Transport and transformation of nutrients and sediment in two agricultural watersheds in Northeast Arkansas

Understanding pollutant transport at different spatial and temporal scales is crucial to agroecosystems management and planning. This study aimed to reduce the knowledge gap between edge-of-field and larger agricultural watersheds. Nutrients and sediment transport and transformation at two small agricultural watersheds, Little River Ditches Basin (LRDB) and Lower St. Francis Basin (LSFB), in Northeast Arkansas, were evaluated. Flow, nutrients, and sediment were measured at 3-5 instream locations in these two contrasting watersheds. These watersheds differed in primary crop, soil type, and size. Differences in sediment and nutrients loads were measured between the two watersheds primarily due to differences in cropping practices and soil type. LSFB was dominated by rice farms and had more pollutant load per unit area but lower concentrations for all measured parameters except nitrate, whereas LRDB was dominated by cotton farms and had less pollutant load per unit area but higher concentrations. Turbidity increased considerably at LSFB, but it did not increase or decrease at LRDB as water traveled downstream. The median nitrate-N concentration at LRDB increased from 1.64 to 2.34 mg L-1 as watershed size increased, in contrast to no increase at LSFB. Total phosphorus (TP) and soluble reactive phosphorus (SRP) concentrations remained constant, but ammonium-N decreased as the water traveled downstream in both watersheds. Nitrate-N were high in spring and late fall at both watersheds. The annual loss of nitrate-N was 9.6 and 8.6 kg ha(-1), sediment was 1604 and 1958 kg ha(-1), and SRP was 0.8 and 0.9 kg ha(-1), respectively from LRDB and LSFB. Source control in spring and late fall could be more effective in reducing agricultural pollution. Published by Elsevier B.V.