Abstract
Terrestrial ecosystems are an important carbon store, and this carbon is vulnerable to microbial degradation with climate warming. After 30 years of experimental warming, carbon stocks in a temperate mixed deciduous forest were observed to be reduced by 30% in the heated plots relative to the controls. In addition, soil respiration was seasonal, as was the warming treatment effect. We therefore hypothesized that long-term warming will have higher expressions of genes related to carbohydrate and lipid metabolism due to increased utilization of recalcitrant carbon pools compared to controls. Because of the seasonal effect of soil respiration and the warming treatment, we further hypothesized that these patterns will be seasonal. We used RNA sequencing to show how the microbial community responds to long-term warming (~30 years) in Harvard Forest, MA. Total RNA was extracted from mineral and organic soil types from two treatment plots (+5°C heated and ambient control), at two time points (June and October) and sequenced using Illumina NextSeq technology. Treatment had a larger effect size on KEGG annotated transcripts than on CAZymes, while soil types more strongly affected CAZymes than KEGG annotated transcripts, though effect sizes overall were small. Although, warming showed a small effect on overall CAZymes expression, several carbohydrate-associated enzymes showed increased expression in heated soils (~68% of all differentially expressed transcripts). Further, exploratory analysis using an unconstrained method showed increased abundances of enzymes related to polysaccharide and lipid metabolism and decomposition in heated soils. Compared to long-term warming, we detected a relatively small effect of seasonal variation on community gene expression. Together, these results indicate that the higher carbohydrate degrading potential of bacteria in heated plots can possibly accelerate a self-reinforcing carbon cycle-temperature feedback in a warming climate.
Highlights
Terrestrial ecosystems play a vital role in the global carbon (C) cycle as soils are estimated to store twice as much C as the atmosphere and all vegetation combined (Schimel, 1995; Scharlemann et al, 2014)
We found no significant differences in the yield of putative mRNA sequences between our treatments, time points (T2 vs. T6), and soil types (Organic vs. Mineral; See Supplementary Data: Appendix 2A)
We did a comparative meta-transcriptome analysis to understand the consequences of long-term warming on terrestrial soil bacterial communities and the related effects on soil C dynamics
Summary
Terrestrial ecosystems play a vital role in the global carbon (C) cycle as soils are estimated to store twice as much C as the atmosphere and all vegetation combined (Schimel, 1995; Scharlemann et al, 2014). Soil C-dynamics is tightly coupled to temperature changes making C sinks vulnerable to global warming. Studies have shown that warming-induced increases in soil respiration have the potential to convert soil from a C-sink to a C-source triggering a self-reinforcing C cycle-temperature feedback (Friedlingstein et al, 2006; Allison and Treseder, 2011; Tucker et al, 2013). We do not yet understand the microbial mechanisms driving warming-induced soil C losses well enough to predict these dynamics in a warming climate
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