Written Paper

Nitrogen sourcing by fast - growing legumes in pure and mixed species fallows in western Kenya  [2000]

Gathumbi, S.M.(Kenya Forestry Research Institute)

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Nitrogen sourcing, accumulation and recycling to crops by tree/shrub/herbaceous legumes in planted fallows are among the innovative features of legume-crop rotation systems (referred to as improved fallows). With increasing concern o¥er the success and adoption of the technology by farmers, mixed species fallows are viewed to have a potential of reducing farmer risks and offer diversified products besides the soil amelioration role. The overriding objective of the study was to investigate the potentials of mixed species fallows compared with monoculture species fallows for enhanced and sustained soil N contribution. Field experiments were conducted on a Kandiudal~c eutrudox in western Kenya and complementary experiments in Wye College greenhouse. Fast-growing woody and herbaceous legumes were evaluated for DM and N accumulation, subsoil nitrogen capture and their N contribution to the overall soil N economy in the fallow system through biological nitrogen fixation (BNF). The feasibility of using 15N isotope techniques in estimating %N derived from BNF and tracing the subsoil N uptake by several legumes was assessed. The main legume species included cajanus (Cajanus cajan), sesbania (Sesbania sesban), crotalaria (Crotalaria grahamiana), tephrosia (Tephrosia vogelii), macroptilium (Macroptilium atropurureum) and arachis (Arachis hypogaea) DM yield for different species in short duration (6-8 months) monoculture fallows were 8, 5-10, 11-12, 13 and 5 t DM ha-l
for cajanus, sesbania, crotalaria, tephrosia and macroptilium respectively. Total N yield ranged between 120-148, 83-100, 178- 230, 150 and 145-186 kg N ha-l for cajanus, sesbania, crotalaria, tephrosia and macroptilium respectively. Foliage N yield constituted the highest proportion (70%) of plant N for the species compared with wood N. Most of the woody legumes in sole stands fixed between 57 - 74 % of their total N yield. This amounted to 72 - 152 kg ha-1 of total N fixed. Tephrosia and crotalaria fixed the largest proportion of their N (66-74%). Crotalaria was the best N2 fixer with the potential of accumulating between 113 to 152 kg ha-l of fixed N in foliage. Total fallow N accumulation was highest for crotalaria and ranged between 199 and 230 kg ha-l. Macroptilium fixed only about 40% (71 kg ha-l) of its N but extracted more N from the soil (87 - 105 kg N ha-l) compared with other species. In mixtures total DM production ranged between 8-10, 7-15, 9-15 and 9-14 t ha-l for cajanus, crotalaria, sesbania and tephrosia mixtures respectively. At 50 : 50 plant density total and foliage DM production of woody legumes was not increased by mixing species. However, undersowing macroptilium into woody fallow species resulted in the highest DM yield compared with other mixtures. Soil N uptake showed that crotalaria + macroptilium, sesbania + crotalaria and cajanus + tephrosia mixed fallows resulted in net depletion of available mineral N. Sesbania in a pure stand and in mixtures with crotalaria and macroptilium reduced soil inorganic N below 100 cm depth. Soil N uptake examined from the recovery of 15N labelled (15N~)2S04 injected at 15, 100 and 200 cm depths showed that well established sesbania had a higher 15N uptake than crotalaria from lower soil depths. Total subsoil 15N recovery was more in sesbania + macroptilium than in sesbania + crotalaria mixtures largely as a result of additional uptake by macroptilium. In contrast, topsoil 15N uptake of sesbania in sesbania + macroptilium was less than that of sesbania + I ", crotalaria mixture suggesting that sesbania uptake activity in the topsoil was stimulated by competition from crotalaria but not macroptilium. Root measurements also revealed that mixing sesbania and crotalaria stimulated development of more roots into the subsoil. Results from this study indicate that mixed species fallows of crotalaria, cajanus, tephrosia, sesbania and macroptilium are promising alternative technologies to pure species fallows for maximization of resource utilization and diversification of fallow benefits.