Phosphorus Export from an Agricultural Watershed: Linking Source and Transport Mechanisms

Abstract

Many source and transport factors control P loss from agricultural landscapes; however, little information is available on how these factors are linked at a watershed scale. Thus, we investigated mechanisms controlling P release from soil and stream sediments in relation to storm and baseflow P concentrations at four flumes and in the channel of an agricultural watershed. Baseflow dissolved reactive phosphorus (DRP) concentrations were greater at the watershed outflow (Flume 1; 0.042 mg L⁻¹) than uppermost flume (Flume 4; 0.028 mg L⁻¹). Conversely, DRP concentrations were greater at Flume 4 (0.304 mg L⁻¹) than Flume 1 (0.128 mg L⁻¹) during stormflow. Similar trends in total phosphorus (TP) concentration were also observed. During stormflow, stream P concentrations are controlled by overland flow–generated erosion from areas of the watershed coincident with high soil P. In-channel decreases in P concentration during stormflow were attributed to sediment deposition, resorption of P, and dilution. The increase in baseflow P concentrations downstream was controlled by channel sediments. Phosphorus sorption maximum of Flume 4 sediment (532 mg kg⁻¹) was greater than at the outlet Flume 1 (227 mg kg⁻¹). Indeed, the decrease in P desorption between Flumes 1 and 4 sediment (0.046 to 0.025 mg L⁻¹) was similar to the difference in baseflow DRP between Flumes 1 and 4 (0.042 to 0.028 mg L⁻¹). This study shows that erosion, soil P concentration, and channel sediment P sorption properties influence streamflow DRP and TP. A better understanding of the spatial and temporal distribution of these processes and their connectivity over the landscape will aid targeting remedial practices.