Nitrogen dynamics of tropical agroforestry and annual cropping systems

0038-0717193$6.00 + 0.00 Copyright 0 1993 Pergamon Press Ltd

Soil Biol. Biochem. Vol. 25, No. 10, pp. 1363-1378. 1993 Printed in Great Britain. All rights reserved

NITROGEN

DYNAMICS OF TROPICAL AGROFORESTRY AND ANNUAL CROPPING SYSTEMS

J. P. HAGCAR,‘* ‘Botany forestry

School, Project

E. V. J. TANNER,’ J. W. BEERS and D. C. L. KASS~

University of Cambridge, Downing Street, Cambridge and 3Agroforestry Coordinator. Centro Agronomic0 Ensenanza, 7170 Turrialba, Costa Rica

CB2 3EA, U.K. 2GTZ AgroTropical de Investigation y

(Accepted 25 March 1993)

Summary-The relative importance of the processes of SOM (maintenance of active soil organic matter) and SYNCHRONY (timing of release of organically-bound nutrients to coincide with crop demand) were assessed for their contribution to the maintenance of crop nitrogen availability in alley cropping. Alley cropping is a system of agroforestry where trees and crops are intercropped, the former being periodically pruned to produce mulch. Two maize alley cropping treatments, with Erythrina poeppigiana and with Gliricidiu sepium, were compared to sole-cropped maize in an 8 yr old experiment at CATIE in Costa Rica. Maize productivity, maize N uptake, and N release from mulch and crop residue decomposition were measured each month during one cropping cycle. The effects of changes in active soil organic matter (SOM) on available N were assessed by measuring field N mineralization and the size of the microbial N pool through the cropping season. Two sub-treatments were introduced to assess the contribution of a current mulch application to maize N uptake (1) removing the mulch, and (2) applying 15N labelled mulch. Monthly sampling of “N in the mulch, microbial biomass, and maize allowed assessment of the SYNCHRONY of mulch N release and crop uptake. Maize biomass and maize N content, N release from mulch and residue decomposition, and N mineralization were all higher in the alley crop than the sole crop by 2.2-, 2.8-, 5.0- and 2.1-fold respectively. Soil microbial N was not significantly different between treatments, but increased by 80% during the cropping season. Maize grown in the alley crop with the mulch removed contained only 3-15% less N at maturity. Similarly 15N labelled mulch only contributed about 10% of crop N. The percentage contribution of mulch lSN to the maize declined from 13-14% 30 days after planting to &II% 100 days after planting. Total recovery of mulch N by the maize was only about IO kg ha-’ and almost all of this was taken up by 60 days after planting. The contribution of mulch N to weed N content declined from 15-24% 7 weeks after mulch application to 24% 9 months after application. Mulch N contributed only 3-5% of the microbial N pool at 40 days and this fell to zero by 105 days. The higher rates of N mineralization under the alley crop compared to rates under the sole crop led to faster establishment of the maize in the alley crop and maintained higher rates of N accumulation thereafter. These higher rates of N mineralization resulted from the build up of readily-mineralizable organic N in the soil over the 7 yrs of tree mulch application. The size of the microbial N pool was not to be related to nitrogen availability nor organic residue inputs. Mulch N released during a cropping season accounted for about 15% of the increase in N uptake by maize. Transfer of mulch N to the crop may have been restricted by the low incorporation of mulch N into the microbial biomass. The long-term build-up of the SOM reserve of mineralizable organic N was more important than the SYNCHRONY of mulch N release and crop uptake in determining the substantially higher productivity and N uptake in the alley crop compared to the sole crop.

INTRODUCTION

The continuous cultivation of annual crops on many humid tropical soils leads to a decline in nitrogen availability and total soil N (Nye and Greenland, 1960; Sanchez et al., 1983). There appears to be two major causes of the decline in soil N availability under continuous annual cropping. (1) Low organic matter inputs to the soil lead to a decline in the mineralization of organic N in the soil (Mueller-Harvey et al., 1985). Ayanaba et al. (1976) found that during the cropping period after a fallow *Present address: Florida, 220 U.S.A.

Department of Botany, University of Bartram Hall, Gainesville, FL 3261 I,

the proportional decline in labile soil N was greater than in total soil N, indicating that the capacity of the soil to supply N declined even more rapidly than the decline in the soil N reserve. (2) Animal crops are characterized by a low efficiency of recovery of soil nutrients at high nutrient availabilities, which leads to high nutrient losses by leaching (Bartholomew, 1977). Van der Kruijs et al. (1988) found low recoveries of “N labelled fertilizer in crops and high losses in leachate. The Tropical Soil Biology and Fertility (TSBF) Programme (Anderson and Ingram, 1989) has defined two main processes by which the management of organic matter inputs may mitigate the fall in soil nutrient availability.

1363

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J. P. HAGGAR ef

(1) The SOM hypothesis that the labile pools of organic-bound nutrients in the soil can be augmented by increasing the input of high quality organic matter. Juo and La1 (1977) demonstrated that returning crop residues to the soil may reduce the decline in soil N during cropping. (2) The SYNCHRONY hypothesis states that the timing of application of organic residues may be critical to the efficient use by the crop of nutrients released by the decomposition of the residues. If large quantities of nutrients are released at a time of low demand by the crop then those nutrients may be lost from the system (Swift et al., 1980). The microbial biomass plays a potentially important role in both the processes of SOM and SYNCHRONY. The microbial N pool is frequently considered as the labile organic N pool of the SOM (Parton et al., 1987). Further the microbial biomass is the mediator of the mineralization of organicallybound nutrients and thus the determinant of the SYNCHRONY of release and uptake. Soil microbial biomass has been suggested as an index of rates of decomposition and mineralization (Alegre et al., 1989). A close relationship between the amounts of organic matter inputs and microbial biomass was shown by Ayanaba et al. (1976). They studied microbial biomass, and N released after fumigation and incubation in soils under different managements. Bush fallow soils had higher N release and microbial biomass than cropped soils; within cropped soils, mulched soils had higher N release and microbial biomass than unmulched soils. Alley cropping is an agroforestry practice that intercrops annual crops with trees planted in lines, the trees are periodically pruned for the production of organic mulch. The aim of this practice is to integrate the soil organic matter sustaining capacity of fallow trees into a continuous annual cropping system. La1 (1989) found that total soil organic C and N concentrations declined less under alley cropping than a plough-till sole crop, but the decline under alley cropping was the same as under the no-till sole crop treatment. Yamoah et al. (1986a) found that total soil N and microbial biomass increased under maize alley-cropped with Cassia compared to sole-cropped maize, but did not change with Gliricidia and Flemingia alley cropping. The N content of mulch produced by pruning the trees in alley cropping may be up to 200 kg ha-’ yr-’ (Young, 1989). Decomposition studies by Yamoah et al. (1986b) showed that the N release from Flemin gia and Cassia prunings was equivalent to 25 and 75%, respectively, of the N demand of the maize. However, “N tracer studies of crop recovery of N from organic residues have shown low recoveries of only l-28% of mulch N in the first crop after application (Seligman er al., 1986; Ladd and Amato, 1986; Sisworo et al., 1990). Substantially higher recoveries of residual N are found when subsequent

al

crops are included, which may reach up to 70% over 3-4 yr. Mulongoy and van der Meersch (1988) found that leaving the prunings from a Leucaena fallow increased N uptake of maize by 37% compared to if the prunings were removed. Nevertheless the increase in N uptake with the prunings represented only 3.2-9.4% of the N released by the prunings. It was suggested that the rest of the N from the prunings would build up the soil organic N pool and become available to future crops. No field assessments of microbial immobilization of labelled N are known to have been carried out in the humid tropics. Estimates of the microbial immobilization of added N from other regions vary from 22% of Medicago littoralis mulch “N applied to a semi-arid calcareous soil in Australia (Ladd et al., 1981) to 337% of “N labelled NH,NO, applied to the Broadbalk Continuous Wheat experimental plots in temperate England (Shen et al.. 1989). Jackson er al. (1989) found that immobilization of “N applied as nitrate or ammonium to an annual grassland in California was five times greater in the microbial biomass than in the plants. It was suggested that the microbial biomass was competitively superior at obtaining available soil N, at least in the short term. Thus although the microbial biomass releases organically-bound nutrients it also immobilizes a proportion of those nutrients within its own biomass and may restrict their availability to plants. Our aim was to evaluate the relative contributions of the processes of SOM (maintenance of soil organic matter) and SYNCHRONY (matching organic nutrient release to crop uptake) in maintaining the nitrogen supply in a 8 yr old alley cropping system. To test SOM soil organic N mineralization rates and soil microbial N were measured in the field. The SYNCHRONY of N release from tree prunings and crop residues was assessed by “N labelling the residues and following fate of the “N through temporal studies of decomposition, uptake by the microbial biomass and transfer to the crop. With a knowledge of the relative capacity of these processes to sustain nutrient availability, priorities can be established as to the best techniques for the development of sustainable agricultural systems in lowland humid tropical soils. SITE DESCRIPTION

The study was carried out on the CATIE (Centro Agronomic0 Tropical de Investigation y Ensenanza), La Montana alley cropping experiment in Costa Rica, established in 1982 (Kass, 1987). The soil was a Typic Humitropept, fine, halloysitic, isohypothermic, with the surface soil (O-15 cm) having pH 4.6, 4.8% organic matter and 2.9 mg total N go ’ (Kass, 1987). The whole experiment is cropped annually with a rotation of maize and beans under a no-till system. The three field treatments were

Nitrogen dynamics in agroforestry

distributed within a randomized with three blocks:

block design,

(1) Sole crop. (2) Alley cropping

with Erythrina poeppigiana (Walpers) 0. F. Cook, tree rows 6m apart, trees 3 m apart within rows. (3) Alley cropping with Gliricidia sepium (Jacq.), tree rows 6 m apart, trees 0.5 m apart within rows. Main plots were 12 x 18 m with each plot split for treatment with N fertilization at 0 or 150 kg ha-’ yr-’ as NH,NO, added to the maize 1 month after planting. The whole experiment receives 71 kg of K, 38 kg of P and 10 kg of Mg ha-’ yr-‘. The present investigations were carried out on the non-N fertilized subplots. Maize (Zea mays, var. Tuxpeno) was planted at 40,000 plants ha-’ and beans (Phaseolus vulgaris, var. Turrialba) at 133,000 plants ha-‘. The trees in the alley-cropped plots were pollarded every 6 months at the beginning of each cropping cycle and the prunings distributed as mulch. All crop residues were left in the field. Production of mulch by pruning the trees (between 42 and 72 months after establishment of the experiment) averaged 11.4 t ha-’ yr-’ for Erythrina and 7.9 t ha-’ yr-’ for Gliricidia, which contained 270 and 220 kg N respectively. Yields of maize in the alley crop were 2-3.5 t ha-’ yr-’ between 1982 and 1989. Sole crop yields were about the same as in the alley crop for the first 6 yr, but then the yield fell to about 1.5 t ha-’ yr-‘. Bean yields (not including the first year when no mulch was applied) averaged 1150kgha-‘yr-’ under alley and cropping 750 kg ha-’ yr-’ under sole cropping (Kass et al., 1989). Only the sole-crop showed a response to N fertilization (by comparison of subplots), indicating that N availability was limiting production of the sole crop but not the alley-crop (Kass et aI., 1989). Total soil N has not differed between the treatments (Kass et a/., 1989). Kass and Diaz-Romeu (1986) demonstrated higher soil K availability under alley cropping than sole cropping. Haggar et al. (1991) found that bioassay plants took up less P from alley-cropped soils than sole-cropped soils, indicating lower soil P availability. Nevertheless, field crop P uptake was higher in the alley crop than the sole crop probably due to overall P availability being increased by the cycling of P contained in the mulch, which was not available in the bioassay. METHODS

Our study was carried out over 4 months in 1989 during one cropping of maize. The trees were pruned on 25 May. The maize was sown on 9 June, emerged 5 days later, reached physiological maturity about 10 September and was harvested at the end of September.

Evaluation

1365

of crop productivity

and N uptake

Maize was sampled every 30 days up to physiological maturity (90 days after sowing) to assess total biomass and N uptake. At each time, in each replicate, a single plant was sampled at 0.5-l .O, I .5-2.0, and 2.5-3.0m from the hedgerow along three randomly chosen 3 m transects perpendicular to the hedgerows. The sole crop maize was sampled in an equivalent fashion. Thus, nine plants were taken per plot per month, and these were combined into one sample per plot. The plant samples were dried at 70°C and weighed. N content was measured by digesting the samples in concentrated HISO (with mercury oxide catalyst) and H,O,, and analysing the digest by standard techniques (Chemlab Instruments Methods sheets CW2-008-11, 1982 and CW2-075-01, 1983). Standard samples, checked by independent laboratories, were analysed with each batch of experimental material. The natural logarithms of biomass and total N content of the maize plants were regressed against time using a second-order orthogonal polynomial. The linear coefficient (b,) measured the mean relative growth rate and the quadratic coefficient (b,) the change in relative growth rate over time (Hunt, 1982). Organic matter and N inputs from tree mulch and crop residues

Standing mulch biomass and N content were measured approximately every 38 days for 115 days, starting from immediately after the pruning of the trees on 25 May 1989 (15 days prior to planting the maize). There was no significant litter fall from the trees for the first 3 months after pruning (and after this new litter can easily be distinguished from old litter). Therefore measurements of changes in standing mulch give an accurate estimate of mulch decomposition. Three 0.5 m2 random quadrat samples of mulch (stratified with respect to distance from the trees) were taken per plot, and combined. Samples taken at 0 and 115 days were divided into leaf, new branch, old residual branch (new and old branch material were only separated at 0 days), crop residue and other material which included dead weeds. Samples taken at 40 and 80 days were not sorted into different materials. Samples were dried (7O”C), weighed and N content measured (same method as for plants). The decomposition constant k of the exponential decay curve of the decline in mulch was calculated by regressing the natural logarithm of mulch biomass and N content against time using a first-order orthogonal polynomial (see analysis of data for further details). Mineralization

of soil organic nitrogen

Concentrations of nitrate and ammonium in the soil and net N mineralization in the field were measured through the cropping season. Measure-

1366

J. P. HAGGAR et al.

ments were taken to establish background conditions immediately before application of mulch (day 0). Further measurements were taken at the emergence of the maize (20 days after application of mulch) and were then repeated approximately every 30 days, for the next 3 months. Paired soil cores 7.5 cm dia and 25 cm deep were taken in the field using a method adapted from Anderson and Ingram (1989). Three pairs of cores were taken from each plot in a stratified random distribution (as for mulch). One of each pair was immediately analysed for nitrate plus ammonium. The other core was isolated in a plastic tube, wrapped inside a plastic bag, and returned to the original sample hole to incubate for 4 weeks, after which it was collected and analysed. Samples taken after mulch application had the mulch removed from the surface of the core. Previous studies had shown that incubated mulch demonstrated irregular changes in nitrate and ammonia contents. Soil nitrate plus ammonium were extracted with 2 M KC1 and measured by titration following steam distillation (nitrate and ammonium were not separated). Net N mineralization was calculated as the difference between the concentrations in the initial and the incubated cores of each pair. The mean difference between the three pairs was calculated for each plot. Differences over time were analysed by orthogonal contrasts between the first two measurements vs the last two, and between the first vs the second, and the third vs the fourth (see analysis of data for more details). Evaluation of N transfer from tree mulch to crop Two sub-treatments were introduced into the main alley crop and sole crop plots in order to assess the contribution of mulch N, applied within a cropping cycle, to crop nutrition: (1) application of 15Nlabelled tree mulch and crop residues to follow the transfer of mulch N to the crop, and (2) maize grown without the application of tree mulch but still with the residual fertility conditions of the main treatments. Tree and crop biomass enriched with “N was produced by applying 10% “N-enriched ammonium sulphate to trees and maize grown in pots in a shade house. Rooted stakes of Erythrina poeppigiana (provenance No. HLAFN 2687 in the CATIE collection) and Giiricidia sepium (provenance No. HLAFN 2752) and seed of Zea mays (cv. Tuxpeno C-7) were sown in large tubs (50 cm dia x 50 cm deep) of river sand and given a complete fertilizer [1.54 g N, 0.3 1 g

Pl, 1.4 g K, 1.6 g Ca and 0.8 g Mg plant -I month ’ for trees and 0.93 g N, 1.O g P, 1.55 g K, 1.44 g Ca and 0.56 g Mg plant-’ month-’ for maize, adapted from methodology of Arnon and Hoagland (1940)]. Sixteen stakes of the trees (one per tub) and 10 plants of maize (two per tub) were grown and all leaf and branch material harvested after 5 months, Only the leaf and stem of the maize plants were harvested, the cobs were removed. The two sub-treatments were established within microplots in the main plots. Two microplots were randomly positioned in each of the three replicates of the three treatments. The alley crop microplots were aligned perpendicular to the rows of trees so that they crossed half the width of an alley and thus covered the grade of influence of the trees. An equivalent distribution was used in the sole crop plots. Microplots in the Erythrina alley crop plots were 3 x 1 m and contained 10 maize plants. Microplots in the Gliricidia alley crop and the sole crop plots were 2 x 1 m and contained 8 maize plants. In one microplot per alley crop plot a proportion of the current tree mulch application (tree had been pruned the previous day) was replaced with 15N amended tree mulch (sub-treatment 1). In the other microplot all of the current mulch application was removed (sub-treatment 2). In the sole crop plots 15N-enriched maize residue was added to one microplot (subtreatment l), and the other microplot was left undisturbed (sub-treatment 2). Thus the application of 15N organic residues in the alley crops was a replacement of mulch already present, whereas in the sole crop it was an extra addition of crop residue mulch. Table 1 gives the quantities and concentrations of “N and total N added in the amended mulch, and the total N in the unamended mulch left in situ in the microplots. The amended tree mulches have a higher percentage N content, due to containing a lower proportion of branch material, than the unamended mulch. 15N amended mulch was added to microplots in only two of the three replicates of the Gliricidia treatment because insufficient material was available. Within the “N amended microplots mulch was sampled at approx. 40, 75 and 110 days after application to assess the quantity of “N released. Two 0.04 m* quadrat samples were taken at random from each microplot and combined for each plot. Samples were then divided into tree mulch and other material (mostly crop and weed residues). Samples were dried at 70°C weighed, and N content measured as for main plot samples.

Table I. Rates of application of “N amended mulch applied to microplots, showing total amount and concentration mulch and total amounts and concentration of N in amended and unamended mulch

Erythrina mulch Gliricidia mulch Maize residue mulch

‘IN (gmm2)

(% “N)

0.671 0.169 0.271

6.26 5.80 5.45

Amended mulch (% N) (gNmm2) 10.72 2.92 4.97

3.3 3.8 I .4

of “N m amended

Unamended mulch left in siru (% N) (gNm+) 4.56 2.6 6.54 2.8 0

Total mulch (gNmm2) 15.29 9.44 4.97

Nitrogen

dynamics

Maize plants were sampled at 30, 60 and 90 days after planting. At each sampling three randomly-chosen maize plants were sampled (stratified with respect to distance from the trees) from each Erythrina alley crop microplot, and two randomly-chosen plants were sampled from each of the other treatment microplots. Plants from within microplots were combined, dried, weighed, and analysed as the mulch. In the 15N amended microplots weed biomass and the later bean crop were sampled to follow the decline in “N over a longer period. Weed biomass was sampled from three randomly positioned 0.5 m2 quadrats at 50, 105 and 245 days after amended mulch application. The bean crop planted after the maize (sown 200 days after mulch application) was sampled in the same way as the weeds at 265 days. Samples from the quadrats within each microplot were combined, dried weighed and analysed as was done for the mulch. Plant and mulch samples from the “N-amended plots were analysed for 15N content by Merlewood Research Station, ITE, Grange-Over-Sands, U.K. using a Dumas oxidation furnace linked to a singleinlet triple-collector mass spectrometer. From the measured “N contents of the plants and mulch, three parameters were calculated: proportional contribution of mulch N to plant N uptake; absolute amount of mulch N taken up by the plants; and the proportion of N released from the mulch that is recovered by the plants. Changes in these parameters for each treatment over time were analysed using first- or second-order orthogonal polynomial regressions within an analysis of variance. Measurement of soil microbial N Within the main plots, and the “N microplots, soil samples were taken at four times: 5 days before application of the mulch (except in “N microplots), and at 40, 70 and 105 days after application of the mulch. Soil cores 2.5 cm dia and 25 cm deep were taken from three randomly-selected 3 m transects within each plot, and one transect within each “N microplot. The transects were aligned perpendicular to the rows of trees, crossing half the width of an alley (an equivalent distribution of transects was used in the sole crop plots). Two cores were taken at each of the following distances from the trees, 75, 150 and 225cm (six cores per transect). The soil from the cores from within each plot was mixed thoroughly, sieved <2 mm to remove roots, and four 30 g (wet wt) subsamples removed. The method used to measure microbial N was based on the fumigation-extraction method of Sparling and West (1988). Two of the subsamples from each plot were fumigated with 0.2 ml of chloroform for 2 h, after which the chloroform was removed by repeated evacuation. Both fumigated and non-fumigated subsamples were then extracted by shaking for 1 h with 100 ml 0.5 M K,SO,. After centrifugation and filtration 20 ml subsamples of the supernatant were

1367

in agroforestry

digested with 2 ml H,SO, and 0.5 ml 190 mM CuSO,. Ammonium N was measured by neutralizing the digests with NaOH, steam distiling, and titrating the ammonia collected in boric acid against 5 mM H,SO,. Standard solutions of ammonium nitrate were distiled with each batch of samples to ensure consistent recovery of ammonia. Values for the two subsamples receiving the same laboratory treatment were averaged. The samples from the 15N-amended microplots were treated as the other samples except that the two subsamples each of fumigated and non-fumigated extracts were combined into one fumigated and one non-fumigated sample, and the whole sample distiled into excess 5 mM H,SO,. The sample was then dried as for the plant and analysed for “N content samples. The above procedure measures extractable soluble organic N and ammonium in the soil. The difference in the extractable organic N and ammonium in the non-fumigated and fumigated samples gives an estimate of the relative sizes of the microbial N pool in the soil (Sparling and West, 1988). Microbial “N content can be calculated from the difference in total 15N in the non-fumigated and fumigated extracts. As the extraction method does not recover all the microbial N, measured microbial N values have been multiplied by factors of between 1.85 and 2.2 in order to estimate actual total microbial N (Sparling and West, 1988; Ocio et al., 1991). In our study measured microbial N recoveries have been multiplied by 2 in order to estimate the recovery of total mulch “N in the microbial biomass. Changes in microbial N over time were analysed by making orthogonal contrasts between the first two measurements vs the last two, and then between the first vs the second, and the third vs the fourth, Soluble organic, and microbial “N concentrations were regressed against time using a linear orthogonal polynomial. In all cases there were three replicate values for each time measurement of each treatment. Analysis of data between treatments and over time Differences between treatments were analysed within an analysis of variance by orthogonal contrasts between the combined alley-cropped treatments and the sole crop, and then a contrast was made between the two alley-cropped treatments. Changes over time were analysed by fitting orthogonal contrasts over time. The significance of the time contrasts was assessed by testing it against the contrast x block interaction (Rowe11 and Walters, 1976). RESULTS

Maize productivity and N uptake Maize biomass and N content at physiological maturity were 2.2- and 2.8-fold higher respectively under alley cropping than sole cropping (Fig. 1). The

J. P. HAGGAR et al

1368 (a)

Maize

biomass

at maturity

time in the alley crop, probably due to the alley crop maize reaching physiological maturity earlier than the sole crop. Mulch

kI L.S

D

organic

N releases

At the beginning of the cropping cycle 140-210 kg ha- ’ (4.4-5.6-fold) more mulch N was present in the alley cropping than the sole cropping

(a)

Maize

mean

relative

growth

rate

i’

Ls

(b)

N concentration

in maize

+/

h

-../../i t

L.S.D.

1

/

I. S.D

I

GA

EA

(c)

Maize

N content

(‘

at maturity (b)

t

EA

GA

11.(15 /I--J 0

c

20

plants were sampled.)

after

I Ill1

X0

\owing ”

0

I

0

Erythrina

alley

crop

0.311~;1

-0

A

Cillricidia

allc)

crop

ll.O_3?2h

-0.l10~17

+

Sole

0 03_77h

(0

0 1102i

0 00~171

crop

s k.

N concentration in the sole-cropped maize was significantly lower than the alley-cropped maize. The mean relative growth rate, and rate of N accumulation of maize between 30 and 90 days, were significantly lower in the Erythrina alley crop than the Gliricidia alley crop and sole crop (Fig. 2). There was a significant negative quadratic function to the relative growth rate and relative rate of N accumulation in the two alley crops, but not in the sole crop. This indicates a decline in the relative growth rates over

611

10

Days

Treatment Fig. 1. Maize biomass and N content at maturity (90 days) under EA = Erythrina alley crop, GA = GIiricidia alley crop and C = sole crop. Columns grouped with different letters were significantly different (P < 0.05). (Each treatment value was the average of 3 replicates, in each of which 9

r;Ltc 01

5.1) r

h

“

Maize mean relative N accumulation

’

cocfficbcnt

only

\lgnlticant

10 I’=

~1llll7X I

0lL05’7)~

0 II)

Fig. 2. Above ground growth and N accumulation of maize under the different systems (on log, scale). Regressions of biomass and N against time were based on 3 replicates per treatment per time (total nine values per regression); points on graph are means of the three replicates. Linear (b, ) and quadratic (b,) coefficients of the orthogonal polynomials of log, biomass and log, N are presented below; coefficients with different letters were significantly different (P < 0.05). Curves presented on the graphs were best fit linear regressions for mean relative growth rate between 30 and 90 days.

1369

Nitrogen dynamics in agroforestry (c) Loss in

(a) Initial dry weight of mulch

over

dry

weight

115 days

15ml

-I I

EA

h

C

GA

EA

Treatment

(b)

Initial

total

GA

C

Treatment

N in mulch

(d)

N loss

over

115

days

0

EA

GA

C

Treatment

C

EA Treatment

Fig. 3. Initial mulch biomass and N content after pruning of the trees (a) and (b), and the loss in mulch dry weight and N content over the subsequent 115 days (c) and (d). Treatments were EA = Ery#zrina alley crop, GA = Gliricidiu alley crop, C = sole crop. Values were divided into components of the mulch: tree leaf (m), new branch material from current pruning (0). old branch remaining from previous prunings (8) (in (c) and (d) new and old branch are combined), crop residue (IIIIJ),and other material including

dead weeds (0). Columns or divisions of columns with different letters were significantly different (P < 0.05).

Of this difference 90-130 N kg ha-’ treatments. (60-65%) was contained in newly-applied tree prunings [Figs 3(a) and (b)]. The remaining extra N in the alley crop mulch came from a greater quantity of residual mulch from the previous prunings. The fresh Erythrina prunings contained about 50% more N than the Gliricidia prunings. During the subsequent 4 months the alley crop mulch released 100-160 kg ha-’ more N than the sole crop mulch, although alley crop mulch biomass declined by only 5@-60% as opposed to a decline of 87% in the biomass of the sole crop mulch [Figs 3(c) and (d)]. About 55% of the N released from the alley crop mulch came from the almost complete decomposition of the leaves in the mulch. The relative rate of biomass decomposition, as indicated by the decomposition constant k, and the relative rate of decline in N content of the mulch were both signifi-

cantly (P < 0.05) higher under the sole crop than the alley crop (Fig. 4). Mineralization of soil organic N

The average concentration of nitrate plus ammonium (available N) in the soil over the whole season was 2.68 pg N g-’ higher (SE + 1.95, significant to P < 0.01) under the alley crops than the sole crop [Fig. 5(a)]. Available N in the soil of the alley crops was on average 4.23 c(g N g- ’ higher (SE & 3.79, significant to P < 0.05) at days 0 and 20 than at days 50 and 80 [Fig. 5(a)]. There was no significant change in the sole crop available N over time. This led to a convergence between the alley crop and sole crop concentrations of available N at 50 and 80 days. The average N mineralization over the season was 4.71 ,ugN g-’ higher (SE & 3.07, a 2.09-fold

J. P. HAGGAR et al

1370

difference, significant to P < 0.05) under the alley cropping treatments than the sole crop [Fig. 5(b)]. The rates of N mineralization were maintained at their original levels throughout the season under all treatments except the Erythrina alley crop, which had significantly lower N mineralization (10.47 pg N g-~’ _t 5.33, P < 0.05) than the Gfiricidia alley crop in the 20 day incubation as compared to the 0 day incubation [Fig. 5(b)]. The latter was due to net immobilization of N in the cores incubated closest to the Erythrina trees, which when averaged with the high rates of net mineralization away from the trees, resulted in a low net mineralization at the plot level.

qf mulch

Contribution

of NO,

+ NH,

in the soil

I=

(b) Net N mineralization

N to plant N uptake

over

30 day periods

across all sampling times, maize in Eryand sole crop microplots without

Averaged

(a) Concentration

14 r q

thrina alley crop

0

SE

A A

(a)

Decline

in mulch

dry weight

+ +

20

0

Days

0

40

20

(b) Decline

20

0

Days Decomposition

x0

in mulch

I 40

I

(1. I

60

i ho

after

100

I

I

x0

IO0

application

of mulch

k. with different (P < 0.05)

constants,

1 I?0

lcttcrs

U

alley crop

-0.OOX4a

-t1.0132a

alley crop

-0.OOh3a

-0.0127a

-0.OlX3h

-0.01Ylh

Dry wcighl

+

Sole crop S.E.

application

x0

of mulch

N content

different

A Gliricidia

I Ml

Fig. 5. (a) Soil available N (NO, + NH.,) concentrations and (b) net N mineralization during one cropping season, under Erythrina alley crop (O), Gliricidia alley crop (A), and sole crop ( + ). (Each point is the average of three replicates each with three subsamples.)

I20

significantly

Erythrina

after

I

40

0.0010

are

N content

0.000 IX

Fig. 4. Decomposition of mulch in terms of decline in dry weight and N content (on lo& scale) under Erythrina alley crop (O), Giiricidia alley crop (A) and sole crop ( + ). (Each point is the average of three treatment replicates, each made up of three subsamples.)

mulch or additional crop residue grew 2445% less (P < 0.05), and contained 33-36% less N (P < 0.05), than maize in microplots with mulch. There were no significant differences between maize with or without mulch in the Gliricidia microplots. At 60 days after sowing, the Erythrina alley crop and sole crop grown without mulch contained 51 and 57% (P < 0.05) less N respectively. After 90 days these values fell to 20 and 11.6% respectively, and were no longer significant (Fig. 6). At maturity (90 days) between 3.5 and 7.5% of maize N had come from the “N-amended mulch (Fig. 7). In the Erythrina alley crop and the sole crop the proportion of the N in the maize derived from the amended mulch significantly declined between 30 and 90 days (Fig. 7). If the unamended mulch left in the microplots released N at the same rate as the amended mulch then the percentage N in the maize from the mulch would be 9.1% in Erythrina alley crop, 11.8% in Gliricidia alley crop, and 7.5% in the sole crop. However, the amended mulch decomposed faster than the unamended mulch as shown by the higher rate of decline in “N than total N in the mixture of amended and unamended mulch. Therefore the figures of total tree mulch

Nitrogen (a)

Biomass

30 days

after

dynamics

sowing

in agroforestry

(d)

1371

N content

30 days

after

sowing

50 r ,

10

1

L.S.D.

1 Erythrina (b)

Biomass

Gliricidia

60 days

Erythrina

Sole crop

after

(e)

sowing

Gliricidia

Sole crop

60 days

N content

after

sowing

500 ; zm z

100

3 2 :

10

.o, al 1 Erythrina

((:)

Biomass

Gliricidia

Erythrina

Sole crop

90 days after

(f) N content

sowing

Gliricidia

Sole crop

90 days after

sowing

5.0

500 1

L.S.D. 1

0.5 10

0.05

1 Erythrina

Gliricidia

Sole crop

Eryrhrina

Treatment

L.S.D.

I

100

Cliricidia

Sole crop

Treatment

Fig. 6. Effect of presence of mulch on biomass and N content (scales lo&) of maize in Erythrina alley crop, Ghicidia alley crop, and sole crop; with mulch (U) and without mulch (0). L.S.D.s (P = 0.05) are for comparison of _+mulch subtreatments.

N transfer to maize must be considered maximum contributions. The maximum estimated total N taken up from the mulch by the maize at maturity (also assuming similar decomposition of amended and unamended mulch) was between 1.I and 1.4 g N mm2 from the tree mulches was about 0.25 g N mm2 from the maize residue mulch (Fig. 8). There was a high rate of uptake of mulch N between 30 and 60 days after

sowing for all treatments; the rate of uptake falls off rapidly after 60 days (Fig. 8). These changes are shown by the significant positive linear and negative quadratic regression coefficients for the change in mulch derived N in the plant over time. The estimated contribution of mulch N (amended plus unamended mulch) to weed N uptake declined from 15-24% at 50 days to 3.2-6.2% at 245 days after amended mulch application (Fig. 9). Fitting an exponential decay model showed that the contribution of mulch N to weed available N was halving every 70-130 days. The amended mulch

application to the maize contributed an estimated l&3.0% of the N uptake of beans sampled 265 days after application.

0

20

40 Days

after

60

80

100

sowing

Fig. 7. The change over 90 days in the proportion of maize N originating from amended mulch under Eryfhrina alley crop (0). Giiricidia alley crop (A), and sole crop ( + ). Regressions against time were based on three replicates (two for Gliricidia) per treatment per time. *Regression coefficients for proportion of maize N from mulch against time significant (P < 0.05).

J. P.

1372

,

I 0

211

ho

‘III

Days

after

I

,

X(1

100

HAGGAR

sowing Linear

coefficient

Ouadratic cocfficicnt

q

Erythrina

allcy

crop

O.O?hS

-0.00094

A

Gliricidia

alley

crop

11.0314

-0.00052

+

Sole crop

0.0201

-0.00

I I5

Fig. 8. Cumulative N uptake by maize from mulch (log, scale) under Erythrina alley crop (IJ), Chicida alleycrop (A), and sole crop ( + ). L.S.D. (P = 0.05) is for comparison between treatments at one time. Regressions against time of log, transformed data were based on three replicates (two for Gliricidia) per treatment per time, points on graph were means of the replicates. Regression coefficients of transformed data were all significant (P < 0.05), but were not significantly different from each other.

Recovery

of mulch

et al

N

Recoveries of N released from the amended mulch by the maize plants after 90 days were 11.7% for the Gliricidia mulch, 8.9% for Erythrina mulch, and 4.9% for maize residue, but the differences between

0

trythrina

A

Gliricidla

+

Sole

100

O

Days

after

700

application

Regression

(decay)

2011

of mulch

coefficient\

0

Erythrina

alley

crop

-0.OOY3

A

Gliricidia

alley

crop

-O.OOhY

+

Control

sole

crop

Fig. 9. Change in the contribution

arc.

-Il.llO5J

of mulch N to weed N uptake (log, scale) over 245 days under Erythrina alley crop (a), Gliricidia alley crop (A), and sole crop ( + ). Regressions against time of log, transformed data were based on three replicates (two for Gliricidia) per treatment per time. All regression coefficients were significant (P < 0.05).

allc!

crop crop

crop

Ouadrat1c c
0 II?

I5

-lr.1~0073

0 OZXY

-I!

i10057

0 II150

mo 00

I I!

Fig. IO. Percentage recovery by maize of N released (log, scale) from the mulch under Erythrina alley cropping (n), Ghicidia alleycropping (A). and sole crop ( + ). L.S.D. (P = 0.05) is for comparison of different treatments at one time. Regressions against time of log, data were based on three replicates (two for Ghicidia) per treatment per time; points on graph are means of the three replicates. Regression coefficients of transformed data were all significant (P < 0.05) but were not significantly different from each other.

mulches were not significant (Fig. 10). The percentage recovery of mulch N by the maize increased rapidly between 30 and 60 days but then levelled off between 60 and 90 days. These changes were shown by the significant positive linear and negative quadratic regression coefficients of the increase in the recovery of mulch N over time. Total recovery of mulch N in the crops and weeds grown over approx. 9 months declined in the order; Gliricidia mulch > maize residue > Erythrina mulch (Table 2). Microbial

I-

allcy

I incilr c
nitrogen

dynamics

There was an 80% increase (P < 0.01 for alley crops, P < 0.05 for sole crop) in microbial N in all treatments between 40 and 70 days after application of the mulch (Fig. 11). At 105 days microbial N was 30% higher in the alley crop treatments than the sole crop (significant to P < 0.06). Microbial N concentrations were not significantly different between treatments at any other time. In all treatments the “N concentrations in the soil soluble organic N and ammonium pool were constant over the period of study (Fig. 12). Microbial “N concentrations at 25 days after mulch application were the same as the soil soluble organic N and NH4 (about 0.1&0.15% excess “N), but then declined significantly (P < 0.05) over the following 2 months to about 0% excess 15N (Fig. 12). There were no significant differences in the rates of decline between treatments.

I373

Nitrogen dynamics in agroforestry Table 2. Recovery of mulch nitrogen in crops and weeds over 9 months (gN m*) Weeds

Weeds Days after application of mulch Erythrina alley crop Gliricidiaalley crop Control sole group

Maize

productivity

Mulch N applied

Total

Beans

% recovered

50

105

II0

245

265

0.274

0.209

I.114

0.106

0.097

1.800

15.29

Il.8

0.324

0.447

1.422

0.274

0.091

2.558

9.46

27.0

0.282

0. I77

0.212

0.219

0.042

0.932

4.97

18.8

DISCUSSION Maize

Weeds

and N availability

The integration of legume trees into the cropping system using an alley cropping technology substantially increased maize biomass production. The higher N concentration of the maize in the alley crop compared to the sole crop suggests that this increase in productivity was due, at least in part, to higher N availability (Fig. 1). The higher biomass and N content of the alley crop compared to the sole crop was already established at 30 days and the subsequent relative growth rates largely just built on that difference (Fig. 2). This was demonstrated by the fact that, although the Gliricidia alley crop had a higher relative growth rate than the Erythrina alley crop and the sole crop, the final biomass and N accumulated were not significantly different between the two alley crop treatments, but very different between the Erythrina alley crop and the sole crop. The difference in productivity between the sole crop and the alley crop appears to be determined by the more rapid establishment of the alley crops. In terms of a logistic growth model the alley crop maize had a shorter lag phase of establishment before attaining exponential growth. The soil N mineralization in both the alley crop and the sole crop soils was considerably higher than the corresponding crop N demand (Fig. 13). Nevertheless the total amount of N mineralized in the sole crop would not have been enough to meet the maize N uptake in the alley crop and the higher N mineralization under the alley crop was necessary to support this. The highest demand for N by the maize was between 30 and 60 days after sowing, as is character-

istic for the species (Hanway, 1962). This period of high N demand in the alley crop can be seen in the fall in available N in the alley crops between 30 and 60 days [Fig. 5(a)]. Over the first 30 days after sowing

(a) 0.2 -

O’: 1

-0.1

S,ope~~L.S~D

0

20

40

60

80

100

@I 0.2 r

A -0.1

t, 0

20

,

I

I

1

1

40

60

80

100

120

(cl 0.2 0 d

30

120

0

0.1 -

r

0

L.S.D.

0 A

Slope

- 0.00226*

*

0 -

+

.

.

-\-\\-\\ A

0

I 31)

10

- IO

Days

after

50

application

70

90

L.S.D

0

I

1

1

20

40

60

Days after application

I IO

of mulch

Fig. 11. Extractable soil microbial nitrogen during one maize cropping period under Erythrina alley crop (o), Gliricidiu alley crop (A) and sole crop (+ ). LAD. (P < 0.05, n = 3) is for comparison of treatment means at

one time. A = maize sown.

-0.1

80

,

I

100

120

of mulch

Fig. 12. Percentage excess “N in microbial N (D), and soluble soil organic nitrogen and ammonium (0) pools under (a) Eryrhrina alley cropping, (b) GIiricidia alley cropping and (c) sole cropping after addition of labelled residues of Eryrhrina, Giiricidia or maize, respectively. Slopes of regressions against time significant to *P < 0.05, **P < 0.01 (SE = 0.000394). L.S.D. (P < 0.05) is for comparison between microbial N and soluble soil N at one time. A = maize sown.

1374

J. (a)

Erythrina

P. HAGGAR

a n 1s 0 0

n

.

. 0

5 -0

al.

would have entered the readily-mineralizable organic N pool in the soil. It was expected that the organic N released from the mulch would increase the N mineralization between 0 and 3 months. This was not observed and indicated that either the N was released from the mulch as ammonium or nitrate (i.e. mineralized in situ) or this N entered the soil organic N pool to be mineralized over a longer period. The latter process would potentially lead to a build-up in

alley crop

20 --

10 -

et

the readily mineralizable N in the soil and result in the higher rates of soil N mineralization observed in the alley crops. The lower maize production in the sole crop relative to the alley crop has only developed after 7 yr of cropping. If this difference in productivity was due to N availability then it indicates that the processes maintaining N availability in the alley crop took about 6 yr to develop a significant effect. This would support the SOM hypothesis that greater organic N inputs in the alley crop increase or maintain the soil pool of readily mineralizable organic N (a mediumterm process occurring over a few years). Factors determining microbial N dynamics

0

20

Days

40 after

60 sowing

80

I00

maize

Fig. 13. Accumulated changes in N for different pools over one cropping season: mulch N released (A); soil N mineralized (0) N in above ground biomass of maize (m).

the absolute amount of N available was also much higher than maize N uptake in all treatments. However, the small root system of young maize plants would not allow them to exploit the total soil volume, and the crop growth may have been N limited. Thus the higher N availability under alley cropping may have increased the speed of establishment of the maize in the alley crop and it certainly maintained the higher absolute rate of N accumulation thereafter (30-60 days). Organic N dynamics

Organic N inputs were substantially higher in the alley crop than the sole crop due to the mulch provided by the pruning of the trees. The mixture of high N concentration leaf material and low quality branch material meant that the rate of decomposition of the tree mulch was no higher than that of the sole crop residue. The greater amount of N released by the alley crop mulch was solely due to the greater biomass, and thus total N content, of the mulch and was not due to a faster rate of release of N. Much of the N released by the decomposition of the mulch may still have been in organic form and

The microbial N pool showed no relationship to the large differences in organic matter inputs and N mineralization rates between the treatments. If the extraction method provided a 50% efficiency of microbial N recovery, then the standing microbial N pool was only about 4 times larger than the standing nitrate plus ammonium pool. The different time responses of the 15N content of the soluble organic N and NH, pool, and the microbial N pool suggested that the soluble organic N and NH, was not the main N source for the microbial biomass (Fig. 12). Equally if the organic matter derived from the mulch was a major source of microbial N, “N concentrations would also have been maintained. Thus it seems unlikely that the size of the microbial N pool was a significant determinant of available N, although a portion of the microbial biomass must have been mediating the mineralization of organic N. The increase in the microbial N pool coincided with the period of rapid growth of the maize; i.e. between 30 and 60 days after sowing (Fig. I 1). At this time organic inputs to the soil from root exudates and root turnover probably increased and could have caused the observed increase in microbial N. The trees in the alley crop had also started to regrow at this time and may similarly have increased root activity in the soil. Cheng and Coleman (1990) found that the presence of an active root system led to higher microbial biomass, and also increased the rate of decomposition of plant residues. In our study microbial biomass may also have responded to changes in soil temperature or moisture content that would have accompanied the increase in plant cover. The decline in microbial N in the sole crop, as compared to the alley crop, at 105 days after mulch

Nitrogen dynamics in agroforestry application coincided with the maturation of the maize and the death of the maize root system at 90 days after sowing. In the alley crop the trees were regrowing vigorously and would have had a high level of root activity that may have sustained microbial populations. Lynch and Panting (1980) attributed similar changes in soil microbial biomass under a winter wheat crop to changes in root activity. In their study microbial biomass increased rapidly when the crop started to grow vigorously in late spring, peaked at the end of June when crop roots attained maximum abundance, and then declined as the wheat ripened. If the microbial biomass was responding to increased root activity then it would suggest that the microbial biomass was limited by carbon substrate availability and not by N availability. A similar response has been found in temperate cropping systems. Shen et al. (1989) found that the microbial N pool in the Broadbalk Continuous Wheat Experiment was larger with low N fertilizer additions (48 kg N ha-’ yr-‘) than with no N fertilizer, but there was no response to higher additions of N fertilizer. Ocio et al. (1991) found little response in microbial N when N fertilizer was added to straw incorporated into the soil. The above observations suggest that caution should be taken when interpreting microbial N pools as a measure of active soil organic N. The microbial N pool showed no relationship with the very different rates of N mineralization. The amount of plant root activity in the soil was probably the most important determinant of the size of the microbial N pool in the tropical cropping systems studied. Root activity was probably more important than the amounts of the above ground plant residue inputs. Mulch N release and recovery in the crop

Tree mulch and maize residue mulch did not differ significantly in their proportional contributions to the N in the crop (about 10%). Sisworo et al. (1990) found that legume crop residues contributed less N than maize residues to a subsequent crop, providing approx. 2&25% and 67% respectively of crop N uptake. This was probably due to lower rates of decomposition of the maize residues. Read et al. (1985) found much lower rates of decomposition of maize residues compared with Leucaena leaves. Our study differs from those cited above in two major respects: (i) the tree legume mulch in our study contained a large amount of branch material, which reduced the average quality of the material and resulted in the proportion of N released from the tree mulch being similar to that for maize residue (Fig. 4); and (ii) soil N mineralization was twice as high under the alley crops than the sole crop so that the same percentage contribution of mulch N to crop uptake would have required only half the amount of N released from the sole crop mulch. Mulongoy and van der Meersch (1988) found even lower crop recov-

1375

eries of mulch N from Leucaena than the mulch N recoveries in our study. The high recovery of Gliricidia mulch N may be correlated with the observed fact that the Gliricidia leaves completely disappeared within a month of application (although a large quantity of branch material remained), which may have led to the Gfiricidia mulch having a higher rate of N release in the soil than the other mulches. The low recovery of maize residue N by the sole crop maize was due to poor growth of the maize. Substantial weed growth in the sole crop increased total N recovery for this treatment. In the alley crops there would also have been substantial recovery of mulch N by the trees. Immobilization of mulch N in the microbial biomass and transfer to maize

At 25-30 days after sowing, mulch N contributed only 2.8-5.0% of the N in the microbial pool as opposed to 12.614.3% of the N taken up by the maize. The different contributions of mulch “N to crop and microbial N pools may be a sampling effect, It is probable that the mulch 15N in the soil was concentrated in the top few centimetres, but the soil was sampled to a depth of 25 cn. Thus microbial N from deeper in the soil may have diluted higher “N enrichments in the surface soil. If the maize takes up the majority of its N from the surface of the soil then it would have a higher mulch 15Nconcentration than was measured in the microbial biomass. Total recoveries of released mulch “N, at 25-30 days after sowing, were the same or greater in the microbial biomass than in the crop (Table 3). Mulch “N uptake by the crop increased to up to 60 days and then declined, while microbial 15Ncontent fell rapidly after 30 days. The apparent delay in the decline in mulch 15N uptake by the crop compared to the decline in 15Ncontent of the microbial biomass may again be a sampling effect; i.e. the crop contained the accumulated r5N up to the time of sampling, whereas the microbial 15Nwas a point measurement in time of a dynamic N pool. The microbial biomass would have been intermediary in the transfer of mulch “N to the crop. The low quantities and concentrations of mulch “N in the microbial N pool show that mulch “N availability to the crop was not limited by immobilization of mulch lSN in the microbial biomass. On the other hand, given that the microbial biomass was intermediary in the transfer of mulch 15Nto the crop, the low uptake of mulch “N by the microbial biomass may itself have limited the mineralization of mulch “N and its subsequent release for plant uptake. The relative importance of SOM and SYNCHRONY in determining crop N uptake

The 2.8-fold higher maize N uptake in the alley crop compared to the sole crop may have been derived from the rapid mineralization of N from the current mulch or the mineralization of residual tree

J. P. HAGGAR et al.

1376

mulch N from previous applications. The lSN tracer study, and the removal of mulch from microplots in the alley crop, has shown that between 10 and 15% of the N in the maize (approx. 0.8-I .Og N mW2) was derived from the current mulch application. Thus this transfer was only large enough to account for a fraction of the greater N uptake by the alley crop maize than that of the sole crop. The timing of the transfer of mulch N to the crop appears to be mainly during the early growth of the crop. The higher proportional contribution of mulch N to crop N in the first 30 days, and the low uptake of mulch N after 60 days, suggest that a flush of available N was released from the mulch during the first 30 days after sowing. Recovery of this flush of mulch N may have been greater if the maize had been sown at the same time as mulch application rather than 15 days after mulch application. Then the flush of mulch N would have been released closer to the time of high maize N demand. It had previously been found that Erythrina and GIiricidiu mulches contained 0.8-1.8 g NmM2 in the form of extractable nitrate and ammonium few days after application. This pool of mulch N could have provided all or most of the mulch N that was transferred to the crop. If N availability during the establishment of the crop was important for subsequent growth, as was suggested earlier, then this flush of mulch N may have been more important than the 10% contribution to maize N would suggest. Over the longer term the mulch N contribution to plant N uptake declined, but continued at least up to 265 days after application. If the rate of decline in the contribution of mulch N to available N remains the same (with a half-life of about 100 days) then after a few years the contribution will be negligible. Thus it does not seem probable that the accumulated effects of the 8 yr of application of tree mulch would be sufficient to have caused a 2-fold increase in the rate of N mineralization under alley croping as compared to sole cropping. On the other hand, Sisworo et al. (1990) found that recovery of N from crop residues,

in a sequence of six crops over 2 yr, fluctuated and did not decline at a constant rate. As well as mulch N, there have been other significant organic inputs associated with the presence of the trees. These include natural litterfall, fine root turnover, and fine root die-back and nodule death that probably occurs after pruning. A larger pool of readily mineralizable soil organic N appears to have been the main cause of the higher N availability under the alley crops, although this was not correlated with a larger microbial N pool. The transfer of N from the mulch to the maize was small, and during the phase of rapid maize growth contributed a declining proportion of the N taken up by the maize. The low level of exploitation of mulch N by the microbial biomass may have limited mulch N transfer to the crop. Thus the SOM process of increasing soil N reserves was more important than the SYNCHRONY of mulch N release and crop uptake in increasing N availability to crops under alley cropping. In the first years of the development of the alley cropping system, before the processes of SOM had time to build up, the timing of the small but significant contribution of mulch N to crop nutrition may have been more important. Sources

of the increase

in available nitrogen

The greater amounts of readily-mineralizable organic N in the soil in the presence of the trees may have been due to several processes: (1) the mobilization of N from the stable organic N pool by increased microbial activity; (2) addition of N from outside the system, e.g. by N, fixation; or (3) reduction of losses from the system, e.g. by leaching or denitrification. Increases in the readily-mineralizable N pool that arise from outside additions of N or mobilization of more recalcitrant soil organic N cannot be distinguished by measuring changes in total soil N. This is because of the small size of the mineralizable N pool compared to total N. Therefore, although all treatments had similar total soil N contents (kgN haa’) this does not mean that the

Table 3. The recovery of the mulch lSN in maize and microbial biomass release from ‘IN from labelled Erythrina, GIiricidia and maize residues applied as mulch under Erythrina alley cropping, Gliricidio alley cropping, and sole cropping respectively Mulch “N released (mgm_?

Maize “N %” (mgm~‘)

Microbial (mgm-*)

!‘N %*

Eryrhrinn alley crop 25-30 days after sowing S-60 days after sowing 90-95 days after sowing

381 498 503

9.0 46.6 46.1

2.4 9.4 9.2

IO.1 6.5 4.1

2.1 1.3 08

Gliricidia alley crop 25-30 days after sowing 55-60 days after sowing 90-95 days after sowing

134 139 I54

3.5 13.4 23.8

2.6 9.6 15.5

7.1 I.3 0.0

5.2 0.9 0.0

Control sole crop 25-30 days after sowing 55-60 days after sowing 90-95 days after sowing

I56 I91 216

3.4 16.2 10.8

2.2 8.5 5.0

13.2 0.0 0.0

8.5 0.0 0.0

“Percentage

of mulch “N released.

1377

Nitrogen dynamics in agroforestry higher N availability under the alley crops could not be due to N added to the system, e.g. by N fixation. Marrs et al. (1983) found that the addition of high N litter from Lupinus arboreus to land reclamation sites increased N turnover and thus productivity, even though the total soil N remained low. Some of the N in the tree biomass will have come from N fixation, but the magnitude of this contribution is not known. Nitrogen fixation by trees can make a significant contribution to associated crops. Sanginga et al. (1986) showed that the N fixed by an inoculated Leucaena fallow (assessed by comparison with an uninoculated Leucuenu fallow) approximately doubled the N uptake by the subsequent maize crop. Nevertheless the trees in the alley crop would be competing with the maize for N. If the alley cropping systems are to be sustainable the tree must produce biomass with an equivalent amount of N for the next application of mulch as was released from the current application. Therefore N availability must be high enough not only to sustain a higher crop production but also to supply the trees. Conclusions

The higher N availability under alley cropping compared to sole cropping was due to an increase in labile soil organic N causing higher soil N mineralization rates. This process was more important than the timing of release of nutrients in increasing maize N uptake and yield. The increase in labile soil organic N was probably only partly due to the above ground organic mulch inputs from the trees, and seemed to have taken several years to develop. The size and dynamics of the microbial N pool did not seem to determine soil N availability, but may have limited mulch N release to the crop. Nitrogen availability and crop N uptake after 7 yr were higher under alley cropping than sole cropping despite the fact that the sole cropping system was zero-tilled and the crop residues were returned, both of which are known to help maintain soil N (Lal, 1989; Juo and Lal, 1977). The low short-term contribution and recovery of mulch N in the crop indicates that timing of organic residues may not be important to maintaining nutrient availability. The ability of the alley cropping system to maintain high N availability longer than the sole cropping system was probably due mainly to the high organic matter inputs from the trees in the alley crop. Acknowledgemenrs-This work was supported by a Research Studentship from the Natural Environmental Research Council, additional support was provided by a Frank Smart Studentship from the Botany School, University of Cambridge, and the CATIE-GTZ and CATIE-AFN Projects in Costa Rica. We would like to give particular thanks to the following personnel from CATIE: Drs C. Ramirez and R. Diaz for the provision of laboratory facilities, to Sr J. Alvarez for his help in managing the experiment, and to Larry Szott for reviewing this paper. We would also like to thank Chris Quarmby, Merlewood

Experimental Station, Cumbria for analysing the 15N samples, Debora Court and colleagues at Soil Science Department, University of Reading for help in the preparation of microbial 15N samples, and Markus Rippin from University of Bonn for collecting the weed and bean samples. REFERENCES

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