Effects of warming on soil microbial communities and feedbacks to nitrogen and carbon fluxes in a mixed temperate forest ecosystem
Soils are the largest repository of organic carbon in the terrestrial biosphere, and climate warming could result in a substantial loss of this carbon to the atmosphere eliciting a positive feedback to climate. The magnitude of this loss depends, in part, on the temperature sensitivity of the soil microbial community to elevated temperatures. Nitrogen deposition is another environmental change stressor that has been documented to alter soil carbon cycling dynamics, often leading to soil carbon accumulation. In many parts of the world, climate warming and atmospheric N deposition are occurring simultaneously and may interact to affect the microbial community and nutrient cycling in unpredictable ways. We have been examining microbial responses to the interactive effects of chronic soil warming and simulated nitrogen deposition for nearly a decade at the Soil Warming x Nitrogen Addition Study at the Harvard Forest Long-Term Ecological Research (LTER) site in Petersham, MA, USA. Parameters measured include soil respiration, N mineralization, extracellular enzyme activities, microbial biomass, substrate utilization, growth rates, efficiency, and community structure.
Soil warming increased soil respiration by 44% over the study period, but the effect of warming was most pronounced in spring and fall. Enhanced soil respiration was concomitant with an increase in decomposition rates and a 30% decline in total organic C stored in the organic horizon. There was a significant reduction in microbial biomass and the microbial utilization of a suite of C substrates which included amino acids, carbohydrates, and carboxylic acids. Heating significantly reduced fungal biomass, with the microbial community shifting towards gram positive bacteria and actinomycetes. Soil metatranscriptomic analysis revealed reduced expression of several genes involved in labile C degradation (e.g. xylanase; hemicellulose degradation), though we observed no changes in overall gene expression of lignin degrading genes (e.g, peroxidases) with soil warming. The efficiency with which soil microorganisms use organic matter was dependent on both temperature and substrate quality, with efficiency declining with increasing temperatures for more recalcitrant substrates. However, the utilization efficiency of a more recalcitrant substrate increased at higher temperatures in soils exposed to almost two decades of warming. Our work suggests that climate warming could alter the decay dynamics of more stable organic matter compounds, thereby having a positive feedback to climate that is attenuated by a shift towards a more efficient microbial community in the longer term.