Rotation effects of grain legumes and fallow on maize yield, microbial biomass and chemical properties of an Alfisol in the Nigerian savanna

Understanding changes in soil chemical and biological properties is important in explaining the mechanism involved in the yield increases of cereals following legumes in rotation. Field trials were conducted between 2003 and 2005 to compare the effect of six 2-year rotations involving two genotypes each of cowpea (IT 96D-724 and SAMPEA-7) and soybean (TGx 1448-2E and SAMSOY-2), a natural bush fallow and maize on soil microbial and chemical properties and yield of subsequent maize. Changes in soil pH, total nitrogen (Ntot), organic carbon (Corg), water soluble carbon (WSC), microbial biomass carbon (Cmic) and nitrogen (Nmic) were measured under different cropping systems. Cropping sequence has no significant (P > 0.05) effect on soil pH and Corg, while WSC increased significantly when maize followed IT 96D-724 (100%), SAMPEA-7 (95%), TGx 1448-2E (79%) and SAMSOY-2 (106%) compared with continuous maize. On average, legume rotation caused 23% increase in Ntot relative to continuous maize. The Cmic and Nmic values were significantly affected by cropping sequence. The highest values were found in legume–maize rotation and the lowest values were found in fallow–maize and continuous maize. On average, Cmic made up to 4.8% of Corg and Nmic accounted for 4.4% of Ntot under different cropping systems. Maize grain yield increased significantly following legumes and had strong positive correlation with Cmic and Nmic suggesting that they are associated with yield increases due to other rotation effects. Negative correlation of grain yield with Cmic:Nmic and Corg:Ntot indicate that high C:N ratios contribute to nitrogen immobilization in the soil and are detrimental to crop productivity. The results showed that integration of grain legumes will reverse this process and ensure maintenance of soil quality and maize crop yield, which on average, increased by 68% and 49% following soybean and cowpea, respectively compared to continuous maize.