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

Abstract

AbstractSupplemental 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.