Riparian land-use impacts on stream bank soil and phosphorus losses from grazed pastures

Abstract

<p>An emerging challenge in watershed-scale research is to quantify the extent of sediment contributed to receiving waters from stream banks versus overland flow from critical source areas, and to develop management strategies to reduce stream bank erosion. The objective of this research was to compare the amounts of sediment and phosphorus loss from critical stream bank source areas to receiving waters via overland flow and stream bank erosion from riparian grazed pasture under different stocking densities (cow-calf pair ha -1). Rainfall simulations were used to calculate sediment and P loads from overland flow from stream-side livestock loafing and access points and lengths of stream banks not directly impacted by livestock (defined as control areas) in central, northeast, and southeast Iowa. Water samples from runoff were analyzed for suspended sediment and total phosphorus. Soil bulk density and moisture samples were also collected around rainfall simulation plots to evaluate differences in compaction between grazing practices. Stream bank erosion rates on these pasture sites were also observed over two years using the erosion pin method. Eroded stream bank length, height and soil bulk density and total soil phosphorus concentration were used to calculate total phosphorus and soil loss via stream bank erosion.;Within 15 m wide strips located on both side of the stream, livestock access paths and loafing areas together accounted for only 2.7% of the total source areas, their suspended sediment and total P contributions to streams accounted for up to 72% (86 kg ha-1), and 55% (78 g ha -1) of total sediment and phosphorus, respectively. Control areas of the riparian source areas contributed high levels of total P in surface runoff accounting for 45% (64 g ha-1) of total P loads. In some cases, significant correlations were found between stocking vi densities and soil bulk density and sediment and total phosphorus loss indicating that use of low stocking density can reduce total phosphorus and sediment loss from surface runoff.;Sediment and phosphorus loss via stream bank erosion did not reveal significant differences among the examined grazing practices. However, a highly correlated parallel relationship was found between the precipitation rates and erosion rates in given pasture sites suggesting that precipitation is one of the major controlling factors of stream bank erosion. Besides precipitation impact, lower stream bank soil bulk density significantly increased soil loss.;Results of this study would suggest that regardless of stocking densities the highly attractive nature of riparian areas within pastures results in significant contributions of sediment and total P to channel waters and the only was to effectively reduce that impact is to exclude direct access of livestock to the channel unless it is carefully armoured.</p>