Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2007
Publication Date: 7/1/2008
Citation: Ingram, L.J., Stahl, P.D., Schuman, G.E., Buyer, J.S., Vance, G.F., Ganjegunte, G.K., Welker, J.W., Derner, J.D. 2008. Grazing and drought impacts on soil carbon and microbial communities in a mixed-grass ecosystem. Soil Science Society of America Journal 72(4):939-948. Interpretive Summary: Rangelands occupy nearly 50% of the world’s land area and are estimated to contain more than two-thirds of the world’s carbon reserves. Good grazing management has been shown to enhance carbon sequestration in rangeland soils and help mitigate elevated atmospheric carbon levels. Our research at the High Plains Grasslands Research Station has shown that 11 years of grazing significantly increase soil organic carbon in the surface 30 cm (12 inch) of the soil. Climatic conditions during this 11 year period include 10 years of average to above normal precipitation. However, during the following 10 years we experienced 7 years of below average precipitation, which resulted in a 30% loss of soil organic carbon in the surface 60 cm (24 inch) of the soil profile in the heavily grazed treatment, with no change occurring in the lightly and non-grazed treatments. Over the years the heavily grazed treatment resulted in a significant plant community shift from one dominated by cool-season grasses to one being dominated by the warm-season grass, blue grama. This species has a much shallower rooting depth which can result in greater decomposition of the root system compared to that of a diverse plant community dominated by cool-season species. Microbial community structure was also affected by heavy grazing. Additional research is necessary to better understand the effects of grazing on microbial community structure and its affects on organic matter decomposition. The long-term potential for rangeland carbon sequestration is significant; however, grazing management and climatic conditions can greatly influence the amount of soil organic carbon storage in these ecosystems.
Technical Abstract: Good management of prairie ecosystems promotes C sequestration and ensures they do not become net sources of CO2. As part of an ongoing study, soil was sampled in 2003 to investigate the long-term effects of different livestock grazing treatments on soil organic C (SOC), total N (TN) and microbial communities. The three treatments studied (no grazing, EX; continuously, lightly grazed [10% utilization], CL; and continuously, heavily grazed [50% utilization], CH) have been imposed on a mixed-grass prairie near Cheyenne, WY, for 21 yrs. In the ten years since treatments were last sampled, the study area has been subject to an ongoing period of drought. In the 0-60 cm depth there has been little change in SOC in the EX or CL treatments between 1993 and 2003, whereas there was a 30% loss of SOC in the CH treatment. We believe this loss was due to plant community changes (from a C3 to a C4 plant dominated community) resulting in C accumulating nearer the soil surface, making it more vulnerable to loss. Soil TN increased in the EX and CL treatments between 1993 and 2003, but declined in the CH treatment. Changes in plant community structure and subsequent C and N losses may have contributed to microbial biomass, respiration, and N-mineralization rates being greatest in CL and least in the CH treatment. While no significant differences were observed in microbial populations based on concentration of phospholipid fatty acid (PLFA) biomarkers, variate analysis of PLFA data revealed that microbial community structure differed among treatments. The CH grazing rate during a drought period altered plant and microbial structure and subsequently negatively impacted biogeochemical C and N cycles.