Simulated Effects of Nitrogen Management and Soil Microbes on Soil Nitrogen Balance and Crop Production

Searching for environmentally friendly N management practices in the midwestern United States is an ongoing task in the agricultural community. Many practices have shown promise in reducing N in tile drainage that may contribute to hypoxia in the Gulf of Mexico. In this study, an agricultural system in central Iowa, managed using a corn ( Zea mays L.)–soybean [ Glycine max (L.) Merr.] rotation from 1996 to 2005 was evaluated with the Root Zone Water Quality Model (RZWQM) for crop yield, grain N, annual N loss in tile drainage flow, and residual soil NO 3 –N under high (H, 199 kg N ha −1 ), medium (M, 138 kg N ha −1 ), and low (L, 69 kg N ha −1 ) N application rates shortly after planting, and a split (S, 69 kg N ha −1 shortly after planting and again at midseason) N treatment. The model adequately simulated the responses of yield and N loss to N application rates. Simulated N losses to drainage flow from 1996 to 2005 were 348, 277, and 228 kg N ha −1 for the H, M, and L treatments, respectively, compared with corresponding measured values of 369, 265, and 201 kg N ha −1 The S treatment had simulated and measured total N losses in drainage flow of 194 and 172 kg N ha −1 , respectively, from 1999 to 2005. The study also demonstrated that RZWQM without soil microbial growth produced very similar simulation results for crop production and soil N and water balances as RZWQM with dynamic soil microbial growth.