Reduced Microbial Resilience after a 17-Year Climate Gradient Transplant Experiment

In 1994, a reciprocal soil transplant experiment was initiated between two elevations (310 m, warmer and drier, and 844 m, cooler and wetter) on Rattlesnake Mountain in southeastern Washington, USA. The original experiment sought to detect whether the microbial and biochemical dynamics developed under cool, moist conditions would be destabilized under hot, dry conditions. In March 2012 we resampled the original transplanted soils, control cores transplanted in situ, and native soils from each elevation, to study longer-term changes in microbial community composition, soil C and N dynamics, and soil physical structure. These resampled cores were randomly assigned to climate-control chambers simulating the diurnal conditions at either the lower or upper sites. We monitored respiration over 100 days, and couple these data with biogeochemical analyses conducted at time-zero, and at the end of the experiment, to examine the consequences of long-term climate change on microbial C cycling under new environmental stresses. All soil types incubated respired more C while in the simulated hotter, drier climate compared with the cooler, moister condition, except for those that had been transplanted from the lower elevation to the upper elevation in 1994, which actually respired less when returned to this, their original climate. These soils also exhibited almost no temperature sensitivity (Q10=1.07, 13-33 °C). Soils incubated in the cooler, moister chamber had greater N-acetylglucosaminidase and β-glucosidase potentials, suggesting that while loss of C as carbon dioxide respiration is reduced under these conditions, internal cycling of C may be enhanced. Ribosomal intergenic spacer analysis was used to fingerprint the bacterial community of all of these soils to identify possible high-level shifts in community composition in the 0-5, 5-10, and deeper depths in these soils. These results suggest that climate change has significantly altered the C dynamics in these soils, and that even after 17 years of adaptation, the soil microbial communities have not recovered to a condition similar to their new environment. These soils also appear to have lost some of their resilience to subsequent climate perturbations, raising more general questions of how current climate change will affect the capacity of soils to buffer against future, different perturbations.