Soil total carbon and nitrogen as indicators of rice response to supplemental fertilizer nitrogen.

Supplemental fertilizer N recommendations for flood‐irrigated rice (Oryza sativa L.) have been developed with tests that quantify plant‐available N within the effective rooting depth. Alkaline hydrolyzable‐nitrogen (AH‐N) soil test methods were recently used to successfully guide supplemental fertilizer‐N management decisions of drill‐seeded, delayed‐flood rice, stimulating interest in evaluating additional indicators of N availability such as soil total carbon (STC) and soil total nitrogen (STN). Our first objective was to evaluate the relationship among STC, STN, and AH‐N to a depth of 60 cm. Our second objective was to correlate STC and STN to rice N uptake and grain yield and develop calibration regression models that identified the yield maximizing N rate (FN 95%RGY). The STC and STN were significantly and positively related to AH‐N when clayey soils were sampled to >15‐cm depth and the coefficient of determination (r2) exhibited a range of.22–.70 for STN and.26–.75 for STC. Overall, STC and STN were significantly related to rice N uptake, unfertilized grain yield (Yield0N), or relative grain yield (RGY) and r2 values tended to be greatest at a 30‐cm depth or only nominally improve when sampling depth exceeded 30 cm. Calibration of STC and STN to predict FN 95%RGY was successfully completed, but all linear regressions had r2 <.40. The influence of sampling depth on the ability of STC and STN to serve as indicators of rice response to supplemental N fertilization demonstrates the utility of sampling within the rice plant's effective rooting depth. Core Ideas: Soil total C and N increased linearly with alkaline hydrolyzable‐N to a 60‐cm depth.Soil total C and N explained <40% of the variability exhibited by rice response parameters.Soil total C and N from >30‐cm depth did not improve prediction of the rice grain yield maximizing fertilizer N rate.Correlation and calibration of soil total C and N to fertilizer N response of rice should consider the extent of rooting depth. [ABSTRACT FROM AUTHOR]