Effect of Nitrogen Fertilizer Application on Corn Residue Decomposition in Iowa

Corn (Zea mays L.) residue is one of the sources of soil organic carbon (SOC) in row cropping systems in the Midwest. Farmers in Iowa apply liquid N to corn residue after harvest, assuming it will increase corn residue decomposition. The objective of this study was to investigate the effectiveness of N application for increasing corn residue decomposition. The study included two fields with three N rates (0, 34, or 67 kg N ha(-1)) of liquid 32% urea ammonium nitrate (UAN) applied after harvest, and two laboratory incubation experiments with three temperatures (0, 25, and 35 degrees C) in 2012 and 2013. The experiment design was a randomized complete block in four replications in a no-tillage system (NT). The average mass of residue organic carbon (OC) after harvest was in the range of 1.6 to 1.7 Mg ha(-1). Residue OC in the field declined sharply in both years, particularly during the first 3 mo, with no significant difference between N treatments. The only difference in field residue decomposition occurred after 6 mo (P = 0.0241) at the Olson location in 2012, where a greater (>21%) amount of remaining OC was associated with 0 kg N ha(-1) than with other N treatments. The incubation study showed an increasing rate of residue decomposition with increasing soil temperature with no N application effects on residue decomposition. These findings show that air and soil temperatures are the driving force for residue decomposition, especially at 25 degrees C, rather than with N addition.