Nitrogen Fertilizer Rate Effects on Soil Organic C in Iowa Continuous Corn and Corn-Soybean Systems.

Nitrogen fertilizer rate is a key factor affecting soil organic C (SOC) in corn-based cropping systems. The objective of this study was to determine the change in SOC in response to long-term N rates for continuous corn and corn-soybean cropping systems at two sites in Iowa. Soil samples were collected to a depth of 15 cm in 1999 and again in 2014 after 15 years of corn N rate treatments ranging from 0 to 269 kg ha^-1. The soil samples were analyzed for total C and N concentrations. For continuous corn at both sites, the average annual change in SOC increased significantly from below 0 Mg ha^-1 yr^-1 where no N was applied, to an optimum of approximately 0.13 Mg ha^-1 yr^-1 at N rates between 150 and 200 kg ha^-1. For corn-soybean rotations, the average annual change in SOC was generally below 0 Mg ha^-1 yr^-1 and increased slightly, but not significantly, with increasing N rate. The results indicate that adequately fertilized continuous corn systems have the potential to accrue more SOC than corn-soybean rotations.