Tradeoffs in microbial carbon allocation may mediate soil carbon storage in future climates

Climate-induced changes in soil microbial physiology impact ecosystem carbon (C) storage and alter the rate of CO2 flux from soils to the atmosphere (Allison et al., 2010). The direction and magnitude of these microbial feedbacks depend on changes in saprotrophic bacterial and fungal C allocation in response to altered temperature, precipitation, and nutrient availability. Soil microbes may differentially allocate C in changing environments by altering processes such as enzyme production, C use efficiency (CUE), or biomass stoichiometry (Figure 1). However, because these mechanisms may operate simultaneously and interact, microbial physiological feedbacks on soil C storage are difficult to predict. For example, initial increases in microbial CUE or biomass C:N may be counteracted by increases in enzyme production to acquire limiting organic nutrients. Few studies have standardized microbial process rates, such as extracellular enzyme production or respiration, to the size of the microbial biomass. Examining process rates alone may obscure the microbial physiological mechanisms that underlie climate-induced changes in soil C cycling, leading to contradictory patterns among different studies. For instance, in a large-scale survey of soil protease activities from climate manipulations, drier and warmer conditions resulted in lower extracellular enzyme activities (EEA) compared to ambient conditions (Brzostek et al., 2012). In contrast, drier soils have also been found to stabilize extracellular enzymes in water films, reducing enzyme turnover rates and increasing potential activities (Lawrence et al., 2009; German et al., 2012).