Randall D. Jackson

Take a Closer Look: Biofuels Can Support Environmental, Economic and Social Goals

The US Congress passed the Renewable Fuels Standard (RFS) seven years ago. Since then, biofuels have gone from darling to scapegoat for many environmentalists, policy makers, and the general public. The reasons for this shift are complex and include concerns about environmental degradation, uncertainties about impact on food security, new access to fossil fuels, and overly optimistic timetables. As a result, many people have written off biofuels. However, numerous studies indicate that biofuels, if managed sustainably, can help solve pressing environmental, social and economic problems (Figure 1). The scientific and policy communities should take a closer look by reviewing the key assumptions underlying opposition to biofuels and carefully consider the probable alternatives. Liquid fuels based on fossil raw materials are likely to come at increasing environmental cost. Sustainable futures require energy conservation, increased efficiency, and alternatives to fossil fuels, including biofuels.

Ecosystem-service tradeoffs associated with switching from annual to perennial energy crops in riparian zones of the US midwest.

Integration of energy crops into agricultural landscapes could promote sustainability if they are placed in ways that foster multiple ecosystem services and mitigate ecosystem disservices from existing crops. We conducted a modeling study to investigate how replacing annual energy crops with perennial energy crops along Wisconsin waterways could affect a variety of provisioning and regulating ecosystem services. We found that a switch from continuous corn production to perennial-grass production decreased annual income provisioning by 75%, although it increased annual energy provisioning by 33%, decreased annual phosphorous loading to surface water by 29%, increased below-ground carbon sequestration by 30%, decreased annual nitrous oxide emissions by 84%, increased an index of pollinator abundance by an average of 11%, and increased an index of biocontrol potential by an average of 6%. We expressed the tradeoffs between income provisioning and other ecosystem services as benefit-cost ratios. Benefit-cost ratios averaged 12.06 GJ of additional net energy, 0.84 kg of avoided phosphorus pollution, 18.97 Mg of sequestered carbon, and 1.99 kg of avoided nitrous oxide emissions for every

Lake to land subsidies: Experimental addition of aquatic insects increases terrestrial arthropod densities

1,000 reduction in income. These ratios varied spatially, from 2- to 70-fold depending on the ecosystem service. Benefit-cost ratios for different ecosystem services were generally correlated within watersheds, suggesting the presence of hotspots – watersheds where increases in multiple ecosystem services would come at lower-than-average opportunity costs. When assessing the monetary value of ecosystem services relative to existing conservation programs and environmental markets, the overall value of enhanced services associated with adoption of perennial energy crops was far lower than the opportunity cost. However, when we monitized services using estimates for the social costs of pollution, the value of enhanced services far exceeded the opportunity cost. This disparity between recoverable costs and social value represents a fundamental challenge to expansion of perennial energy crops and sustainable agricultural landscapes.

Plant nitrogen and phosphorus limitation in 98 North American grassland soils

Aquatic insects are a common and important subsidy to terrestrial systems, yet little is known about how these inputs affect terrestrial food webs, especially around lakes. Mývatn, a lake in northern Iceland, has extraordinary midge (Chironomidae) emergences that result in large inputs of biomass and nutrients to terrestrial arthropod communities. We simulated this lake-to-land resource pulse by collecting midges from Mývatn and spreading their dried carcasses on 1-m2 plots at a nearby site that receives very little midge deposition. We hypothesized a positive bottom-up response of detritivores that would be transmitted to their predators and would persist into the following year. We sampled the arthropod community once per month for two consecutive summers. Midge addition resulted in significantly different arthropod communities and increased densities of some taxa in both years. Detritivores, specifically Diptera larvae, Collembola, and Acari increased in midge-addition plots, and so did some predators and parasitoids. Arthropod densities were still elevated a year after midge addition, and two years of midge addition further increased the density of higher-order consumers (e.g., Coleoptera and Hymenoptera). Midge addition increased arthropod biomass by 68% after one year and 108% after two years. By manipulating the nutrient pulse delivered by midges we were able to elucidate food web consequences of midge deposition and spatial and temporal dynamics that are difficult to determine based on comparative approaches alone. Resources cross ecosystem boundaries and are assimilated over time because of life-history strategies that connect aquatic and terrestrial food webs and these systems cannot be fully understood in isolation from each other.

Nitrous oxide emissions during establishment of eight alternative cellulosic bioenergy cropping systems in the North Central United States

The availability of nutrients is a critical determinant of ecological dynamics in grasslands, but the relationships between soil resource availability and nutrient limitation across ecosystems are not clear. To better understand how soil nutrient availability determines nutrient limitation in vegetation, we grew the same species of grass (Schizachyrium scoparium) in 98 North American grassland soils and fertilized them factorially with nitrogen (N) and phosphorus (P). On average adding N, P, and the two nutrients together increased biomass relative to unfertilized plants by 81%, 22%, and 131%, respectively. Plants grown on low-P soils were not primarily limited by P. Instead, these plants were colimited by N and P, while plants grown on high-P soils were primarily limited by N and only secondarily limited by P. Limitation was not predicted by total soil N. The preponderance of colimitation between N and P on low-P soils suggests that low P availability alters the N cycle to constrain supplies to plants such that N and P are made available in proportion to their demand by plants.

Cellulosic biofuel contributions to a sustainable energy future: Choices and outcomes

Greenhouse gas (GHG) emissions from soils are a key sustainability metric of cropping systems. During crop establishment, disruptive land‐use change is known to be a critical, but under reported period, for determining GHG emissions. We measured soil N2O emissions and potential environmental drivers of these fluxes from a three‐year establishment‐phase bioenergy cropping systems experiment replicated in southcentral Wisconsin (ARL) and southwestern Michigan (KBS). Cropping systems treatments were annual monocultures (continuous corn, corn–soybean–canola rotation), perennial monocultures (switchgrass, miscanthus, and poplar), and perennial polycultures (native grass mixture, early successional community, and restored prairie) all grown using best management practices specific to the system. Cumulative three‐year N2O emissions from annuals were 142% higher than from perennials, with fertilized perennials 190% higher than unfertilized perennials. Emissions ranged from 3.1 to 19.1 kg N2O‐N ha−1 yr−1 for the annuals with continuous corn > corn–soybean–canola rotation and 1.1 to 6.3 kg N2O‐N ha−1 yr−1 for perennials. Nitrous oxide peak fluxes typically were associated with precipitation events that closely followed fertilization. Bayesian modeling of N2O fluxes based on measured environmental factors explained 33% of variability across all systems. Models trained on single systems performed well in most monocultures (e.g., R2 = 0.52 for poplar) but notably worse in polycultures (e.g., R2 = 0.17 for early successional, R2 = 0.06 for restored prairie), indicating that simulation models that include N2O emissions should be parameterized specific to particular plant communities. Our results indicate that perennial bioenergy crops in their establishment phase emit less N2O than annual crops, especially when not fertilized. These findings should be considered further alongside yield and other metrics contributing to important ecosystem services.

Spring-water Nitrate Increased with Removal of Livestock Grazing in a California Oak Savanna

The promise of cellulose Cellulosic bioenergy, obtained from the lignocellulose that makes up nearly half of plant biomass, has considerable potential as an environmentally friendly energy source, but it still requires substantial resources to produce. Robertson et al. review the trade-offs between the use of cellulosic biofuels and climate mitigation, biodiversity, reactive nitrogen loss, and water use to direct more effective policies for their production. Growing native species on unfarmed land is a promising way forward. Science, this issue p. eaal2324 BACKGROUND Cellulosic biofuels offer environmental benefits not available from grain-based biofuels and are a cornerstone of efforts to meet transportation fuel needs in a future low-carbon economy, even with electrified vehicles and other advances. Bioenergy with carbon capture and storage (BECCS) is also key to almost all Intergovernmental Panel on Climate Change mitigation scenarios that constrain end-of-century atmospheric CO2 to 450 parts per million. Some cellulosic feedstocks can come from industrial and agricultural by-products or from winter cover crops, but a substantial fraction must come from cellulosic biomass crops—perennial grasses and short-rotation trees planted for this purpose. Land requirements, however, are substantial and raise crucial questions about the environmental sustainability of a future bioenergy economy. First, if planted on existing croplands, will biofuel crops increase food prices or lead to the establishment of new cropland elsewhere, with concomitant climate harm? Second, will planting biofuel crops diminish biodiversity, especially if non-native or invasive species are cultivated on land with existing conservation value? Third, might perennial biofuel crops use more water than the vegetation they replace, leading to lower water tables and reduced surface water flows? And finally, if crops are fertilized, how much additional reactive nitrogen might be added to a biosphere already overburdened? ADVANCES Recent empirical findings have shed considerable light on these questions. Broad generalizations are difficult, but we know now, for example, that planting perennial cellulosic biofuel crops on marginal lands—that is, land not currently used for food production because of low fertility, environmental sensitivity, or other reasons—can potentially avoid food-fuel conflict and indirect land-use change effects while providing substantial climate benefits. The direct carbon costs of establishing crops on such lands can be minimized by avoiding tillage and by avoiding land with large existing carbon stocks, such as forests and wetlands. Diverse plantings provide multiple ecosystem services including wildlife conservation, pollination, and pest protection that can benefit neighboring crops; relatively little plant diversity can provide disproportionately large benefits. Biofuel crops can be planted that require little if any nitrogen fertilizer, thus avoiding its environmental impact. And although different crops have different water-use efficiencies, most crops examined appear to evapotranspire about the same proportion of growing season rainfall, suggesting little impact on landscape water balances in humid temperate regions. It is also clear that there is no best crop for all locations even within a single region, and that all choices involve trade-offs. For example, highly productive non-native species can maximize climate benefits but harm biodiversity. Balancing trade-offs entails societal choices. OUTLOOK Many questions about cellulosic biofuel sustainability remain. Still needed is an integrated understanding of the entire field-to-product enterprise sufficient to leverage synergies and to avoid trade-offs that can diminish environmental benefits. More specifically, and of particular importance, is the need for knowledge to facilitate the successful cultivation of highly productive native species on marginal lands, where plant growth is often limited by abiotic stressors. Harnessing the plant microbiome to help ameliorate environmental stress is a major untapped frontier, as is the potential for microbiome-assisted soil carbon gain. The promise of cellulosic biofuels for helping to create a more sustainable energy future is bright, but additional effort is required, including policies and incentives to motivate farmers to grow appropriate crops in appropriate places in sustainable ways. We must be careful to facilitate genuine climate mitigation that enhances rather than diminishes other ecosystem services. The planet deserves no less. Switchgrass (Panicum virgatum) at daybreak in the U.S. Midwest. Switchgrass is one of several promising cellulosic biofuel species that are native and can provide high yields and greenhouse gas mitigation as well as other ecosystem services associated with nitrogen and water conservation and insect and wildlife biodiversity, especially when grown in species mixtures. PHOTO: K. STEPNITZ, MICHIGAN STATE UNIVERSITY Cellulosic crops are projected to provide a large fraction of transportation energy needs by mid-century. However, the anticipated land requirements are substantial, which creates a potential for environmental harm if trade-offs are not sufficiently well understood to create appropriately prescriptive policy. Recent empirical findings show that cellulosic bioenergy concerns related to climate mitigation, biodiversity, reactive nitrogen loss, and crop water use can be addressed with appropriate crop, placement, and management choices. In particular, growing native perennial species on marginal lands not currently farmed provides substantial potential for climate mitigation and other benefits.

Diversity, productivity and landscape-level effects in North American grasslands managed for biomass production

We characterized spatial and temporal changes in nitrate concentrations of the leachate from annual grasslands and subsequently emergent spring-waters and tested the effect of livestock grazing removal on them. Nitrate patterns indicated that annual grassland soils are a likely N source to spring-fed wetlands, which appear to intercept and transform N along its hydrologic path from upland soils to spring-fed, headwater streams. Aboveground biomass and soil N extractions suggested that removal of livestock grazing from these wetlands impaired this function by allowing dead plant material to accumulate inhibiting plant production (hence, plant N demand), resulting in elevated stream-water nitrate (NO3−) concentrations. Nitrous oxide (N2O) fluxes indicated that grazing removal may increase the relative importance of this N-loss pathway. Microbial biomass varied with season but was not affected by grazing treatments suggesting that N2O losses were related to differences in NO3− availability rather than grazing effects on microbial community composition or their activity. Spring-fed wetlands provide important ecosystem services such as plant uptake and denitrification at transition zones between terrestrial and aquatic ecosystems. These N-retention and transformation functions may be enhanced through biomass harvesting by livestock.

Influence of corn, switchgrass, and prairie cropping systems on soil microbial communities in the upper Midwest of the United States.

Expanding markets for bioenergy will increasingly shape the composition and configuration of crop production. Growing interest in second-generation biofuels (e.g., lignocellulosic ethanol) is driving a discussion about the most productive and appropriate cropping systems. Interest in perennial grasslands as a bioenergy source has many people asking about the importance of plant community diversity in bolstering productivity, resistance to pest and pathogen pressure and wildlife habitat, among other ecosystem services. We review the current understanding of diversity–productivity relationships across multiple spatial scales, but also emphasize perspectives that have received less attention in the literature.

Grazing effects on spring ecosystem vegetation of California's hardwood rangelands.

Because soil microbes drive many of the processes underpinning ecosystem services provided by soils, understanding how cropping systems affect soil microbial communities is important for productive and sustainable management. We characterized and compared soil microbial communities under restored prairie and three potential cellulosic biomass crops (corn, switchgrass, and mixed prairie grasses) in two spatial experimental designs – side‐by‐side plots where plant communities were in their second year since establishment (i.e., intensive sites) and regionally distributed fields where plant communities had been in place for at least 10 years (i.e., extensive sites). We assessed microbial community structure and composition using lipid analysis, pyrosequencing of rRNA genes (targeting fungi, bacteria, archaea, and lower eukaryotes), and targeted metagenomics of nifH genes. For the more recently established intensive sites, soil type was more important than plant community in determining microbial community structure, while plant community was the more important driver of soil microbial communities for the older extensive sites where microbial communities under corn were clearly differentiated from those under switchgrass and restored prairie. Bacterial and fungal biomasses, especially biomass of arbuscular mycorrhizal fungi, were higher under perennial grasses and restored prairie, suggesting a more active carbon pool and greater microbial processing potential, which should be beneficial for plant acquisition and ecosystem retention of carbon, water, and nutrients.