Agronomy Journal 92:501-511 (2000)
© 2000 American Society of Agronomy
SOIL AND CROP MANAGEMENT
Decomposition and Nitrogen Release of Prunings from Hedgerow Species Assessed for Alley Cropping in Haiti
Lionel Isaac,
C.Wesley Wood and
Dennis A. Shannon
Department of Agronomy and Soils, 202 Funchess Hall, Auburn University, Auburn, AL 36849-5412 USA
wwood{at}acesag.auburn.edu
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ABSTRACT
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Decomposition and N release patterns from prunings of eight tree species were studied under field conditions. Leaves and stems (<1 cm diam) from 4-yr-old hedgerows were sealed in separate litter bags and placed on soil surface at low and high elevation (1150 m) sites in Haiti. Leaves decomposed faster than stems and leaf decomposition was described by a two-pool exponential model. At the low elevation, leaf C loss was highest (82%) in gliricidia [Gliricidia sepium (Jacq.) Kunth ex Walp.] and lowest (42%) in flamboyant [Delonix regia (Bojer ex Hook) Raf.] after 48 wk. At the high elevation, leaf C loss after 48 wk was 48% in leucaena [Leucaena leucocephala (Lam.) De Wit] and 29% in Acacia angustissima (Mill.) Kuntze. Initial N concentrations correlated with leaf C loss (Phase I) at low elevation while C:N and lignin:N correlated with leaf C loss (Phase II) at low and high elevations, respectively. Nitrogen release resembled carbon loss. At the low elevation, gliricidia, Leucaena shannonii Donn. Sm., and leucaena released >50 kg N ha-1 during the first 4 to 6 wk, whereas at high elevation A. angustissima contributed <40 kg N ha-1 during the period. Leaf N release correlated negatively with (lignin + polyphenol):N at low elevation. At the low elevation, gliricidia, L. shannonii, and leucaena provided adequate N for maize production in alley cropping. At the high elevation, A. angustissima contained adequate N, but N release rate may not meet peak N demands of an associated crop.
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INTRODUCTION
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DECLINE in soil productivity and environmental quality and progressive deterioration of natural resources in the tropics have led to a search for new methods to sustain crop production via more efficient nutrient cycling. In Haiti, soil degradation and decreasing soil fertility from continuous cropping on sloping lands are responsible for the growing interest in developing farming systems based on efficient use of internal resources. The challenge resides in sustaining crop production while maintaining soil fertility through supply and efficient management of organic residues.
Cropping systems based on perennial trees, preferably legumes, with proper management practices have been used as a viable means of sustaining crop productivity in low input agricultural systems (Wilson and Kang, 1981). Alley cropping, which is production of food crops between rows of leguminous trees or shrubs that are periodically pruned to provide organic materials, is receiving considerable attention as a profitable alternative, owing to its potential for sustaining crop yields in the tropics (Kang et al., 1984). In alley cropping, addition of hedgerow prunings can maintain soil organic matter and assure a slow and continuous release of nutrients (Budelman, 1988), improve moisture retention, reduce soil erosion, and prevent weed seed germination (Bannister and Nair, 1990; Chirwa et al., 1994; Yamoah et al., 1986a). However, success of alley cropping depends upon its capacity to replenish or sustain reserves of nutrients that are removed or lost from soil through appropriate choice of woody species and proper hedgerow management.
Suitability of hedgerow species for alley cropping has been tested in many tropical regions. Species such as leucaena and gliricidia were found to perform well in trials in humid lowland tropics (Kang et al., 1984), while flemingia [Flemingia macrophylla (Willd.) Merr.] was reported to have good potential on a slightly acid soil (Budelman, 1988). Leucaena diversifolia (Schltdl.) Benth. was reported to have good potential in high elevation, semi-arid conditions (Balasubramanian and Sekayange, 1991). In Haiti, attempts to introduce hedgerows were made with leucaena and other species including gliricidia, Senna siamea (Lam.) H.S. Irwin & Barneby, Erythrina variegata L., Calliandra calothyrsus Meisn. and Moringa oleifera Lam. However, few of these species have been assessed in a systematic fashion.
Important considerations in assessing hedgerow species for alley cropping are the amount and rate of N released that can benefit the companion crop. Although research has shown that prunings of legume species can be an effective source of N for crop production (Yamoah et al., 1986b; Kang and Wilson, 1987) little is known regarding factors that govern the N release patterns under different agroecological conditions. Rates of mulch decomposition and N release are dependent on climate, nutrient availability, and resource quality (Scholes et al., 1994). Myers et al. (1994) reported that environmental conditions (soil type, nutrient status, and climate) and management practices may alter chemical composition of legume species, affecting N mobilization and release from residues. Contents of N, lignin and polyphenol are chemical factors controlling degradability of plant materials added to soil (Constantinides and Fownes, 1994; Fox et al., 1990). Initial N concentration of plant materials is often the best predictor of rate of N mineralization when contents of lignin or polyphenol are not high enough to lead to immobilization initially (Constantinides and Fownes, 1994; Fox et al., 1990; Melillo et al., 1982; Palm and Sanchez, 1991). Lignin content or lignin:N ratio were shown to be chemical factors affecting degradability in nonlegume species (Melillo et al., 1982). Jama and Nair (1996) observed that mulch decomposition and N release from leucaena and C. siamea were related more to their C:N ratios than to polyphenol contents under tropical semiarid conditions. It is hypothesized that management practices and environmental conditions may affect chemical composition of hedgerow species altering the decomposition rates of their prunings. An understanding of the impact of agroecological conditions on quality of plant materials and consequently on decomposition and N dynamics will result in better assessment of tree species suitable for alley cropping under specific environments.
The objectives of this study were to (i) evaluate the suitability of leucaena, L. diversifolia, leucaena hybrid (L. leucocephala x L. diversifolia), L. shannonii, gliricidia, flamboyant, A. angustissima, and flemingia for alley cropping in terms of residue decomposition and N release rates under field conditions and (ii) determine the effect of chemical composition of the residues on their decomposition and N release patterns under specific environments.
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Materials and methods
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The study was conducted from September 1995 through August 1996 at a low and a high elevation in Haiti. The soil at the low elevation site (18°13' N, 73°42' W; 70 m above sea level) is classified as a loamy, mixed (calcareous), iso-hyperthermic Typic Troporthents (Guthrie et al., 1995). Average temperature is 26.5°C and mean annual rainfall is estimated at 1600 mm. The rainfall pattern is bimodal with rains occurring from March until mid-June and from late August through November. Selected soil physical and chemical properties of the upper 20 cm were: 24% sand, 56% silt, 20% clay, 26 cmolc kg-1 soil cation exchange capacity (CEC), 43 g kg-1 organic matter (OM), and pH (H2O) 8.0. Particle size was determined by the pipette method and by sieving. Cation exchange capacity was determined by extraction with 1 M ammonium acetate and organic mater was determined by Walkley–Black method (Soil Survey Staff, 1991). The high elevation site (18°29' N, 72°15' W; 1150 m above sea level) has a mean annual temperature of 22°C and a mean rainfall of 1200 mm distributed as at the low elevation site. Total measured rainfall during the period of the experiment was 1507 and 1060 mm at the low and high elevation sites, respectively (Fig. 1)
. The soil at the high elevation is classified as a clayey-skeletal, kaolinitic, iso-hyperthermic Typic Hapludalfs (Guthrie et al., 1995). The topsoil (0–20 cm) has the following characteristics: 11% sand, 38% silt, 51% clay, 45 cmolc kg-1 soil CEC, 44 g OM kg-1, and pH (H2O) 8.1.
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Fig. 1 Monthly rainfall during the study period (September 1995–August 1996) at a low elevation and a high elevation site in Haiti
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The decomposition study was carried out on promising species from a larger experiment to assess suitability of 35 hedgerow trees and shrubs for alley cropping under different agroecological conditions. The hedgerows were established during the first rainy season of 1991 in plots consisting of single rows 5 m long (low elevation) and 6 m long (high elevation). Hedgerow rows were spaced 3 to 4 m apart with plants spaced 10 cm within row at both locations. Four replicates of each species were arranged in a randomized block design at the low elevation. Variability in soil depth induced by terracing mandated use of 16 incomplete blocks of four plots each at the high elevation site. Following establishment (approximately 11 mo after planting) hedgerows were pruned to a 50 cm height three to four times a year at each site.
Species included in this study were selected based on their mean annual biomass production and their capacity to tolerate repeated prunings. At the low elevation site, prunings of approximately 4-mo regrowth of leucaena variety K636, gliricidia variety HYB, leucaena hybrid variety KX3, L. shannonii, and flamboyant were used to evaluate decomposition and N release patterns. At the high elevation, prunings (4-mo regrowth) of leucaena variety K636, leucaena variety K156, leucaena hybrid variety KX3, A. angustissima, and flemingia were used. Hedgerows were pruned to a 50 cm height on 11 September and 5 October 1995 at the low elevation and high elevation sites, respectively. Prunings were separated into leaves and stems <1 cm diameter.
Decomposition and N release patterns of the prunings were monitored in the field using nylon mesh bags measuring 20 by 10 cm with 50 to 60 µm openings. Fresh leaves and stems (<1 cm diameter) harvested from hedgerows were air-dried before being placed in the bags. Subsamples were oven dried at 71°C for 48 h to account for moisture. The quantity of biomass included in the litter bags was calculated based upon the biomass yield of the species in Mg ha-1 and the surface area of the bag. This amounted to 5.1 to 7.8 g or 3.0 to 7.3 g oven-dry leaf at the low and high elevation sites, respectively. Stem biomass sealed in each bag ranged from 1.8 to 3.3 g or 1.3 to 3.1 g at the low and high elevation sites, respectively. These were equivalent to a leaf dry weight ranging from 1.6 to 4.0 Mg ha-1 and a stem dry weight ranging from 0.7 to 1.8 Mg ha-1. For each species, 20 bags of each type (leaves or stems) were placed in the field on 15 September 1995 at the low elevation and on 9 October 1995 at the high elevation site, respectively. The bags were arranged in a randomized block design with four replicates within each site. Following placement on the soil surface, four bags of each type of prunings per species were retrieved from the field at 2, 4, 8, 16, and 48 wk. At each sampling time, soil and debris adhering on bag surfaces were carefully removed before drying the sample at 71°C for 48 h. Soil contamination was accounted for by ashing a 1-g subsample in a muffle furnace at 400°C for 12 h and converting the data to an ash-free basis. The ash-free dry weight was calculated using the following formula (Cochran, 1991):
 | (1) |
Contents of each bag were weighed, ground to pass a 1 mm sieve and analyzed for total C and N using LECO CHN-600 analyzer (Leco Corp., St. Joseph, MI). Initial materials also were analyzed for total C and N, lignin, cellulose, and polyphenol. Lignin and cellulose concentrations were determined via acid-detergent fiber analysis after Anderson and Ingram (1989). Polyphenol was extracted as total soluble polyphenolics in 50% aqueous methanol for 1 h in a water bath at 80°C and determined colorimetrically using the Folin–Denis method (Anderson and Ingram, 1989).
Percent C and N remaining on an ash-free basis and data on chemical characteristics of plant materials were analyzed using the General Linear Model (GLM) procedure (SAS Institute, 1990) for a randomized complete block design within each site. Percent original C and N remaining at each sampling were regressed on time using nonlinear regression models (Neter et al., 1996; SAS Institute, 1990). The single three-parameter or the double four-parameter models were used to calculate decomposition and N release rate constants (k) (Wieder and Lang, 1982). The best fit model was determined based on lowest mean square error values. General forms of the equations were as follows:
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 | (3) |
where Y is the percent of initial C or N remaining at sampling time t, 
0 is the recalcitrant pool fraction (Eq. [2]) or the easily decomposable fraction (Eq. [3]), 1 is the difference 100 - 0, k1 and k2 are decomposition or N release constants, and t is the time in weeks. The first model assumes one rate of decomposition or N release from the plant materials while the second model defines an initial rapid phase followed by a slower phase of decomposition or N release.
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Results
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Pruning Chemical Characteristics
Dry matter production was determined to estimate N accumulation potentially available for a companion crop. At the low elevation site, leaf dry matter was highest for leucaena and lowest for L. shannonii, whereas stem biomass was greatest for leucaena and lowest for gliricidia (Table 1)
. Leaf N concentrations ranged from 36.5 g kg-1 (leucaena) to 25.9 g kg-1 (flamboyant). Except for flamboyant, leaf N concentrations were similar across species. Trends for stem N were slightly different, but the lowest N concentration still occurred in flamboyant. Therefore, N content was mostly dependent on species production, leucaena being the highest with a leaf N content of 147 kg ha-1. Relatively high leaf N concentrations led to C:N ratios ranging from 11.7 (gliricidia) to 15.3 (flamboyant), whereas C:N ratios in stems were all above 36. Cellulose and lignin concentrations were greater in stems than in leaves, whereas contents of polyphenol were higher in leaves (Table 1). Residues from gliricidia were rich in cellulose, but contained significantly lower concentrations of polyphenol, while leaves from the leucaena hybrid were higher in lignin and polyphenol.
At the high elevation site, biomass yield was highest for A. angustissima and lowest for leucaena (Table 2)
. Leaf N concentrations varied from 34.7 g kg-1 in A. angustissima to 26.1 g kg-1 in flemingia leading to a higher C:N ratio (17.8) in the latter species. The apparently higher N in stems at the high elevation as compared with those at the low elevation resulted in lower C:N values for these materials. As for species grown at the low elevation, cellulose and lignin concentrations were higher in stems, whereas leaves contained greater concentrations of polyphenol (Table 2).
Pruning Decomposition Patterns
Differences in pruning decomposition were apparent within each site (Fig. 2)
. At the low elevation site, a rapid decrease in percent C remaining in leaves was observed during the first 8 wk after bag placement (Fig. 2a). The most rapid decline was observed for gliricidia, followed by L. shannonii. Leaves from flamboyant showed significantly lower decomposition rates than other species. Carbon loss from leaves ranged from 82% in gliricidia to 42% in flamboyant at the end of the decomposition period. At this site, C loss from leaves decreased in the order of gliricidia = L. shannonii > leucaena = leucaena hybrid > flamboyant. At the high elevation site, leaves decomposed slower than at the low elevation. Carbon loss after 48 wk was highest for leucaena (48%) and lowest for flemingia and A. angustissima (29%) (Fig. 2c). Patterns of C loss from leaves at the high elevation site occurred in the order of leucaena = leucaena hybrid = L. diversifolia > A. angustissima = flemingia. Except for flemingia, the two-phase model best described C remaining in decomposing leaves at both sites (Table 3)
, with k values ranging from 0.12 to 0.79 wk-1 (Phase I) and from 0.002 to 0.014 (Phase II).
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Fig. 2 Carbon remaining from decomposing leaves (a and c) and stems (b and d) of species assessed for alley cropping at low and high elevation sites in Haiti (bars are SE)
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Table 3 Equations for percent carbon remaining from prunings of hedgerow species decomposing in litter bags under field conditions at low and high elevation sites in Haiti (t is in weeks)
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Carbon remaining in stems followed a different pattern than was expressed in leaves. At the low elevation, C loss from stems after 48 wk was 59% in gliricidia and averaged 37% in the other species (Fig. 2b). Similarly to leaves, stems decomposed more slowly at the high elevation site and less variation was found among species over the whole period (Fig. 2d). Patterns of stem decomposition were best characterized by a two-pool model (except for flamboyant) at the low elevation and by a single exponential model (except for leucaena) at the high elevation site (Table 3). The k values were consistently higher for species grown at low elevation and lower for stems than for leaves.
Nitrogen Release Patterns
Leaf N remaining, expressed as percent initial N, followed a pattern similar to C remaining at the low elevation site (Fig. 3a) . At this site, leaves released most of their N during the first 8 to 16 wk of decomposition, gliricidia being ranked first while its leaf mulch still retained 26% of initial N after 16 wk. Between 8 and 16 wk, N immobilization apparently occurred in flamboyant and after 16 wk, all species mineralized their leaf N at considerably slower rates. At the high elevation site, patterns of leaf N release differed slightly from that of C loss. Percent N release was greatest for L. diversifolia and A. angustissima after 48 wk. However, species grown at this site retained more than 65% of their leaf N at the end of the study period (Fig. 3c). Patterns of leaf N remaining were best described by a two-pool model at the low elevation site and by a one pool model (except for L. diversifolia) at the high elevation site (Table 4)
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Fig. 3 Nitrogen release from decomposing leaves (a and c) and stems (b and d) of species assessed for alley cropping at low and high elevation sites in Haiti (bars are SE)
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Table 4 Equations for perecent N remaining from prunings of hedgerow species decomposing in litter bags under field conditions at low and high elevation sites in Haiti (t is in weeks)
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Nitrogen release from stems at the low elevation site showed an increase during the first 8 to 16 wk followed by a very slow mineralization in gliricidia and N immobilization in the other species (Fig. 3b). Stems from flamboyant released their N faster than those of other species. At the high elevation site, stem N release was consistently higher in leucaena hybrid whereas N release was similar in the other species (Fig. 3d). The single exponential model was successfully fitted to stem N remaining (except for flemingia) in both sites (Table 4).
Nitrogen release from pruning residues was calculated by multiplying the percentage estimate of N release from the respective equations by the initial N content and adding results for leaves and stems together according to species and site. Cumulative N release during the first 16 wk is shown in Fig. 4
. At the low elevation, leucaena had a significantly greater initial N content (Table 1), but its amount of N release was similar to that of gliricidia during the first 16 wk. At the end of this period, gliricidia released 71% of its N content whereas leucaena has released only 46% (Fig. 3). The low N content combined with a slow rate of decomposition for flamboyant resulted in lower N contributions (36.4 kg N ha-1) over 16 wk (Fig. 4a). At the high elevation site, the slower decomposition of hedgerow species led to very limited N contributions, A. angustissima being the highest with 46 kg N ha-1 (29% of N content) and flemingia the lowest with 9 kg N ha-1 (16% of N content) after 16 wk (Fig. 4b).
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Fig. 4 Cumulative N release from decomposing prunings of hedgerow species at low and high elevation sites in Haiti
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Relation of Pruning Chemical Characteristics and Rate Constants
The coefficients of determination of linear regressions between pruning chemical characteristics and rate constants of decomposition and N release are shown in Table 5
. At the low elevation site, initial leaf N concentrations correlated positively with C loss (r2 = 0.75; P < 0.05) and N release rate constants (r2 = 0.76; P < 0.05) during Phase I (Table 5 and Fig. 5)
, whereas initial C:N and (lignin + polyphenol):N ratios correlated with C loss (r2 = 0.89; P < 0.01) and leaf N release (r2 = 0.92; P < 0.01) during Phase II, respectively (Table 5 and Fig. 5). Although lignin concentration appears to influence the decomposition of stems (Table 5), the relationship of decomposition and N release with lignin concentration in stems did not test significant at the 5% level. At the high elevation site, there was no clear indication as to which chemical factor influenced decomposition of pruning residues except that lignin:N was negatively correlated (r2 = 0.99; P < 0.001) with rate constants of leaf C remaining during Phase II (Table 5 and Fig. 6)
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Table 5 Coefficient of determination (r2) of linear regressions between pruning chemical characteristics and rate constants (k) of decomposing leaves and stems at a high and a low elevation site in Haiti
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Fig. 5 Relationship between rate constants and chemical characteristics of prunings of hedgerow species decomposing at a low elevation site in Haiti; NS, *, **, nonsignificant at the 0.05 and significant at the 0.05 and 0.01 probability levels, respectively
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Fig. 6 Relationship between rate constants and chemical characteristics of prunings of hedgerow species decomposing at a high elevation site in Haiti; NS, ***, nonsignificant at the 0.05 and significant at the 0.001 probability levels, respectively
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Discussion
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Continuous supply of organic materials from hedgerows is of particular interest in maintaining alley cropping systems (Kang et al., 1984). Therefore, sustained biomass yield (data not reported) under an intensive pruning regime (four times a year) was the primary reason for inclusion of the species in this study. On average, species yielded more than 3.6 Mg ha-1 and 2.5 Mg ha-1 dry mulch biomass at the low and high elevations, respectively. These amounts of biomass are adequate to provide both N and organic materials in an alley cropping system. Similar biomass productivity for gliricidia, leucaena, and flemingia has been reported (Budelman, 1988).
As expected, differences in chemical composition were apparent between species within site and between sites for the same species. Besides intrinsic characteristics, soil nutrient content, and climatic conditions are likely to contribute to these differences. Concentrations of N, lignin, and cellulose were comparable with values previously found in prunings of gliricidia and leucaena species (Handayanto et al. 1994; Jama and Nair, 1996; Lehmann et al., 1995). Although variations were observed between sites, leaf N concentrations and C:N ratios of species used were within the range for N mineralization to occur. In both sites, polyphenol concentrations of the leucaena used were slightly higher than values previously reported (Jama and Nair, 1996; Palm and Sanchez, 1991; Tian et al., 1992). Differences in age of prunings used and soil fertility are probably the factors behind the differences in our study.
Comparison of results at both sites showed consistent differences in prunings decomposition between the species. The most important difference at the low elevation site was the resistance of flamboyant leaf mulch to decomposition. The large lignified central rachis of its leaves may have contributed to its higher C:N ratio resulting in slower rates of decomposition. However, persistence of leaf mulch of flamboyant may be an advantage where the primary goal is to reduce soil erosion. Rapid decomposition of leaves of gliricidia and L. shannonii may be related to their high quality (high N and low polyphenol). Similarly, Budelman (1988) reported faster rates of decomposition of leucaena and gliricidia under similar conditions. In contrast, Lehmann (1995) found greater mass loss from leaves of gliricidia incubated for shorter period. The size openings of the mesh bags (5 mm) used by Lehmann (1995) may have allowed intense soil faunal activity on the mulch, increasing its mass loss over time. With respect to stems, the slower decomposition as compared with leaves concurred with previous observations from decomposing twigs of leucaena (Jama and Nair, 1996) and gliricidia (Lehmann et al., 1995). However, stems decomposed faster in our study than previously observed (Jama and Nair, 1996; Lehmann et al., 1995). Amount and frequency of rainfall (low elevation site) are likely the factors responsible for this rapid decomposition.
Decomposition of prunings from species grown at the high elevation site occurred at a slower rate than at the low elevation (Table 3). Although no terms in the equation models take into account the influence of climatic factors, it is likely that temperature and humidity may have contributed to differences in decomposition rates between sites. Since the mesh size of the litterbags used may exclude larger soil-dwelling fauna from feeding on the mulch, the speed of decomposition was expected to be site-specific and to relate to temperature, availability of water, and chemical composition of a particular species. The higher temperature and fairly well distributed rainfall (Fig. 1) during the weeks following bag placement at the low elevation site are optimum situations for fast decomposition. Vanlauwe et al. (1995) found positive correlation between number of rainfall events and dry matter loss from decomposing leaves of leucaena and Senna siamea. The reverse conditions (low temperature and erratic rainfall) combined with higher contents of lignin and polyphenol of species grown at the high elevation site are likely the cause for the slow decomposition.
Analysis of N release must be based upon intrinsic capabilities of the species to release N and amount of residues applied. At the low elevation site, gliricidia and L. shannonii released their N faster than leucaena, but the higher dry matter productivity of the latter species may ensure higher levels of annual N contributions (Fig. 4). Similar N contributions by gliricidia and leucaena have been reported (Tian et al., 1993; Yamoah et al., 1986b). Based on N recommendations for maize (Zea mays L.) (70–80 kg ha-1) in the region where the study was conducted, the N released from gliricidia and leucaena prunings at the low elevation would be adequate for maize production without needs for fertilizer N. At the high elevation, even though leaf N concentrations were relatively high, the slow decomposition limited the capability of hedgerow species to contribute sufficient amounts of N for crop needs. With respect to stems, their slow decomposition combined with low N content at both sites indicated that these materials are not an important source of N in alley cropping systems. However, if stems are applied together with high quality leaf materials, they may slow the decomposition rate, leading in some cases to more synchronous N release with crop demand.
Percent C and N remaining was described satisfactorily by one or two-pool models previously used by Wieder and Lang (1982). The rate constants (k) of decomposition and N release were estimated directly from the models. Values for k1 (Tables 3 and 4) were slightly higher than those reported by Tian et al. (1992) under similar conditions but k2 values were comparable with those reported by Jama and Nair (1996). The significant correlation found with the k constants emphasized the role of chemical composition of a specific material on its decomposition. Initial N concentration or C:N has been considered to be an important factor influencing the degradability of organic residues added to the soil (Melillo, 1983). Such a trend was found for the leaf materials incubated at the low elevation site (Fig. 5). The relationship between decomposition rate and initial N (Phase I) and C:N ratio (Phase II) was best described via linear regression. With respect to N release, the high and variable polyphenol concentration of the species used was expected to influence the rates of N release in early stages of decomposition. The present results showed that initial N concentrations correlated well with N release rates at Phase I but the (lignin + polyphenol):N ratio was most successful in explaining the differences in N release rates between species at Phase II (Fig. 5). Similarly, Handayanto et al. (1994) found that the ratio (lignin + polyphenol):N was consistently the best quality factor to predict weight loss and N released from prunings of four legume tree species incubated in litterbags. Also, Jama and Nair (1996) observed that rate of Phase II mulch decomposition and N release of leucaena and Cassia siamea appeared to be influenced by lignin and polyphenol contents.
Low rates of decomposition of leaf materials with narrow C:N have been related to the polyphenol or polyphenol:N ratios and to the lignin + polyphenol:N ratio (Fox et al., 1990; Oglesby and Fownes, 1992; Palm and Sanchez, 1991). At high elevation, neither of these factors appeared to influence rates of decomposition and N release of the different species. The erratic rainfall may not have been adequate to allow soluble polyphenols affect the decomposition and N release rates early during the process. At this site, parameters that regulate decomposition and N release rates during different phases were not strongly identified due probably to the use of species with similar N and polyphenol concentrations. However, the lignin:N ratio of the prunings was found to correlate with decomposition rates of leaves during the second phase. Jama and Nair (1996) noted that a high lignin:N ratio (20:1) was involved in the low rate in the second phase of decomposition of cassia (Cinnamomum aromaticum Nees) mulch under semi-arid conditions. High concentrations of lignin and polyphenol from pruning materials at the high elevation site, combined with climatic conditions, may explain the overall low decomposition from prunings of species grown at this location.
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Conclusions
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The pattern of N release is as important in assessing suitability of hedgerow species for alley cropping as is the total N provided. At the low elevation site, gliricidia and L. shannonii decomposed faster and provided a substantial amount of N in a short period of time. However, risk for N losses from these species may be important where prunings are applied before planting the associated crop. Patterns of decomposition and N release of leucaena combined with its high productivity under conditions found at the low elevation site indicated this species has greater potential to provide both N and organic materials on a sustained basis in an alley cropping system. Although flamboyant yields a high quantity of biomass, its low N concentration and slow release pattern may hamper its successful use in alley cropping unless N fertilizer also is applied. However, flamboyant may be an advantage where the primary aim is to reduce soil erosion. At the high elevation site, A. angustissima showed best potential in terms of total N, but N release may not be sufficient to meet peak N demands of associated crops.
Initial N concentrations and C:N and (lignin + polyphenol):N ratios were the factors influencing leaf decomposition and N release from species grown at the low elevation site. However, further research may be needed at the high elevation site to better understand factors influencing patterns of leaf N release. The lignin:N ratio seemed to regulate the decomposition and N release patterns of stems, but the relationship was not significant. Perhaps a wider range of substrate quality needs to be considered before a relationship can be established.Melillo Aber 1983
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ACKNOWLEDGMENTS
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The authors are grateful for the funding provided for this research by the United States Agency for International Development (USAID) through the following projects: Soil Management Collaborative Research Support Program (CRSP) Project, Soil Management Practices for Sustainable Production on Densely Populated Tropical Steeplands (Grant No. LAG-G-00-97-00002-00), the USAID/Haiti Productive Land Use Systems Project (Contract No. 521-0217-C-00-5031-00), and the Agroforestry II Project (Contract No. 521-0217-C-00-0004-00).
Received for publication August 10, 1999.
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REFERENCES
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ABSTRACT
INTRODUCTION
