Trade or scavenge? Miscanthus alters its nitrogen acquisition strategy in relation to soil properties

Miscanthus x giganteus (Miscanthus) is a robust bioenergy crop, able to withstand the harsh conditions found on many anthropogenically disturbed lands. Mounting evidence suggests that this robustness is related to the ability of Miscanthus to partner with soil microorganisms to gain nutrients. Of particular interest is how Miscanthus may alter microbially-mediated nitrogen (N) cycling (e.g., N-fixation and mineralization) resulting in efficient N acquisition across a range of soil abiotic conditions. However, a mechanistic understanding of how Miscanthus responds to and influences soil nitrogen cycling under the variable soil properties that emerge from land disturbance is lacking. Further, it remains unclear whether these plant-growth promoting microbial functions can be bolstered by directly managing microbial community composition (i.e., biofertilization). To address these knowledge gaps, we performed a greenhouse experiment with Miscanthus grown in soil from two sites with different disturbance histories and soil properties (e.g., soil texture, inorganic nitrogen, and C/N). We also tested whether a commercially available biofertilizer or a soil transplant from a mature Miscanthus stand altered Miscanthus growth or plant-microbe interactions. We discovered that initial soil properties outweighed biofertilization in determining microbial community composition and function. Specifically, we observed that when Miscanthus was grown in nitrogen poor soil from a highly disturbed site, rates of N fixation by free-living N-fixers increased. This increase was predicted strongly by concomitant changes in arbuscular mycorrhizal fungi (AMF) abundance suggesting a synergism between N fixation rates and AMF that facilitates plant nitrogen acquisition. These effects of Miscanthus on the soil microbiome composition and function were less pronounced in the less disturbed soil with more nitrogen. This suggests a scenario where, under nitrogen limitation, Miscanthus invests in nutrient trading by investing in partnerships with nitrogen fixing bacteria and AMF. Alternatively, when nitrogen is more plentiful, Miscanthus may scavenge nutrients, obtaining nitrogen released from organic matter by saprotrophs. These observations aid in illuminating how Miscanthus is able dynamically shape microbial functions occurring in its rhizosphere which more broadly increases our understanding of plant microbe interactions and bioenergy agroecosystems.