OOS 16-6
Microbial responses to chronic soil warming in a temperate forest
Current carbon-climate models predict increased soil organic matter decomposition as global climate warms, with higher than normal soil carbon dioxide fluxes to the atmosphere eliciting a positive feedback to climate. However, results from several field studies demonstrate that although soil respiration is initially stimulated by warming, this effect often diminishes over time, with elevated soil respiration in chronically warmed soils returning to ambient levels within a few years. Microbial decomposition of soil organic matter is responsible for as much as half or more of the carbon dioxide released from soils, and so a thorough understanding of how soil microorganisms respond to temperature is needed to accurately predict how climate warming may alter soil carbon fluxes. We have been examining microbial responses to chronic soil warming for over a decade at the Harvard Forest Long-Term Ecological Research (LTER) site where soils have been continuously warmed to 5°C above ambient. This talk will synthesize our current understanding of how the microbial community responds to an altered temperature regime.
Results/Conclusions
Soil warming increased soil respiration by 44%, with the effect of warming being 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 forest floor (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. There is some evidence that cord-forming basidiomycete fungi grow faster under warmer conditions and out-compete other litter-dwelling fungi, reducing diversity. 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.