Determining bacterial and nutrient concentrations and loadings of surface runoff from differing grazer access and vegetative cover in north-central Oklahoma

Abstract

Water bodies in Oklahoma are primarily fed by runoff, making bacterial and nutrient contamination of surface runoff a significant water quality concern. The purpose of this thesis was to determine the impacts of grazing and vegetation cover on bacterial and nutrient concentrations and loadings in surface runoff at the field scale in northcentral Oklahoma. To address these concerns, I measured Escherichia coli (E. coli), total Kjeldahl nitrogen, total phosphorus, and ortho-phosphate concentrations and loadings from 10 experimental watersheds at the Cross Timbers Experimental Range, Stillwater, Oklahoma. Results showed that E. coli concentrations of runoff from the grazed prairie watershed with the higher stocking rate, were significantly greater than concentrations from ungrazed watersheds. Total Kjeldahl nitrogen concentrations were greatest from eastern redcedar (Juniper virginiana) woodland watersheds compared to all other land uses measured in the study, but small sample sizes created problems with detecting statistical significance. Total phosphorus concentrations and loading were lowest from switchgrass (Panicum virgatum) watersheds, but this was not the case with measurements for Ortho-P. Loading values were influenced by runoff volume because the more volume associated with an event, the more nutrients in total mass that are carried downstream. In cases where watershed covers of forest and grassland differ substantially, differences in runoff volume dictate loading differences rather than land use. Using concentration to compare water quality between watersheds in these instances should be implemented. These results indicate that cattle grazing and eastern redcedar impact the water quality of runoff, and land management practices such as biomass feedstock production systems have added benefits by reducing total phosphorus concentrations of runoff and loadings to streams.