The demand for energy from biofuel production is increasing, prompting concerns about environmental impacts and long-term sustainability. Recent analyses have focused on carbon (C) budgets, with less attention paid to nitrogen (N) dynamics, especially in marginal lands that are unsuitable for food crops. We know that marginal land N availability is the result of two primary processes: N-fixation and the depolymerization of SOM (N-mineralization), yet the relationship between these two processes is relatively unexplored. Recent work has documented N-fixation associated with bioenergy crops and this potentially critical source of N may decrease need for external fertilizer inputs, dramatically enhancing sustainability. Both N-fixation and N-mineralization are energy intensive and in the rhizosphere this energy (i.e. C) comes directly from the plant in the form of root exudates. Here, we focus on the perennial bioenergy crop switchgrass (Panicum virgatum v. Cave-In-Rock) to explore this resource exchange. We ask the question: Do plants allocate photosynthate to exudates, rather than biomass accrual, to gain nutrients through microbial N-fixation or N-mineralization?
Results/Conclusions
During the growing season (April-October) in 2016-2018 we collected soils and switchgrass tissue and metrics every 2 or 4 weeks from fertilized (56 kg N ha-1) and unfertilized subplots at two of the Great Lakes Bioenergy Research Center Marginal Land Experiment locations in Michigan, Lux Arbor and Lake City. We measured N availability, potential rates of N-fixation, N-depolymerization and N-mineralization, microbial biomass N, plant tissue N, plant height, specific leaf area, root mass and specific root length, and photosynthetic efficiency. Fertilizer N had no effect on switchgrass growth and total harvest biomass, no effect on microbial biomass N and only effected N availability and N-cycling processes just after application. At the peak of the growing season, we found that N-fixation was positively correlated with plant available N and that N-fixation rates did not differ with and without N fertilizer. Additionally, net N-mineralization was almost always negative, indicating N immobilization. At Lake City, N-fixation was positively correlated with soil moisture and plant available NHâ‚„. At both sites, plant height and specific leaf area were strongly correlated with net N-mineralization. Peptidase enzyme activities (N depolymerization) did not vary by fertilizer treatment. Ongoing analyses will allow us to determine further relationships between switchgrass growth dynamics and microbial processes that make N available. These data demonstrate high variability in N dynamics and highlight the need for studies across multiple years and locations.