Nitrogen and Phosphorus Concentrations and Export from an Ozark Plateau Catchment in the United States

Abstract

In 1993–1995, the Beaver Lake Basin contained about 2000 poultry houses producing about 200 000 Mg yr −1 of poultry litter, and 8000 and 4000 Mg yr −1 of nitrogen (N) and phosphorus (P), respectively. Most of the poultry litter was land applied as a fertiliser to meet forage N requirements, making it susceptible to transport from the landscape during episodic precipitation events. Nitrogen and P concentrations were measured in four sub-watersheds of Beaver Lake, a reservoir on the White River in Arkansas, USA, to assess possible relationships between pasture land use and stream nutrient concentrations and export. Surface water samples were collected 17 times annually for 2 years from ten total stream sites within the four watersheds. Samples were analysed for soluble reactive P (SRP), total P (TP), ammonium-N (NH 4-N), nitrate-N (NO 3-N), total Kjeldhal N (TKN) and total N (TN). Discharge was measured at four gauged stream stations, and nutrient export was calculated using the US Geological Survey ESTIMATOR software and non-biased re-transformation from log space. Stream SRP, NO 3-N and TN concentrations (geometric-mean) increased linearly with per cent of pasture in watersheds, whereas N and P export coefficients increased exponentially with pasture land use. Nutrient export (kg yr −1) increased with basin size, but nutrient yield (kg km −2 yr −1) decreased with basin size. Nutrient yield was from three times to over 10 times greater than nutrient yields observed in regional undeveloped streams and the average of the Hydrologic Benchmark Network of the US Geological Survey. It is apparent that pasturelands in this basin affect stream nutrient concentrations and export to Beaver Lake and its tributaries. This investigation emphasises the need to carefully manage poultry litter because small losses of nutrients compared to the total amount of nutrients produced in a basin may still impact stream nutrient concentrations and export.