Nitrogen cycling assessment in a hedgerow intercropping system using

Nitrogen (N) cycling was determined in monocultures of Sorghum bicolor (L.) Moench and alley cropped sorghum with Acacia saligna (Labill.) H. Wendl. in semiarid Northern Kenya. N input through biological N2 fixation of the acacia, N transfer from the legume to the intercrop and losses of applied N through harvest and 15 leaching were estimated using N enrichment. The biological N fixation and N transfer estimates clearly 2 15 demonstrated the limitations of N enrichment techniques in field experiments showing even higher transfer than 15 actually fixed N. Therefore, N transfer in the hedgerow intercropping system could not be determined by the N 15 15 dilution methodology. The N balance approach, however, yielded reliable results even 1.5 years after N 15 application. 74 to 88% of the applied N was recovered after three cropping cycles, most of it in the soil (0–1.2 15 m). Only about 10% of the N was taken up by the above-ground vegetation of both monoculture and 15 agroforestry. The trees took up more of the applied N (8.4%) than the sorghum (1.3%) in the agroforestry system, indicating nutrient competition between tree and crop. Leaching losses below 1.2 m depth were low in this 15 semi-arid environment with 3 and 6% of the applied N in the monoculture and agroforestry system, respectively. N losses from leaching were 2.5 times higher in the alley than under the tree row. Incorporating the leguminous 15 tree into the sorghum cropping system had no effect on total leaching and total uptake of applied N in above-ground biomass. Introduction performance in alley cropping systems is available (Palm 1995), but the below-ground N fluxes and Nitrogen is one of the most important nutrients limitinteractions between trees and crops have not been ing crop production in tropical smallholder agriculture sufficiently addressed up to now. (Sanchez 1976). Whereas numerous studies exist In addition to increasing the N input, N losses must about N management in humid areas, information also be minimized to improve the N benefit to the about N cycling in dryland agriculture is scarce (Vlek crop. It has been frequently hypothesized that trees et al. 1981). reduce nutrient leaching and form a safety-net under Integrating leguminous trees into farmland may the root zone of the annual intercrop (e.g.Van Noordimprove the N supply of the cropping system through wijk et al. (1996)). Strong evidence exists that trees biological N fixation (Dakora and Keya 1997). Highare able to reduce nutrient leaching in comparison to 2 er crop yields, however, can only be achieved if the sole cropped annuals (Hartemink et al. 1996; fixed N can actually be utilized by the annual crop Seyfried and Rao 1991). How leguminous trees planted between the trees. Some information about affect N cycling in simultaneous tree-crop associathe effects of mulched tree prunings on intercrop tions is less clear. Agroforestry systems may have