NITRATE IN RIVER AND SUBSURFACE DRAINAGE FLOWS FROM AN EAST CENTRAL ILLINOIS WATERSHED

Abstract

The objective of this study was to quantify the effects of crop management systems on the movement of nitrate to a drinking water reservoir in a watershed with intensive row-crop agriculture and sub-surface drainage in eastern Illinois (USA). Nitrate concentrations and sub-surface tile flow were monitored for six years at fields with various tillage and cropping management practices; the eight tile systems drained areas varying from 3 to 21 ha. Nitrate concentrations were also determined at various locations along the main stream of the watershed at locations that defined watershed sizes from 6910 to 48 900 ha. Concentrations of nitrate-N in the river followed a pronounced seasonal cycle, with maximum concentrations between 10 and 15 mg/L occurring in the spring and minimum concentrations of 0 to 5 mg/L occurring in the autumn. There were no significant differences in nitrate concentrations among specific sampling locations along the river, suggesting that the entire watershed is contributing more or less equally to the nitrate load. Nitrate-N concentrations from the tile drains varied from 0 to 39 mg/L and differed among field and cropping management systems used. Seasonal variations of nitrate concentrations in tile drain effluent were not as well defined as observed in the river. Nitrate-N concentrations in tile drain effluent were higher from fields where greater amounts of nitrogen fertilizer were applied, particularly when the fertilizer was applied prior to planting. Pre-plant application systems with average nitrogen application of 107 kg-N/ha/yr yielded a mean concentration of nitrate-N of 16.8 mg/L in the tile drain outflow which was significantly greater than the mean concentration of nitrate-N of 10.2 mg/L from the side dress and manure application systems that had received an average nitrogen application of 93 kg-N/ha/yr. The mean concentration of nitrate-N in tile drain discharge from continuous grass was 1.0 mg/L. Nitrate-N losses from cropped fields ranged from 14 to 35 kg/ha/yr depending upon the management system, which were 14 to 36% of the nitrogen applied. Losses from the grassed system were 3.8 kg/ha/yr and at the most upstream river station were 11 kg/ha/yr of nitrate-N.