Soil organic matter and the indigenous nitrogen supply of intensive irrigated rice systems in the tropics

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 (N_i) of the soil-floodwater system and soil organic carbon (SOC) or total N (N_t) in surface soil of long-term fertility experiments (LTFEs) at 11 sites, in 42 farmer's fiels with similar soil type, and in the same field in ten consecutive rice (Oryza sativa L.) crops. The N_i was estimated by crop N uptake from plots without applied N (N_o plots) under otherwise favorable growth conditions. There was a tight linear correlation between yields and N uptake in N_o plots and tremendous variation in both parameters among LTFE sites, farmer's fields, and in the same field over time. Correlation between N_i and SOC or N_t 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 N_i of tropical lowland rice systems would contribute to the development of crop management practices that optimize utilization of indigenous N resources.