Subirrigation systems to minimize nitrate leaching

Nitrate leaching from corn production systems and the subsequent contamination 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 subirrigation and intercropping will reduce leaching losses from cultivated corn and minimize water pollution. Nitrate leaching under subirrigation at a depth of either 0.7 m or 0.8 m below the soil surface was compared with leaching under free drainage. The cropping systems investigated were corn \I(Zea mays\N L.) monoculture and corn intecropped with annual italian ryegrass \I(Lolium multiflorum\N Lam. \Icv.\N Barmultra). The effects of three fertilizer application rates (0, 180, and 270 kg N ha\u–¹) on leaching were investigated in the freely drained plots. The greatest annual loss of NO\d3-–N in tile drainage water (21.9 kg N ha\u–¹) occurred in freely draining, monocropped plots fertilized with 270 kg N ha\u–¹. Monocropped plots fertilized with 270 kg N ha\u–¹, with subirrigation at 0.7 m depth, resulted in annual nitrate losses into tile draining of 6.6 kg N ha\u–¹, 70% less than under free drainage. Annual soil denitrification rates (60 kg N ha\u–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 NO\d3-–N compared with monocropping. Off-season (November 1, 1993,to May 31, 1994) tile drainage losses of NO\d3-–N (7.8 kg N ha\u–¹) from freely draining monocropped plots accounted for 30% of the annual tile drainage losses.

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