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SOIL & WATER MANAGEMENT & CONSERVATION

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

^a USDA-ARS, Agricultural Systems Research Unit, Fort Collins, CO 80526
^b USDA-ARS, National Soil Tilth Lab., Ames, IA 50011
^c USDA-ARS, Arid-land Agricultural Research Center, Maricopa, AZ 85238

^* Corresponding author (Liwang.Ma{at}ars.usda.gov).

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 NO₃–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.

Abbreviations: FWANC, flow-weighted annual nitrogen concentrations • ME, Nash–Sutcliffe model efficiency • RMSE, root mean square error, RZWQM, Root Zone Water Quality Model