The Influence of Gross Nitrogen Mineralization on Corn Nitrogen Uptake.

A long-term (12 yr) cropping systems experiment in central Iowa demonstrated that diversified cropping systems can enhance corn (Zea mays) yields and reduce nitrogen losses. Corn yields have been 4% greater in two diversified cropping systems receiving composted manure compared to a simpler system relying solely on inorganic N fertilizer, while nitrate leaching losses were 55% lower in the diversified systems. Nitrogen fertilizer inputs and soil inorganic N pools were also lower in the diverse systems, suggesting that cycling of organic N plays a greater role in crop N nutrition in these systems. In order to examine the rate of production of inorganic N from soil organic N and its relationship to crop N uptake we measured gross ammonification, crop N content, and soil organic carbon (SOC) over a growing season in the three cropping systems. Corn N uptake was greater in the diversified systems compated to the simple system. SOC was a good predictor of gross ammonification rate but not corn N uptake. At a given SOC level, gross ammonification was greater in the diversified systems. However, only a weak relationship between gross ammonification and crop N uptake rate was observed. Additionally, gross ammonification rate in the top 20cm of soil was consistently larger than corn N uptake rate in all three systems, so the supply of inorganic N from the soil organic N could potentially satisfy crop N demand. Our hypothesis that organic N is a more important source of N for corn in diversified systems was supported by our finding that gross ammonification is enhanced in the diversified systems. However, the weak correlation between crop N uptake rate and gross ammonification rate in any of the systems suggests that gross ammonification rates were not a major factor determining plant N uptake. Future research should focus on improving understanding of the outcome of competition events between plants and microbes for newly mineralized inorganic N.