Soil microbial communities are exposed to a variety of carbon (C) compounds which affect biogeochemical processes, and community composition. Current research has put considerable focus on dissolved organic C (DOC) from root exudates and leaf litter leachates which are an important carbon source for soil microbes. Recently, volatile organic compounds (VOCs) have emerged as another important C source in soil. Like DOC, VOCs can affect microbial community composition, and soil biogeochemistry. Unlike DOC, VOCs can diffuse through air filled pore space, suggesting that VOCs may be important C sources in the bulk soil where exudates and leachates are less abundant, and dry soils that have minimal water connectivity. In order to compare the effects of DOC and VOCs on soil chemistry and microbial communities under different moisture regimes, we performed a 28-day microcosm experiment with five levels of moisture (25%, 35%, 45, 60%, and 70% of water holding capacity), and five levels of C amendment: a no carbon control, two dissolved compounds (glucose, and oxalic acid), and two volatile compounds (methanol, and α-pinene). At the end of the experiment we measured soil DOC, microbial biomass C, NO3-, and NH4+, and characterized the microbial community using marker gene sequencing.
Results/Conclusions
We used NMDS to visualize bacterial communities with Bray–Curtis distances, and found statistically significant separation of communities based on an interaction between soil moisture and carbon amendment (PERMANOVA: F16 = 1.41, P < 0.001). NMDS axis 1 was negatively correlated with soil NH4+, and positively correlated with NO3-. Both soil moisture and C amendment affected the abundance of individual taxa. Notably, methanol amendment resulted in greater relative abundance of Proteobacteria at all moisture levels (12-23%), driven largely by an increase in the relative abundance of the family Methylobacteriaceae. Additionally, the relative abundance of the nitrogen cycling phylum Thaumarchaeota decreased by 20% under methanol amendment (averaged across all moisture levels), while the phylum Nitrospirae decreased by 34%, under methanol amendment at 70% water holding capacity but was largely unaffected at lower moisture levels. This study suggests that VOCs may have an equivalent or greater effect on microbial communities than DOC. Since VOCs are likely more transient in nature than DOC - i.e. they are present during decomposition of litter but can rapidly diffuse through the soil matrix – it is possible that VOCs create hot spots and hot moments which lead to blooms or inhibition of specific taxa.