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 rela tionship 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 fields with similar soil type, and in the same field in ten consecuti ve rice (Oryza sativa L.) crops. The N-i was estimated by crop N uptak e from plots without applied N (N-o plots) under otherwise favorable g rowth 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. Co rrelation between N-i and SOC or N-t explained Little of this variatio n. Factors likely to contribute to the poor correlation were: (1) inpu ts of N from sources other than N mineralization of SOM in surface soi l, (2) degree of congruence between soil N supply and crop demand, whi ch is sensitive to soil drying, length of fallow, crop rotation, and r esidue management, and (3) differences in SOM quality related to inten sive cropping in submerged soil. Better understanding of the processes governing the N-i of tropical lowland rice systems would contribute t o the development of crop management practices that optimize utilizati on of indigenous N resources.