OOS 38: Ecological Dimensions of Biofuel Production

Abstract

The session was organized by S. C. Davis, E. H. DeLucia, and K. Anderson-Teixeira, and held on 5 August 2010 during the ESA Annual Meeting in Pittsburgh, Pennsylvania. The report was written by Sarah C. Davis, Energy Biosciences Institute, Department of Plant Biology, Institute of Genomic Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois. E-mail: davissc@illinois.edu 1) Gelfand, I., T. Zenone, J. Poonam, J. Chen, S. Hamilton, and G. P. Robertson. “Conversion of grasslands to annual cropping systems for bioenergy production causes large CO2 emissions.” 2) Zeri, M., K. Anderson-Teixeira, M. Masters, G. Hickman, E. DeLucia, and C. Bernacchi. “Measurements of the ecosystem impacts of four biofuel crops on the cycles of carbon, water, and nitrogen in Central Illinois.” 3) Anderson-Teixeira, K. J., P. K. Snyder, and E. H. DeLucia. “Quantifying the climate impacts of land use change.” 4) Quinn, L. D., and J. R. Stewart. “Assessing the invasiveness of Miscanthus sinensis, a potential bioenergy crop.” 5) Kent, A. “Ecology of nitrogen-fixing bacteria associated with a perennial grass biofuel crop.” 6) Heaton, E., J. Singer, F. Dohleman, and S. Long. “Managing perennial monocultures for ecosystem services.” 7) Hamburg, S. “Carbon implications of using forest biomass for bioenergy.” 8) Vadeboncoeur, M. A., R. D. Yanai, S. P. Hamburg, and J. D. Blum “Large variation in stand-scale sustainability of forest biomass harvesting in central New Hampshire.” 9) Ripley, C. E., C. M. Clark, B. Bierwagen, A. Chen, S. D. LeDuc, B. B. Lin, A. Mehl, R. A. Simmons, and D. A. Tobias. “The potential and pitfalls of biofuels: how knowledge gaps may impair a comprehensive assessment.” Evaluating the ecological implications of biofuel production is an example of one way the Ecological Society of America can accomplish its mission to “ensure the appropriate use of ecological science in environmental decision making.” Despite emerging evidence that describes the potential benefits of carefully managed biofuels (e.g., Robertson et al. 2008, Tilman et al. 2009, Dale et al. 2010), the ecological community remains divided about whether or not science supports decision making that would incentivize biofuel production. In fact, the halls of the David L. Lawrence Convention Center (Pittsburgh, Pennsylvnia) echoed disapproval of biofuels this past August from some ecologists who cite scenarios of land use change and greenhouse gas emissions associated with first-generation grain-based biofuels, but are less aware of recent advances toward cellulosic fuel sources. The ecological benefits of biofuels have been debated in scientific literature, but there is much new information that suggests there are gains in ecosystem services that can be made with second-generation cellulosic feedstock crops. The population that made their way to this session were exposed to some of the most recent findings in biofuels research, beyond the first-generation perspective. The emphasis in biofuels research has indeed shifted to second-generation lignocellulosic crops (Somerville et al. 2010) because of the reduced environmental impacts relative to first-generation annual grain crops. Dr. Elya Gelfand (1) presented yet another line of evidence for the negative impacts associated with converting land to intensely managed annual grain crops. Gelfand measured terrestrial greenhouse gas (GHG) balances, using both static chambers and eddy covariance, to calculate the carbon debt associated with converting grasslands managed under the Conservation Reserve Program (CRP) to continuous corn or corn–soybean agriculture. After conversion from CRP grassland, which formerly was a net sink of GHG, Gelfand estimated that the carbon debt was unlikely to be repaid if conventional corn–soybean agriculture were established on the same land. If no-till management were used with corn–soybean, the debt might be repaid after 18 years. These results support those previously published by others (e.g., Shapouri et al. 2003, Pimental and Patzek 2005) and underscore the importance of prioritizing research on second-generation biofuel crops. The majority of the presentations focused on second-generation cellulosic feedstocks, covering topics that ranged from the microbial communities that mediate nutrient cycling of perennial feedstock crops to the biofuel-related policies that affect forest carbon cycling. Four of the scheduled speakers presented research funded by the Energy Biosciences Institute (EBI) at the University of Illinois. EBI is the home institute for the session organizers, but it is also a leader in the field of biofuels research, housing scientists of many disciplines that collectively study the full production chain for bioenergy. Other speakers represented institutions across the United States, including the Great Lakes Bioenergy Research Center, another leader in bioenergy research, Michigan State University, Iowa State University, University of New Hampshire, the Environmental Defense Fund, and the Environmental Protection Agency. The session began with Dr. Marcelo Zeri (2) introducing the field experimental design that has been established by EBI in Urbana, Illinois, to resolve the carbon, nitrogen, and water biogeochemical cycles of alternative bioenergy agro-ecosystems. A replicated block design with side-by-side field trials of corn–corn–soybean rotation agriculture, high-diversity prairie communities, switchgrass, and miscanthus plots has been established (Fig. 1); each treatment is fully instrumented with separate eddy covariance towers, micro-meteorological stations, automated chambers to measure CO2 fluxes, N2O flux chambers, soil moisture and temperature probes, minirhizotron tubes, and separate tile drainage systems. The project represents the first large-scale and relatively long-term (10 years) experiment to test four bioenergy cropping systems under identical conditions. Established in 2008, data from the plots are still preliminary because the perennial grasses require three years to attain mature crop yields, but Zeri demonstrated the extensive data sets that are being compiled to account for water, C, and N cycling continuously in each cropping system. Energy Farm at the University of Illinois (Urbana, Illinois, USA) in October of 2010 after corn and soybean harvests (photo taken by Tim Mies, Energy Biosciences Institute). Beyond site-level GHG fluxes of bioenergy agro-ecosystems, there are also GHG fluxes associated with land use change that are caused by introducing new cropping systems. There is much uncertainty associated with estimates of land use change caused by biofuel crops and debates about indirect land use change especially persist in scientific literature. Dr. Kristina Anderson-Teixeira (3) introduced a concise method for calculating the GHG value of ecosystems (Anderson-Teixeira et al. 2010), an approach that could be used as a standard for estimating the impact of land conversion on climate change. Anderson-Teixeira asserts that GHG accounting should include direct fluxes, fluxes offset by the ecosystem replaced in a land use change event, and radiative forcing associated with all gas fluxes, as well as the physical properties of an ecosystem canopy. Estimates of GHG fluxes resulting from land use change will always be subject to the uncertainty in data that characterize the change. Nevertheless, standardized methods for quantification of GHG fluxes provide guidelines for measurements that are needed to resolve effects of land use change. Other speakers associated with EBI described studies on the invasiveness of miscanthus and microbial communities that mediate nutrient cycling in perennial bioenergy crops. Dr. Lauren Quinn (4) observed that traits favorable for bioenergy crops (rapid growth, pathogen resistance, high nutrient use efficiency, etc.) are also traits that are common to invasive species. Quinn introduced the invasive characteristics of Miscanthus sinensis, which has been classified as invasive in several states of the United States (USDA 2010). M. sinensis is a close relative of the sterile hybrid Miscanthus × giganteus, which has been introduced as a biofuel crop with great potential due to its high yields and low input requirements. Although M. sinensis has greater cold and drought tolerances that could be advantageous for crop success in northern sites, Quinn's results demonstrate that there would be a greater risk of invasion associated with plantations of M. sinensis relative to the sterile and self-incompatible hybrid Miscanthus × giganteus (Quinn et al. 2010). One of the perceived beneficial traits of Miscanthus × giganteus as a bioenergy feedstock is the unique microbial community associated with this crop that includes nitrogen-fixing bacteria (Davis et al. 2010), as described by Dr. Angela Kent (5). There have been an increasing number of studies that document endophytic and free-living bacteria that fix nitrogen in perennial grass species. Kent's recent work contrasts the microbial communities associated with Miscanthus spp. at different sites, both in native habitats (in Japan) and in Miscanthus agro-ecosystems in the United States. Diazotroph populations were similar among native and cultivated Miscanthus varieties, but assemblages were unique relative to other perennial grasses (e.g., switchgrass). Nitrogen-fixing bacteria were most abundant in warmer, wetter sites than in colder, drier sites. These results are indicative of the geographic variance that may exist in the sustainability of biofuel agro-ecosystems. A limitation to cultivating Miscanthus × giganteus is the need to propagate by rhizomes, as it is a sterile hybrid that does not produce seed. Rhizome planting requires wider spacing that results in less uniform coverage during establishment than seeded grasses, and therefore necessitates greater herbicide use for weed control. Dr. Emily Heaton (6) tested the hypothesis that nurse crops would increase the success of establishment and reduce the ecological impact associated with herbicide treatments. Heaton found that the benefits of this practice include soil protection, but there is a trade-off in yield losses. M. × giganteus appears to be affected by competition from the intended nurse crops. These results are a contribution to accumulating data that will eventually define best management practices for perennial biofuel crops. An emerging area of biofuel research that requires the engagement of ecologists to properly address policies and educate policy-makers is forest biomass removal. Dr. Steven Hamburg (7) presented an analysis of carbon emissions associated with harvesting biomass from forests in New England. Hamburg theorized that forests eventually reach carbon saturation, and that harvesting biomass can have lower emissions than fossil fuel burning after that point. The time required to reach carbon saturation and the subsequent break-even point with fossil fuel emissions is site specific. Hamburg also asserted that residue removal from forests already harvested for timber is a carbon-neutral source of biomass, because the residues in New England timberlands decompose quickly, releasing carbon back to the atmosphere. Matt Vadeboncoeur (8) continued the analysis of effects of biomass removal on forests by questioning the timing of nutrient limitation in harvested forests. His on-going research involves measuring macronutrient availability of forests that are subject to different frequencies and intensities of harvest. Ultimately, Vadeboncoeur's research should facilitate the identification of sustainable harvesting practices for hardwood forest in New England. The variation in biogeochemistry across sites prevents absolute estimates of sustainable harvesting, but policies are nevertheless being established to incentivize biomass removal from forests. The session concluded with research from Dr. Caroline Ridley (9) that analyzed the recent surge of scientific literature addressing biofuels. Ripley found there has been an exponential increase in the number of publications about biofuels in the last decade, with ~1500 expected in 2010. Despite the unprecedented research interest, there are key areas of research that are lacking. Most notably for the ecological community, there is a dearth of literature that addresses biodiversity responses to biofuel production. Clearly, the scientific debate about effects of land use change (e.g., Searchinger et al. 2008, Plevin et al. 2010) has implications for biodiversity, but there has been greater attention given to the effects of land conversion on GHG fluxes. To synthesize the diverse research that was represented in the topical session on biofuels, there are three emergent messages for ecologists. First-generation biofuels that use intensive conventional management practices have ecological costs that outweigh the benefits. Second-generation biofuel crops that are high yielding with low input requirements offer the ecological benefits of increased terrestrial carbon sequestration and reduced GHG emissions, but the land that is used to cultivate cellulosic feedstocks must be chosen carefully to reduce the risk of negative impacts. And finally, ecosystem research that characterizes spatial variance in biogeochemistry, biodiversity, and resilience has an important role for informing policies that incentivize biomass harvest for energy. Ecologists who are not directly engaged in bioenergy research will probably find it difficult to stay informed about all advances in the field because of the increasing volume of scientific discovery in energy biosciences. Regardless of the difficulty, the mission of ESA, as referenced in the introduction, is a call to “ensure the appropriate use of ecological science in environmental decision making,” and energy-related policies are some of the most important for ecological sustainability. The effects of emerging bioenergy production systems are not simple, and the science of ecology is a necessity in this research field. In the midst of ongoing debates about biofuels in both science and policy, opportunities for reversing the ecological impacts of fossil fuel consumption exist, and should not be overlooked.