Relating preseason soil nitrogen to maize yield in tree legume-maize rotations

Legumes can be an important source of N for cereals in tropical, subsistence farming systems that use little or no fertilizers. Our objective was to identify measures of soil N availability, following growth of diverse legumes, that correlated with yield of a subsequent unfertilized maize (Zea mays L,) crop. Unfertilized maize followed nine 3-yr-old systems (six monocultnres of planted trees legumes, a groundnut [Arachis hypogea L.]-maize-soybean [Glycine max (L) Merr.] rotation, an uncultivated fallow with natural regrowth of vegetation, and unfertilized maize monoculture) in two experiments on a Ustic Rhodustalf in eastern Zambia, Total soil C and N before maize planting were not related to grain yield of maize. Preseason soil inorganic N (NO3 + NH4), aerobic N mineralization, and light-fraction N (N in macroorganic matter, 150-2000 pm and <1.37 Mg m(-3)) at 0- to 15-cm depth correlated (P < 0.01) with maize grain yield, Preseason inorganic N combined with light-fraction N accounted for 59% of the variance in maize grain yield. The predictability of maize yield was slightly improved (64% of the variance) by including the population of the parasitic weed striga [Striga asiatica (L,) Kuntze] with preseason inorganic N and Light-fraction N. The results suggest that yield of unfertilized, rain-fed maize following different rotational systems on a N-limiting soil in an area with monomodal rainfall was strongly related to the additive effects of preseason soil inorganic N and a soil N fraction related to N mineralization.