Empirical Evidence for the Potential Climate Benefits of Decarbonizing Light Vehicle Transport in the U.S. with Bioenergy from Purpose-Grown Biomass with and without BECCS

Climate change impacts of bioenergy technologies: A comparative consequential LCA of sustainable fuels production with CCUS

The use of sustainable biomass can be a cost-effective way of reducing the greenhouse gas emissions in the maritime and aviation sectors. Biomass, however, is a limited resource, and therefore, it is important to use the biomass where it creates the highest value, not only economically, but also in terms of GHG reductions. This study comprehensively evaluates the GHG reduction potential of utilising forestry residue in different bioenergy technologies using a consequential LCA approach. Unlike previous studies that assess GHG impacts per unit of fuel produced, this research takes a feedstock-centric approach which enables comparisons across systems that yield diverse products and by-products. Three technologies-combined heat and power plant with carbon capture, hydrothermal liquefaction, and gasification-are assessed, while considering both carbon capture and storage (CCS) or carbon capture and utilisation (CCU). Through scenario analysis, the study addresses uncertainty, and assumptions in the LCA modelling. It explores the impact of energy systems, fuel substitution efficiency, renewable energy expansion, and the up/down stream supply chain. All technology pathways showed a potential for net emissions savings when including avoided emissions from substitution of products, with results varying from -111 to -1742 kgCO2eq per tonne residue. When combining the bioenergy technologies with CCU the dependency on the energy system in which they are operated was a significantly higher compared to CCS. The breakpoint was found to be 44 kg CO2eq/kWh electricity meaning that the marginal electricity mix has to be below this point for CCU to obtain lower GHG emissions. Furthermore, it is evident that the environmental performance of CCU technologies is highly sensitive to how it will affect the ongoing expansion of renewable electricity capacity.

Climate vs Energy Security: Quantifying the Trade-offs of BECCS Deployment and Overcoming Opportunity Costs on Set-Aside Land

Bioenergy with carbon capture and storage (BECCS) sits at the nexus of the climate and energy security. We evaluated trade-offs between scenarios that support climate stabilization (negative emissions and net climate benefit) or energy security (ethanol production). Our spatially explicit model indicates that the foregone climate benefit from abandoned cropland (opportunity cost) increased carbon emissions per unit of energy produced by 14-36%, making geologic carbon capture and storage necessary to achieve negative emissions from any given energy crop. The toll of opportunity costs on the climate benefit of BECCS from set-aside land was offset through the spatial allocation of crops based on their individual biophysical constraints. Dedicated energy crops consistently outperformed mixed grasslands. We estimate that BECCS allocation to land enrolled in the Conservation Reserve Program (CRP) could capture up to 9 Tg C year-1 from the atmosphere, deliver up to 16 Tg CE year-1 in emissions savings, and meet up to 10% of the US energy statutory targets, but contributions varied substantially as the priority shifted from climate stabilization to energy provision. Our results indicate a significant potential to integrate energy security targets into sustainable pathways to climate stabilization but underpin the trade-offs of divergent policy-driven agendas.

Bioenergy sorghum stem growth regulation: intercalary meristem localization, development, and gene regulatory network analysis

Bioenergy sorghum is a highly productive drought tolerant C₄ grass that accumulates 80% of its harvestable biomass in approximately 4 m length stems. Stem internode growth is regulated by development, shading, and hormones that modulate cell proliferation in intercalary meristems (IMs). In this study, sorghum stem IMs were localized above the pulvinus at the base of elongating internodes using magnetic resonance imaging, microscopy, and transcriptome analysis. A change in cell morphology/organization occurred at the junction between the pulvinus and internode where LATERAL ORGAN BOUNDARIES (SbLOB), a boundary layer gene, was expressed. Inactivation of an AGCVIII kinase in DDYM (dw2) resulted in decreased SbLOB expression, disrupted IM localization, and reduced internode cell proliferation. Transcriptome analysis identified approximately 1000 genes involved in cell proliferation, hormone signaling, and other functions selectively upregulated in the IM compared with a non-meristematic stem tissue. This cohort of genes is expressed in apical dome stem tissues before localization of the IM at the base of elongating internodes. Gene regulatory network analysis identified connections between genes involved in hormone signaling and cell proliferation. The results indicate that gibberellic acid induces accumulation of growth regulatory factors (GRFs) known to interact with ANGUSTIFOLIA (SbAN3), a master regulator of cell proliferation. GRF:AN3 was predicted to induce SbARF3/ETT expression and regulate SbAN3 expression in an auxin-dependent manner. GRFs and ARFs regulate genes involved in cytokinin and brassinosteroid signaling and cell proliferation. The results provide a molecular framework for understanding how hormone signaling regulates the expression of genes involved in cell proliferation in the stem IM.

Land-based climate solutions for the United States

Meeting end-of-century global warming targets requires aggressive action on multiple fronts. Recent reports note the futility of addressing mitigation goals without fully engaging the agricultural sector, yet no available assessments combine both nature-based solutions (reforestation, grassland and wetland protection, and agricultural practice change) and cellulosic bioenergy for a single geographic region. Collectively, these solutions might offer a suite of climate, biodiversity, and other benefits greater than either alone. Nature-based solutions are largely constrained by the duration of carbon accrual in soils and forest biomass; each of these carbon pools will eventually saturate. Bioenergy solutions can last indefinitely but carry significant environmental risk if carelessly deployed. We detail a simplified scenario for the United States that illustrates the benefits of combining approaches. We assign a portion of non-forested former cropland to bioenergy sufficient to meet projected mid-century transportation needs, with the remainder assigned to nature-based solutions such as reforestation. Bottom-up mitigation potentials for the aggregate contributions of crop, grazing, forest, and bioenergy lands are assessed by including in a Monte Carlo model conservative ranges for cost-effective local mitigation capacities, together with ranges for (a) areal extents that avoid double counting and include realistic adoption rates and (b) the projected duration of different carbon sinks. The projected duration illustrates the net effect of eventually saturating soil carbon pools in the case of most strategies, and additionally saturating biomass carbon pools in the case of forest management. Results show a conservative end-of-century mitigation capacity of 110 (57-178) Gt CO₂ e for the U.S., ~50% higher than existing estimates that prioritize nature-based or bioenergy solutions separately. Further research is needed to shrink uncertainties, but there is sufficient confidence in the general magnitude and direction of a combined approach to plan for deployment now.

Environmental outcomes of the US Renewable Fuel Standard

The Renewable Fuel Standard (RFS) specifies the use of biofuels in the United States and thereby guides nearly half of all global biofuel production, yet outcomes of this keystone climate and environmental regulation remain unclear. Here we combine econometric analyses, land use observations, and biophysical models to estimate the realized effects of the RFS in aggregate and down to the scale of individual agricultural fields across the United States. We find that the RFS increased corn prices by 30% and the prices of other crops by 20%, which, in turn, expanded US corn cultivation by 2.8 Mha (8.7%) and total cropland by 2.1 Mha (2.4%) in the years following policy enactment (2008 to 2016). These changes increased annual nationwide fertilizer use by 3 to 8%, increased water quality degradants by 3 to 5%, and caused enough domestic land use change emissions such that the carbon intensity of corn ethanol produced under the RFS is no less than gasoline and likely at least 24% higher. These tradeoffs must be weighed alongside the benefits of biofuels as decision-makers consider the future of renewable energy policies and the potential for fuels like corn ethanol to meet climate mitigation goals.

The effects of microalgae-based fertilization of wheat on yield, soil microbiome and nitrogen oxides emissions

Overuse of agrochemicals is linked to nutrient loss, greenhouse gases (GHG) emissions, and resource depletion thus requiring the development of sustainable agricultural solutions. Cultivated microalgal biomass could provide such a solution. The environmental consequences of algal biomass application in agriculture and more specifically its effect on soil GHG emissions are understudied. Here we report the results of a field experiment of wheat grown on three different soil types under the same climatic conditions and fertilized by urea or the untreated biomass of fresh-water green microalga (Coelastrella sp.). The results show that neither soil type nor fertilization types impacted the aboveground wheat biomass, whereas, soil microbiomes differed in accordance with soil but not the fertilizer type. However, wheat grain nitrogen (N) content and soil N oxides emissions were significantly lower in plots fertilized by algal biomass compared to urea. Grain N content in the wheat grain that was fertilized by algal biomass was between 1.3%-1.5% vs. 1.6%-2.0% in the urea fertilized wheat. Cumulative soil nitric oxide (NO) emissions were 2-5 fold lower, 313-726 g N ha^-1 season^-1 vs. 909-3079 g N ha^-1 season^-1. Cumulative soil nitrous oxide (N₂O) emissions were 2-fold lower, 90-348 g N ha^-1 season^-1 vs. 147-761 g N ha^-1 season^-1. The lower emissions resulted in a 4-11 fold lower global warming impact of the algal fertilized crops. This calculation excluded the CO₂ cost from the algae biomass production. Once included algal fertilization had a similar, or 40% higher, climatic impact compared to the urea fertilization.

Subsampling and DNA pooling can increase gains through genomic selection in switchgrass

Genomic selection (GS) can accelerate breeding cycles in perennial crops such as the bioenergy grass switchgrass (Panicum virgatum L.). The sequencing costs of GS can be reduced by pooling DNA samples in the training population (TP), only sequencing TP phenotypic outliers, or pooling candidate population (CP) samples. These strategies were simulated for two traits (spring vigor and anthesis date) in three breeding populations. Sequencing only the outlier 50% of the TP phenotype distribution resulted in a penalty of <5% of the predictive ability, measured using cross-validation. Predictive ability also decreased when sequencing progressively fewer TP DNA pools, but TPs constructed from only two phenotypically contrasting DNA samples retained a mean of >80% predictive ability relative to individual TP sequencing. Novel group testing methods allowed greater than one CP individual to be screened per sequenced DNA sample but resulted in a predictive ability penalty. To determine the impact of reduced sequencing, genetic gain was calculated for seven GS scenarios with variable sequencing budgets. Reduced TP sequencing and most CP pooling methods were superior to individual sequence-based GS when sequencing resources were restricted (2,000 DNA samples per 5-yr cycle). Only one scenario was superior to individual sequencing when sequencing budgets were large (8,000 DNA samples per 5-yr cycle). This study highlights multiple routes for reduced sequencing costs in GS.

Phosphorus availability and leaching losses in annual and perennial cropping systems in an upper US Midwest landscape

Excessive phosphorus (P) applications to croplands can contribute to eutrophication of surface waters through surface runoff and subsurface (leaching) losses. We analyzed leaching losses of total dissolved P (TDP) from no-till corn, hybrid poplar (Populus nigra X P. maximowiczii), switchgrass (Panicum virgatum), miscanthus (Miscanthus giganteus), native grasses, and restored prairie, all planted in 2008 on former cropland in Michigan, USA. All crops except corn (13 kg P ha⁻¹ year⁻¹) were grown without P fertilization. Biomass was harvested at the end of each growing season except for poplar. Soil water at 1.2 m depth was sampled weekly to biweekly for TDP determination during March–November 2009–2016 using tension lysimeters. Soil test P (0–25 cm depth) was measured every autumn. Soil water TDP concentrations were usually below levels where eutrophication of surface waters is frequently observed (> 0.02 mg L⁻¹) but often higher than in deep groundwater or nearby streams and lakes. Rates of P leaching, estimated from measured concentrations and modeled drainage, did not differ statistically among cropping systems across years; 7-year cropping system means ranged from 0.035 to 0.072 kg P ha⁻¹ year⁻¹ with large interannual variation. Leached P was positively related to STP, which decreased over the 7 years in all systems. These results indicate that both P-fertilized and unfertilized cropping systems may leach legacy P from past cropland management.

Restoring Soil Fertility on Degraded Lands to Meet Food, Fuel, and Climate Security Needs via Perennialization

A continuously growing pressure to increase food, fiber, and fuel production to meet worldwide demand and achieve zero hunger has put severe pressure on soil resources. Abandoned, degraded, and marginal lands with significant agricultural constraints—many still used for agricultural production—result from inappropriately intensive management, insufficient attention to soil conservation, and climate change. Continued use for agricultural production will often require ever more external inputs such as fertilizers and herbicides, further exacerbating soil degradation and impeding nutrient recycling and retention. Growing evidence suggests that degraded lands have a large potential for restoration, perhaps most effectively via perennial cropping systems that can simultaneously provide additional ecosystem services. Here we synthesize the advantages of and potentials for using perennial vegetation to restore soil fertility on degraded croplands, by summarizing the principal mechanisms underpinning soil carbon stabilization and nitrogen and phosphorus availability and retention. We illustrate restoration potentials with example systems that deliver climate mitigation (cellulosic bioenergy), animal production (intensive rotational grazing), and biodiversity conservation (natural ecological succession). Perennialization has substantial promise for restoring fertility to degraded croplands, helping to meet future food security needs.

Bioenergy landscapes drive trophic shifts in generalist ants

Changes in trophic niche-the pathways through which an organism obtains energy and nutrients-are a fundamental way in which organisms respond to environmental conditions. But the capacity for species to alter their trophic niches in response to global change, and the ways they do so when able, remain largely unknown. Here we examine food webs in three long-term and large-scale experiments to test how resource availability and nutritional requirements interact to determine an organism's trophic niche in the context of one of the largest global trends in land use-the rise in bioenergy production. We use carbon and nitrogen stable isotope analyses to characterize arthropod food webs across three biofuel crops representing a gradient in plant resource richness (corn monocultures, fields dominated by native switchgrass and restored prairie), and to quantify changes in the trophic niche of a widespread generalist ant species across habitats. In doing so, we measure the effects of basal resource richness on food chain length, niche breadth and trophic position. We frame our results in the context of two hypotheses that explain variation in trophic niche-the niche variation hypothesis which emphasizes the importance of resource diversity and ecological opportunity, and the optimal diet hypothesis which emphasizes dietary constraints and the availability of optimal resources. Increasing plant richness lengthened food chains by 10%-20% compared to monocultures. Niche breadths of generalist ants did not vary with resource richness, suggesting they were limited by optimal diet requirements and constraints rather than by ecological opportunity. The ants instead responded to changes in plant richness by shifting their estimated trophic position. In resource-poor monocultures, the ants were top predators, sharing a trophic position with predatory spiders. In resource-rich environments, in contrast, the ants were omnivores, relying on a mix of animal prey and plant-based resources. In addition to highlighting novel ecosystem impacts of alternate bioenergy landscapes, our results suggest that niche breadth and trophic diversification depend more on the presence of optimal resources than on ecological opportunity alone.

The soil microbial carbon pump: From conceptual insights to empirical assessments

The global soil carbon (C) pool is massive, so relatively small changes in soil organic carbon (SOC) stocks can significantly alter atmospheric C and global climate. The recently proposed concept of the soil microbial carbon pump (MCP) emphasizes the active role of soil microbes in SOC storage by integrating the continual microbial transformation of organic C from labile to persistent anabolic forms. However, the concept has not been evaluated with data. Here, we combine datasets, including microbial necromass biomarker amino sugars and SOC, from two long-term agricultural field studies conducted by large United States bioenergy research programs. We interrogate the soil MCP concept by investigating the asynchronous responses of microbial necromass and SOC to land-use change. Microbial necromass appeared to preferentially accumulate in soil and be the dominant contributor to SOC accrual in diversified perennial bioenergy crops. Specifically, ~92% of the additional SOC enhanced by plant diversity was estimated to be microbial necromass C, and >76% of the additional SOC enhanced by land-use transition from annual to perennial crops was estimated to be microbial necromass. This suggests that the soil MCP was stimulated in diversified perennial agroecosystems. We further delineate and suggest two parameters-soil MCP capacity and efficacy-reflecting the conversion of plant C into microbial necromass and the contribution of microbial necromass to SOC, respectively, that should serve as valuable metrics for future studies evaluating SOC storage under alternative management in changing climates.

Cost and Life-Cycle Greenhouse Gas Implications of Integrating Biogas Upgrading and Carbon Capture Technologies in Cellulosic Biorefineries

Gaseous streams in biorefineries have been undervalued and underutilized. In cellulosic biorefineries, coproduced biogas is assumed to be combusted alongside lignin to generate process heat and electricity. Biogas can instead be upgraded to compressed biomethane and used as a transportation fuel. Capturing CO₂-rich streams generated in biorefineries can also contribute to greenhouse gas (GHG) mitigation goals. We explore the economic and life-cycle GHG impacts of biogas upgrading and CO₂ capture and storage (CCS) at ionic liquid-based cellulosic ethanol biorefineries using biomass sorghum. Without policy incentives, biorefineries with biogas upgrading systems can achieve a comparable minimum ethanol selling price (MESP) and reduced GHG footprint ($1.38/liter gasoline equivalent (LGE) and 12.9 gCO_2e/MJ) relative to facilities that combust biogas onsite ($1.34/LGE and 24.3 gCO_2e/MJ). Incorporating renewable identification number (RIN) values advantages facilities that upgrade biogas relative to other options (MESP of $0.72/LGE). Incorporating CCS increases the MESP but dramatically decreases the GHG footprint (-21.3 gCO_2e/MJ for partial, -110.7 gCO_2e/MJ for full CCS). The addition of CCS also decreases the cost of carbon mitigation to as low as $52-$78/t CO₂, depending on the assumed fuel selling price, and is the lowest-cost option if both RIN and California's Low Carbon Fuel Standard credits are incorporated.

Simulated Biomass Sorghum GHG Reduction Potential is Similar to Maize

Policy support for cellulosic biofuels is contingent on their achieving much greater reductions in life-cycle greenhouse gas emissions than corn starch ethanol. Biomass sorghum has been suggested as a genetically and agronomically tractable feedstock species to augment near-term cellulosic feedstock production. This study used DayCent modeling to investigate biomass sorghum production emissions relative to corn with and without stover utilization at 3,265 across the rainfed United States. Sorghum produced greater average feedstock dry matter (15.6 ± 1.4 vs 14.8 ± 2.2 Mg ha^-1 yr^-1) and slightly lower estimated ethanol energy yields (10.6 ± 1.0 vs 11.8 ± 2.9 MJ m^-2 yr^-1) as corn grain with 75% stover collection. The high biomass removals in both the sorghum and corn stover scenarios led to soil organic carbon losses on 90 and 100% of sites, respectively. Average feedstock production emissions intensities were similar between sorghum and corn with 75% stover removal (17.6 ± 2.8 vs 18.8 ± 3.0 g CO₂e MJ^-1), but were notably lower under sorghum for sites in the southwestern study region (13.6 ± 3.0 vs 22.5 ± 3.1 g CO₂e MJ^-1). These results suggest that biomass sorghum produces cellulosic feedstock with similar emissions to corn grain and at current yield levels is unlikely to meet the Renewable Fuel Standard emissions reduction threshold for cellulosic biofuels.