Effect of N Application on Corn Residue Decomposition.

Crop residue can be a source of organic carbon and nitrogen for soil microorganisms. The process of residue decomposition is complex and influenced by both climatic and soil conditions.  Our working hypothesis is that applying liquid nitrogen over corn residue after harvest will not increase residue decomposition. To answer this question, field and laboratory experiments were conducted in 2012 and 2013 to investigate the effect of N application on corn residue decomposition.  The field experiments were conducted at two different sites in Iowa.  One experiment site was on poorly-drained soil types, Clyde silty loam (Fine-loamy, mixed, superactive Typic Endoaquous) and Readlyn loam (Fine-loamy, mixed, mesic Aquic Hapludolls). The second site was on well-drained soil type, Tama silty clay loam (Fine-silty, mixed, superactice Typic Argiudolls). The experimental design was a randomized complete block design with four replications in a no-tillage (NT) system, where three N rates of 32 % UAN at 0, 34, and 67 kg N ha^-1 were applied on corn residue. Residue samples were collected, weighed, and transferred to nylon bags, which were left on the field and samples collected every three months for 12 months period. The laboratory incubation study was conducted under three temperature (0^oC, 25^oC and 35^oC) with the same corn residue samples treated with same N rates as in the field. The results of field and laboratory experiments did not show any significant differences in the rates of residue decomposition with different N rates. However, the rate of residue decomposition in the laboratory was highest at 35^oC and the lowest at 0^oC. In the field, the average decline in residue weight over two years after 3 months at the poorly-drained site was 25% compared to 30%  at the well-drained site. After 12 months residue decomposition at the poorly-drained and well-drained sites was 58 and 60%, respectively. The results of field and laboratory studies confirm that N application does not increase residue decomposition, but air and soil temperature has an effect regardless of N rate.