Feedstock, climate, and management effects on sustainability of biofuel crops

Stephanie Juice, Christopher A. Walter and Edward R. Brzostek, Department of Biology, West Virginia University, Morgantown, WV, Kara E. Allen, Manaaki Whenua--Landcare Research, Lincoln, New Zealand, Danielle Berardi, Forest, Rangeland, and Fire Science, University of Idaho, Moscow, ID

Background/Question/Methods
Biofuels represent one possible sustainable energy source to reduce fossil fuels and limit climate change to 2oC. They could additionally increase soil organic carbon (SOC), providing climate and soil health benefits, depending on plant traits and agricultural management. For example, perennial feedstocks (e.g. switchgrass, miscanthus) may sequester more C than annuals (e.g. corn, soy) through root-rhizosphere interactions and reduced soil disturbance. Understanding SOC response to feedstock traits given current and future climates will improve C cycle predictions under expanding biofuel production.
However, bioenergy models typically incorporate outdated SOC dynamics that cannot capture SOC sequestration in perennial feedstock systems. Specifically, they lack microbial turnover, SOC protection, and realistic tillage mechanisms. We therefore adapted a microbially-explicit model with SOC protection for bioenergy, including a realistic tillage mechanism. FUN-CORPSE (Fixation and Uptake of Nitrogen- Carbon, Organisms, Rhizosphere, and Protection in the Soil Environment) dynamically predicts plant C investment to gain nitrogen, and downstream effects on microbial activity and SOC. We drove the model with DayCent plant inputs and validated it using field data from the University of Illinois Energy Farm. We then ran model experiments to assess the influence of feedstock traits, warming temperatures, and agricultural practices on SOC stocks in bioenergy systems.
Results/Conclusions
The model accurately represented SOC pools at the UI Energy Farm following 3000 years of prairie spin up and a historical period in which agriculture was first introduced. Perennial bioenergy crop systems, including switchgrass and miscanthus, stored more SOC than conventional corn cultivation, and reversion from annual to perennial crops restored a significant fraction of SOC lost in the transition from prairie to agriculture. This was due to larger residue remaining and altered rhizosphere C dynamics in perennials vs. corn. Warming temperatures increased SOC losses in all crops, with the highest losses occurring in annual crops. SOC pools, and the ratio of unprotected to protected SOC, was highly sensitive to agricultural practices and varied with model representation of tillage. Our results collectively demonstrate the importance of including plant-microbial interactions and SOC protection along with realistic tillage mechanisms in bioenergy models to accurately project the future C balance of biofuel use. They further illustrate the importance of perennial feedstocks to achieving sustainability goals surrounding bioenergy cultivation.