Soil organic carbon (SOC) stocks represent a large component of the global carbon cycle that is sensitive to warming. However, model projections of SOC changes are still highly uncertain. New models of SOC cycling are explicitly incorporating the role of microbial dynamics in decomposition along with the temperature-dependence of microbial processes. However, trophic interactions such as predation of microbial decomposers by other organisms, which can be highly temperature-dependent, have not yet been incorporated into quantitative soil carbon models. Here, we incorporated a microbial predator into two models of SOC cycling to determine how predation would affect soil community population dynamics and biogeochemistry. First, we modified a tri-trophic standard population ecology model and to investigate how the presence of predators affected SOC stocks and their dependence on substrate input rates. Next, we incorporated the predator trophic level into a global-scale, predictive soil carbon model to determine how trophic interactions would affect temperature sensitivity of SOC stocks across climate gradients.
Results/Conclusions
In population-based model simulations, the presence of predators increased SOC stock dependence on carbon input rates. The response of SOC stocks to warming depended on predator temperature sensitivity, with temperature-insensitive predators causing SOC stocks to increase under warming due to enhanced top-down control on microbial biomass. In global model simulations, the presence of predators increased soil carbon stocks, particularly in warm climates. Predator impacts in colder regions depended on predator temperature sensitivity, with temperature-insensitive predators increasing high-latitude SOC stocks and temperature-sensitive predators not having a significant impact on high latitude SOC stocks. Under warming, predator activity decreased SOC losses, but the strength of this effect was highly dependent on the temperature sensitivity of the predators. Our results suggest that the presence of higher trophic levels can reduce the sensitivity of SOC to warming, but that temperature sensitivity across trophic levels may be a key determinant of warming response. Furthermore, these results suggest that measurements of microbial physiology may not be directly applicable to soil decomposition models unless they are understood in the context of other trophic interactions.