Long-term warming affects specific lignin-associated soil bacteria

Background/Question/Methods

Soils store more carbon than the vegetation and atmosphere combined, yet we still know relatively little about how the soil microbes mediating the formation and loss of organic matter in the pedosphere will respond to elevated global temperatures. After 20+ years of experimental 5°C warming in mixed deciduous stands at the Harvard Forest in central Massachusetts, it is clear that purely kinetic or substrate availability effects cannot explain observed changes in soil respiration, and that adaptation of the bacterial community is likely to play a role. Specifically, a previously-observed decrease in the fungal:bacterial ratio and increased bacterial diversity with warming at this site indicate that bacteria may be playing an increasingly important role in recalcitrant carbon decomposition.

Here we set out to more directly evaluate this hypothesis by incubating unamended or lignin-baited Bio-Sep beads, an inert porous medium, at two depths in the organic horizon of these experimental plots, deployed in both heated and control (unheated) plots. We assessed the fungal:bacterial ratio using qPCR, and subsequently identified changes in the bacterial and fungal communities using high-throughput sequencing of the 16S ribosomal RNA gene and TRFLP of the internally transcribed spacer region, respectively.

Results/Conclusions

Warming treatment increased richness, evenness, and diversity of the bacterial community, consistent with whole-soil results from this site. Beta diversity of the bacterial community was driven primarily by bead amendment, with warming treatment playing a secondary role. Lignin-amended beads were enriched in bacterial genera containing known lignin-degrading bacteria when compared to unamended beads, but showed reduced oxidative enzyme activity. We also identified a number of OTUs specifically enriched for or depleted by warming on lignin amended beads, scattered over the phylogenetic tree. Fungal:bacterial ratio was unaffected by warming in lignin-amended beads, but decreased in the absence of lignin due to a loss of fungi.

Although sequencing results indicated a subset of the specifically lignin-enriched bacterial OTUs increased in relative abundance with warming treatment, potential enzyme activity assays failed to support this. It is possible that these bacteria are not primarily decomposing lignin, but rather the lower molecular weight decomposition products produced by fungi. Further physiological assays of highly dominant, heat-labile isolates resulting from this study will help us identify whether the strong warming response of the most abundant lignin-degrading bacteria is biologically meaningful.