CHEMICAL AND BIOLOGICAL ASSESSMENT OF NITROGEN AVAILABILITY TO LOWLAND RICE (CALIFORNIA)

Soil N availability to lowland rice was investigated using different soil types collected from northern California. The nitrogen supplying capacity of these soils was evaluated by different chemical and biological techniques. Rice dry matter yield and N uptake correlated highly with the N supplying capacity determined by anaerobic incubation methods involving incubation of soils at different temperatures for various lengths of time. Of these methods soil N released after 2 weeks of anaerobic incubation at 25(DEGREES)C seems to be the best index for soil N availability, since it correlated highly with rice dry matter yield (r = 0.914) and N uptake (r = 0.913). Soil biomass-N correlated highly with rice dry matter yield (r = 0.878) and N uptake (r = 0.857), but the length of time required for this index determination make it less favorable as an index for soil N availability.;Combining chemical and biological N availability indices in a single multiple linear regression equation did not improve their prediction of available soil N.;Under flooded conditions soil characteristics, such as pH, available P, CEC, organic C, total N and C/N ratio did not account for all the variations in soil mineralizable N determined by anaerobic incubation for 2 weeks at 25(DEGREES)C.;Under field flooded conditions high grain yield and grain N uptake were obtained with soils that released high amounts of mineralizable N, determined by biological or chemical means.;Among the chemical indices, available N determined by the oxidative release of soil N by alkaline permanganate and acid dichromate provides a good index of soil N availability to rice. These indices correlated highly with rice dry matter yield and N uptake.;The availability to rice of N released from soils or mineralized from ('15)N-labeled plant residues increased with increasing levels of applied N. The percent contribution of N released from ('15)N-labeled plant residues to rice N nutrition was important at low levels of applied N especially in soils low in their mineralizable N.;The difference in response of japonica rice varieties to levels of soil N seems related to morphological and physiological characteristics. Differences in root to shoot ratios and N uptake by roots or shoots did not account for the variations in grain yield and grain N uptake.