Biomass allocation and nodulation of

Gliricidia sepiumunder two cut-and-carry forage production regimes

P. NYGREN* and P. CRUZ

INRA Centre Antilles-Guyane, Unité Agropédoclimatique, B.P. 515, F-97165 Pointe-à-Pitre,Guadeloupe (* Corresponding author: Department of Forest Ecology, Tropical Silviculture Unit,Box 28, FIN-00014 University of Helsinki, Finland; E-mail: pekka.nygren@helsinki.fi)

Keywords: fodder trees, legume trees, nitrogen harvest, nodule biomass, pruning, silvopastoralsystems

Abstract. The effects of two pruning regimes on the above-ground biomass allocation and nodu-lation of Gliricidia sepium (Jacq.) Walp. (Leguminosae: Robinieae) were studied in a cut-and-carry forage production system under humid tropical conditions in Guadeloupe, French Antilles.The grass layer composed of a mixture dominated by Paspalum notatum Flügge (80%) andDigitaria decumbens Stent. The pruning regimes were partial pruning (ca. 50%) every twomonths and complete pruning every six months. The complete pruning caused an almost completeturnover of N2 fixing nodules. The nodule biomass decreased after the partial pruning, but theturnover was not complete. The nodule to foliage biomass ratio followed the same pattern underboth treatments, and the values of the ratio converged towards the end of the experimental period.The maxima of standing nodule biomass were 7.2 and 13.0 kg ha–1 in the partially and com-pletely pruned trees, respectively. The cumulative leaf fodder harvest was higher under partialpruning management, due to smaller litter loss. The branch biomass production was higher undercomplete pruning management. Grass production was not affected by the pruning pattern of G.sepium. It was concluded that the partial pruning management produces more fodder in thestudied association, and the nodulation probably adjusts to the canopy N requirements. Thepotential N release to soil in the turnover of nodules of G. sepium (max. 0.82 kg ha–1) isnegligible compared to the N export in tree and grass fodder harvest, 190 and 215 kg ha–1 inpartially and completely pruned plots, respectively.

Introduction

Trees and shrubs are estimated to form the basic feed source for 75%of tropical livestock (FAO, 1985). A wide range of tree fodder productionsystems from extensive semi-nomadic browsing to intensively managedlegume tree protein banks is found throughout the tropics. A speciallypromising alternative is to produce legume trees and forage grass on the sameplot (e.g. Benavides et al., 1989; Ezenwa et al., 1995; Cruz, 1997). The systemoffers two important benefits: increased land-use efficiency (Benavides etal., 1989; Cruz et al., 1993), and balanced diet with the legume browse asthe principal protein source and grass as energy source (Camero et al., 1993;Smith et al., 1995).

The directly grazed legume tree – grass associations are assumed to besustainable, because the nutrients are efficiently recycled in the cattle excreta

Agroforestry Systems 41: 277–292, 1998. 1998 Kluwer Academic Publishers. Printed in the Netherlands.

(Catchpoole and Blair, 1990c), provided that the grazing intensity is not toohigh to impede the resprouting of trees (Mochiutti, 1995). The sustainabilityof the cut-and-carry fodder production systems, where both trees and grassare periodically harvested and carried to stabulated animals, is more ques-tionable. Very high amounts of nutrients may be exported from the plot inthe harvested forage; e.g. in a protein bank of Gliricidia sepium (Jacq.) Walp.(Leguminosae: Robinieae) in Queensland, Australia, ca. 740 kg ha–1 a–1 ofnitrogen was exported in tree prunings (Peoples et al., 1996). The directnitrogen transfer from legume trees to the associated grass seems to be ofminor importance (Catchpoole and Blair, 1990b; Rao and Giller, 1993). Thus,only the foliage litter and decaying roots and nodules may recycle N and othernutrients from trees to grass.

The cutting interval of fodder trees is usually quite short, because the pro-portion of nitrogen-rich foliage out of total harvest is high in the youngresprouts (Catchpoole and Blair, 1990a), and the nutritive value of the youngleaves is high (Mochiutti, 1995). Short cutting interval reduces forage lossin litterfall, and consequently, also nutrient recycling in the foliage litter.Complete pruning was observed to cause the complete turnover of nitrogenfixing nodules in the legume tree Erythrina poeppigiana (Walpers) O.F. Cook,and renodulation started two months after the pruning (Nygren and Ramírez,1995). The nodule turnover may have two opposing effects on the nitrogeneconomy of a cut-and-carry forage production system: (i) the decaying nodulesmay release tree N to the soil, and (ii) the unnodulated trees may competewith the grass for the soil N. The production and turnover rate of nodules maydepend both on the frequency and intensity of the pruning management. Thus,it is important to understand the effects of different pruning regimes on thedynamics of N2 fixing nodules in the cut-and-carry forage production.

Gliricidia sepium is perhaps the most widely cultivated multipurpose treein the tropics after Leucaena leucocephala (Lam.) de Wit. It is an importantcomponent of cut-and-carry forage production systems in Southeast Asia, SriLanka and in the Caribbean, while living fence systems are preferred in itsnative range in Central America (Simons and Stewart, 1994). In Guadeloupe,French Antilles, this introduced fodder tree is common in living fences whichare browsed by cattle. Due to the lack of pastureland in Guadeloupe and otherislands of the Lesser Antilles, the development of intensive cut-and-carryforage production systems is an attractive alternative for small-scale animalhusbandry. The aim of the field research reported here was to study the effectsof two different harvesting regimes on the above-ground biomass allocation,litter production and dynamics of N2-fixing nodules of G. sepium associatedwith perennial C4 grasses in a cut-and-carry forage production system underhumid tropical conditions.

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Materials and methods

The study was carried out at the experimental farm of the Antillean researchcentre of the Institut National de la Recherche Agronomique (INRA) in Prised’Eau, Guadeloupe (16

°12¢ N, 61°39¢ W, 125 m a.s.l.). The soils are alluvialUltisols with low acidity and relatively high cation exchange capacity, mostlydue to the abundance of calcium. The soil is quite uniform to the depth of0.5 m (Table 1). The climate is humid tropical. During the field measurementsof the present study, from September 1995 through February 1996, the averagedaily temperature maxima varied from 30.4 ± 0.7 °C in October to 27.7 ±0.6 °C in February, and the minima varied from 19.3 ± 1.5 °C in January to22.6 ± 1.1 °C in September. The rainfall during the six-month-period was1736 mm. Precipitation exceeded potential evapotranspiration also in the driestmonth, January (99.0 mm vs. 91.9 mm).

The experiment was established in May 1993. Gliricidia sepium wasplanted using stakes cut from a near-by living fence. The trees were plantedin rows with 0.7 m between trees and 3 m between rows, totalling 4 760 treesper hectare. The total area of the experiment is 0.4 ha. The between-row alleyswere initially planted with Pangola grass (Digitaria decumbens Stent.), butBahia grass (Paspalum notatum Flügge) soon invaded the plot. At the timeof the present study, P. notatum was dominating in the grass layer (ca. 80%of grass biomass), with D. decumbens being second most common species.Small amounts of Stenostaphrum secundatum O. Kuntze were also observed.

Between May 1993 and March 1995 the whole experiment was manageduniformly by means of partial prunings every 3–6 months according to thegrowth of trees, in order to avoid excessive shading of the grass layer.Complete fertilisation (60 kg ha–1 N, 50 kg ha–1 P and 100 kg ha–1 K) wasprovided at the beginning of the experiment. An additional N fertilisation wascarried out six months later and at the end of 1994 to promote grass growth.Another complete fertilisation (145 kg ha–1 N, 45 kg ha–1 P and 90 kg ha–1 K)was carried out in March 1995.

In April 1995, a 800-m2 area composing of nine rows of G. sepium andeight grass alleys was separated from the main experiment for this study. The

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Table 1. Chemical characteristics of the alluvial Ultisols of the study site in Prise d’Eau,Guadeloupe, in January 1996.

Soil pH Organic matter Total N Available P2O5 CEC Cadepth cm in water g kg–1 of dry matter meq 100g–1

00–10 6.0 39.2 ± 3.17a 2.6 ± 0.21 0.078 ± 0.030 13.5 ± 1.17 11.3 ± 0.94 11–30 6.0 35.0 ± 3.16 2.3 ± 0.26 0.058 ± 0.017 13.5 ± 1.08 11.6 ± 0.86 31–50 6.1 26.1 ± 5.84 1.9 ± 0.35 0.051 ± 0.023 11.9 ± 0.91 10.3 ± 0.97

a The figures are means ± standard deviation of 18 sample points.

trees were left untouched until August 1995 to assure a good nodulation atthe beginning of the two treatments. In September 1995, the trees were prunedcompletely and the experiment was divided into six 11 ´ 12 m plots, eachextending over four G. sepium hedgerows and three grass alleys. The twotreatments were assigned randomly to the plots. One treatment consisted ofpartial prunings every two months; about 50% of foliage were removed bycutting entire branches. The other treatment was left untouched for six months.Both treatments were pruned completely at the end of the experimentationon 29 February 1996. Both treatments were fertilised at the rate of 25, 32and 15 kg ha–1 of P, K and Mg, respectively, in October 1995 to promote nodu-lation. Thereafter, no fertilisations were carried out.

At each pruning, the above-ground biomass of G. sepium was estimatedby applying the pipe model theory (Nygren et al., 1993). The diameter of allbranches of ten trees per treatment was measured below the lowest green leaf.The fresh leaf area, and dry leaf and branch mass (48 h at 70 °C) was deter-mined for three branches per sample tree in the partial pruning treatment,and regression equations passing through the origin were fit to the data toestimate these variables as a function of branch cross-sectional area. Theregression coefficients varied between the prunings, especially for branchbiomass, and separate coefficient were applied for each pruning (Table 2). Theproportion of explained variance (R2) of the regressions varied from 0.94 to0.98. Four 30 ´ 30 cm litter collectors were placed on each plot after ten weeksof regrowth, when the litterfall of G. sepium begun. Thereafter, litter wascollected weekly, and its dry mass (48 h at 70 °C) was determined. The totalN concentration of the biomass compartments was determined by means ofthe Kjelldahl method at each pruning date.

Nodule biomass of G. sepium in the uppermost 10 cm soil layer wassampled three weeks after the initial pruning, and thereafter just before andtwo weeks after each partial pruning (9 November 1995 and 5 January 1996)and the final complete pruning (29 February 1996). Four undisturbed soilblocks of 1 dm3 were removed at the distances of 25, 75 and 125 cm fromthe hedgerows, on both sides of the central tree rows of each plot. This givesthe total of 16 samples per distance and plot. The soil blocks were washed

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Table 2. Regression coefficients applied to estimate the leaf area, leaf biomass and branchbiomass of a branch of Gliricidia sepium as a function of the branch cross-sectional area inmm2 in Prise d’Eau, Guadeloupe.

Pruning date Weeks of regrowth Leaf area Leaf biomass Branch biomassdm2 g g

9 Nov. 1995 09 0.382a 0.203 0.1315 Jan. 1996 17 0.411 0.296 0.324 29 Feb. 1996 25 0.240 0.217 0.742

a None of the regressions includes intercept term. Sample size for all regressions is 90 branches.

on a 1.5 mm sieve. The recovered nodules were cut to check the leghemo-globin activity; the nodules with pink or red interiors were considered active,N2-fixing, and the others inactive, non-N2-fixing. The nodules were pooled byactivity, distance and plot for the determination of biomass. They were driedfor 48 h at 70 °C, weighted and ashed for 6 hours at 650 °C. The ash-freedry mass was used in this study. On 5 January 1996, a subsample of bothactive and inactive nodules for each plot was analysed for total carbon andnitrogen content by means of dry combustion (Carlo Erba Instruments, Milan,Italy).

Grass was cut every three weeks to a height of 2 cm. The grass biomasswas sampled from two randomly situated temporary 0.4 m2 subsample plotsin each plot, giving a total of six repetitions for both pruning treatments. Thebiomass was determined after drying for 48 h at 70 °C, and the total N con-centration in the grass was determined by the Kjelldahl method. The grassspecies were not separated in the biomass estimations.

Results

Above-ground biomass production

The cumulative foliar biomass production in six months, i.e. the sum of thestanding biomass at the end of the experiment, litter fall and biomass removedin the prunings during the experimentation, was about the same under bothpruning regimes of G. sepium (Figure 1), but 30% more forage was harvestedin the partial pruning management. The total leaf fodder harvest was 1050kg ha–1 during the six-month-period in the partially pruned G. sepium, and800 kg ha–1 in the final complete pruning of the untouched trees. The totallitter fall during the period was 190 and 510 kg ha–1 in the partially prunedand unpruned plots, respectively. The total foliage biomass of the unprunedtrees peaked at 1400 kg ha–1 of standing biomass and 50 kg ha–1 of litter after17 weeks of regrowth.

The unpruned G. sepium produced more branch biomass than the partiallypruned trees (Figure 2); the cumulative branch biomass production was2750 and 1520 kg ha–1 in the unpruned and partially pruned trees, respectively.Practically no branch turnover was observed during the six-month-period,except the shedding of tiny dominated twigs at the beginning of regrowth.

The grass biomass production was unaffected by the management of G.sepium, although the shading leaf area in the unpruned hedgerows was abouttwo times higher than in the partially pruned hedgerows (Figure 3). The totalgrass harvest was 7360 and 6720 kg ha–1 in the alleys between unpruned andpartially pruned hedgerows, respectively. Following the initial peak produc-tion after eight weeks of tree regrowth, the grass harvest remained relativelystable throughout the experimentation.

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Nodulation

Very low nodulation, below 1 kg ha–1, was observed at three weeks after theinitial complete pruning (Figure 4). Thereafter, the nodulation recovered at

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Figure 1. Cumulative foliar biomass production and its distribution to standing green foliage,pruned green foliage and litter in partially pruned (top) and unpruned (bottom) Gliricidia sepiumhedgerows associated with perennial C4 grass at the time of prunings in Prise d’Eau, Guadeloupe.

the same rate under both pruning regimes until the second partial pruning after17 weeks regrowth. After that, the nodule biomass of the partially pruned G.sepium dropped, but the unpruned trees were in a phase of vigorous nodulegrowth. The highest nodule mass in the unpruned G. sepium (13.0 kg ha–1)was observed after 25 weeks regrowth and after 17 weeks regrowth in thepartially pruned hedgerows (7.2 kg ha–1). The proportion of N2-fixing nodulesout of total nodule mass varied between 80% and 90% in both treatments, andsenescent nodules formed the major part of the non-fixing nodules. Anexception was observed in unpruned hedgerows after 17 weeks regrowth: thepercentage of N2-fixing nodules was only 61%, because recently formed, butnot yet N2-fixing, nodules were very abundant, and almost no senescentnodules were observed.

The complete pruning at the end of the experimentation caused a highnodule turnover in all trees (Figure 4), down to 1.4 and 2.7 kg ha–1 in thepartially pruned and unpruned treatments, respectively. The proportion of N2-fixing nodules out of total nodule biomass was only 42% in the unprunedtreatment.

The nodule to foliage biomass ratio followed the same pattern under bothmanagement regimes (Figure 4), but was slightly higher in the partially prunedG. sepium, except before the first partial pruning after nine weeks regrowth.The foliage biomass shortly after the complete prunings (3 and 27 weeks

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Figure 2. Cumulative branch biomass production in partially pruned (filled circles) andunpruned (open circles) Gliricidia sepium hedgerows associated with perennial C4 grass andits distribution to standing and pruned biomass at the time of prunings in Prise d’Eau,Guadeloupe.

regrowth in Figure 4) was too low for the computation of a meaningful noduleto foliage biomass ratio.

The nodules of G. sepium were found everywhere in the three metreswide alleys between the hedgerows (Figure 5). The nodulation progressedgradually towards the alley centres, and after 25 weeks regrowth, the noduleswere most abundant in the alley centres under both pruning regimes. Thenodule density reduced close to the unpruned hedgerows after 17 weeksregrowth.

The nitrogen concentration of both N2-fixing and non-fixing nodules of G.sepium was quite high (Table 3). The higher ash content in the non-fixingnodules may be partially caused by small soil particles, which were impos-sible to wash out completely from the mushy senescent nodules. The washingof solid N2-fixing nodules was easier, and their ash content probably betterreflects the mineral content of the nodules of G. sepium. The carbon contentof both nodule groups was very similar.

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Figure 3. Harvested grass (mixture of Paspalum notatum and Digitaria decumbens) biomass(circles) in the 3 m wide alleys between partially pruned and unpruned Gliricidia sepiumhedgerows and the leaf area index of G. sepium hedgerows (squares) in Prise d’Eau, Guadeloupe.

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Figure 4. Development of ash-free nodule biomass (top) and nodule to foliage biomassratio (bottom) in partially pruned and unpruned Gliricidia sepium hedgerows associated withperennial C4 grass during six months regrowth (from 8 September 1995 to 29 February 1996)following the initial complete pruning in Prise d’Eau, Guadeloupe. The prunings are markedwith arrows.

Nitrogen harvesting

No significant differences were observed between the pruning regimes in thenitrogen concentrations of the biomass of G. sepium or grass. The leaves of

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Figure 5. Spatial distribution of nodules of partially pruned (top) and unpruned (bottom)Gliricidia sepium in 3 m wide alleys of perennial C4 grass after 9, 17 and 25 weeks regrowthfollowing the initial complete pruning in Prise d’Eau, Guadeloupe.

G. sepium were quite rich in N, and about two thirds of the leaf N was stillpresent in the litter (Table 4). The branches had much lower N concentra-tion. The N concentration in the grass was lower than in the green foliage orlitter of G. sepium. However, because of the high grass biomass production,the grass harvest contributed about 70% to the total N export from the systemunder both pruning regimes (Table 5). Although the branch N concentrationwas low, considerable amounts of N were exported in the branches ofuntouched G. sepium at the final complete pruning. Leaf N contributed 65%to the N export in the harvest of G. sepium under the partial pruning man-

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Table 3. Chemical characteristics of N2-fixing and non-N2-fixing nodules of Gliricidia sepiumassociated with perennial C4 grass on an alluvial Ultisol in Prise d’Eau, Guadeloupe.

Ashes Na Ca

mg g–1 mg g–1 g g–1

N2-fixing 068.3 ± 4.13b 63.0 ± 5.91 0.476 ± 0.006Non-fixing 104.7 ± 24.93 53.0 ± 7.78 0.483 ± 0.015

a C and N concentrations are corrected to ash-free nodule mass.b The figures are means ± standard deviations of six samples.

Table 4. Nitrogen concentrations in above-ground biomass compartments of Gliricidia sepiumand grass (mixture of Paspalum notatum and Digitaria decumbens) grown in a cut-and-carryforage production association on an alluvial Ultisol in Prise d’Eau, Guadeloupe.

G. sepium (n = 18) Grass (n = 72)

Leaves Branches Litter Leaves

33.5 ± 0.7a 12.3 ± 0.8 23.8 ± 1.9 20.7 ± 2.1

a All figures are average values of N concentrations (mg g–1) ± standard deviation.

Table 5. Nitrogen export in harvested biomass of Gliricidia sepium and grass (mixture ofPaspalum notatum and Digitaria decumbens), and nitrogen cycling in litterfall of G. sepiumduring six months in a cut-and-carry forage production association on an alluvial Ultisol in Prised’Eau, Guadeloupe.

G. sepium Grass Total Litterharvested

Leaves Branches Total

Pruned 35.3a 19.0 54.3 135.6 189.9 4.7± 2.7 ± 1.3 ± 3.9 ± 9.8 ± 6.1 ± 1.3

Unpruned 26.8 35.2 61.9 152.9 214.8 12.8± 8.4 ± 11.1 ± 19.6 ± 20.0 ± 2.5 ± 0.6

a All figures are average values of N content (kg ha–1) ± standard deviation per biomasscompartment in three plots.

agement. The amount of N recycled in the litter of G. sepium was very low,being about 2.5% and 6.0% of the N export in the forage harvest in thepartially pruned and unpruned plots, respectively.

Discussion

The foliar, branch and nodule biomass was uniform in all plots after nineweeks of regrowth (Figures 1, 2 and 4) when the partial pruning treatmentbegun. This indicates the uniformity of the trees in the experimental area. Alsothe soil appeared to be quite uniform (cf. Table 1). Thus, we concluded thatthe differences in the above-ground biomass allocation and nodulationobserved later on during the experiment were caused by the differences inpruning management.

The above-ground biomass estimation was based on the pipe model rela-tionships of individual branches. This method has been shown to give reliableestimates of leaf area and leaf biomass of periodically pruned trees if entirebranches are removed or left on the tree (Nygren et al., 1993), as was donein this study. The proportion of explained variance of the regressions appliedin this study was high for both estimated biomass compartments and leaf area,indicating that the method can be applied for G. sepium. Differences in theregression coefficients between the pruning dates were observed, and theaccuracy of the biomass estimate was assured by applying the specific regres-sion for each sampling date.

The highest standing foliage biomass observed in the unpruned trees (1400kg ha–1) was higher than foliage biomass in final pruning + litterfall (800 +510 kg ha–1; cf. Figure 1). This difference was probably caused by the recir-culation of carbohydrates and other dry matter from the senescing leaves toother tree organs, e.g. new leaves and branches, before shedding; followingto that, the dry mass of litter may have been lower than the total dry matterloss from senescing foliage. In February, the litterfall was not compensatedby the production of new leaves in the unpruned hedgerows, because the treeswere starting the reproductive cycle. This caused a reduction of the standingfoliar biomass. However, only a tiny amount of flowers (ca. 3.0 kg ha–1) wereblossoming at the time of the final pruning, which interrupted the reproduc-tive cycle.

The tree fodder harvest was about 20% higher under partial pruning man-agement than in the complete pruning after six months regrowth. This dif-ference may be mainly attributed to the lower leaf biomass loss in litter.Further, the fodder availability during the whole growing cycle is better underpartial pruning management. The total fodder harvest (tree + grass) was aboutthe same under both pruning regimes due to the steady grass production. Underseasonal climatic conditions in Nigeria, complete pruning every three monthsgave both highest tree foliage (mixture of G. sepium and L. leucocephala) andGuinea grass (Panicum maximum Jacq. cv. Ntchisi) harvest under the same

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or lower tree density than in our experiment (Ezenwa et al., 1995). In thehumid tropics of Costa Rica, the G. sepium fodder harvest was the best with33% pruning every 50 days or 66% pruning every 75 days. Complete pruningat short cutting interval caused high tree mortality, and at long interval theforage production was low. The fodder quality was the best with the shortestpruning interval (Mochiutti, 1995). These results indicate that short pruninginterval is beneficial for G. sepium fodder production.

The grass production and N content were not affected by the pruning patternof G. sepium, although the shading leaf area was higher in the unprunedhedgerows. Shading has two opposing effects on pasture growth: it reducesthe photosynthetic production and, consequently, carbon available for growth,and it may improve soil water status and nitrogen mineralisation, especiallyunder dry conditions (Wilson, 1996). The combined effect is often increasedabove-ground growth and N uptake of grasses in tropical silvopastoral systems(Wilson et al., 1990; Cruz, 1997). Because we did not measure the grass rootbiomass, it is possible that the total biomass production was lower underunpruned trees, but the above-ground biomass was not affected due to theincreased shoot to root ratio (cf. Wong, 1991).

The nodule biomass of G. sepium was quite low under both pruningregimes, with the peak value of only 13.0 kg ha–1. High nodule turnover wasobserved after both complete prunings (8 September 1995 and 29 February1996), but the partial pruning had only a minor effect on nodulation. Thenodule to foliage biomass ratio followed the same pattern under both man-agement regimes, and was higher in partially pruned trees, especially between9th and 17th week of regrowth. Considering that the nodulation may followthe N requirements of the canopy in N2-fixing trees (Nygren, 1995), as wellas in other legume plants (Parsons et al., 1993), this result suggests that thenodule mass, and consequently the N2 fixation, may have been adjusted tothe N use in the G. sepium canopy.

The nodulation of inoculated G. sepium under controlled conditions hasbeen abundant on different soils (Kadiata et al., 1996) and under differentfertilisation regimes (Cobbina et al., 1992). Low nodulation was observed onlyon a very acid Amazonian Ultisol (Moreira, 1995). The highest nodule tofoliage biomass ratio is reached in young saplings (Kadiata and Mulongoy,1992). Under field conditions the nodulation seems to be less abundant;Liyanage et al. (1994) found 170–430 mg tree–1 of nodules in a pure G. sepiumplantation of 5000 trees ha–1 in Sri Lanka (0.85–2.15 kg ha–1 of nodules), andSuarna et al. (1989) report 25–54.6 kg ha–1 of G. sepium nodules in fodderproduction associations with 11 000 trees ha–1 in Bali. Our results fall betweenthese two observations. Higher relative nodulation (nodule to foliage biomassratio of 33 mg nodules per g foliage) was observed in small G. sepium seedlingin the field in Nigeria (Mulongoy and Owoaje, 1992).

Comparison of the nodulation in the greenhouse or field grown seedlingswith the observations on established trees in our site, and in the two earlierstudies, indicates the difficulty of extrapolating the seedling data on rhizo-

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bial symbiosis to big trees. G. sepium seems to require more N in the estab-lishment phase, and the relative allocation in the root system (cf. Parrottaand Singh, 1992) and nodules is higher than in established trees.

In spite of the low nodulation, G. sepium seems to be an active N2 fixer.The low nodulating G. sepium in Sri Lanka supplied on average 55% (range52–64% in different genotypes) of the tree N by means of N2 fixation(Liyanage et al., 1994). Active fixation has also been observed in G. sepiummanaged by periodic prunings; the proportion of fixed N out of total N varied31–54%, depending on season, in six complete prunings of an alley croppingassociation in the Philippines (Ladha et al., 1993), and 56–89% in the fodderremoved in seven complete prunings of a protein bank in Queensland (Peopleset al., 1996). If the nodule biomass of G. sepium in these associations is aboutthe same as in our site, or in the two other field sites studied (Suarna et al.,1989; Liyanage et al. 1994), the G. sepium – Rhizobium sp. symbiosis appearsto be a very efficient N2 fixer.

Because of the low biomass, the nodules of G. sepium are of minor impor-tance for the N recycling in agroforestry associations. In our site, the completeturnover of the 13.0 kg ha–1 of nodules with N concentration of 63 g kg–1 inthe unpruned trees would have released only 0.82 kg ha–1 of N to the soil.This is only 6.4% of the N recycling in litterfall under the same managementregime, and negligible compared to the N export in fodder harvest (Table 5).It is also low compared to the potential N supply in the decomposing nodulesof Erythrina poeppigiana following complete pruning, which is estimated tobe 2.3–8.9 kg ha–1, depending on genotype (Nygren and Ramírez, 1995).

Our results, together with the few other field observations on the nodula-tion of G. sepium (Suarna et al., 1989; Liyanage et al., 1994), permit toconclude that the species has a low nodulation level. The nodule turnover afterpruning seems to be caused by the adjustment to the canopy N requirements:the nodule to foliage biomass ratio followed the same pattern in partiallypruned and unpruned trees, and the ratio converged to the same value towardsthe end of the study period. The complete pruning caused an almost completeturnover of N2 fixing nodules as observed earlier in E. poeppigiana (Nygrenand Ramírez, 1995). The role of nodule turnover in the N recycling of thestudied G. sepium – grass association is negligible. However, according to thereviewed literature, G. sepium seems to be an active N2 fixer, and may thusprovide a considerable proportion of its own N requirements by means of theN2 fixation. Frequent partial prunings gave a better forage yield than acomplete pruning during the study period, but the medium- to long-termsustainability of the management regimes requires further study. Research isalso needed on the effects of different pruning regimes on the N2 fixationitself.

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Acknowledgements

We thank Simon Leinster, Franck Solvar and Saint-Ange Sophie for themanagement of the experiment, and the whole group of field and laboratorytechnicians of the Agropedoclimatic Unit for participation in the nodulesampling. The contribution of P.N. was financed by a post-doc scholarshipfrom the Agronomy Department of INRA.

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