Nitrate contamination of groundwater under irrigated coastal plain soils

To develop best management practices (BMP) for agricultural land to protect groundwater, data is needed on the leaching of N from irrigated coastal plain soils treated with Poultry manure. This study was conducted to determine the vertical and seasonal patterns of NO3 leaching under such soils. Four commercially farmed corn (Zea mays L.) fields were studied, two receiving only fertilizer N (240 to 360 kg N ha-1 over a 2-yr period) and two with a continuing history of poultry manure applications (25-29 Mg ha-1 over 2 yr). In each field, a transect of four monitoring wells was installed 4 to 8 m deep (1 m below the seasonally low water table). Three additional wells were installed in forestland adjacent to three of the fields. Groundwater and soils (to 1.5-m depth) were periodically sampled for analysis of NO3(-)N. Under the unmanured field, groundwater concentrations averaged 15.1 mg L-1 during August through November 1986, while the corresponding figure for the manured fields was not significantly different at 18.3 mg L-1. Two months after spreading manure in November and December, as much as 104 mg NO3(-)N was measured in the groundwater under the manured fields. From December 1986 through September 1987 the groundwater under the manured fields had significantly higher NO3(-)N concentrations than did that under the unmanured fields (43.7 vs. 18.1 mg L-1, respectively). Only for one well site with a buried A horizon did high Cl to NO3(-)N ratios and low N)3(-)N concentrations indicate rapid denitrification. The forestland groundwater always contained < 1 mg NO3(-)N L-1, and high Cl to NO3(-)N ratios, suggesting that NO3 in the cropland groundwater was lost after entering the forested areas, and that forests may therefore protect waterways from subsurface N contamination.