2.4 Nitrogen mineralization
Grass and legume residues (0.2 gm on a dry weight basis; <1mm size) mixed with topsoil (20 gm; <2mm) were packed in a plastic container and incubated at 280C for 14 weeks to measure N-mineralization. Soil moisture content was maintained at 50% of field capacity throughout the experiment. Soil samples without plant residue were also incubated as control. The incubated soil samples were extracted with 100 ml 1M KCl solution by shaking for 1 h in an orbital shaker and filtered through Whatman #42 filter paper at an interval of 1, 2, 3, 4, 6, 8, 10, 12 and 14 weeks. The inorganic N concentration in the soil extract was analyzed by the steam distillation method (Keeney & Nelson, 1983).