Carbon, Water and Land-use in Conservation Reserve Program Lands of the Shortgrass Prairie
Niall P. Hanan1, Jack A. Morgan2, Keith H. Paustian1, Ingrid C. Burke3
1Natural Resource Ecology Lab, 2USDA-Agricultural Research 3Forest Science Department,
Colorado State University Service, Fort Collins Colorado State University
1. Introduction
Semi-arid grasslands and shrublands are some of the most widespread vegetation formations on Earth, covering approximately one eighth of the global land surface. As such they are second only to forests in terms of total (biome-wide) annual carbon exchange with the atmosphere (Scholes and Hall, 1996 ). The combination of large area and potential for the soils of these regions to gain or loose organic matter, depending on the fine balance of climate, management and ecological conditions (Ojima et al., 1993; Parton et al., 1993; Parton et al., 1995 ), means that the semi-arid regions of the world could play a significant role in the sequestration, or liberation, of carbon in the future.
In North America, the prairie ecosystems of the Great Plains extend east from the Rocky Mountains across the western Mississippi basin, and from southern Canada to New Mexico and Texas, covering an area of approximately 1.3 million km2 (Riebsame, 1990 ). The climate of the Great Plains is continental, with a pronounced decline in precipitation from east to west and temperature from south to north. Vegetation types follow these gradients, with tallgrass species (e.g. Andropogon, Panicum, Sorghastrum) more common in the mesic eastern regions, and short- and mid-grass species (Bouteloua, Buchloa, Stipa, Pascopyrum) more important in the semi-arid west. Much of the Great Plains still comprises natural grasslands and grazed rangelands, but large areas have been converted to crops (primarily wheat, corn, sorghum and soybeans), particularly in the eastern tallgrass prairies.
The semi-arid shortgrass steppe (SGS), which extends from the Colorado-Wyoming border, south to New Mexico and western Texas, is dominated by warm-season C4 grasses such as Bouteloua gracilis and Buchloe dactyloides. The SGS ecosystem is adapted to low and variable rainfall and herbivory by wild ungulates. The combination of low rainfall and resilience to grazing has protected the region from large-scale conversion to crops, with more than 50% of the original SGS still supporting “natural” shortgrass species, although now grazed by domestic livestock rather than wild herbivores (Lauenroth and Milchunas, 1991 ). Consequently, while grazing lands are important throughout the Great Plains, they are a relatively larger component of the agricultural landscape of the shortgrass region.
Agricultural practices in the Great Plains and, in particular, Federal and State agricultural policies and subsidy systems, have the potential to produce rapid changes in landuse. For example, the USDA Conservation Reserve Program (CRP), that pays farmers to return cropland to natural or semi-natural grassland, has active contracts on more than 2 million acres of shortgrass prairie in eastern Colorado alone. Equally large, or larger, areas of CRP land exist in the other Great Plains states. The CRP requires farmers to replant long-term, “resource-conserving” covers, especially native grasses, to improve soil, water, and wildlife resources on their land, and to leave the land unused for the duration of the contract. In the shortgrass prairie regions of eastern Colorado, many contracts were initiated in 1992-93 for 10-year terms, with a big extension to the Program in 1996-98 (http://www.fsa.usda.gov/dafp/cepd/crp.htm). In the next few years, large areas of CRP land will reach the end of their contracts. Given changing economic conditions and uncertainty regarding the future of the Conservation Reserve Program, it is likely that much of this land will either return to agriculture or be turned over to grazing (Figure 1).
The CRP lands of eastern Colorado are located primarily in SGS regions where the land-use options available to farmers are relatively limited. Grazing on CRP land is not allowed, and many farmers view CRP land as a wasted grazing resource. It is likely that a significant portion of CRP will be turned over to grazing at the end of the contracts. Irrigated agriculture is important in the region, but the potential for expansion of the irrigated acreage is severely limited by the growing urban and agricultural demands on a limited water supply. Dry-land agricultural rotations are common and this will likely also be a common fate of the CRP lands. Past research on the impacts of rangeland conversion to conventional agricultural rotations is extensive (Janzen et al., 1997; Juma et al., 1997; Paul et al., 1997; Peterson and Westfall, 1997; Rasmussen et al., 1998; Bowman et al., 1999; Campbell et al., 1999 ). Thus we have good information on the impacts of this change on soil carbon dynamics, productivity and nutrients. Much less information is available on the impacts of range conversion to minimum-till agricultural systems, but it is commonly assumed that both carbon and water conservation will be much better than in conventional systems (e.g. Kessavalou et al., 1998 ). With increasing accessibility of minimum-till and no-till technologies, and the associated benefits in soil fertility and productivity, and given the possibility that farmers might be able to benefit from carbon-credit trading in the future, it is likely that conversion to no-till agricultural rotations may also be a popular choice for CRP land.
Figure 1. Origin (solid arrows) and futures (dashed arrows) of Conservation Reserve Program (CRP) land in Great Plains prairie ecosystems (P=grazed prairie, A=crop rotation, C=CRP)
Changes in the management of CRP land will alter the vegetation structure and phenology in ways that impact both the timing and intensity of short-term carbon, water and energy exchange and long-term carbon sequestration and hydrologic balance (Cole et al., 1989; Hanan et al., 1998; Reeder et al., 1998 ). For example, while the timing of carbon and water dynamics in grazed rangeland may be similar to ungrazed rangeland, the magnitude of carbon and water fluxes will likely be altered because of changes in productivity, leaf area and water use efficiency. Grazers can positively impact both productivity and water balance of grassland ecosystems because grazing stimulates shoot production and reduces build-up of old and senescent leaves, while at the same time reducing the transpiring leaf area (LeCain et al., 2000 ). By contrast, a winter wheat crop has a markedly different phenology, with growth concentrated in the cool season, high productivity because of fertilization, and very different transpiration dynamics because of the changes in timing of leaf area vis a vis rainfall and atmospheric water demand.
Thus at short time-scales, changes in land use will impact patterns of productivity, evapotranspiration and water use efficiency across the region. Research using coupled biosphere-atmosphere models (incorporating interactive land-surface biogeochemistry and biophysics in a regional atmospheric dynamics model) shows that atmospheric dynamics are very sensitive to changes in terrestrial vegetation via effects on surface energy balance and the partition of energy between sensible and latent heat fluxes (Pielke et al., 1998; Stohlgren et al., 1998; Pielke et al., 1999 ). The potential magnitude of land-use change associated with CRP land in the Great Plains could lead to changes in precipitation patterns, near-surface temperature and humidity, which will in-turn feed-back on the water balance, growth and biogeochemistry of the vegetated landscape. At seasonal and inter-annual time-scales, the conversion of ungrazed prairie to grazed rangeland or dry-land agriculture will likely impact trends in long-term carbon sequestration and may alter landscape trajectories towards more mesic, or more arid, conditions.
We propose to develop a new eddy covariance site in the shortgrass steppe of the Great Plains region, with a focus on the impact of grazing and minimum-till agriculture on prairie dynamics and the fluxes of carbon, water and energy at time-scales from hourly to inter-annual. Three long-term eddy covariance systems will be established in large, adjacent, parcels in eastern Colorado that have been in the CRP for 10 years or more. After an initial comparison period (of at least 2 months), one parcel will be opened to cattle grazing at moderate intensity, a second parcel will be converted to minimum-till agriculture, while the third parcel will remain in CRP. The CRP site will provide a control for the transition sites, and long-term information on the behavior of CRP grasslands that have been left undisturbed for periods of 10 or more years. The new sites in shortgrass steppe ecosystems will contrast and complement the existing (and historical) Ameriflux sites in tallgrass prairie systems (Verma et al., 1992; Ham and Knapp, 1998; Bremer and Ham, 1999; Burba and Verma, 2001; Suyker and Verma, 2001 ).
The project will be closely linked to several existing research programs at the NSF-funded Shortgrass Steppe-Long Term Ecological Research site (SGS-LTER), including the wealth of historical research and several on-going research programs. Investigators Indy Burke and Jack Morgan are Principal Investigators of the SGS-LTER research program. In addition, we will work closely with the “Consortium for Agricultural Soil Mitigation of Greenhouse Gases” (CASMGS) on which Investigator Keith Paustian is a lead PI. The CASMGS project plans a network of measurement sites to monitor carbon and trace gas dynamics. In the case of carbon, CASMGS will make detailed measurements of the change in carbon stocks in soils over periods of 3-5 years, to infer carbon sequestration and liberation using a mass balance approach. If this proposal to NIGEC is successful, our new sites will be included in the CASMGS monitoring programs. The combination of long-term eddy covariance measurements with comprehensive mass balance measurements will provide separate and independent measurements of carbon sequestration/liberation rates and processes. We feel this combination will lead to major gains in our understanding of the rates and mechanisms of carbon sequestration in shortgrass steppe ecosystems.
The measurement program planned in this project will be fully integrated and coordinated across three land-use types (with three eddy covariance systems). The field measurement program will also be coupled with detailed biosphere-atmosphere exchange modeling. The model to be used for this purpose has been developed under existing funding to simulate vegetation dynamics and biogeochemistry, land surface-atmosphere interactions and atmospheric dynamics. The close linkage of the field program with a modeling component will provide, on the one hand, site-based datasets against which biogeochemical and biophysical components of the model will be tested. On the other hand, the modeling component of this work will provide a powerful tool for scenario testing to explore the impact of land-use changes on carbon and water dynamics in the coupled biosphere-atmosphere system of the Great Plains region (Figure 1).
2. Objectives
As large areas of CRP land approach the end of 10-year contracts, and given the uncertain future of the CRP program, farmers may look to alternative options for their land. What are the consequences of these various options, in terms of seasonal and long-term carbon and water balance of shortgrass prairie ecosystems and interactions with the regional atmosphere? Specific aims of this proposal include:
· To measure and contrast the carbon and water dynamics, at hourly to inter-annual time-scales, associated with a range of CRP land use trajectories in shortgrass prairie ecosystems.
· To determine the impacts on carbon sequestration, hydrological dynamics and water-use efficiency associated with grazing of shortgrass prairies and conversion to minimum-till agricultural systems.
· To examine the effect of alternative land-use options in the Great Plains on the two-way interactions between terrestrial ecosystems and the regional atmosphere in an integrated modeling system.
· To contribute to, and benefit from, parallel research activities focused on regional and continental carbon balance and the mechanisms of carbon sequestration in managed ecosystems.
Through a combination of direct measurements of ecosystem-scale fluxes of carbon, water and energy over a period of at least 2-3 years, detailed process measurements in vegetation and soils, and model analyses of integrated biosphere-atmosphere interactions, we will address the following hypotheses:
H0,1: Alternative land-use options have no impact on short-term carbon balance or long-term carbon sequestration in shortgrass prairie systems
H1,1: Return of CRP land to pasture will increase the seasonal productivity of the prairie because of reduced litter cover and increased physiological capacity of remaining leaf area. This will contribute to increased carbon sequestration in the medium- to long-term (1-3 years) as net production is transferred to slow turnover soil carbon pools.
H2,1: Conversion of CRP land to a minimum-till agricultural rotation will increase seasonal carbon uptake because of fertilizer stimulation of production, and increase sequestration in soils through the combination of high productivity and low soil disturbance characteristic of minimum-till agriculture. (By contrast, conventional tillage results in net loss of carbon, as documented in research cited above)
H0,2: Alternative land-use options have no impact on water balance and energy partitioning in prairie systems
H1,2: Return of CRP land to pasture will reduce transpiration during the growing season because of reduced leaf area and thus reduce the frequency/severity of intra-seasonal drought and extend the length of the growing season compared to the CRP control. The combination of reduced albedo and reduced evapotranspiration during the growing season will markedly increase the partition of energy to sensible heat flux, with likely effects on convective cloud formation and precipitation, and feedbacks on ecosystem biogeochemistry.
H2,2: Conversion of CRP land to a minimum-till agricultural rotation will significantly alter the timing and intensity of evapotranspiration because of the different phenology of the crop species and shift of the growing period to cool season conditions. Under winter wheat crop the peak growth and transpiration will occur in the cool season when atmospheric demand is relatively reduced (vis a vis summer). Summer evaporation from the maturing crop will be low, resulting in improved water balance at annual time-scales, and increased partition of energy to sensible heat flux during the summer.
H0,3: Alternative land use options have no impact on the water-use efficiency of prairie systems
H1,3: Return of CRP land to pasture will result in much improved water-use efficiency because of coupled increases in productivity and reduced transpiration. Given the temporal synchrony in semi-arid systems between plant growth, soil decomposition and soil-water availability, increased water-use efficiency during the growing season will lead to increased long-term soil carbon sequestration.
H2,3: Conversion of CRP land to a minimum-till agricultural rotation will also improve water-use efficiency because of high productivity (related to fertilization and high-yield crops) coupled with the shift of the growing season to cool season months during which transpiration is reduced. The combination of high water-use efficiency and low soil disturbance will result in the maximization of both water-use efficiency and soil carbon sequestration in the minimum-till agricultural system.
3. Relevance to Great Plains Regional Center strategic vision
The proposed research will contribute to the primary goals of the GPRC that seek to "increase basic understanding of how agricultural and grassland ecosystems exchange carbon with the atmosphere and how environmental change is likely to impact key ecosystems in the region".
In particular, we will address GPRC Goal 1 ("to increase understanding of the net carbon exchange and the processes involved, and effects of environmental change on ecosystems"). This will be accomplished by measurement of carbon and water fluxes at ecosystem-level, coupled with detailed physiological and biogeochemical measurements that will allow us to understand the mechanisms controlling pool sizes and transfer processes. When land-use change is included as a driver of "environmental change", we will further address the changes in carbon and water dynamics that result when shortgrass prairie is co-opted by human populations for grazing by domestic animals and crop production.
We will also address GPRC Goal 2 ("to evaluate tools needed to determine impacts of environmental change on social, biological and physical systems"). The measurement program will contribute invaluable data for testing biophysical models describing high-frequency exchanges of mass and energy between the land surface and the regional boundary layer, and the lower frequency models that simulate vegetation growth and biogeochemistry that contribute to and control the high-frequency fluxes. In the regional climate model context, our simulations will allow us to examine the two-way interactions between land surface processes and atmospheric processes, and the ways in which current and future land-use may affect the regional climate, with cascading effects on regional ecosystems.
4. Methods
4.1. Eddy covariance and micrometeorological measurements
The experimental design will examine the carbon and energy balance consequences of three possible CRP land-use transitions, and track those systems over a period of at least three years. Bowen ratio (BR) flux stations are already operating in a grazed rangeland at the CPER and a conventional wheat crop on a nearby commercial farm. These measurements, and the physiological and biogeochemical studies associated with them, provide important background information on the functioning of grazing and wheat-fallow systems under varying seasonal and annual climate conditions. However, reduced reliability of the BR method under certain conditions (Verma and Rosenberg, 1975; Ohmura, 1982 ) and lack of direct measurements of the turbulent exchange process limits the comprehensiveness and utility of these measurements. In this project we will initiate a program of direct measurements of turbulent fluxes using the eddy covariance technique that will contrast the short- and medium-term consequences of land-use transitions in CRP lands. We hypothesize that the initial years following a change in land use, are the times when carbon emission and sequestration are most active and that much of that dynamic will be captured during the 3-year course of this research. The project will initiate long-term measurements in three adjacent treatments in the shortgrass prairie region using the experimental design shown in Figure 2.
Eddy covariance systems (indicated by the hour-glass symbols in Figure 2) will be established in 3 adjacent parcels (each > 12 Ha) of CRP land that is at or near the end of the CRP contract. We will design the measurements systems to include the core measurements, and most of the desired ancillary measurements, of the Ameriflux network (http://public.ornl.gov/ameriflux). A sonic anemometer and open path CO2/H2O gas analyzer will be mounted at a reference height (zr) of 4.0 meters (3.5 meters above maximum canopy height) on a tower. Incident and reflected/emitted shortwave and longwave radiation will be measured using pyranometers and pyrgeometers installed at 4-5 meters on separate mounts to avoid interference from the primary tripod in the reflected solar measurements. Air temperature and relative humidity will be measured using shielded probes mounted in profiles from near the soil surface to zr, with [CO2] profiles measured at the same heights using a gas analyzer drawing air through a switched tube-sampling system. Soil moisture and temperature will be measured in 2-3 profiles from near the soil surface to 1.5 meters. Soil heat flux will be measured using 4-6 resistive heat-flux plates installed at 0.01 and 0.05 meters depth. Power will be provided by three 75-Watt solar panels charging a 450 Ahr battery array. With the relatively low power consumption of the open-path system, this power configuration is sufficient for the expected range of summer and winter insolation conditions in eastern Colorado. This assessment was based on analysis of daily Colorado Front Range surface radiation data available at the NASA Langley Research Center Atmospheric Sciences Data Center (http://eosweb.larc.nasa.gov/sse). Datalogger and control systems will be mounted in an instrument enclosure at the base of the tower, with raw flux measurements recorded at 10 Hz and meteorological measurements measured every 30 seconds and averaged to 30 minutes for storage. Personnel will visit at weekly intervals, or more frequently when necessary, for site maintenance and data retrieval.
Figure 2. Experimental design to examine the carbon and water consequences of land use change in the shortgrass prairies of Eastern Colorado, from CRP land (C) to grazing (P) and minimum-till agriculture (A).
Measurements will be made for an initial period of at least 2-3 months in the undisturbed parcels to establish a baseline comparison under CRP. Given similar landuse history (for 10 years, at least), and site selection for areas of similar soil and geomorphology, we expect that these baseline measurements will indicate similar functional behavior in the 3 parcels. After the initial baseline comparison, one parcel will be opened to grazing by domestic cattle at moderate stocking rate and schedule (Klipple and Costelo, 1960 ), while a second parcel will be cleared of native vegetation (using the usual method of chemical herbicide) and planted to winter wheat. The third parcel will remain in the unmanaged (CRP) condition to provide a control comparison and a measurement of CRP functioning after 10-15 years. Given the likely timing of this project (start date for all proposals July 2002), we anticipate installing meteorological and profile instruments in March 2003, with flux measurements starting on all three sites in early 2003. Thus grazing would begin on the grazing treatment in Spring 2003. Clearance and planting of the minimum-till site would occur at the normal times for winter wheat crops (September-October 2003).
The CRP land for this project will be selected, in so far as possible, on flat terrain and consistent soil type. Our budget request includes funds for land rental at rates slightly above the average CRP cost per acre for the region. In eastern Colorado, in the years 1992-93, CRP contracts were issued on some 5000 acres in the vicinity of the SGS-LTER and CPER sites (in Weld, Morgan, Washington, and Logan counties). These counties are within easy reach of CSU (<2 hours by car). In addition during those years, approximately 15,000 acres came into the CRP in Yuma, Kit Carson and Cheyenne counties. Locations in these counties would be more distant from CSU, but travel on a daily basis would still be quite feasible. We will benefit from existing research infrastructure at the SGS-LTER and CPER sites that will contribute much of the instrumentation for one of the eddy covariance systems. Continuation of measurements beyond the initial 3-year period is anticipated from core funding or new proposals.
4.2. Physiology, biogeochemistry and biophysics
The eddy covariance systems will measure total ecosystem CO2 and H2O fluxes, but interpretation of treatment effects on whole-ecosystem exchange will require ancillary measurements of the soil and plant attributes which respond to treatments and ultimately determine the fluxes. Measurements of single-leaf photosynthesis will be conducted on representative species in each treatment using a steady-state, portable gas analysis system already available to the project (CIRAS-1, PP Systems, United Kingdom). The measurements will consist of in situ CO2 and H2O exchange measurements performed throughout the growing season on both recently expanded and older canopy leaves. More intensive CO2 and light response curves of photosynthesis will be conducted at critical growth stages to document changes in leaf photosynthetic capacity due to treatment and environment. Leaf samples will be harvested following gas exchange measurements to analyze for total C and N. To document treatment effects on water relations, measurements of leaf water potential will be made during mid-day when all leaf gas exchange measurements are taken. Two recently expanded leaves of representative species will be sampled in each plot, using a PMS pressure chamber (Plant Measurement Systems, Corvallis, OR, USA).
Carbon stocks will be measured on the three study sites using the protocols to be established for the CASMGS network. Replicate soil samples (12 or more, depending on spatial variability) will be collected to 1.0 meter depth in each treatment. The soil cores will be separated into five depth increments (0-5, 5-10, 10-20, 20-50, 50-100 cm) and sieved to remove rocks and coarse roots. Total organic carbon and nitrogen content will be determined by combustion on a Leco CHN autoanalyzer. Active C, silt- and clay-associated C, and aggregate-associated C will be determined by sequential laboratory fractionations (Six et al., 2001 submitted). These measurements will be repeated at the beginning and near the end of the project to determine whether total soil organic C content changed and to assess the dynamics of the different soil C fractions in each treatment. Changes in soil C fractions will be used to interpret the source/sink relationships that may be driving net ecosystem liberation/sequestration, respectively. In addition to measurement of soil carbon dynamics, trace-gas flux measurements (N2O, CH3 and CO2) using chamber measurements repeated at 2-4 week intervals throughout the year are planned as part of the CASMGS project. It is anticipated that the sites to be established under this project will be included in that program of measurements such that we are able to assess soil contributions to net ecosystem CO2 exchange, and the loss/gain of carbon in methane production/consumption.
Destructive samples of above-ground phytomass will be collected at bi-weekly to monthly intervals (depending on growth rates) and separated by species into live and standing dead phytomass. Sub-samples of above-ground phytomass will be run through a leaf area meter to estimate total leaf area index (LAI). Harvested samples will be oven-dried and used for calculation of above-ground NPP. The leaf area and NPP measurements will be used to test and parameterize the integrated system model described below. Light bar measurements, at regular intervals during the growing season at each site, will be conducted to establish relationships between LAI and canopy light interception so that canopy level CO2 exchange can be interpreted and modeled in part as a function of light interception. Periodic samples of root phytomass will be used with existing information on root turnover (Milchunas and Lauenroth, 1992 ) to estimate total system NPP for comparisons with net ecosystem carbon exchange.
4.3. Integrated regional biosphere-atmosphere system modeling
The field programs, including the eddy flux measurements, biogeochemical, biophysical, and physiological measurements, will provide direct information on the net ecosystem exchanges under varying land-use, and the mechanisms that result in differences. However, to understand the role of land-use changes in the coupled land-atmosphere system at regional scales, and to extrapolate from relatively small sites to the larger shortgrass prairie region, requires integration of the measurements in a regional modeling context.
We will benefit from close linkages with an Integrated Research Challenge (IRC) grant from NSF on which Hanan is a Co-Investigator (Dr. Dennis Ojima of NREL is PI). The IRC project is developing a comprehensive object-oriented modeling system for research on continental-scale carbon balance. Components of the model are based on existing and well-tested models (e.g. the Century model, Schimel et al., 1990; Ojima et al., 1993; Parton et al., 1998 ; Biome BGC, Running and Hunt, 1993 ; CLM, Bonan, 1995 ; SiB2, Sellers et al., 1996 ) but adapted and recoded to conform to the stringent scientific and computer-science requirements of the new model. Much of the work towards this new model has already been completed, and a version of the model suitable to our needs for studies in the Great Plains region (i.e. not yet including sub-models describing succession or long-term biogeographical vegetation change) will be available for use in early 2002. The land surface components are integrated in a modified version of the Regional Atmosphere Modeling System (RAMS, Pielke et al., 1992 ) such that the primary biosphere processes are internally generated and consistent. The biosphere and atmosphere are fully and dynamically interactive and land surface-atmosphere exchanges are translated into varying fluxes and scalar concentrations in the regional atmosphere. Predicted regional fields of scalar concentrations are amenable to direct validation against aircraft measurements. We note that, while we do not plan aircraft flights in this project, such measurements may well be available in the near future under NIGEC or other funding (e.g. the request for a GPRC/MRC synthesis program in the NIGEC RFP; the Cobra Project, and the North American Carbon Plan).
In our proposed project, measurement and modeling programs will be well integrated and provide mutual benefits. The site-based measurements will provide excellent datasets for verification of model simulations of plant growth, plant and soil biogeochemistry, biophysics and surface-atmosphere exchanges. On the other hand, the model will place the site measurements in their regional context, and allow exploration of the real significance of different land-use types in the coupled biosphere-atmosphere system. The integrated biosphere-atmosphere modeling will further facilitate scenario-testing, where the impact of various land-use trajectories on regional carbon sequestration, water balance, and biosphere control of boundary layer processes and regional climate will be explored.
The regional model will be parameterized for the shortgrass prairie and adjacent regions using soil, vegetation and land-use maps collated for the LTER site and surrounding regions. The model will run with 20 vertical levels (of varying depth), horizontal grid increment of 4km and domain size of approximately 125 x 100 cells. The model domain will therefore incorporate the entire shortgrass steppe region, from the Front Range of the Rocky Mountains east across Colorado, into western Nebraska and Kansas, and from southern Wyoming to northern New Mexico. Soil physical and chemical properties will be defined using information on soil type and long-term steady-state simulations (i.e. "spin-up") of a biogeochemical model (e.g. Century; Parton, 1996 ). The climate driver data within, and at the boundaries of, the model domain will be obtained from assimilated weather analyses available from NOAA and other sources (e.g. RUC data from NOAA, LDAS data from NASA). Model simulations will be tested by comparison of predicted weather conditions at the three ground sites against measured radiation, temperature, humidity, etc. (note that the model grid increment will mean that all three sites will likely fall within the same grid element). Furthermore, predicted CO2, water and heat exchange over the three treatments will be tested in both offline simulations (i.e. for a single point using measured weather drivers) and in the coupled model (i.e. in cells defined as CRP, grazing and no-till, using predicted weather drivers in the fully coupled model).
The model scenarios will include "actual vegetation" runs using our best estimate of the current distributions of CRP, grazed prairie, conventional-tillage crops and no-till crops, and model experiments involving, for example, a large expansion in grazing of the CRP areas, or conversion to no-till agricultural rotations. This will allow the investigation of the impact of these landuse types on the coupled biosphere-atmosphere system in the shortgrass prairie region.
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