Effect of co-applied cowdung and inorganic nitrogen on microbial respiration in soil under laboratory conditions

A laboratory incubation study was carried out for 8 weeks at 30°C to assess microbial respiration and nitrogen (N) transformation in a sandy loam, Iwo series (Oxic Tropudalf) amended with both inorganic N and cowdung. The treatments consisted of 3× 3 factorial combinations of manure and inorganic N. Cowdung rates were 0, 12, 24 t ha−1 and rates of (NH4)2SO4 were 0, 25, and 50 mg N kg−1 (0, 60, and 120 kg N ha−1) in soil. Manure, N and their interactions had significant (P=0.05) effects on soil microbial respiration and N transformation. While inorganic N alone increased acidity, cowdung increased soil pH and thus ameliorated the acidity produced as a result of nitrification. Microbial respiration increased with the rate of cowdung with and without N addition. While N immobilization was evident at the end of the 2nd week, there was net mineralization at the end of the 8th week of incubation. The increases in inorganic N concentrations between the 2nd and 8th week of incubation were 514 and 551% for the cowdung, and between 107 and 329% for the inorganic N amended treatments. In soil that received inorganic N, N immobilization increased with increase in the rate of cowdung addition. Addition of (NH4)2SO4 caused priming of indigenous organic nitrogen. Microbial respiration was stimulated by factors of 2.6, 2.8, and 1.7 by cowdung, co-applied cowdung and N, and inorganic N, respectively. Terminal pH and inorganic N concentrations were positively correlated (r = 0.69*; 0.89* *) with cumulative CO2 evolved from the treatments. When cowdung was added, the adverse effects of inorganic N application on soil biological and physico-chemical properties could be attenuated. Increasing the stability of manure C through composting and applying it and inorganic N to opposite sides of the seedling should minimize N immobilization and enhance N uptake.