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Title: Carbon, water, and heat flux responses to experimental burning and drought in a tallgrass prairie

item FISHER, MARC - Lawrence Berkeley National Laboratory
item TORN, MARGARET - Lawrence Berkeley National Laboratory
item BILLESBACH, DAVID - University Of Nebraska
item DOYLE, GEOFFREY - Science Application International Corporation(SAIC)
item Northup, Brian
item BIRAUD, SEBASTION - Lawrence Berkeley National Laboratory

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/23/2012
Publication Date: 12/15/2012
Citation: Fisher, M.L., Torn, M.S., Billesbach, D.P., Doyle, G., Northup, B.K., Biraud, S.C. 2012. Carbon, water, and heat flux responses to experimental burning and drought in a tallgrass prairie. Agricultural and Forest Meteorology. 166-167:169-174. DOI:10.1016/j.agrformet.2012.07.011.

Interpretive Summary: Fire has been regularly applied to southern tallgrass prairie of Oklahoma in managing plant communities to improve hunting, clear land, and other purposes. Current prescriptions for grazing management in these grasslands regularly include spring burns to improve forage production for cattle grazing. However, such burns are also a source of the greenhouse gasses carbon dioxide and carbon monoxide at the time of burning, and are perceived as a large loss of C to the atmosphere. We undertook a study to compare the carbon (C) balance of pastures in response to a prescribed spring burn. Two, 84-acre pastures were chosen, amounts of plant materials present were measured, and a burn was applied to one pasture in March 2005. We then installed equipment to measure amounts of C in the air and C respired by the soil, and established plots in both pastures to measure amounts of C in plant tissues produced during 2005 and 2006. Average precipitation was 69% of the long-term average (23 versus 32 inches). The two pastures averaged 3450 lb/acre C in plant tissues before the burn. The spring burn removed 38% of existing material and C from the burned pasture. This loss was followed by a 1600 lb/acre increase (45%) in C stored in plant growth of the burned pasture by the end of 2005. This resulted from a large increase in C movement from the atmosphere into plant materials, compared to the unburned pasture. In comparison, dry growing conditions in 2006 resulted in both pastures having similar amounts of C in plant materials (4200 lb C/acre), and losses of soil C to the atmosphere by both pastures 115 to 400 lb/acre/yr. These results suggest growing conditions following prescribed spring burns can have greater effects on C cycling by southern tallgrass prairie than applied burning treatments.

Technical Abstract: Both natural fire and prescribed burns are disturbances to biogeochemical cycles of grassland ecosystems in the southern Great Plains. This study examined eddy covariance whole ecosystem-atmosphere flux in carbon (C) exchange in response to a prescribed burn. Two, paired 34 ha paddocks in central Oklahoma were assigned treatments (burned and unburned) in 2005, eddy covariance and soil respiration systems were established, and 10 experimental plots per paddock were identified in 2005. A prescribed spring burn was applied to one paddock (March 2005), and covariance, respiration and aboveground biomass were measured in 2005 through 2006. Annual precipitation (600 mm/yr) during the study was lower than the site average (860 mm/yr). The paddocks contained 520±60 and 360±40 g C/m^2 (burned and unburned, respectively), in aboveground biomass prior to the burn. The fire removed ~38% of aboveground biomass from the burned paddock, and left a charred residual litter layer. Maximum green biomass during 2005 was 450±60 and 270±40 g C/m^2, and cumulative net C exchange as CO2 totaled -330±30 and -150±30 g C/m^2, respectively, in burned and unburned paddocks. This result indicated the burned paddock recovered slightly more C than was removed by the burn. Ecosystem respiration averaged 1850±100 g C/m^2 across paddocks, as measured by eddy covariance and respiration chambers. Maximum green biomass during 2006 was 210±30 g C/m^2 and 140±30 g C/m^2 in the burned and unburned paddocks, due to dry growing conditions. Net C exchange was reduced to +45±20 and +13±20 g C/m^2, indicating near zero net exchange, and ecosystem respiration was reduced to 1250±50 and 1130±50 g C/m^2. Results indicate available soil moisture was a greater limitation to C storage and energy response by native prairie than the applied burn at multi-seasonal time scales.