Nutrient Management Research Database

Efficient management of rice (Oryza sativa) nutrition across soils ranging from organic to mineral soils varies widely because of 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 soil organic carbon (SOC) content of the rice paddy soil increased, evaluated across a wide range of SOC content. The INS, defined as N uptake from N omission plots, was estimated from 54 plots over a 3-yr period at two locations in the Sacramento-San Joaquin Delta over a range of SOC from 6 to 232 g SOC kg-1. Additionally, 10 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 to 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.

• Large amounts of nutrients can be supplied by the soil independent from nutrients supplied from fertilization.
• Differences in the level of N present in the soil (indigenous N supply) can complicate determining efficient rates on fertilization.
• This 3 year study of rice paddy management in the Sacramento-San Joaquin Delta examined the relationship between soil organic carbon levels and indigenous N supply.

• Soils with moderate N levels had the highest levels of indigenous soil N.
• Soils with lower organic C levels and soils with higher organic C levels exhibited lower levels of indigenous N than those with moderate levels.
• This study shows that indigenous soil N can be predicted from soil organic carbon content/
• Therefore, soil organic carbon content should inform N management plans in rice paddies in the Sacramento-San Joaquin Delta.