Nutrient Budgets for Agricultural Watersheds in the Southeastern Coastal Plain

Abstract

Watershed—level agroecosystem studies are essential to relate land management to the external environmental effects produced by agricultural nutrients and to enhance our understanding of agricultural nutrient cycles. Inputs and outputs of N, P, K. Ca, Mg, and Cl were determined for four subwatersheds of the Little River in the Georgia Coastal Plain from 1979 through 1981. The four watersheds had 40, 36, 54, and 50%, respectively, of their land in agricultural uses (row crop and pasture). Prescipitation inputs and streamflow outputs were determined by field sampling of water volumes and nutrient concentrations. Agronomic inputs (from fertilizer and symbiotic N—fixation) and outputs in harvested material were estimated from land use data; countrywide averages of fertilizer applications and crop yield, and plot studies on peanuts and soybeans. All elements except Cl had greater inputs than outputs on each watershed each year. The general order of streamflow loads was Cl > Ca > K > Mg> N > P. Fertilizer inputs exceeded precipitation inputs for all elements on all watersheds. Outputs on N, P, and K in harvest generally exceeded streamflow loads, but harvest outputs of Ca, Mg, and Cl were generally lower than streamflow loads. The two watersheds with more agricultural land had consistently higher loads of N, K, Ca, Mg, and Cl in streamflow and had No3—N loads 1.5 to 4.4 times higher than loads from the less agricultural watersheds. Streamflow loads on the Little River watersheds were similar to those on other Coastal Plain agricultural watersheds with comparable land use and discharge volumes. Budgets for the upland portion of one of the watersheds indicated that large amounts of N, P, K, Ca, and Mg were not accounted for. about 56 kg°ha—1@?yr—1 of N were retained or lost to gaseous emissions from the uplands. Apparently, a large percentage of the nutrients applied to these watersheds was being retained somewhere in the watershed or being lost in some unquantified way.