Landscape-Level Trace Gas and Energy Fluxes on Grazed and Ungrazed Tallgrass Prairie

Clenton Owensby, Jay Ham, Eduardo Santos, Lisa Auen, and Nate Brunsell

Kansas State University and University of Kansas

We used relaxed eddy conditional sampling , open path eddy correlation, and remotely sensed land surface fluxes in heterogeneous terrain with large aperture scintillometry to obtain landscape-level carbon, energy, and water vapor fluxes on grazed and ungrazed tallgrass prairie. Our results can be used to extend experimentally-derived information from research to construct and validate models for estimating grazing effects on Global Climate Change, primarily using carbon balance determinations. We previously conducted research over a 9-yr. period which examined the effect of elevated atmospheric CO₂. A primary goal of research with elevated CO₂ is to quantify carbon sequestration for the major terrestrial biomes.That previous work showed improved water use efficiency and increased soil carbon in tallgrass prairie soils under elevated CO₂. In order to scale those results to the landscape, we measured the exchange of energy, water, trace gases, and biotic materials between the atmosphere and the terrestrial environment in real-world settings, which include exposure to fire and/or long-term grazing pressure, which are typical in grasslands across the globe. Therefore, Our objective was to quantify the impact of grazing on the carbon, water, and energy balance of the tallgrass prairie. and to examine how inter- and intra-annual variation in environmental factors (especially precipitation) interact with grazing pressure to impact carbon fluxes. Determine the governing leaf level and rhizosphere processes that impact the system's response to grazing.

Project Summaries

Results

Clenton Owensby, Jay Ham, and Lisa Auen. Fluxes of CO₂ from grazed and ungrazed tallgrass prairie. Rangeland Ecology and Management 59:111-127. Authors are Professor of Range Management, Professor of Environmental Physics,, and Assistant Scientist in Range Management, Department of Agronomy, Kansas State University, Manhattan, KS 66506-5501. .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 and ungrazed 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 because of 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) for the first half of the grazing season (May 1 to July 15) and the area was 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 GR and UG tallgrass prairie appeared to be carbon-storage neutral for the 3 years of data collection (1998 ungrazed: -31 g Cm²,1998 grazed: -5 g Cm2; 1999 ungrazed: -40 g Cm²2, 1999 grazed:-11 g Cm²2; 2000 ungrazed: +66 g Cm², 2000 grazed: 0 g Cm².

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Brunsell, N.A., J.M. Ham, and C.E. Owensby. 2008. Assessing the multi-resolution information content of remotely sensed variables and elevation for evapotranspiration in a tallgrass prairie environment. Remote Sensing and the Environment 112:2977-2987. Authors are Professor of Range Management, Professor of Environmental Physics,, and Assistant Scientist in Range Management, Department of Agronomy, Kansas State University, Manhattan, KS 66506-5501. .PDF

Abstract

Understanding the spatial scaling behavior of evapotranspiration and its relation to controlling factors on the land surface is necessary to accurately estimate regional water cycling. We propose a method for ascertaining this scaling behavior via a combination of wavelet multi-resolution analysis and information theory metrics. Using a physically-based modeling framework, we are able to compute spatially distributed latent heat fluxes over the tall-grass prairie in North-central Kansas for August 8, 2005. Comparison with three eddy-covariance stations and a large aperture scintillometer demonstrates good agreement, and thus give confidence in the modeled fluxes. Results indicate that the spatial variability in radiometric temperature (a proxy for soil moisture) most closely controls the spatial variability in evapotranspiration. Small scale variability in the water flux can be ascribed to the small scale spatial variance in the fractional vegetation. In addition, correlation analysis indicates general scale invariance and that low spatial resolution data may be adequate for accurately determining water cycling in prairie ecosystems.

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Quintana de la rocha, Adolpho, Eduardo A. Sanchez, and Clenton E. Owensby. 2022. Partitioning evapotranspiration and carbon flux in ungrazed and grazed tallgrass prairie. Agriculture, Ecosystems and Environment.. ttps://doi.org/10.1016/j.agee.2022.108285114. .PDF

Abstract

The effect of cattle grazing on water and carbon cycling in grasslands is not fully understood. The goal of this study was to evaluate how grazing affects carbon and water cycles in a tallgrass prairie. Eddy covariance measurements were carried out from 2003 to 2005 growing seasons (from May to September) in Manhattan, Kansas, U.S., in grazed (GR) and ungrazed (UG) tallgrass prairie paddocks. Net ecosystem CO₂exchange (NEE) was partitioned into gross primary productivity (GPP) and ecosystem respiration (R eco) using the relationship between the air temperature and nighttime NEE data. Evapotranspiration (ET) was partitioned into transpiration (T) and evaporation (E) using an approach based on the concept of underlying water use efficiency. Our results revealed that grazing reduced maximum aboveground iomass (AGB) and green leaf area index (GLAI) by 22.5% and 24.3%, respectively. Grazing also reduced GPP, NEE and T in the middle of the growing seasons but enhanced these fux components at the end of the growing seasons compared to the UG paddock. Grazing reduced the growing season cumulative NEE by 11.9% and enhanced ET by 3.3% compared to the UG paddock. The reduction in NEE at UG was associated with an increase in R eco (9.4%). In turn, changes in ET in response to grazing were driven more by the increase in E 26.6%) than by the decrease in T (5.9%). Despite the reduction in AGB and GLAI caused by grazing, similar cumulative GPP values at the canopy scale at both paddocks suggest that there are compensatory mechanisms that maintain similar CO₂ assimilation at UG and GR. Our findings suggest that the grazing regime adopted in this tallgrass prairie impacts the carbon cycle more strongly than the water cycle, given the higher reduction in NEE compared to the slight increase in ET on GR compared to UG. These results can be useful to outline strategies to improve management practices aimed at improving the efficiency of rangelands.

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Lindsay C. Morris,¹ Chinthaka Weerasekara,² Nathan Malarich ³, Fabrizio R. Giorgetta,^3,4 Daniel I. Herman,^3,4 Kevin C. Cossel,³Nathan R. Newbury,³ Clenton E. Owensby,² Stephen M. Welch,² Brett D. DePaola,¹ Ian Coddington,³ Eduardo A. Santos,² and Brian R. Washburn³Using Open-Path Dual-Comb Spectroscopy to Monitor Methane Emissions from Simulated Cattle Grazing .PDF

1 Kansas State University, Department of Physics, Manhattan, KS, 66506 2 Kansas State University, Department of Agronomy, Manhattan, KS, 66506 3 National Institute of Standards and Technology, Communications Technology Division, Boulder, CO, 80305 4 University of Colorado, Boulder, Department of Physics, Boulder, CO . 80309

* brian.washburn@nist.gov Abstract:

A controlled methane release study was performed using dual comb spectroscopy to simulate cattle emissions at a grazing system. The DCS system can measure a 10 nmol/mol methane enhancement above the atmospheric background concentration.

Work of the US Government and not subject to copyright.

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