Nitrogen, Phosphorus, and Organic Carbon in Streams Draining Two Grazed Catchments

Abstract

Abstract High concentrations of N, P, and organic C in water cause problems such as excessive eutrophication in reservoirs and bacterial growth in distribution systems. The aim of this study was to measure the export of N, P, and organic C in streams draining two grazed catchments with no point sources in the Mt. Lofty Ranges, South Australia, during 1 yr. Retreat Valley catchment (1.3 km 2 ) is grazed by sheep, has steep topography, and loam over clay soils. Lawless catchment (3.0 km 2 ) is flatter, grazed by dairy cattle, and has sand over clay soils. Rainfall and specific runoff were similar. Annual exports were 924 kg suspended solids ha −1 , 9.0 kg N ha −1 , 1.0 kg P ha −1 , and 55 kg organic C ha −1 from Retreat Valley; and 40 kg suspended solids ha −1 , 6.6 kg N ha −1 , 1.1 kg P ha −1 , and 66 kg organic C ha −1 from Lawless. In Retreat Valley, 33% of the N, 86% of the P, and 65% of the organic C was in particulate forms resulting from erosion; and 45% of the N was nitrate. In Lawless, 82% of the N, 76% of the P, and 92% of the organic C was dissolved. Of the dissolved N, approximately 93% was organic. Concentrations of P in both streams, and dissolved organic C in Lawless, exceeded desirable limits. A combination of management options, including management of grazing and fertilizer application, soil amendment, and subsurface drainage, could lead to reductions in nutrient exports.