Influence of grazing and precipitation on ecosystem carbon cycling in a mixed-grass prairie

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.