Climate Change, Microbes, and Soil Carbon Cycling

Microbial responses to climate change will partly control the balance of soil carbon storage and loss under future temperature and precipitation conditions. We propose four classes of response mechanisms that can allow for a more general understanding of microbial climate responses. We further explore how a subset of these mechanisms results in microbial responses to climate change using simulation modelling. Specifically, we incorporate soil moisture sensitivity into two current enzyme-driven models of soil carbon cycling and find that moisture has large effects on predictions for soil carbon and microbial pools. Empirical efforts to distinguish among response mechanisms will facilitate our ability to further develop models with improved accuracy. Introduction There is twice as much carbon in soils as in the atmosphere ( Jenkinson et al., 1991), making below-ground responses to climate change an important aspect of ecosystem responses and feedbacks to climate. Nevertheless, below-ground responses to climate change remain a large source of uncertainty (Solomon et al., 2007), such that earth system models poorly predict current soil carbon pools (Todd-Brown et al., 2013). This is probably due, in part, to historical assumptions of purely abiotic controls of soil carbon cycling and the lack of a strong mechanistic framework for how soil microbes respond to environmental change and the resulting impacts on the fate of soil carbon (Chapin III et al., 2009; Ogle et al., 2010). Soil respiration is the main pathway for the transfer of carbon from terrestrial to atmospheric pools (Schlesinger and Andrews, 2000). Soil microbes may also make a larger contribution to the building of soil organic carbon than previously thought (Kindler et al., 2006; Liang and Balser, 2008, 2011; Potthoff et al., 2008). For example, mycorrhizal fungi can be the dominant pathway through which carbon from plants enters the soil pool, with hyphal turnover representing ~60% of soil organic matter inputs and the remaining ~40% due to fine root turnover and leaf litter (Godbold et al., 2006). Furthermore, the type of mycorrhizal fungus can determine soil carbon: Averill et al. (2014) found that ecosystems dominated by plants colonized by ectomycorrhizal fungi stored 70% more soil carbon per unit nitrogen than ecosystems dominated by plants associated with arbuscular mycorrhizal fungi. Thus, the effects of climate change on the activity and physiology of the soil microbes will partly determine what proportion of annual soil carbon input is respired versus stored in the longterm reservoir of soil organic carbon (Chapin III et al., 2002). Shifts in microbial community composition, abundance and function have been observed in climate change experiments manipulating temperature, precipitation, carbon dioxide and their interactions (e.g. Castro et al., 2010; Cheng et al., 2012; Harper et al., 2005; Hawkes et al., 2011; Horz et al., 2004, 2005; Lindberg et al., 2002; Liu et al., 2009; Staddon et al., 2003; Zogg et al., 1997). Although results appear to be site specific, some broader patterns can be gleaned from metaanalyses. Based on 32 experimental temperature manipulations, warming increased soil respiration by 20% and net nitrogen mineralization by 46% (Rustad et al., 2001). Blankinship et al. (2011) analysed 75 experimental climate studies and found