Separate Drivers for Microbial Carbon Mineralization and Physical Protection of Carbon

While the effects of temperature and moisture on soil microbial activity are relatively well-understood, and it is well-recognized that microbial byproducts are a critical source of soil C, our understanding of how temperature and moisture affect physical protection of soil C is lacking. We performed a 6-month incubation of soil and plant litter under varying temperature and moisture. Using 13C-depleted plant litter, we used stable isotope partitioning to trace plant litter C into various aggregate fractions after 30, 50, and 60% of plant C had been respired. In addition, we evaluated microbial biomass C, enzyme activity and bacterial community composition using 16S rRNA genes. While warmer temperatures increased C mineralization rate and enzyme activity and decreased microbial biomass, soil aggregation was enhanced under drier conditions irrespective of temperature. Bacterial community composition shifted over time and with temperature and moisture, and a subset of the bacterial community was associated with the drier conditions that promoted aggregation. These results indicate that physical protection of C in aggregates is correlated to changes in moisture regime while microbial C mineralization is more responsive to temperature. Predictions of how management and climate will affect soil C storage should incorporate separate responses to temperature and moisture for aggregate and microbial C pools.