Soil nitrogen dynamics and relationships with maize yields in a gliricidia-maize intercrop in Malawi

Many soils of southern Africa are severely N deficient, but inorganic ferti lizers are unaffordable for most subsistence farmers. Rotations and intercr ops of legumes with crops may alleviate N deficiency through biological N-2 fixation and redistribution of subsoil N to the surface. We monitored soil inorganic N dynamics for two seasons in a gliricidia [Gliricidia sepium (J acq.) Walp.] - maize (Zea mays L.) intercrop in the unimodal rainfall area of southern Malawi. One maize crop per year was grown with or without inter planted gliricidia, in factorial combination with three rates of N (0, 24 o r 48 kg N ha(-1)). Application of gliricidia prunings increased (p < 0.001) topsoil (0 to 20 cm) inorganic N at the end of the dry season and during t he early rains. Differences between plus and minus gliricidia treatments we re less when total inorganic N to 1-m depth was summed. A greater proportio n of the total inorganic N to 1-m depth occurred in the topsoil (0 to 20 cm ) when gliricidia was present, suggesting that redistribution of subsoil N to the surface accounted for part of the N increase by gliricidia. Gliricid ia lowered (p < 0.05) subsoil water content during drier periods. Gliricidi a plots accumulated more (p < 0.01) ammonium-N during the dry season. Nitra te-N remained constant during the dry season but rose rapidly in gliricidia plots after the onset of rains. A 2-factor model including preseason inorg anic N and anaerobic N mineralization potential accounted for 84% of the va riability in maize yields for the two seasons' data combined. The combinati on of preseason inorganic N and potential N mineralization appears to provi de a good estimate of N supply to maize in systems receiving both organic a nd inorganic sources of N.