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Soil organic matter (SOM) has been proposed as an index of N supply in paddy soils although field validations are few. We evaluated the relationship between the indigenous N supply (Ni) of the soil-floodwater system and soil organic carbon (SOC) or total N (Nt) in surface soil of long-term fertility experiments (LTFEs) at 11 sites, in 42 farmer's fields with similar soil type, and in the same field in ten consecutive rice (Oryza sativa L.) crops. The Ni was estimated by crop N uptake from plots without applied N (No plots) under otherwise favorable growth conditions. There was a tight linear correlation between yields and N uptake in No plots and tremendous variation in both parameters among LTFE sites, farmer's fields, and in the same field over time. Correlation between Ni and SOC or Nt explained little of this variation. Factors likely to contribute to the poor correlation were: (1) inputs of N from sources other than N mineralization of SOM in surface soil, (2) degree of congruence between soil N supply and crop demand, which is sensitive to soil drying, length of fallow, crop rotation, and residue management, and (3) differences in SOM quality related to intensive cropping in submerged soil. Better understanding of the processes governing the Ni of tropical lowland rice systems would contribute to the development of crop management practices that optimize utilization of indigenous N resources.

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Field observations indicate a long-term decrease in crop uptake of N derived from soil organic matter under continuous production of irrigated lowland rice (Oryza sativa L.). Decreased availability has been associated with an accumulation of phenolic lignin residues in soil organic matter, which can chemically bind N. To evaluate the hypothesis that the decrease in N availability results primarily from anaerobic decomposition of incorporated crop residues, 15 N-labelled fertilizer was applied three times during one growing season in a field study that compared anaerobic decomposition with aerobic decomposition for annual rotations of rice (Oryza sativa L.)-rice and rice-maize (Zea mays L.). Contents of 15 N and total N during the growing season were measured in humic fractions and total soil organic matter. Results indicated an inhibition of N mineralization for the rice-rice rotation with anaerobic decomposition of crop residues, both for 15 N that was immobilized after application and for total N. The inhibition was strongest for 15 N that was applied at planting. It became more evident as the season progressed and reached significant levels during mid-season stages of plant growth when crop demand for N peaks. These results were clearest for a young, phenolic-rich humic fraction that was active in 15 N immobilization and remineralization. Comparable but less significant trends were evident for a more recalcitrant humic fraction and for soil organic matter. Trends in crop-N uptake associated the combination of riceÀrice rotation and anaerobic decomposition with inhibited uptake of soil organic N but uninhibited uptake of fertilizer N. Increased aeration of rice soils through aerobic decomposition of crop residues or crop rotation is a promising management technique for improving soil N supply in lowland rice cropping.

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