Indigenous Nitrogen Supply of Rice is Predicted by Soil Organic Carbon

Efficient management of rice (Oryza sativa) nutrition across soils ranging from organic to mineral soils varies widely due to large contributions of nutrients, including N, from the indigenous supply. This study tested the hypothesis that the indigenous N supply (INS) would increase if the SOC content of the rice paddy soil increased, evaluated across a wide range of soil organic carbon (SOC) content. The INS, defined as N uptake from N omission plots, was estimated from 54 plots over a three year period at two locations in the Sacramento-San Joaquin Delta over a range of SOC from 6 to 232 g SOC kg-1. Additionally ten N rate trials (0 to 160 kg N applied ha-1) were conducted concurrent with the N omission plots. The INS did not increase as SOC increased across the entire SOC gradient, and instead exhibited a concave quadratic trend across the SOC gradient; greatest in the 110--170 g SOC kg-1 range and lower in sites with less than 110 g SOC kg-1 or more than 170 g SOC kg-1. Consequently, positive yield response to N fertilizer was observed in soils with low INS, with no positive yield response on soils with high INS. This study indicates that the INS can be predicted by the SOC content, hence fertilizer-N recommendations should include considerations for SOC content.