsresulted from a higher rate of residue N mineralizationin 1991 and from a larger addition of residue N in 1992(Table 5). The N in plant residues with narrow C/Nratios generally mineralize faster than that with widerC/N ratios (Becker et al., 1994). Although, the residueswere not analyzed for their C/N ratio, it is likely thatthe clitoria residues had a narrower C/N ratio than thatof the crotalaria and siratro residues.

FALLOWCROTALARIACUTCRIASIRATRO

July 03 July 12 July 19 July 26 Aug02

20

10 - - .

June 25 July 11 July 20 July 30Dates of sampling

Fig. 2. Total 2 M KC1 extractable N in 0- to 30-cm soil during thefirst one month period of permanent flooding in a Philippine ricelowland as influenced by recycling of residues of three dry seasonlegumes. Vertical lines above mean bars indicate standard errors.

Dry Season Crop Residue Effects on Yieldand Nitrogen Uptake of Wet Season Rice

Incorporation of weed residues into fallow plots didnot alter grain yield and N uptake of the succeeding ricecrop. Incorporation of the DS legume residues, however,significantly increased yield and N uptake of WS ricein both years (Table 6). The legume residues were eitherequally or more effective than 25 to 50 kg fertilizer Nin stimulating yields. Even when aboveground residuesof the DS crops had been removed, rice yield and Nuptake were larger in the legume than in the weed-freefallowed plots. This could possibly be attributed to the Ncontribution from belowground residues of the legumes.Such contributions were calculated to have ranged from17 to 33 kg N ha"1 in 1991 and 13 to 36 kg N ha"1 in1992 (Table 7). From the amounts of residue N appliedand N accumulation in rice, apparent crop recovery of theresidue N was computed for both years. Such recoveriesranged from 15 to 31% (Table 8).

The year-to-year variation in crop recovery of N fromthe legume residues could be due to variation in theirapplication rates and possibly influenced by the chemicalcompositions of residues (not determined in this presentstudy). Legume residues that contain large amounts oflignin and polyphenolic compounds for instance areknown to be relatively resistant to decomposition (Foxet al., 1990; Palm and Sanchez, 1991; Becker et al.,


Table 6. The effects of fallow or legume cropping during dry seasons (DS) and recycling of the DS crop residues on grain yield and Nuptake of wet season (WS) rice in a Philippine rice lowland.

DS crop

Grain yield of WS rice N uptake by WS rice

+ residue - residue Difference + residue — residue Difference

PigeonpeaCrotalariaClitoriaDesmanthusSiratroWeedy fallow

O N25 NSONMean

5.2 abt5.6 a5.2 ab5.1 ab4.8 ab

2.8 c4.1 b4.5 ab4.6

. t ha-' -

3.9 ab3.9 ab4.5 a3.9 ab3.5 ab

2.7 b3.5 ab4.5 a3.8

19911.3**1.6**0.7*1.2**1.3**

0.1 ns0.5ns0.0 ns0.8

98 ab90 ab

110 a89 ab82 ab

43 c69 be80 ab83

1992

kg ha-'

70 a63 a77 a64 a61 a

44 a56 a74 a64

28**27**33**25**21**

-1 ns13*6 ns

19

PigeonpeaCrotalariaClitoriaDesmanthusSiratroFallow^Weed freeWeedy

Mean

5.0 ab5.1 ab5.3 a4.8 abc4.8 abc

4.4 c4.4 c4.8

4.5 a3.8 b4.5 a3.5 be4.3 a

2.5 d3.3 c3.8

0.5*1.2**0.8**1.4**0.5*

1.9**1.2**1.1

92 ab87 b

101 a82 be72 c

60 d76 c81

70 a51 b56 b47 b54 b

34 c51 b52

21**36**44**35**18**

26**25**29

*, ** Significant at P < 0.05 and 0.01, respectively; ns = nonsignificant.t Under each year, means in a column followed by a common letter are not significantly different (P < 0.05) by Duncan's multiple-range test.t The subplot treatments were 50 and 0 kg fertilizer N ha~', instead of + and - residue, respectively.

1994). Regardless of the mechanisms, the larger croprecovery of N from the Crotalaria and clitoria residuesobserved in both years suggested their superiority overthe other three as N sources for rice in lowland rice-basedcropping systems.

Legume Effects on ApparentSoil Nitrogen Balance

For legumes to play a positive role in the maintenanceof soil N fertility, they must leave behind more N fromN2 fixation than the amount of soil N they remove.We demonstrated that this could be achieved with thedual-purpose legumes included in our study (Table 7).

For example, it was estimated that crotalaria provided

a BNF input of 199 to 223 kg N ha"1. Despite substantialN removal through forage harvests (159-162 kg N ha"1

in five clippings), soil mineral N levels under crotalariaapparently increased by 37 to 64 kg ha"1 at the end ofthe season (Table 7). Additional contributions of thebelowground residues to WS rice ranged from 17 to 19kg N ha"1 (Table 7). Moreover, legumes appear to assistin reducing the potential for soil N losses by assimilatingsoil NO3-N when the NOs" levels were high and subjectto leaching and denitrification (Fig. la and Ib) and byretaining it in an organic form that is less susceptible toloss during the dry-to-wet transition (George et al.,1994). These observations suggest that dual-purpose le-gumes incorporated into rice-based cropping systems as

Table 7. The contributions of one grain and four forage legumes grown during dry seasons to apparent soil N balance in a Philippinerice wetland, t

Total N accumulationin aboveground

Legumes biomass



154277256251132

235279306283178

N fixedby legumes!

Ill22320019691

167199240226132

N returned to soilSoil N removal , through aboveground

by legumes legume residues

————————— kg ha"1 -1991

4354565541

19926880665746

11311811413781

14311716216199

Apparent Nbalance in soil

+ 70+ 64+ 58+ 82+ 40

+ 75+ 37+ 96

+ 104+ 53

Apparent Ncontribution oflegume roots

2619332017

3617221320

t Data calculated from average percentage of total N derived from N2 fixation (%Ndfa) values derived from Table 4 as N fixed = 1/100 (%Ndfa x totalN). The fourth column (Soil N removal by legumes) is the difference between the second and third columns. The sixth column (Apparent N balance insoil) is the difference between the fourth and fifth columns. The seventh column (Apparent N contribution of legume roots) represents the difference inN uptake (see Table 6) by rice from legume (without aboveground residue) and fallow (weedy with ON in 1991 and weed free in 1992) plots.

t Estimated by N difference and "N natural abundance methods with Setaria and Paragrass as nonlegume reference plants.


Table 8. Apparent recovery of legume residue N in rice in aPhilippine lowland.

LegumeResidue

N applied

Residue Naccumulated

in ricetCrop recoveryof residue NJ

- kg ha-1991



113 ab§118 a114 ab137 a81 b

143 ab117 be162 a161 a99c

28 ab27 ab33 a25 ab21 b

199221 c36 b44 a35 b18 c

2523291826

1531272218

t Estimated by the N difference method.t Data derived from second and third columns.§ Under each year, means in a column followed by a common letter are

not significantly different (P < 0.05) by Duncan's multiple-range test.

DS components contribute not only to increased produc-tivity but also to the maintenance and improvement ofsoil fertility in rainfed lowlands by virtue of their capacityto fix large amounts of atmospheric N and reduce soilN loss.

CONCLUSIONFive dual-purpose legumes grown during the DS

yielded 2 to 4 t ha"1 of forage or forage-plus-grain andyet left behind more N from BNF than the amount ofsoil N they removed during both years of this test.Legume residues incorporated into the soil supplied Nto WS rice and produced benefits comparable with thatof 25 to 50 kg fertilizer N. Such DS legumes that improveannual productivity of rainfed ricelands might be attrac-tive to farmers who are generally reluctant to growlegumes solely for green manures on their farms. Sincethe benefit of a steady increase in soil N and soil fertilityis clear, work must now be directed to removing barriersto adoption of appropriate components of the technology,so that the long-term sustainability of rice-based croppingsystems is assured. To ensure a positive role of forage andgrain legumes in sustaining soil N fertility, N removedthrough forage cuttings and grain harvests must not ex-ceed N fixed during the season. Location-specific studiesshould be initiated to identify suitable dual-purpose le-gumes of farmers' choice and to determine the optimumnumber and time of clippings for individual forage le-gumes.

ACKNOWLEDGMENTSThe authors acknowledge E.T. Rebancos, M.C. Duqueza,

E.G. Castillo, and Ruben M. Guevarra for their skilled techni-cal assistance in the field or laboratory.


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legume productivity and soil nitrogen dynamics in lowland rice-based cropping systems

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