Predicting potential anaerobic nitrogen mineralization of rice-rice and rice-wheat soils of Asia

Nitrogen (N) supplying capacity of soils in rice (Oryza sativa L.)based cropping systems has been reported to be in decline in some sites in Asia. Studying the relationship between N mineralization, rice cropping intensity and soil properties may help us understand changes in N supplying capacity. We conducted short- and longterm laboratory incubations in flooded, anaerobic conditions to measure N mineralization in a range of rice-rice and rice-wheat (Triticum aestivum L.) cropped soils. Our objectives were to determine: (1) what soil properties affect potential anaerobic N mineralization, and (2) whether potential anaerobic N mineralization is affected by rice cropping system. Several soil properties, especially total organic N (TON) and total organic C (TOC), were Positively correlated with N mineralization potential at 30 degreesC (N(0)) and with 7-d production of NH(4) at 40 degreesC (N-7-d). Nitrogen mineralization was only weakly correlated with TON or TOC if the initial large flush during the first 14 d was ignored. Rice-rice soils on average had greater N mineralization than rice-wheat soils. mostly due to higher TON concentrations. Seven-day production of NH(4) at 40 degreesC was so similar to N(0) estimated after 84 d of anaerobic incubation at 30 degreesC that the 7-d incubation could possibly substitute for the long-term one in developing N fertilizer recommendations.