Evaluation of nitrogen fixation using 15N dilution methods and economy of a maize-tepary bean intercrop farming system in semi-arid SE-Kenya

Abstract

Tepary bean has become popular among poor small-scale farmers in semi-arid Kenya, where it is intercropped with maize. This study aimed at i) evaluating the N-economy of maize/tepary bean intercrop versus sole crop using natural abundance and 15N enriched fertilizer methods, and ii) assessing the contribution of fixed N2 by tepary bean to the total N balance in the intercrops and sole cropping systems assessed from harvested seed and residues. Experiments were carried out during the short rains of 2001/2002 and long rains of 2003 at Kenya Agriculture Research Institute (KARI) Kiboko, Kenya. Randomised block design was used with one block devoted to the 15N natural abundance (–N), the other 15N labelled fertilizer (+N), replicated 4 times. Above ground biomass and total N were determined in sole crops or intercrops (–N or +N). Tepary bean received 53–69% of its N supply from N2-fixation with N2-fixation slightly affected by intercropping or N fertilizer application. N2-fixation of tepary in greenhouse experiment was lower (36–66%) than in the field study and more affected by N supply. Budgets for N were estimated for field intercrops based on above-ground seed yields, return of crop residues, input of fixed N and fertilizer N. N2-fixation was 59 kg N ha-1 in plots receiving no N fertilizer, and 73 kg N ha-1 in plots receiving N as urea. Corresponding fixation by sole tepary was high (87 and 82 kg N ha-1, respectively), but this advantage was outweighed by greater land use efficiency in intercrop than sole crop