Kinetics of wheat straw decomposition and nitrogen mineralization in rice field soil

The decomposition of wheat straw in the soil in a rice field was studied by nylon mesh bag technique and mineralization of carbon and nitrogen was measured. The rate of decomposition of the wheat straw was relatively faster in the beginning but it slowed down after four weeks. By the end of 22nd week, 82 to 86 per cent of the wheat straw had decomposed. The first order model showed better fitness than zero and second order models for the wheat straw decomposition. Sesbania green manuring before rice transplanting appeared to accelerate the wheat straw decomposition in the rice field. The rate constant for the wheat straw decomposition was worked out to be 0.0130 and 0.0116 d−1 in the green manured plot and without green manure plot, respectively, and the half-life of the wheat straw was found to be 53 and 60 days for these two plots. During decomposition of the wheat straw carbon content decreased, N content increased and C:N ratio decreased with time. There was practically no N mineralization from the wheat straw during 2nd to 10th week after the straw incorporation but thereafter N mineralization proceeded gradually. The results suggested that the wheat straw should be incorporated into the soil about 7–8 weeks before rice transplanting to escape the adverse effects of immobilization of inorganic nitrogen on rice growth.