Seasonal changes in soil microbial biomass nitrogen under an irrigated wheat-maize cropping system

Seasonal changes in soil microbial biomass nitrogen (N-mic) were followed under an irrigated wheat-maize cropping system receiving urea and/or farmyard manure (FYM) at 0-200 kg N ha(-1) year(-1) for the past ten years. The N-mic was maximurn (109-218 kg N ha(-1)) at the wheat stem elongation stage and minimum (80-148 kg N ha(-1)) at the wheat tillering stage. Fertilizer application significantly increased the N indicating that the belowground processes were limited by N availability. Averaged across sampling dates, the N-mic ranged from 93 kg N ha(-1) (in unfertilized) to 176 kg N ha(-1) (in FYM applied at 32 t ha(-1) year(-1)), whereas the annual N-mic flux ranged from 57 kg N ha(-1) (in unfertilized) to 118 kg N ha(-1) (in FYM applied at 16 t ha(-1) year(-1)). Crop dry matter and N yields increased due to fertilizer application. At an equivalent N application rate, urea applied alone produced maximum yields, followed by Urea combined with FYM, whereas FYM applied alone yielded minimum. The size and flux of N-mic were poor indicators of the crop N availability, whereas the soil mineralizable N determined by alkaline permanganate method at the crop sowing stage was significantly correlated with the soil mineral N, and with the crop dry-matter and N yields.