Plant Vs. Environmental Influences on Soil Respiration in a Mixed-Grass Prairie Under Cattle Grazing: A Path Analysis.

Despite heavy study in past decades, soil respiration and its regulation in agricultural fields are not well understood. This is primarily due to the fact that soil respiration is influenced by multiple factors operating at different time/spatial scales in the soil-plant system. Novel methods combining field experimentation and statistical testing are needed for improved understanding. The aim of this research was to clarify to what extent soil respiration in a grazing rangeland was influenced by plant growth and by environmental factors. The study was conducted in a mixed-grass prairie under long-term moderate and heavy grazing by beef cattle. Within each grazing pasture, a land area of 15 x 15 m2 in dimension was selected, a rain-out-shelter 3x6 m2 in dimension was used to provide drought treatment and soil trenching was used to exclude live plant contribution to soil respiration. Sixteen permanent soil collars were installed in the experimental plots and soil respiration was measured weekly during the growing seasons from 2007 to 2011. Soil respiration rates were linked to plant variable (leaf area index) and environmental variables (soil temperature and moisture at 7.5 cm depth) through a path model and the bootstrap resampling was used to test the plant vs. environmental controls on soil respiration. Results show that physiological control of soil respiration was more prominent under moderate than under heavy grazing only in the natural condition (p=0.0005) but not under the drought condition (p=0.316). Our data also show that regardless whether or not the plots were trenched, the effect of soil temperature on soil respiration always dominated that of soil water under the natural condition, but the reverse was true under drought (p<0.008). These results suggest that soil water status strongly dictates which factor(s), physiological or environmental, dominate the regulation of soil carbon efflux in the mixed-grass prairie ecosystem.