Submitted to: Agriculture Ecosystems and the Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/20/2003
Publication Date: 10/20/2003
Citation: McDowell, R.W., Sharpley, A.N., Folmar, G.J. 2003. Modification of phosphorus export from a catchment by fluvial sediment phosphorus inputs. Agriculture Ecosystems and the Environment. 99:187-199. Interpretive Summary: The release of phosphorus from sediments that have been deposited in stream and river bottoms (fluvial sediments) is central to the onset of eutrophication. Eutrophication is a process of accelerated growth of weeds in water that impair fishing, navigation, and the quality of drinking water. The amount of phosphorus on these sediments and its release to stream water is related to a series of complex chemical and physical processes that can reflect the sediments' origin and history of land use. The chemistry of phosphorus in fluvial sediments is the end result of many land use factors in a watershed, such as manure management, erosion control measures, and dominant topography, as well as how fast the stream or river flows. Research was conducted to see if we could link land use and/or chemical and physical processes with the behaviour of P in fluvial sediments and stream waters for an agricultural watershed in central Pennsylvania. A second objective was to identify where in the watershed it would be most effective to place remedial measures to decrease the loss of phosphorus. The study identified the major areas of the watershed where phosphorus was being moved from the land to the stream. This knowledge will help make it possible to target remedial measures to diminish prelease from these main sources without the expense, labor and short-term variation inherent in taking a broad-based, "blanket" watershed approach to mitigating P loss.
Technical Abstract: Phosphorus (P) export from agricultural watersheds can accelerate freshwater eutrophication. Landscape-based remedial measures can reduce edge-of-field P losses. However stream channel hydraulics and fluvial sediment properties can modify the forms and amounts of P exported by the time it reaches the watershed outlet. This study examines if land use, fluvial sediment properties, and stream flow P are interdependent within a mixed land-use watershed in Pennsylvania, USA (WE-38), so that remedial strategies can be most effectively targeted to mitigate watershed P export. Samples of the top 2 - 3 cm of stream-bed sediments (n = 40) were collected in April 2002, before and after tributary confluences and in areas of likely deposition. Stream water samples were collected at each of 23 sub-watershed outlets during base (n = 7) and storm flow (n = 3) events between 2000 and 2002. Fluvial sediment P and sorptive properties varied among tributaries and flow regimes. Total P of sediments in tributaries (322 mg/kg) was greater than at confluences (239 mg/kg), whereas P release rate was lower (92 and 166 mg P/L/min, respectively). This was attributed to physical disturbance by turbulent mixing and presence of more sand-sized particles at confluences (747 g/kg) than tributaries (707 g/kg). The percent cropped (r = 0.51) and forest (r = -0.57) land was related to the Mehlich-3 extractable P concentration of outflow sediment for each sub-watershed. This in turn influenced sediment P release, which was related to base flow P, when sediments establish quasi-equilibrium with flowing water within the watershed. However, storm flow P was not related to any sediment P properties but to percent of each sub-watershed in cropland (r = 0.58), reflecting the importance of erosion in P transport. Storm flow suspended sediment was related to sub-watershed area in crop (r = 0.78). To gain a better understanding of processes controlling P transport within and from a watershed and, thus, mitigation of losses, fluvial sediment properties as well as landscape management must be considered.