Nitrogen, phosphorus and organic carbon in streams draining two grazed catchments

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 km2) is grazed by sheep, has steep topography, and loam over clay soils. Lawless catchment (3.0 km2) 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 loads.