Date of Award
MS in Agriculture - Soil Science
Earth and Soil Sciences
Dr. Marc Horney
Planned grazing management in rangelands may improve carbon sequestration potential of soils by increasing plant biomass and the rate of nutrient cycling, which might mitigate global warming. The effects of high-intensity, short-duration planned grazing of sheep on several soil and ecosystem properties were investigated on a mixed grassland in San Luis Obispo, CA. The objectives of this study were to (a) identify soil properties related to soil C sequestration in rangelands; (b) determine if planned grazing improved soil carbon sequestration; (c) quantify changes in identified variables in grazed and rested plots; and (d) analyze any changes in plant species composition attributable to grazing. Total rest exclosures and short-duration grazing by sheep at average stocking densities of 115,000 pounds per acre were applied at two sites with clay-loam soils: a rangeland site that had been rested for over 50 years (REX), and an adjacent site that was previously rested for over 50 years, and subsequently grazed for six years (GR). Bare soil, live plants, plant litter, and perennial and annual plant densities were not different between sites. Soil organic carbon (SOC) was higher at the GR site, but total nitrogen was not different between sites, resulting in higher C:N ratios at the GR site. Soil pH was lower at the GR site, moisture was higher at the GR site, and bulk density and aggregate stability were not different between sites. There was a higher incidence of black soils at the REX site, suggesting perhaps a difference in soil mineralogy which may impact SOC. Treatment did not have an effect on any of the variables investigated except perennial plant diversity, in which grazed plots had lower species diversity than rested plots. Site aspect may have an effect on the results, as the GR site was northwest-facing while the REX site was south-facing. It is possible that higher SOC at the GR site is due to topography, rather than grazing management. Further investigation is required, but if grazing can be used in California as a strategy for increasing soil carbon sequestration, the rates of desertification may be slowed and damage caused to the ecosystem by global warming may be reduced.