Abstract
Grasslands sequester and store large amounts of soil carbon, which is primarily controlled by herbivory and precipitation. However, few studies have examined the combined effects of these two factors and quantified how they control carbon cycling in temperate grasslands. The objective of this study was to quantify how grazing intensity affects the magnitudes and patterns of net CO2 exchange in the mixed-grass prairie, the largest native grassland ecosystem in North America. The study was conducted during two contrasting precipitation years (dry vs. wet summer), which allowed investigation of the interaction between precipitation and grazing intensity on the magnitudes and patterns of net CO2 exchange. Our three grazing regimes have been in place for 20 years and consist of light and heavy grazing and ungrazed exclosures. Ecosystem CO2 exchange rates were strongly influenced by changes in summer precipitation. Decreasing summer precipitation reduced ecosystem respiration (RE) by 45%, gross ecosystem production (GEP) by 75%, and net ecosystem exchange (NEE) by 70%. The lightly grazed pastures had the greatest rates of RE, GEP, and NEE during the wet summer; however, NEE did not differ between grazing treatments in the dry summer. These results indicate that grazing intensity and precipitation interact to influence carbon cycling on mixed-grass prairie ecosystems.
Highlights
Understanding the factors controlling the exchange of CO2 between the biosphere and the atmosphere and the sequestration of carbon (C) by landscapes has become a central concern for science, policy, and management (Follett et al 2000; Kaiser 2000; Schulze et al 2000; Sims et al 2008; Morgan et al 2010; Polley et al 2010)
Grasslands represent more than 40% of the global landscape, accrue and store large amounts of soil C, and are influenced by precipitation and grazing intensity (Sala et al 1988; Amthor et al 1998; Flanagan et al 2002; Fay et al 2008)
It is vital that we develop a better understanding of the patterns and magnitudes of CO2 exchange between grasslands and the atmosphere and how those exchanges may be influenced by grazing regimes and precipitation
Summary
Understanding the factors controlling the exchange of CO2 between the biosphere and the atmosphere and the sequestration of carbon (C) by landscapes has become a central concern for science, policy, and management (Follett et al 2000; Kaiser 2000; Schulze et al 2000; Sims et al 2008; Morgan et al 2010; Polley et al 2010). Net CO2 exchange and C sequestration is the net effect of C fixation by plants, heterotrophic and autotrophic respiration, and soil C storage All of these processes are potentially sensitive to land use such as grazing intensity (Schuman et al 1999; LeCain et al 2000; Welker et al 2004a; Ingram et al 2008), abiotic factors such as precipitation or temperature (Briggs and Knapp 1995; Chimner and Welker 2005; Bradford et al 2006; Chimner et al 2010; Polley et al 2010), and soil nitrogen (N) processes (Schulze et al 2000). Our understanding of how these factors directly and indirectly affect the magnitudes and patterns of CO2 exchange on rangelands is still rudimentary (Kelly et al 2002; Smith et al 2002; Hunt et al 2004) and requires quantification if we are to develop realistic and effective C management options on rangelands (Allen-Diaz 1996; Kaiser 2000; Wofsy and Harriss 2002; Ingram et al 2008)
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