Volatile and Dissolved Organic Carbon Sources Have Distinct Effects on Microbial Activity, Nitrogen Content, and Bacterial Communities in Soil.

Variation in microbial use of soil carbon compounds is a major driver of biogeochemical processes and microbial community composition. Available carbon substrates in soil include both low molecular weight-dissolved organic carbon (LMW-DOC) and volatile organic compounds (VOCs). To compare the effects of LMW-DOC and VOCs on soil chemistry and microbial communities under different moisture regimes, we performed a microcosm experiment with five levels of soil water content (ranging from 25 to 70% water-holding capacity) and five levels of carbon amendment: a no carbon control, two dissolved compounds (glucose and oxalate), and two volatile compounds (methanol and α-pinene). Microbial activity was measured throughout as soil respiration; at the end of the experiment, we measured extractable soil organic carbon and total extractable nitrogen and characterized prokaryotic communities using amplicon sequencing. All C amendments increased microbial activity, and all except oxalate decreased total extractable nitrogen. Likewise, individual phyla responded to specific C amendments-e.g., Proteobacteria increased under addition of glucose, and both VOCs. Further, we observed an interaction between moisture and C amendment, where both VOC treatments had higher microbial activity than LMW-DOC treatments and controls at low moisture. Across moisture and C treatments, we identified that Chloroflexi, Nitrospirae, Proteobacteria, and Verrucomicrobia were strong predictors of microbial activity, while Actinobacteria, Bacteroidetes, and Thaumarcheota strongly predicted soil extractable nitrogen. These results indicate that the type of labile C source available to soil prokaryotes can influence both microbial diversity and ecosystem function and that VOCs may drive microbial functions and composition under low moisture conditions.

Global patterns in the growth potential of soil bacterial communities.

Despite the growing catalogue of studies detailing the taxonomic and functional composition of soil bacterial communities, the life history traits of those communities remain largely unknown. This study analyzes a global dataset of soil metagenomes to explore environmental drivers of growth potential, a fundamental aspect of bacterial life history. We find that growth potential, estimated from codon usage statistics, was highest in forested biomes and lowest in arid latitudes. This indicates that bacterial productivity generally reflects ecosystem productivity globally. Accordingly, the strongest environmental predictors of growth potential were productivity indicators, such as distance to the equator, and soil properties that vary along productivity gradients, such as pH and carbon to nitrogen ratios. We also observe that growth potential was negatively correlated with the relative abundances of genes involved in carbohydrate metabolism, demonstrating tradeoffs between growth and resource acquisition in soil bacteria. Overall, we identify macroecological patterns in bacterial growth potential and link growth rates to soil carbon cycling.

Volatile organic compounds from leaf litter decomposition alter soil microbial communities and carbon dynamics.

Investigations into the transfer of carbon from plant litter to underlying soil horizons have primarily focused on the leaching of soluble carbon from litter belowground or the mixing of litter directly into soil. However, previous work has largely ignored the role of volatile organic compounds (VOCs) released during litter decomposition. Unlike most leaf carbon, these litter-derived VOCs are able to diffuse directly into the soil matrix. Here, we used a 99-d microcosm experiment to track VOCs produced during microbial decomposition of 13 C-labeled leaf litter into soil carbon fractions where the decomposing litters were only sharing headspace with the soil samples, thus preventing direct contact and aqueous movement of litter carbon. We also determined the effects of these litter-derived VOCs on soil microbial community structure. We demonstrated that the litter VOCs contributed to all measured soil carbon pools. Specifically, VOC-derived carbon accounted for 2.0, 0.61, 0.18, and 0.08% of carbon in the microbial biomass, dissolved organic matter, mineral-associated organic matter, and particulate organic matter pools, respectively. We also show that litter-derived VOCs can affect soil bacterial and fungal community diversity and composition. These findings highlight the importance of an underappreciated pathway where VOCs alter soil microbial communities and carbon dynamics.

Soil Bacterial and Fungal Communities Exhibit Distinct Long-Term Responses to Disturbance in Temperate Forests.

In Appalachian ecosystems, forest disturbance has long-term effects on microbially driven biogeochemical processes such as nitrogen (N) cycling. However, little is known regarding long-term responses of forest soil microbial communities to disturbance in the region. We used 16S and ITS sequencing to characterize soil bacterial (16S) and fungal (ITS) communities across forested watersheds with a range of past disturbance regimes and adjacent reference forests at the Coweeta Hydrologic Laboratory in the Appalachian mountains of North Carolina. Bacterial communities in previously disturbed forests exhibited consistent responses, including increased alpha diversity and increased abundance of copiotrophic (e.g., Proteobacteria) and N-cycling (e.g., Nitrospirae) bacterial phyla. Fungal community composition also showed disturbance effects, particularly in mycorrhizal taxa. However, disturbance did not affect fungal alpha diversity, and disturbance effects were not consistent at the fungal class level. Co-occurrence networks constructed for bacteria and fungi showed that disturbed communities were characterized by more connected and tightly clustered network topologies, indicating that disturbance alters not only community composition but also potential ecological interactions among taxa. Although bacteria and fungi displayed different long-term responses to forest disturbance, our results demonstrate clear responses of important bacterial and fungal functional groups (e.g., nitrifying bacteria and mycorrhizal fungi), and suggest that both microbial groups play key roles in the long-term alterations to biogeochemical processes observed following forest disturbance in the region.