Nitrogen budgets in legume based cropping systems in northern Malawi

Smallholder farmers in sub Saharan Africa (SSA) operate in a risky environment characterized by low soil fertility, unpredictable weather and markets. Identification of technologies that optimize crop yields in a variable climate, while building soil fertility, can contribute to sustainable cropping systems. Participatory on-farm trials were conducted in Ekwendeni of northern Malawi to evaluate performance and yield of legume diversified cropping systems. Prior to implementation of trials, household and farm field surveys were conducted to characterize cropping systems and soils. Soil fertility among farms was highly variable and largely coarse textured with very low organic matter (12+/-3.7 g kg-1). There was no evidence of cropping systems effect on nutrient levels except for inorganic P which was lower in legume diversified fields than in maize fields. A survey showed that farmers valued a wide range of legume traits that included food, yield, maturity period, post harvest handling, soil fertility, market potential and pest resistance.;On-farm trials evaluated maize-based cropping with a range of legume growth types and planting arrangements (groundnut representing an annual grain legume and pigeonpea representing a semi-perennial grain legume, planted as sole and intercrop systems rotated with maize). The trials were conducted over two years and showed that interspecific competition, inorganic P and plant density markedly influenced crop growth and biological nitrogen fixation (BNF). The type of species present in the intercrop -- legume or cereal - did not alter the BNF response. On area basis, there was no evidence of higher N fixation rate by groundnut-pigeonpea intercrop (GNPP) "doubled-up legumes" compared to sole stands of either species. Overall performance of intercrops vs sole crops was superior in terms of grain yield produced in the first year of the rotation, as indicated by calculation of a land equivalent ratio (LER). The LERs were 1.50 and 1.56 for GNPP and MZPP compared to sole crops, indicating that intercropped species were more efficient at utilizing resources than sole stands.;Performance over the two years of the cropping system was evaluated in 2008/2009. Maize was planted on fields previously planted to sole or intercropped legumes. Indicators of N status (chlorophyll and biomass) showed that maize growth in year two was influenced by cropping system. In contrast, soil inorganic N did not show a response to cropping system. A previous crop of sole or intercropped legumes increased maize grain yields by 21-62% compared to a previous crop of maize. Integrated soil fertility management (ISFM) was also evaluated, comparing all cropping systems with the addition of 24 kg N ha-1 inorganic fertilizer to a continuous maize N-rate study (0, 24 and 92 kg of N ha-1 fertilizer). This allowed estimation of a N-fertilizer equivalency for ISFM maize in year two, which varied from 18 to 55 kg of N ha-1. Overall, legume presence increased maize yield by 69-200% compared to sole crop, unfertilized maize.;A farmer preference survey showed a preference for GNPP/maize rotation even though this system did not optimize yield, followed by pigeonpea/maize rotation, and lastly MZPP/maize systems. Farmers' choices were based on cropping systems that provide multiple benefits.