Fate of nitrogen derived from 15N-labeled plant residues and composts in rice-planted paddy soil

Pot experiments that lasted for 3 y were conducted to investigate the dynamics of nitrogen derived from plant residues (rice root, hull, straw, corn root, and rapeseed pod-wall), and composts (rice straw compost, cattle manure compost, and cattle manure sawdust compost), which were labeled with ¹⁵N. The rates of nitrogen uptake by rice (=N efficiency), denitrification, and immobilization derived from the organic materials incorporated before the first year of cultivation were investigated throughout 3 y of cultivation. At the end of the first year of cultivation, relatively high rates of N efficiency were obtained for rapeseed pod-wall (24.6%), rice straw (19.1%), and rice hull (18.6%), while corn root and cattle manure sawdust compost displayed a noticeably high denitrification rate. Corn root, cattle manure sawdust compost, rice hull, and rapeseed pod-wall exhibited remarkably high N mineralization rates ranging from 60 to 75% of the organic materials N applied. Cumulative rates of N efficiencies from the organic materials applied before the first year of cultivation fitted well to a first-order kinetic model and their asymptotes were compared among the organic materials. The asymptotic rates of N efficiency tended to depend on the rates at the end of the first year of cultivation.