Ecological Sustainability of Energy Cane as a Biofuel Feedstock
In the southeastern United States, the proposed change in land use associated with an expanding biofuel industry is from grazed pasture to energy cane. During the transition, the project team predicts large initial ecosystem losses of carbon and nitrogen associated with cultivation, but with the establishment of the perennial energy cane, it's anticipated that the retention of atmospheric C02 as recalcitrant organic matter in the soil will increase. In addition, the transition from pasture grazed by cattle to fields of energy cane should greatly reduce methane flux to the atmosphere. To test these hypotheses, the team is developing coordinated measurements of CO2, CH4 and water vapor fluxes, as well as potential changes in soil carbon and N03 leaching in pasture, representing current land use, and energy cane, representing future land use (started in 2012).
We parameterized the biogeochemical model DayCent to infer potential yields of energy cane and how changing land from grazed pasture to energy cane would affect ecosystem greenhouse gas (GHG; CO2, CH4 and N2O) dynamics and soil C pools. As land conversion in Central Florida Highlands County has been occurring on Histosols and Spodosols, the model was used to simulate energy cane production and changes in GHG dynamics and soil C pools in both soil types as affected by land conversion. Our results showed that energy cane was productive on both soil types (46-76 Mg dry mass yield per Ha). Overall, converting pasture to energy cane created a sink for GHGs on Spodosols and reduced the size of the GHG source on Histosols.
The scarcity on specific field observations on GHG dynamics and C storage in energy cane and pastures on both soil types complicate model simulations. Therefore, a major focus of our research during 2012 was to improve our ability to predict the impact of land conversion on the environment by characterizing ecosystem GHG dynamics in energy cane crops and pastures on Histosols. Our results suggest that energy cane plantations are larger N2O sources than pastures during the wet season. Under drier conditions, the N2O source strength of energy cane crops and pastures was similar, but energy cane crops were stronger sources of N2O after fertilization. Soils in energy cane and grazed pasture ecosystems were not significant sources of CH4, implying that pastures are a substantial source of CH4 due to the presence of cattle. An important new finding is that the activity of soil C decomposers was higher in pastures than in energy cane crops, implying that energy cane plantations may have strong potential to restore depleted C stored in soils.