Microbial Determinants of Soil Carbon Response to Climate Warming.
Teresa Balser, Univ of Wisconsin-Madison, 1525 Observatory Dr., Dept of Soil Science, Madison, WI 53706
The issue of whether soil will act as a net carbon source or sink in response to climate warming is currently a matter of intense interest in global change policy and research communities Because global soil organic carbon concentration is greater than twice that of the atmosphere, even small changes in flux can have a significant impact on atmospheric CO2. In particular, the sensitivity of recalcitrant (‘older') carbon to temperature is a critical parameter for predicting the role of soil as a feedback agent in climate warming. While there is a reasonable level of agreement that younger (labile) C will generally display a predictable pattern of response to temperature (e.g. it has a Q10 of approximately 2.4 and increasing rate of mass loss as temperature rises), the dynamics of older carbon largely remain a mystery. The sensitivity of recalcitrant C to rising temperature has been predicted to increase, or remain invariant. This variability is likely due to the web of interacting factors that influence carbon stability in soil. As litter transforms to ‘soil organic matter', and then ages to stable (humic) forms it becomes increasingly chemically altered and associated with soil minerals. Further, litter of differing chemical quality will vary in its transformation, as will the availability of organisms to degrade it. As a result, temperature sensitivity of older carbon is not a simple function of enzyme response, but instead is the product of a complex suite of interactions among the varying temperature responses of competing processes such as activation energy, altered substrate diffusion, mineral sorption or occlusion, historical carbon input and land use, and acclimation of the decomposer community. To date, studies including or focusing on more than one of these factors at a time are rare, and consequently, results of existing studies often appear idiosyncratic or surprising. If we desire a predictive understanding of the potential of soil to feedback to climate warming, it will be necessary to increase our understanding of the factors controlling the temperature sensitivity of recalcitrant carbon, and advance our conceptual as well as quantitative models beyond the biologically simplistic assumptions embedded in current temperature sensitivity approximations. And although soil mineralogy does play an important role in control, and dissolved organic species are an important loss vector, ultimately an understanding of carbon cycling in soil must be predicated upon an understanding of microbial ecophysiological responses (adaptive responses to environmental change). It is no longer sufficient to assume that microorganisms are passive catalysts whose enzymatic abilities and behaviors are environmentally determined (i.e. it is no longer sufficient to assume that “everything is everywhere and the environment selects”). We now have both the awareness and the tools necessary to question this assumption and move toward a more sophisticated treatment of microbial behavior and temperature sensitivity in our ecosystem and global models. In this talk, we synthesize a set of experiments investigating the compositional and functional response of soil microorganisms to temperature, and present a conceptual model for microbial control over recalcitrant soil carbon temperature sensitivity. We propose that the microbial community will respond in two primary ways to increased temperature: 1) in the short-term, activity will change as extant enzymes are affected, and labile carbon is depleted; and 2) in the longer term, there will be a change community composition, potentially resulting in changing carbon use as microbial degradation ‘potential' shifts. We investigate the consequences of these two types of change for loss of older carbon from soil. We ask first, what is the impact of short-term changes in microbial behavior/activity? There is a growing body of work indicating that the size and quality of the carbon pool accessed by microorganisms changes as temperature changes. Models that define decomposition or respiration rate coefficients (k) as a function of temperature assume constant substrate pool size, and uniform substrate preference. However, the pool size of carbon substrate available to the microbial community can vary substantially with temperature. In addition, several researchers have found that not only does the size of the carbon pool accessed change with temperature, but microbial use of specific substrates also changes. We present results from longer-term studies of temperature change and carbon utilization to assess change in microbial community composition under altered climatic regimes.