Microbial Biomass Nitrogen as an Index of N Availability in Acidic Soils of North East India

A reliable estimate of soil’s nitrogen (N) supplying capacity is essentialto improve N fertilizer efficiency and crop productivity.In the study reported here, we evaluated the utility of soil microbial biomass N (SMB-N) vis-a-visalkaline KMnO 4 extractable-Nas an index of N availability in acidic soils of northeast India. These indices were evaluated based on their correlation with available N obtained through two standard incubation methods viz., aerobic incubation (AI-N) and anaerobic incubation (ANI-N), and also with dry matter yield, N concentration and N uptake (PNU) in maize (Zea mays L.). SMB-N correlated significantly better with AI-N and ANI-N (r = 0.836 ** and 0.811 ** respectively) compared to alkaline KMnO 4 -N(r =0.394 ** and 0.548 ** respectively). Correlations of plant parameters were also stronger with SMB-N (r=0.707** for PNU) than with alkaline KMnO 4 -N (r= 0.625**).Based on the significantly stronger relationship of SMB-N with the standard biological indices and the plant responses, we envisage SMB-N as a reliable index of N availability in acidic soils of northeast India.