Modeling soil microbial dynamics in carbon and nitrogen cycling

Goal To determine the relative importance of different mechanisms that influence carbon loss from soil under variable soil-moisture conditions. The soil contains more carbon than the atmosphere and plants combined (Scurlock and Hall 1998). This carbon is important for maintaining soil fertility, controlling erosion, and the long-term storage of carbon that could be released into the atmosphere. As greenhouse-gas concentration in our atmosphere increases, the rate at which this carbon is released into the atmosphere as carbon dioxide (decomposition) has been given new importance. Specifically, if predicted shifts in climate increase carbon release, positive feedbacks with soil-climate-atmosphere could emerge and alter the trajectory of climate change (Singh et al. 2010). These scenarios call for clarification of relationships between new climates and soil processes, and for a better understanding of the mechanisms that control these patterns. This information could help decrease uncertainty in current ecosystem models and better predict carbon-flux and climate-change trajectories and adaptive strategies. One particular circumstance in which overall carbon-dioxide (CO 2) flux has been shown to deviate from rates predicted using empirical relationships arises in response to drying and rewetting cycles (Yuste et al. 2007; Borken and Matzner 2009). The loss of CO 2 from soil may be higher when soils are exposed to fluctuation in soil moisture than when soil moisture is more constant (Clein and Schimel 1994; Miller et al. 2005; Jarvis et al. 2007). Understanding this phenomena, called the " Birch effect " (Birch 1958) is especially important because future climates are expected to be characterized by more variable rainfall (IPCC 2007), including longer drought and larger rewetting events. Identifying the mechanisms that control CO 2 dynamics under drying and rewetting could improve estimates of global carbon budgets and reduce error in predictions of soil CO 2 flux under future climates. Although the large pulse of CO 2 that occurs after rewetting a dry soil has long been described , the mechanisms that underlie the Birch effect are still unclear. Most discussions divide mechanisms into those controlled by physical factors and those influenced by the biology of the soil microbial community (Xiang et al. 2008; Navarro-García et al. 2012; Manzoni et al. 2012). Physically, moisture alters carbon and nitrogen diffusion rates, which can prevent microbial access to substrate. In addition, a large rain event can break up large soil particles, which contain stored, but not accessible, carbon. Precipitation changes are also likely to alter …