What controls carbon and nutrient cycling in soil? Microbial growth as the fundamental driver of soil biogeochemistry.

Growth and death of microorganisms is central to the understanding of almost all global element cycles. To grow, heterotrophic microorganisms need to assimilate organic carbon, and to sustain a flux of readily available organic carbon, they need to depolymerize and deconstruct soil organic matter. When soil organisms die, their remnants become part of the soil organic matter. Thus, the processes that lead to both the decomposition and the accumulation of organic matter in terrestrial environments, are driven by the growth, activity and turnover of heterotrophic microbial communities in soil. Yet, little is known about how microbial growth, turnover, and activity is controlled in the current and in a future climate. In this lecture, I will share insights from a range of experiments that aimed at understanding the effects of soil warming, elevated CO_2, and drought, alone or in combination on microbial growth, turnover, and carbon use efficiency. I will draw on examples not only from my own work, but those of others, covering different levels of resolution, from the growth of microbial communities to that of individual bacterial taxa. I will argue that activity, growth, and turnover of microorganisms are the fundamental units of biogeochemical functioning in soils and that we need to move beyond the commonly reported metrices in soil ecology, and even beyond measuring rates of decomposition and mineralization themselves, if we are to understand the effects of climate changes on soil processes.