Soil microbial biomass carbon (MBC) and nitrogen (MBN) reflect microbial size and soil fertility status, and they serve as the living nutrient pool in soil. The study was conducted to assess the effect of various organic sources including poultry manure (PM), farmyard manure (FYM), compost (CM) and biochar (BC) with varying levels of mineral fertilizers on soil microbial dynamics and nutrient(N, P) mineralization under laboratory incubation experiments. The organic amendments were applied based on 0.25, 0.5 and 1.0 % C of soil (w/w) in combination with 75, 50 and 25 % of recommended doses of NPK (120:90 and 60 kg N, P₂O₅ and K₂O ha^-1), respectively. After application of amendments the soil was incubated (16 days) for CO₂ emissions and 28 days for N, P mineralization. The rate of CO₂ evolution significantly increased with C levels but declined with time and the highest CO₂ evolution at any time was exhibited by compost followed by PM, FYM and BC that could be associated with higher readily available fraction of C. Soil microbial biomass C and N significantly increased in compost at 1% C level in combination with inorganic fertilizer. Nitrogen and phosphorous mineralization was increases as the incubation period increases (28 days). However, the net release of N and P decreased with an increase in C levels that could be associated both with immobilization as well the fact that higher levels received lower (25%) NPK levels. However, compost and PM treated soils maintained higher N and P which is associated with their higher fraction of labile nutrients.  These results suggest that continuous application of organic sources improves the N and P mineralization, SMB and their activity. Thus, regular application of carbon sources could be used as a strategy to improve the soil health of less fertile alkaline calcareous soil.

Key words: Farm yard manure, Poultry manure, Biochar, Compost, Microbial biomass carbon, Microbial biomass nitrogen.