This doctoral dissertation improvement project will study how climate warming affects the movement of carbon dioxide from soils in boreal forests to the atmosphere. In particular, it will test whether warming affects the decomposition of decay resistant organic matter more than that of easily decomposed organic matter. This work will provide a more complete understanding of the processes by which organic carbon is sequestered in or released from boreal forest soils.
Increased movement of carbon dioxide from terrestrial soils to the atmosphere in response to climate warming represents a potentially important feedback to climate change. Understanding the fate of soil carbon and how it is affected by warming temperatures is important for improving models of greenhouse gas inputs to the atmosphere.
The decomposition of soil carbon is expected to increase with climate warming, as the reaction rates of soil microbial enzymes that degrade carbon increase with temperature. Moreover, because soils store large amounts of carbon, approximately twice as much carbon as the atmosphere, even a relatively small increase in decomposition could release enough CO2 to contribute substantially to enhancing the greenhouse effect in a warmer world. This research award examines two mechanisms that might control the response of soil carbon decomposition over long time scales. First, soil microbial communities are expected to adjust to the warmer temperatures, reducing the response of soil carbon decomposition to warming. Secondly, the decomposition of more recalcitrant (less easily decomposed) soil carbon is expected to respond more to temperature than the decomposition of labile (more easily decomposed) soil carbon. Because recalcitrant soil carbon comprises up to 75% of total soil carbon, even a small increase the decomposition of these pools could lead to a large release of carbon dioxide to the atmosphere. This study aimed to improve upon ongoing dissertation research that uses an open-air warming experiment established at two boreal forests sites in Minnesota, and has found that boreal forests soils could be a signficant source of CO2 in a warmer world. In order to examine soil microbial community adjustment to warming, we examined the effect of experimental warming on the temperature sensitivity of extracellular enzyme activity in these southern boreal forest soils. Enzymes important in carbon decomposition were chosen for analyses. In order to examine the temperature sensitivity of labile versus recalcitrant soil carbon decomposition, we measured the effect of warming on the radiocarbon age of soil CO2 efflux (recalcitrant carbon will have a older radiocarbon age). In preliminary analyses we found that experimental warming had a significant effect on the temperature sensitivity of enzyme activity, indicating that warming alters carbon decomposition by the microbial community. The effect of warming, however, differed based on the enzyme measured. In addition, we found that the warming treatment increased the radiocarbon age of soil CO2 efflux (marginally), indicating that recalcitrant soil organic matter appears to be more sensitive to warming. Together these results suggest that recalcitrant carbon might be an important source of CO2 to the atmosphere under a warmer climate, but that changes in the soil microbial community need to be considered in predictions of the potential feedback of soil carbon decomposition to warming. This research achieved broader impacts through contributions to the understanding of the mechanisms that underlie the response of terrestrial ecosystems to climate change. In particular, this study provides important process-level information of the mechanisms in influencing the dynamics of the response of soil respiration to warming. Such information is critical to improving models of future climate change. Moreover, this award provided important training for a graduate student (William Eddy), helped to advance relatively new techniques (radiocarbon and soil extracellular enzyme kinetics) in the fields of ecosystem ecology and biogeochemistry, and provided methodical resources for future researchers at the University of Minnesota.