Soil microbial oligotrophs respond to long-term warming in temperate forest soil

Background/Question/Methods

Earth's climate is warming, and if increased temperature accelerates SOM decomposition then carbon (C) stored in soils will transfer to the atmosphere, resulting in a self-reinforcing (positive) feedback to the climate system. Though soil microbes are major drivers of soil C cycling, we lack an understanding of how long-term warming affects them. Ongoing field studies in the Harvard Forest (Petersham, MA) have experienced 5°C above ambient soil temperatures at three replicated sites for 5, 8 and 20 years, which now correspond to three distinct phases of CO₂ emissions observed in the oldest field study site. Previous studies have suggested warming-induced decreasing abundance of fungi and adaptation of microbial communities to more efficiently decompose recalcitrant soil C in these sites. To analyze changing diversity among bacterial communities, we sequenced the 16S ribosomal RNA genes using Illumina technology and performed quantitative PCR to understand changing absolute abundances of groups. These results are then compared to metagenomic analyses of the long-term warming plots to determine how warming might affect microbial functional potential and terrestrial climate feedbacks in a warmer world.

Results/Conclusions

Only organic soils exposed to 20 years of warming showed changes in bacterial community structure and diversity and decreased abundance of fungi. We applied a copy number correction to the data and found no effect on the warming trend, suggesting that taxa with low or single copies of the 16S ribosomal RNA gene were responsible for driving the bacterial warming effect in the long-term warmed, organic soils. These taxa are considered to be metabolic specialists, or K-selected species. The dominant taxa abundant at 0.1% or greater represented 0.3% of the richness but nearly 50% of the observations (sequences). Communities were also strongly uneven, with individual members of the Actinobacteria, Acidobacteria and Alpha-proteobacteria responding strongly to warming. Long-term warming appears to be creating more niche space in organic layer soils, which is manifested as increased bacterial alpha diversity, shifting beta diversity. Metagenomic data suggests that warming is associated with decreased carbon use efficiency, increased genes associated with recalcitrant C decomposition, and altered N and P cycling. Because a warmer world will induce changes in plant communities, changes in soil bacterial and fungal potential functions will affect plant-microbial interactions and terrestrial feedbacks to the climate system. Further metatranscriptomic studies promise to illuminate the active C cycling populations and pathways in these communities that are responding to warming and associated changes in microbially available carbon.