Gross primary production (GPP) and ecosystem respiration (Re) are the fundamental drivers of carbon exchanges between terrestrial ecosystems and the atmosphere (Chapin and others, 2009), although other exchanges of carbon, for example, export or direct oxidation (Lovett and others, 2006) can modify net ecosystem production (NEP). The long-term accumulation of carbon in terrestrial ecosystems results in systems in which carbon contents of soil organic matter (SOM) often exceeds that of biomass (Post and Kwon, 2000). This SOM pool exists as a steady state between GPP and Re in ecosystems unless drivers change or perturbations (for example, climate) occur. As illustrated by Wilhelm and others (2010), conversion of grasslands to agriculture and cultivation practices can result in reduced soil carbon with the release of CO2 to the air by stimulated oxidation, contributing to higher Re. Specific land-use and management practices, therefore, influences NEP with additional reactions caused by irregular climate conditions (Luo, 2007). The isotopic status of the SOM reflects the net inputs by C3 and C4 systems and therefore in native prairies documents climate change (von Fischer and others, 2008) through the Holocene.
The recent concerns and questions being raised over issues such as climate change and alternative energy have driven significant changes in land management practices, especially in the highly agricultural Midwestern U.S. (Wilhelm, and others, 2010). It is important to insure the sustainability of these and other land management practices and to be aware of the potential impacts that such practices can have on NEP or exchanges of carbon with the atmosphere (Anderson-Teixeira and others, 2009). Since the mid-1990s, a highly advanced and growing network of micrometeorological towers has been utilizing eddy covariance methods to measure the exchanges of carbon dioxide, water vapor, and energy between terrestrial ecosystems and the atmosphere. These towers, also known as flux towers, are being strategically placed throughout North America in an effort to effectively represent major ecosystems and to make these data available to the scientific community. Such a dataset offers a unique and valuable resource for use in the study and quantification of carbon exchanges between terrestrial ecosystems and the atmosphere and can ultimately lead to answering the questions raised about resource sustainability.
The focus of this study has been defined as the North American Great Plains, a region primarily consisting of grassland and cultivated cropland (Figure 1). Currently, more than 100 site-years of flux-tower measurements, represented by over 30 individual cropland or grassland sites throughout the Great Plains, have been accumulated and are being analyzed in conjunction with applicable remotely sensed data. Given the terrestrial composition of the focus area, it is essential to account for both grassland and cultivated cropland ecosystems to achieve a comprehensive quantification of NEP. Recent studies have shown that, through the use of complex regression tree modeling, flux tower measurements and remotely sensed data can be utilized to quantify and map NEP in grassland ecosystems across the Great Plains (Zhang and others, 2011) and the dramatic affect that annual climate and land use has on NEP.
|Figure 1. Flux tower locations used by Zhang et al. (2011) for Great Plains grassland flux mapping. |
Applying similar quantification methods to the cropland ecosystems of the Great Plains will allow for further expansion of NEP quantification and mapping of the region. Such an application first required that major crop types commonly grown in the Great Plains, such as corn, soybeans, and wheat were known with relatively high spatial and temporal resolution. We developed and implemented a crop type classification model, based primarily on weekly time series normalized differential vegetation index (NDVI) data, to account for these major crop types. The models were originally developed for the Greater Platte River Basin, but have the capability to be expanded to cover larger regions, such as the Great Plains. Our efforts are progressing in the area of cropland NEP quantification in the Great Plains and still require additional acquisition and processing of source flux tower data and the development of carbon flux algorithms for the major crop types in the region. Attaining these lingering aspects of carbon fluxes in the Great Plains will greatly increase our ability to comprehensively quantify NEP in the region.
Through all of our research and development in this area, we have also devised an approach that effectively identifies and maps areas within the Great Plains which are poorly represented by the current flux tower distribution. This information could be utilized for future management and planning purposes of the flux tower network.
We integrate our flux quantification with detailed documentation of the carbon isotope status of soil organic matter (SOM) throughout the soil profile and in various particle size fractions. This allows us to quantify C3 and C4 contributions to the SOM, and our analyses across the latitudes of native prairies in North America allow a reconstruction of past systems from which climate information is derived.