Subirrigation systems to minimize nitrate leaching

Nitrate leaching from corn production systems and the subsequent contaminat ion of ground and surface waters is a major environmental problem. In field plots 75 m long by 15 m wide, the writers tested the hypothesis that subir rigation and intercropping will reduce leaching losses from cultivated corn and minimize water pollution. Nitrate leaching under subirrigation at a de pth of either 0.7 m or 0.8 m below the soil surface was compared with leach ing under free drainage. The cropping systems investigated were corn (Zea m ays L.) monoculture and corn intercropped with annual Italian ryegrass (Lol ium multiflorum Lam, cv. Barmultra). The effects of three fertilizer applic ation rates (0, 180, and 270 kg N ha(-1)) on leaching were investigated in the freely drained plots. The greatest annual loss of NO3--N in tile draina ge water (21.9 kg N ha(-1)) occurred in freely draining, monocropped plots fertilized with 270 kg N ha(-1). Monocropped plots fertilized with 270 kg N ha(-1), with subirrigation at 0.7 m depth, resulted in annual nitrate loss es into tile drainage of 6.6 kg N ha(-1), 70% less than under free drainage . Annual soil denitrification rates (60 kg N ha(-1)) with subirrigation at 0.7 m were about three-fold greater than under free drainage. Intercropping under free drainage resulted in a 50% reduction in tile drainage loss of N O3--N compared with monocropping. Off-season (November 1, 1993, to May 31, 1994) tile drainage losses of NO3--N (7.8 kg N ha(-1)) from freely draining monocropped plots accounted for 30% of the annual tile drainage losses.