Water table management, nitrogen dynamics, and yields of corn and soybean

Concern about NO(3)- contamination of surface waters has prompted the development of agricultural water table management systems to reduce NO(3)- loss in subsurface drainage outflow by subirrigating through the existing subsurface drainage lines during the growing season and controlling off-season outflows. We hypothesized that soil N pools, crop yields, and N uptake in a corn (Zea mays L.)- soybean (Glycine max.) rotation differ between subirrigation (water table at 40 cm) with controlled drainage (SI/CD) vs. subsurface drainage (SD) alone on Omulga silt loam (Aeric Fragiaqualfs). Mean microbial biomass N, potentially mineralizable N, dissolved organic N, and ammonia N were not affected by the water table management system. Mean NO(3)- -N was not affected by the water table management system at 0- to 15-cm and 15- to 30-cm depths, but the 2-yr mean soil NO(3)- concentration at the 30- to 75-cm depth was 46% lower in SI/CD compared with SD. The average corn yield was 19% greater, and the average soybean yield was 64% greater, in SI/CD plots, compared with SD. Corn N uptake was 13% greater and soybean N uptake was 62% greater with SI/CD, compared with SD. The SI/CD water table management system increased plant N uptake and reduced deep-profile NO(3)- concentrations, thereby reducing the amount of NO(3)- potentially available to move via drains to surface waters.