Trees generally form symbioses with either arbuscular (AM) or ectomycorrhizal (EcM) fungi, where differences in litter qualities and mycorrhizal nutrient acquisition strategies can affect forest soil decomposition and nutrient cycling. However, the degree to which substrate quality or fungal community composition affects microbial functions like enzyme production and microbial carbon use efficiency (CUE) has yet to be disentangled. Many EcM fungi can directly produce extracellular enzymes, a key step in decomposition and nutrient liberation, whereas AM fungi may only indirectly affect decomposition by priming the saprotrophic community. Given these differences, changes in the fungal community along a gradient of EcM basal area should affect how the soil microbes function. Concurrently, increasing substrate quality (defined as a decrease in the carbon to nitrogen ratio (C:N)) should also increase CUE and affect enzyme activities. To test this distinction, we sampled the organic horizon and 10 cm of the mineral horizon in northern hardwood forest soils along a gradient of EcM association. We measured potential activities of five hydrolytic and two oxidative enzymes, and CUE. We also measured the fungal community by extracting DNA from soil samples and calculating both the diversity (species richness) and EcM percent abundance, and the C:N ratio of each sample.
Results/Conclusions
We found that the C:N ratio and EcM percent abundance was positively correlated with percent EcM basal area, allowing us to relate these predictors to soil enzyme activities and CUE. Our results showed that fungal richness or EcM percent abundance were important predictors of microbial CUE, several nutrient-targeting enzymes, and oxidative enzymes, whereas the C:N ratio was important for carbon-targeting and chitin-targeting enzymes. With the exception of carbon-targeting enzymes and peroxidase, all enzyme activities were positively associated with EcM percent abundance, suggesting an important contribution to enzymes that target nutrient-containing molecules. A PCA analysis showed that several nutrient-targeting enzymes had opposite associations with richness relative to microbial CUE, which decreased with richness, indicating potential tradeoffs between enzyme production growth efficiency. In order to infer effects of diversity, we investigated the relationship between species richness, fungal functional guilds, and environmental variables; we found that species richness was negatively associated with EcM percent abundance and positively associated with pH, indicating EcM as a potential mechanism to explain how species richness affects soil microbial functions. Overall, our findings indicate that mycorrhizal association affects soil microbial function more so than substrate quality regarding CUE and nutrient-targeting enzymes.