Owensby, Clenton E., Jay M. Ham, and Lisa M. Auen. 2005. Fluxes of CO₂ from grazed and ungrazed tallgrass prairie. Rangeland Ecology and Management 59:111-127. .PDF

Abstract

To determine the impact of seasonal steer grazing on annual CO₂ fluxes of annually-burned native tallgrass prairie, we used relaxed eddy accumulation on adjacent pastures of grazed (GR) and ungrazed (UG) tallgrass prairie from 1998 to 2001. Fluxes of CO₂ were measured almost continuously from immediately following burning through the burn date the following year. Aboveground biomass and leaf area were determined by clipping biweekly during the growing season. Carbon lost due to burning was estimated by clipping immediately prior to burning. Soil CO₂ flux was measured biweekly each year using portable chambers. Steers were stocked at twice the normal season-long stocking rate (0.81 ha steer-1) for the first half of the grazing season (~May 1 to July 15) and the area left ungrazed the remainder of the year. That system of grazing is termed "intensive early stocking". During the early growing season, grazing reduced net carbon exchange relative to the reduction in green leaf area, but as the growing season progressed on the grazed area, regrowth produced younger leaves that had an apparent higher photosynthetic efficiency. Despite a substantially greater green leaf area on the ungrazed area, greater positive net carbon flux occurred on the grazed area during the late season. Net CO₂ exchange efficiency was greatest when grazing utilization was highest. We conclude that with grazing the reduced ecosystem respiration, the open canopy architecture, and the presence of young, highly photosynthetic leaves are responsible for the increased net carbon exchange efficiency. Both grazed and ungrazed tallgrass prairie appeared to be carbon storage neutral for the three years of data collection (1998: UG -31 gC m-2, GR -5 gC m-2; 1999: UG -40 gC m-2, GR -11 gC m-2; 2000: UG +66 gC m-2, GR 0 gC m-2).

Current Carbon, Water Vapor, and Energy Flux Research:

Impact of Abusive Grazing, Moderate Grazing and No Grazing on Carbon Fluxes and Storage. Grasslands that have been grazed for an extended period at high utilization rates have reduced annual net primary productivity and greatly reduced belowground biomass. Changing grazing rate to recommended levels increases productivity and belowground allocation. The rate of C storage following a switch to good management is not known and difficult to measure directly in the short term. Eddy covariance flux measurements are being used to quantify C sequestration by comparing C balances between heavily-grazed areas and areas where stocking rate has been changed to the recommended rate. The site is located in pristine tallgrass prairie in the Kansas Flint Hills. Vegetation is dominated by the C4 grasses, Andropogon gerardii Vitman and Sorghastrum nutans (L.) Nash. Fluxes are being monitored at a site which is being subjected to heavy grazing (2x the normal rate), at a moderately-grazed site, and at a comparable ungrazed site. Surface-atmosphere exchange and meteorological conditions has been measured at tower sites within each treatment. Net C exchange and water vapor flux was measured by eddy covariance using a triaxial sonic anemometer and an open-path gas analyzer. Open path gas analyzers will be calibrated every week using a gas standard and a portable dew point generator. Aboveground net primary production and leaf area will be monitored by harvesting 4 randomly placed 0.25 m2 plots. Plots will be harvested once every two weeks in the first half of the growing season and monthly during the last half. Aboveground biomass will also be determined prior to and immediately after annual burns in April to quantify C released by the fires. An annual carbon balance will be calculated for each grazing regime. Results will quantify the potential for loss of C from the system as a result of abusive grazing rates.

Objectives:

* To measure the impact of heavy grazing on carbon dynamics of tallgrass prairie.

* To quantify annual carbon fluxes to produce a carbon budget on ungrazed, moderately grazed, and heavily grazed tallgrass prairie.

Approach:

Measuring Net Carbon Exchange, Evapotranspiration, and the Surface Energy Balance. Surface-atmosphere exchange and meteorological conditions are being measured at tower sites within each treatment area. Net carbon exchange and water vapor flux are being measured by eddy covariance using a triaxial sonic anemometer and a open-path gas analyzer. The eddy flux equipment will be positioned 2.5 to 3 m above the surface and sampled at 10 Hz using a Campbell 23X datalogger. Velocities, gas concentrations, and virtual temperatures sampled at 10 Hz also will be routed to laptop computer interfaced with the 23X datalogger. Access to streamwise data will allow spectral correction using low pass filtering or analytical methods if necessary (Massman and Lee, 2001). Covariances and fluxes (i.e., CO₂, H20, and sensible heat) are being computed every 30 minutes after coordinate rotation and corrections for simultaneous fluxes of heat and water vapor. Estimates of sensible heat flux, computed from sonic-based speed-of-sound data, include cross wind corrections. Initially, fluxes for H20 and CO₂ will be adjusted using the energy-balance closure approach. This technique forces energy balance closure [net radiation (Rn) - soil heat flux(G) = latent heat flux(LE) + sensible heat flux(H)] by first assuming the eddy covariance measurements of the Bowen ratio (H/LE) are correct. The eddy flux estimate of H/LE is used with Rn and G to compute adjusted values of LE and H – resulting in energy balance closure. It is then assumed that the fractional adjustment in CO₂ flux is the same as that used for LE. This correction technique is appealing in grasslands and crops because Rn and G in the eddy-flux source area can be measured very accurately. This technique will probably not be used at night when Rn is negative and Rn-G is small. Hand-held chambers, autochambers and modeling techniques will be employed to verify soil CO₂ fluxes and nighttime fluxes of CO₂. Measuring the energy balance and correcting the eddy flux data will require detailed measurements of soil heat flux (G) and net radiation (Rn). Net radiation will be measured with a net radiometer positioned 3 m above the surface. Periodically, a second, portable net radiometer and tripod will be positioned along a transect in the eddy flux source area to check for positional variability. Soil heat flux plates (3 to 5 transducers) will be installed at 5 cm and the rate change in storage above the plates will be measured automatically using dual probe heat capacity sensors. Dual probes will also be used to measure soil water content at 2.5 and 10 cm depths. Ancillary instrumentation will include: a tipping-bucket rain gauge, relative humidity and temperature probe (Vaisala, Helsinki, Finland), and a pyranometer and PAR sensor. Throughout the year, the open path gas analyzers will be calibrated every week using a gas standard and a portable dew point generator. All data will be transmitted to the campus of Kansas State University using a wireless data system.

An animation of the autochamber used to measure hourly ecosystem respiration can be viewed by clicking on this link

Biomass and Leaf Area. Biomass and leaf area for grasses and herbaceous dicots is determined biweekly during the first half of the growing season and monthly until the end of the growing season by harvesting four 0.25 m2 samples in the source area of the tower. Green leaf area for grasses and herbaceous dicots is measured with an area meter. Aboveground biomass is determined gravimetrically after samples had been dried for 72 h at 55C. Aboveground biomass will also be determined just prior to the annual burn by harvesting four 0.25 m2 samples in the source area of the tower. Carbon content of the aboveground biomass is measured using a C/N Analyser. Carbon loss during the burn is determined by multiplying the carbon content of the aboveground biomass (~42%) by the carbon content. Export of carbon from the system will be estimated by multiplying the carbon content of beef cattle times the weight gain of the livestock on the grazed areas.

Results to Date: The original research plan called for an area that had been grazed heavily in the past. An exhaustive search in the Manhattan vicinity found several sites, but no owner was willing to let us do the research there. As an alternative, we chose to create an overgrazed situation on the Rannells Flint Hills Prairie which could be compared to existing flux sites which would leverage the output from the project. The flux towers were set up on a range site comparable to the existing flux sites at the beginning of the growing season following the burn. By accumulating fluxes for the year, we can determine the extent of the carbon loss associated with overgrazing and compare that to the carbon balances from ungrazed and moderately-grazed areas. Two flux towers were placed in the heavily-grazed area and following 3 seasons of heavy use, in 2007 the area will be stocked at moderate rates to determine the recovery of carbon losses due to heavy grazing.