4.4

Initial impacts of overgrazing on carbon dioxide flux on a southern mixed-grass prairie

Phillip L. Sims, USDA/ARS, Woodward, OK; and J. A. Bradford

North American temperate terrestrial ecosystems may play a significant role in the balance of global carbon budgets. The objective of these studies was to determine the net CO₂ fluxes and the impact of livestock grazing on CO₂ fluxes on a Southern Plains mixed-grass prairie. Two studies were conducted on the Southern Plains Experimental Range, Woodward, OK. Initially, CO₂ fluxes and ET, calculated at 20-minute intervals, were measured in 1995, 1996, and 1997 on grassland and sagebrush-dominated plant communities using Bowen ratio/energy balance instrumentation. Between 1998 and 2001 CO₂ fluxes were measured on two grassland sites, one that was grazed moderately at about 60 Animal Unit Days ha^-1 and another that was grazed heavily at about 90 Animal Unit Days ha^-1. Above and below ground plant biomass, leaf area, and plant height were measured at both studies. In the first study, season-long ET averaged 564 and 515 mm for the grass and sagebrush-dominated sites, respectively. The daily ET rates for the two treatments were similar, averaging 2.28 and 2.30 mm d^-1, respectively. Seasonal CO₂ flux averaged 404 g CO₂ m^-2 yr^-1 for the grassland site and 20 g CO₂ m^-2 yr^-1 for the sagebrush-dominated site. Annual CO₂ fluxes were 257 and 23 g CO₂ m^-2 yr^-1, respectively, for these treatments. The grassland site had higher rates of seasonal and annual CO₂ fluxes, 1.54 and 0.70 g CO₂ m^-2 yr^-1, respectively, than the sagebrush site, 0.01 and 0.06 g CO₂ m^-2 yr^-1, respectively. In the second study, moderately grazed, mixed-grass prairie had a growing season flux ranging from 82 to 712 g CO₂ m^-2 compared to a range of 200 to 325 g CO₂ m^-2 on the heavily grazed site. Estimates of the annual CO₂ flux for the moderately grazed site ranged between -30 and 181 g CO₂ m^-2 yr^-1 compared to a net flux of -480 to -542 g CO₂ m^-2 yr^-1 for the heavily grazed prairie. By the third season of heavy grazing, the standing herbage on the moderately grazed site averaged about 3,000 ± 540 kg ha^-1 compared to about 1,930 ± 400 kg ha^-1 on the heavily grazed site.

  Extended Abstract (28K)

Session 4, ARS Network
Tuesday, 21 May 2002, 8:30 AM-11:15 AM

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