The role of microbial dispersal in overcoming factors that constrain soil respiration response to climate change

The largest remaining uncertainty in the terrestrial carbon cycle is whether soil will become a carbon source or sink in future decades. Better ecological understanding of the soil microbial communities that regulate soil respiration and carbon decomposition can improve parameterization of global carbon models. Here we investigate whether climatic legacy effects, which can constrain microbial function in the face of environmental change, can be overcome by effective dispersal. To address this gap in knowledge, we performed a lab microcosm experiment using 15 combinations of soil communities to mimic potential dispersal outcomes and maintained these under wet or dry conditions. Soils were taken from three sites at the drier western and three sites at the wetter eastern ends of a precipitation gradient in central Texas with a similar geomorphic profile and known legacies dependent on historical rainfall. Soil origin treatment was created from two western sites, two eastern sites, or a mixture of one western and one eastern site. Dispersal treatment was based on the amount of each soil origin in the mixtures (0:100, 15:85, 50:50, 85:15, 100:0). The mixtures were created by adding live soil inoculum (2.5 g) to autoclaved background soil (22.5 g) comprised of a mixture of soils from all six sites. We assessed whether changes in soil respiration were due to moisture regime or dispersal treatments over 12 weeks. Contemporary soil moisture was the primary driver of respiration, with 878% more respiration in wet vs. dry treatments. Soils that were evenly mixed western communities differed from the other soil dispersal treatments, likely because of intrinsic functional limitations or increased biotic interactions. Legacy effects were substantially weaker here compared to other studies in the same system. We speculated that the resources provided by autoclaved background soils might have disrupted the historical contingency of soil moisture in this system. Community composition data must be used to resolve whether the lack of functional differences were due to microbial communities sharing similar taxa or physiological plasticity between differing communities.