Critics respond to study claiming poor GHG performance of cellulosic biofuels

The study led by University of Nebraska-Lincoln assistant professor Adam Liska, funded through a three-year, $500,000-grant from the US Department of Energy, used carbon dioxide measurements taken from 2001 to 2010 to validate a soil carbon model that was built using data from 36 field studies across North America. Among their findings were that using corn crop residue to make ethanol and other biofuels reduces soil carbon and under some conditions can generate more greenhouse gases than gasoline.

Liska said that his team tried, without success, to “poke holes” in their study. However, in its 4 November issue, Nature Climate Change is publishing several responses from other researchers doing their own hole-poking in the Liska study. The criticism largely is based on two factors: that Liska et. al’s model ignored some of the carbon benefits associated with cellulosic ethanol byproducts, and that it used extreme parameters.

Niclas Scott Bentsen, Søren Larsen and Claus Felby from the University of Copenhagen commented that:

We do not dispute the main findings that harvest of residues has a negative impact on SOC levels and that this impact should be addressed when evaluating the potential benefits of cellulosic biofuels. We do, however, find that the conclusion, that cellulosic biofuels increase CO₂ emissions, builds on an incomplete analysis and that the analysis could have reached the opposite conclusion had it been more complete.

Bentsen and colleagues note that consequential lifecycle analysis requires that mass balances are closed and if not, some impact allocation must take place. In cellulosic ethanol production based on agricultural residues, 20–25% of the carbon in the biomass ends up in ethanol and half of that amount in CO₂. Approximately 40% is retained in the lignin residue and the rest (~20–30%) in molasses/vinasse.

Liska et al. surprisingly disregard a considerable part of that carbon mass and attribute all CO₂ interactions between the product system and the atmosphere to ethanol. The lignin fraction is not accounted for in the main comparison between cellulosic and fossil fuels.

Further, they note, while the IPCC and the European Union both recommend a 20-year perspective for such analysis—and while much LCA work applies a 100-year time perspective—Liska et al. used a 5-10 year perspective.

Applying any of these [longer] time perspectives to the analysis of Liska et al. would reduce the greenhouse-gas impact of cellulosic biofuel and render cellulosic biofuels capable of reducing CO2 emissions and perhaps even meeting the Renewable Fuel Standard reduction target.

Loss of SOC from biofuel production is a critical issue for greenhouse-gas emissions and soil quality, and it should be addressed in both science and management. But it is highly important that all biogenic carbon is included in greenhouse-gas analyses and that relevant time frames are applied, which is not the case for the analysis by Liska and co-authors.

—Bentsen et al.

John J. Sheehan, Mark Easter, William Parton, Keith Paustian and Stephen Williams from Colorado State University; and Paul R. Adler and Stephen J. Del Grosso from the US Department of Agriculture Agricultural Research Service (ARS) wrote:

The claim by Liska et al. that corn stover-derived ethanol can be worse than gasoline has generated lots of media interest, but offers little value to the research community or to policymakers. They have merely demonstrated that if you model an irresponsible and unsustainable scenario, the results will look irresponsible and unsustainable. No one who has given serious thought to crop residues for biofuels would find their proposed across-the-board 6 Mg ha−1 collection rate in the US Corn Belt at all reasonable.

In an earlier work, Sheehan and his colleagues then used soil carbon and LCA and showed show that using corn stover for ethanol production would only make sense if farmers simultaneously adopted conservation tillage practices and constrained removal rates to account for local yield, soil, climate and topographical conditions. Other field and modelling studies have shown that soil carbon levels can be maintained with conservation tillage and moderate stover removal.

In contrast, Liska et al. applied a very simplistic soil carbon model that ignores important variables such as soil moisture and soil texture—making regional extrapolations highly questionable—and doesn’t allow for varying management practices. This is an important shortcoming, as farmers can reduce their tillage intensity with stover harvest, saving money, without compromising yields. … Adoption of conservation tillage reduces soil carbon losses to almost zero, while including a cover crop can yield net soil carbon increases.

The problems with the scenario of Liska et al. do not end at the farm. They took another extreme position by ignoring the carbon savings associated with coproduct electricity in the conversion facility.

… The study by Liska et al. is symptomatic of a broader problem in the realm of LCA. Had they followed International Organization for Standardization standards and engaged stakeholders in the design of this study, instead of unilaterally making extreme and unsustainable assumptions, they might have ended up evaluating more useful scenarios. This is a mistake all too commonly found in the LCA literature.

—Sheehan et al.

A third comment came from G. Philip Robertson from Michigan State University and the Great Lakes Bioenergy Research Center at Michigan State University; Peter R. Grace from Queensland University of Technology (Australia); R. César Izaurralde from The Great Lakes Bioenergy Center, University of Maryland and Texas A&M; William P. Parton from Colorado State University; and Xuesong Zhang from the Joint Global Change Research Institute, Pacific Northwest National Laboratory and University of Maryland.

Robertson and his colleagues noted that the first-order kinetics soil organic carbon model used in the Liska study was tested with data from a single site that is “largely unrepresentative” of other Corn Belt soils: a highly productive irrigated no-till corn experiment in Mead, Nebraska, where SOC even with very high yields and all residue retained lost 4.6% of its SOC stock over a four-year period.

The model was then used to extrapolate results across the US Midwest to conclude that stover removal in general reduces SOC in Midwest soils.

These findings diverge sharply from predictions made by process-based SOC models that also incorporate soil texture and water content to resolve SOC changes with crop management. These models, well constrained and broadly tested in a wide variety of soils, climates and cropping systems in the Midwest and elsewhere, almost universally predict stable or increasing SOC with full residue retention under no-till management in Midwest soils. Soil inventory models also show stable or increasing SOC across the Midwest, as do global inventories.

We are unprepared to explain the basis for the anomalous behaviour of the Mead, Nebraska site, but a very low root-shoot ratio of ~0.07, which likely underestimates root carbon inputs, may explain part of the difference, as might irrigation, which can accelerate decomposition.

… While we agree with Liska et al. that full residue removal without cover crops will likely deplete SOC over the long term—although at a much lower rate than they estimate—this is an unrealistic scenario: we are not aware of any management practices for corn grain production that prescribe 100% stover removal.

… while we agree with the motivation that underlies their analysis … we believe that this is best achieved with efforts that are based on our full understanding of carbon turnover in agricultural soils, and not on models that unduly simplify important relationships.

—Robertson et al.

The defense. Liska and his colleagues responded to the criticism by defending their SOC model and by asserting that:

The question for LCA is also not: how could these systems be in the future? The question is, however: how are these systems performing now, and how are they going to perform in the near term? The lignin coproduct is burned to provide energy for biofuel processing, and currently no electricity exports or other coproducts exist in the Poet’s Liberty project.

Standards for LCA are under development and in a state of flux. Owing to the complexity of LCA, a wide range of values can be produced in these assessments due to arbitrary variability in spatial and temporal parameter values, modelling assumptions, timeframes and system boundaries. Consequently, our analysis focused on quantifying uncertainty in one primary variable: net SOC loss to CO₂ from residue removal. The 30-year time interval precedent set by Searchinger et al. is arbitrary and biases results in favour of biofuel producers. Precedents used by the US Environmental Protection Agency may not favour near-term emissions reductions, and existing precedents will probably be revised.

To accurately represent current climatic conditions and SOC dynamics, temperature measurements from 2001 to 2010 were used, because older data do not represent increased temperatures and future projections are more uncertain. The model, however, was also used to estimate SOC changes from 2010 to 2060 with estimated increases in crop yields and temperatures from the IPCC’s Fifth Assessment Report climate simulations.

… to dilute SOC emissions over 30 years or more does not represent actual CO₂ emissions over the first 10 years, and presenting longer-term lower values can be deceptive. … If residue is removed for biofuel, these systems could produce more CO₂ emissions than gasoline for more than 10 years and then possibly reduce emissions in 20 to 30 years, after agricultural SOC stocks have significantly decreased and crop yields have probably declined. Alternatively, SOC loss from residue removal can be widely recognized, and appropriate management can be used to compensate for lost carbon and increased CO₂ emissions.

—Liska et al.

Resources

Niclas Scott Bentsen, Søren Larsen & Claus Felby (2014) “CO₂ emissions from crop residue-derived biofuels,” Nature Climate Change 4, 932 doi: 10.1038/nclimate2401

John J. Sheehan, Paul R. Adler, Stephen J. Del Grosso, Mark Easter, William Parton, Keith Paustian & Stephen Williams (2014) “CO₂ emissions from crop residue-derived biofuels,” Nature Climate Change 4, 932-933 doi: 10.1038/nclimate2403

G. Philip Robertson, Peter R. Grace, R. César Izaurralde, William P. Parton & Xuesong Zhang (2014) “CO₂ emissions from crop residue-derived biofuels,” Nature Climate Change 4, 933-934 doi: doi:10.1038/nclimate2402

Adam J. Liska, Haishun Yang, Matthew P. Pelton & Andrew E. Suyker (2014) “Reply to ‘CO₂ emissions from crop residue-derived biofuels’” Nature Climate Change 4, 934–935 doi: 10.1038/nclimate2423