Climate Benefits of Increasing Plant Diversity in Perennial Bioenergy Crops

Abstract

Bioenergy from perennial grasses mitigates climate change via displacing fossil fuels and storing atmospheric CO2 belowground as soil carbon. Here, we conduct a critical review to examine whether increasing plant diversity in bioenergy grassland systems can further increase their climate change mitigation potential. We find that compared with highly productive monocultures, diverse mixtures tend to produce as great or greater yields. In particular, there is strong evidence that legume addition improves yield, in some cases equivalent to mineral nitrogen fertilization at 33–150 kg per ha. Plant diversity can also promote soil carbon storage in the long term, reduce soil N2O emissions by 30%–40%, and suppress weed invasion, hence reducing herbicide use. These potential benefits of plant diversity translate to 50%–65% greater life-cycle greenhouse gas savings for biofuels from more diverse grassland biomass grown on degraded soils. In addition, there is growing evidence that plant diversity can accelerate land restoration. Bioenergy from perennial grasses mitigates climate change via displacing fossil fuels and storing atmospheric CO2 belowground as soil carbon. Here, we conduct a critical review to examine whether increasing plant diversity in bioenergy grassland systems can further increase their climate change mitigation potential. We find that compared with highly productive monocultures, diverse mixtures tend to produce as great or greater yields. In particular, there is strong evidence that legume addition improves yield, in some cases equivalent to mineral nitrogen fertilization at 33–150 kg per ha. Plant diversity can also promote soil carbon storage in the long term, reduce soil N2O emissions by 30%–40%, and suppress weed invasion, hence reducing herbicide use. These potential benefits of plant diversity translate to 50%–65% greater life-cycle greenhouse gas savings for biofuels from more diverse grassland biomass grown on degraded soils. In addition, there is growing evidence that plant diversity can accelerate land restoration. More than 84% of global energy production is derived from fossil fuels, including natural gas, petroleum, and coal.1Höök M. Tang X. Depletion of fossil fuels and anthropogenic climate change—A review.Energy Policy. 2013; 52: 797-809Crossref Scopus (359) Google Scholar Energy production from these non-renewable resources releases ∼35 billion Mg of CO2 per year; if trends do not change, annual emissions will exceed 43 billion Mg by 2050.2EIAInternational Energy Outlook 2019 with Projections to 2050. Energy Information Administration, U.S. Department of Energy, 2019Google Scholar Fossil-fuel-based CO2 emissions are the primary cause of climate change, which is expected to result in average global temperature increases between 1.5 and 4°C by 2050.3Pachauri RK Allen MR Barros VR Broome J Cramer W Christ R Pachauri R Meyer L Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. IPCC, Geneva, Switzerland2014Google Scholar The Intergovernmental Panel on Climate Change (IPCC) warns that even the lowest predicted increases could trigger disastrous consequences, including sea level rise, mass population displacement, increased frequency of extreme weather, and mass species extinctions.3Pachauri RK Allen MR Barros VR Broome J Cramer W Christ R Pachauri R Meyer L Climate Change 2014: Synthesis Report. 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Goh C.S. Sustainable bioenergy deployment in East and South East Asia: notes on recent trends.Sustain Sci. 2019; (Published online June 29, 2019)https://doi.org/10.1007/s11625-019-00712-wCrossref Scopus (0) Google Scholar The chemical composition of various biomass types makes them suitable for different bioenergy conversion systems. Biomass sources with high lignin content (e.g., wood) can be directly combusted for heat and power in place of fossil fuels like coal. Biomass rich in starch and sugars (e.g., maize and sugarcane) is well-suited for conversion to liquid ethanol. Biomass from perennial grassland plants contains both lignin and cellulose and can be directly combusted or converted to ethanol. Cultivating multi-species mixtures of perennial grassland plants for biomass offers a unique suite of ecosystem services.16Quijas S. Schmid B. Balvanera P. Plant diversity enhances provision of ecosystem services: A new synthesis.Basic Appl. Ecol. 2010; 11: 582-593Crossref Scopus (81) Google Scholar In addition, plant diversity is associated with many other benefits, described below, that may further increase the GHG mitigation potential of these perennial grassland systems. This review summarizes how enhanced diversity of perennial bioenergy grassland plants can augment their life-cycle GHG benefits through increased soil C sequestration, increased aboveground biomass production, reduced nitrous oxide emissions, and improved weed suppression. Numerous field experiments that manipulated plant diversity have shown that increasing diversity can lead to a wide range of ecological and environmental benefits.17Isbell F. Adler P.R. Eisenhauer N. Fornara D. Kimmel K. Kremen C. Letourneau D.K. Liebman M. Polley H.W. Quijas S. et al.Benefits of increasing plant diversity in sustainable agroecosystems.J. Ecol. 2017; 105: 871-879Crossref Scopus (72) Google Scholar Grassland ecosystem productivity increases with plant diversity18Tilman D. Reich P.B. Knops J. Wedin D. Mielke T. Lehman C. Diversity and productivity in a long-term grassland experiment.Sci. 2001; 294: 843-845Crossref PubMed Scopus (0) Google Scholar, 19van Ruijven J. Berendse F. Diversity-productivity relationships: Initial effects, long-term patterns, and underlying mechanisms.Proc. Natl. Acad. Sci. USA. 2005; 102: 695-700Crossref PubMed Scopus (0) Google Scholar, 20Cardinale B.J. Wright J.P. Cadotte M.W. Carroll I.T. Hector A. Srivastava D.S. Loreau M. Weis J.J. Impacts of plant diversity on biomass production increase through time because of species complementarity.Proc. Natl. Acad. Sci. USA. 2007; 104: 18123-18128Crossref PubMed Scopus (0) Google Scholar because more diverse communities have greater diversity of traits to foster species complementarity and to access resources partitioned in time.21Loreau M. Hector A. 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Questions have been raised about whether the benefits of plant diversity observed in grassland biodiversity experiments would hold true in bioenergy crop production.31Dickson T.L. Gross K.L. Can the Results of Biodiversity-Ecosystem Productivity Studies Be Translated to Bioenergy Production?.PLoS One. 2015; 10: e0135253Crossref PubMed Scopus (12) Google Scholar, 32Griffith A.P. Epplin F.M. Fuhlendorf S.D. Gillen R. A Comparison of Perennial Polycultures and Monocultures for Producing Biomass for Biorefinery Feedstock.Agron. J. 2011; 103: 617-627Crossref Scopus (29) Google Scholar, 33Russelle M.P. Morey R.V. Baker J.M. Porter P.M. Jung H.-J.G. Comment on “Carbon-Negative Biofuels from Low-Input High-Diversity Grassland Biomass.”.Sci. 2007; 316: 1567Crossref PubMed Google Scholar In particular, the species used in biodiversity experiments are randomly selected from a large pool of native species,19van Ruijven J. Berendse F. 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Mitchell R. Heaton E.A. Perennial biomass crop establishment, community characteristics, and productivity in the upper US Midwest: Effects of cropping systems seed mixtures and biochar applications.Eur. J. Agron. 2018; 101: 121-128Crossref Scopus (3) Google Scholar,37Butler T.J. Muir J.P. Huo C. Guretzky J.A. Switchgrass Biomass and Nitrogen Yield with Over-Seeded Cool-season Forages in the Southern Great Plains.BioEnergy Res. 2013; 6: 44-52Crossref Scopus (10) Google Scholar Even in biodiversity experiments, although more diverse systems tend to be more productive on average, diverse mixtures do not always outperform the most productive monocultures.20Cardinale B.J. Wright J.P. Cadotte M.W. Carroll I.T. Hector A. Srivastava D.S. Loreau M. Weis J.J. Impacts of plant diversity on biomass production increase through time because of species complementarity.Proc. Natl. Acad. Sci. USA. 2007; 104: 18123-18128Crossref PubMed Scopus (0) Google Scholar Furthermore, grassland biodiversity experiments are generally carried out at a small scale (e.g., plot size of ∼10 m × 10 m) and species diversity is well maintained through, for example, regular weeding.22Tilman D. Reich P.B. Knops J.M.H. Biodiversity and ecosystem stability in a decade-long grassland experiment.Nature. 2006; 441: 629-632Crossref PubMed Scopus (968) Google Scholar In contrast, bioenergy crop production will be carried out at large farm scale and species diversity may not be as well maintained. These differences raise questions about the extent to which the benefits of biodiversity can be translated to bioenergy cropping systems.31Dickson T.L. Gross K.L. Can the Results of Biodiversity-Ecosystem Productivity Studies Be Translated to Bioenergy Production?.PLoS One. 2015; 10: e0135253Crossref PubMed Scopus (12) Google Scholar, 32Griffith A.P. Epplin F.M. Fuhlendorf S.D. Gillen R. A Comparison of Perennial Polycultures and Monocultures for Producing Biomass for Biorefinery Feedstock.Agron. J. 2011; 103: 617-627Crossref Scopus (29) Google Scholar, 33Russelle M.P. Morey R.V. Baker J.M. Porter P.M. Jung H.-J.G. Comment on “Carbon-Negative Biofuels from Low-Input High-Diversity Grassland Biomass.”.Sci. 2007; 316: 1567Crossref PubMed Google Scholar Here, we present a critical review of perennial bioenergy cropping experiments to gain a better understanding of the potential effects of increasing plant diversity. With a focus on grassland biomass, we reviewed ∼50 studies that covered a wide range of perennial grassland systems that are candidates for bioenergy crops, including many warm-season (C4) grasses (e.g., switchgrass, big bluestem, indiangrass, and bermudagrass) as well as cool-season (C3) grasses and legumes (e.g., intermediate wheatgrass, tall fescue, orchardgrass, and alfalfa). The studies included ∼90 experiments carried out in ∼70 different sites, most in the US (Figure 1; see Box 1 for details on literature search). The experiments included different diversity treatments, e.g., comparing productive monocultures like switchgrass with more diverse mixtures, interseeding legume species in C4 or C3 grasses, or manipulating plant species richness from low- to high-diversity mixtures selected from a pool of productive plant species (see Supplemental Information). A synthesis of these studies provides an opportunity to examine the question of whether increasing plant diversity can benefit perennial bioenergy cropping systems.Box 1Literature SearchThe aim of this review is to examine the effects of plant diversity on perennial bioenergy grass production systems, emphasizing (1) aboveground biomass yield, (2) soil C storage, (3) nitrous oxide emissions, (4) nitrate leaching, and (5) weed suppression. Thus, the studies reviewed (1) focused on grasses that are candidate bioenergy feedstocks instead of random species as in biodiversity experiments, (2) included multiple diversity levels (e.g., from monoculture to low- and high-diversity mixtures or legume-grass versus grass only), and (3) investigated any of the 5 aspects.A search was conducted on ISI Web of Science and Google Scholar for journal articles in English published prior to June 2019 using the following search terms:“aboveground biomass OR yield OR forage OR total biomass OR soil carbon storage OR soil carbon stocks OR soil organic matter OR N2O emissions OR nitrous oxide OR N fertilizer OR nitrate leaching OR nitrate runoff OR weed growth OR weed suppression OR weed competition AND plant diversity OR biodiversity OR species diversity OR mixture OR monoculture OR single species OR polyculture OR richness OR composition AND perennial bioenergy crop OR perennial grass OR grassland biomass”Results were evaluated as to whether the primary species studied are bioenergy feedstock candidates, such as switchgrass, big bluestem, little bluestem, indiangrass, Miscanthus, intermediate wheatgrass, alfalfa, orchard grass, reed canarygrass, and tall fescue. References within the remaining articles were searched as well. From the articles that met the initial criteria, those that reported only single-year results and those that did not compare treatments with a control were excluded. In some cases, experiments were reported in multiple studies; only one was included in the review. A total of 54 studies were selected, including 89 experiments across ~70 different sites located mostly in the US. The aim of this review is to examine the effects of plant diversity on perennial bioenergy grass production systems, emphasizing (1) aboveground biomass yield, (2) soil C storage, (3) nitrous oxide emissions, (4) nitrate leaching, and (5) weed suppression. Thus, the studies reviewed (1) focused on grasses that are candidate bioenergy feedstocks instead of random species as in biodiversity experiments, (2) included multiple diversity levels (e.g., from monoculture to low- and high-diversity mixtures or legume-grass versus grass only), and (3) investigated any of the 5 aspects. A search was conducted on ISI Web of Science and Google Scholar for journal articles in English published prior to June 2019 using the following search terms: “aboveground biomass OR yield OR forage OR total biomass OR soil carbon storage OR soil carbon stocks OR soil organic matter OR N2O emissions OR nitrous oxide OR N fertilizer OR nitrate leaching OR nitrate runoff OR weed growth OR weed suppression OR weed competition AND plant diversity OR biodiversity OR species diversity OR mixture OR monoculture OR single species OR polyculture OR richness OR composition AND perennial bioenergy crop OR perennial grass OR grassland biomass” Results were evaluated as to whether the primary species studied are bioenergy feedstock candidates, such as switchgrass, big bluestem, little bluestem, indiangrass, Miscanthus, intermediate wheatgrass, alfalfa, orchard grass, reed canarygrass, and tall fescue. References within the remaining articles were searched as well. From the articles that met the initial criteria, those that reported only single-year results and those that did not compare treatments with a control were excluded. In some cases, experiments were reported in multiple studies; only one was included in the review. A total of 54 studies were selected, including 89 experiments across ~70 different sites located mostly in the US. Because a primary goal of bioenergy is to mitigate climate change, our review focused on 5 main areas: aboveground biomass (or yield), soil carbon (C) storage, soil N2O emissions, nitrate leaching, and weed control. In other words, we evaluated whether more diverse systems would produce greater yields, store more soil C, have lower rates of nitrogen losses, and be more resistant to weed invasion, especially as opposed to monoculture systems that are commonly studied.38Wang D. Lebauer D.S. Dietze M.C. 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