The sustainable intensification of agriculture relies on the efficient use of ecosystem services, particularly those provided by the microbial community. Managing for these ecosystem services can improve plant yields and reduce off-site impacts. For instance, increasing plant diversity is linked to positive effects on yield, and these beneficial effects are often mediated by the microbial community and the nutrient transformations it carries out. My dissertation has aimed to elucidate the mechanisms by which plant diversity improves agricultural production. In particular, I have focused on how changes to the amount and diversity of carbon (C) inputs affects soil microorganisms involved in the nitrogen (N) cycle. My work spans multiple scales of observation: from a global meta-analysis to mechanistic studies utilizing denitrification as a model system. In a global meta-analysis, I found that increasing plant diversity through intercropping yields a net increase in extracellular enzyme activity. This effect varied by plant species and soil type suggesting that increases in the quality of nutrient inputs mediates these positive effects on microbial activity. Then, I looked at how intercropping cover crops into corn affects soil nutrient pools and microbial activities in a field experiment. No effect of interseeding cover crops into corn was found on soil nutrient pools or microbial activities. However, by analyzing differences in relationships between nutrient pools and microbial activities at two locations throughout Michigan, I was able to describe how the availability of dissolved organic C (DOC) drives differences in microbial N-cycling processes. I then investigated how C availability drives activity in microbial hotspots within the soil by comparing differences in denitrification potential in bulk soil versus the rhizospheres of corn and interseeded cover crops. Here, I found that denitrification rates were increased in the rhizospheres of all plant types, and this effect varied depending on the species of plant. I was able to further differentiate the impact of DOC and microbial biomass C on the rhizosphere effect and found that C availability was the primary driver of differences in denitrification rates between rhizospheres. Since plants provide many different forms of C to soil microbes, it is important to understand how the chemistry of C inputs affects microbial activity. I used a series of C-substrate additions to determine how C chemistry affects denitrifiers. I found that amino acids and organic acids tended to stimulate the most nitrous oxide (N2O) production and reduction. Although management and site affected overall rates of denitrification, C-utilization patterns of microbes were mostly similar between locations. To identify the mechanisms responsible for these effects, I performed a final experiment to track how denitrifiers utilized different C compounds. The C substrates that stimulated the most complete reduce of N2O also were utilized with the lowest C-use efficiency (CUE). This suggests possible trade-offs between N2O reduction and CUE, with important implications for how to manage microbial communities. Overall, my work demonstrates that land management can impact microbial community activity by influencing the identity of soil C inputs. While the importance of increasing soil C inputs has been known, this dissertation supports the notion that the chemical identity of C inputs can exert significant controls on microbial activity. Moreover, by comparing microbial traits I highlight the importance of trade-offs in how microbially mediated C- and N cycling are coupled.
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