SEASONAL SOIL NITRATE DYNAMICS IN CORN AS AFFECTED BY TILLAGE AND NITROGEN-SOURCE

A better understanding of seasonal soil NO3- dynamics in corn (Zea mays L.) as affected by different tillage and N sources is essential to refine N management options for more profitable production and less negative impact on water resources. We conducted an experiment in central Pennsylvania to study seasonal soil NO3- dynamics in corn managed with conventional tillage (CT) or net-till (NT) and supplied with commercial N fertilizer or legume green manures of red clover (Trifolium pratense L.) or hairy vetch (Vicia villosa Both) double-cropped with winter wheat (Triticum aestivum L.). The legume cover crops and winter fallow after the wheat harvest were followed by two corn crops. Soil samples taken biweekly during the corn growing seasons were analyzed for NO3- concentrations in the 0- to 25- (Ap horizon) and 25- to 45-cm (upper B horizon) soil layers. In the first corn crop, soil NO3- concentrations peaked about 4 wk after planting in all treatments. Where no broadcast N fertilizer was applied, mid-to late-season soil NO3--N concentrations in the Ap horizon averaged 6, 25, and 1.3 mg kg(-1) in the clover CT, vetch CT, and fallow CT treatments, respectively, while the NO3--N concentrations in all NT 0 N treatments were similar (1.3-1.9 mg kg(-1)). After the first corn harvest, the clover and vetch CT 0 N treatments had 113 and 124 kg NO3--N ha(-1) in the 120-cm soil profile, slightly higher than that estimated for the fallow treatment with N fertilizer at the economic optimum rate (99 kg NO3--N ha(-1)). No-tillage also reduced the amount of NO3- accumulated in the 0- to 120-cm soil profile to one-half of the NO3- levels in the corresponding CT treatments, regardless of N source. Both tillage and N source strongly affected seasonal soil NO3- dynamics in the first corn season, with the major difference appearing in the Ap horizon.