Riparian land-use impacts on stream and gully bank soil and phosphorus losses with an emphasis on grazing practices
The tall-grass prairies, wetlands and forests that dominated the Iowa landscape have been replaced by annual row-crops and grass pastures that occupy more than 90% of the landscape today. Because of these changes, water reaches streams and gullies much faster and has led to incised streams and an extensive growth of gully networks. Stream and gully banks are major contributors of non-point source sediment and phosphorus. Reducing these pollutants is a priority to maintaining healthy streams. The objective of this study was to compare stream and gully bank soil and phosphorus losses under different land-use practices in three Iowa regions. The hypothesis was that stream and gully bank erosion would decrease in the following order: annual row-cropped fields, continuous pastures, rotational pastures, intensive rotational pastures, pastures with cattle excluded from the stream, grass filters and riparian forest buffers. In rotational and intensive rotational grazing the pastures are divided into paddocks. One paddock is grazed at a time while the others are rested. To estimate soil losses for each treatment, erosion pins were used for stream and gully bank rates and all the severe eroding site areas were measured. Estimating soil total phosphorus concentrations from the stream and gully bank faces allowed us to estimate phosphorus losses. In addition phosphorus concentrations in riparian areas, stream and gully beds and loafing areas were measured. Riparian forest buffers, grass filters and pastures with the cattle excluded from the stream had the lowest soil and phosphorus losses from stream and gully bank erosion. Row-cropped fields had high soil and phosphorus losses, similar to the continuous pastures. There are some indications that moving to rotational and intensive rotational pastures soil and phosphorus losses could decrease bank soil and phosphorous losses compared to continuous pastures. The effectiveness of rotational and intensive rotational pastures will depend on the number of paddocks along the stream, stocking rates, grazing in the stream paddocks during wet periods and how well landowners follow the pasture system guidelines. Finally cow entry points and loafing areas should require special attention because they can be major sources of soil and phosphorus to streams.