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 cro pping systems has been reported to be in decline in some sites in Asia. Stu dying the relationship between N mineralization, rice cropping intensity an d soil properties may help us understand changes in N supplying capacity. W e conducted short- and longterm laboratory incubations in flooded, anaerobi c conditions to measure N mineralization in a range of rice-rice and rice-w heat (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 cropp ing system. Several soil properties, especially total organic N (TON) and t otal organic C (TOC), were Positively correlated with N mineralization pote ntial at 30 degreesC (N-0) and with 7-d production of NH4 at 40 degreesC (N -7-d). Nitrogen mineralization was only weakly correlated with TON or TOC i f the initial large flush during the first 14 d was ignored. Rice-rice soil s on average had greater N mineralization than rice-wheat soils. mostly due to higher TON concentrations. Seven-day production of NH4 at 40 degreesC w as so similar to N-0 estimated after 84 d of anaerobic incubation at 30 deg reesC that the 7-d incubation could possibly substitute for the long-term o ne in developing N fertilizer recommendations.