Tuesday, November 14, 2006
201-4
Grazing Management Effects on Potential Sediment and Phosphorus Loss from Streambanks.
Shelly Moeller1, John Kovar2, James R. Russell3, and Mathew M. Haan1. (1) Iowa State Univ, 4112 Lincoln Swing #111, Ames, IA 50011, (2) USDA-ARS Natl. Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011-4420, (3) Iowa State University, 337 Kildee, Ames, IA 50011
Animal grazing on lands near streams has the potential to contribute sediment and nutrients to surface waters. To minimize the impact of theses systems, we must understand of the interactions of grazing systems on streambank erosion. In this study, we used six 12-ha grass pastures that were bisected by a 195 m stream segment. Two replications of three management treatments [continuous stocking with unrestricted stream access (CSU), continuous stocking with restricted stream access (CSR), and rotational stocking (RS)] were randomly assigned to the six pastures. Pastures were stocked with 15 fall-calving Angus cows from May to October of 2005. To estimate stream bank erosion within each of the grazing treatments, erosion pins were driven into the ground at 1-m intervals from the stream’s edge to the top of the bank on both sides of the stream along 10 equidistant transects in each pasture. Pin length was measured before grazing, monthly during the grazing season, and after grazing ceased to determine sediment erosion and deposition. To determine P content of the streambank soils, samples were collected less than one meter from the stream channel in each of the pastures. Three cores were collected from each pasture, divided by soil horizon, and composited for analysis. Total soil P values, in conjunction with soil bulk density and erosion pin measurements, will allow us to estimate how much P is being lost to the stream through erosion of the stream banks. Net erosion of stream banks was greatest in September from the CSU treatment, compared with either the CSR or RS management treatments. Total soil P in stream bank soil samples ranged from 200 to greater than 500 mg/kg. Therefore, soil lost to erosion associated with large rainfall events could significantly increase P levels in the stream.