Nitrogen Fertilizer Effects on Irrigated Conventional Tillage Corn Yields and Soil Carbon and Nitrogen Pools

Conventional plow tillage (CT) is a common soil management practice under irrigated continuous corn (Zea maysL.) in the semiarid central Great Plains that requires a relatively high N fertilizer rate to optimize grain yield and economic returns. This study investigated how four rates of N input (0, 67, 101, and 224 kg N ha−1) under irrigated CT management affects aboveground corn production, stover characteristics, and crop biomass. We also examined soil quality parameters under these treatments, including total soil carbon (TSC) and soil particulate organic matter (POM), soil organic carbon (SOC) and total soil nitrogen (TSN) stocks in the 0‐ to 7.6‐, 0‐ to 15.2‐, and 0‐ to 30.4‐cm soil depths of a clay loam soil. Nitrogen fertilization significantly increased corn grain, stalks, cob and stover yields as well as C and N content of the biomass. The C/N ratios of stalks and stover decreased with increasing N rate, but cob C/N ratios did not change. Increasing N rate increased TSN concentration in 0‐ to 7.6‐ and 7.6‐ to 15.2‐cm depths and decreased C/N ratio in all soil layers between 1999 and 2008. We found no significant change in SOC and POM‐C stocks under increasing N fertilizer addition after 10 yr of irrigated, CT continuous corn despite higher stover yields, and the stover C and N returned to the soil after harvest with increasing N rate. The lack of increased SOC storage with increasing residue C inputs suggests that decomposition processes controlled the SOC stocks.