https://www.nature.com/articles/s41558-024-02111-1 Skip to main content Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. Advertisement Advertisement Nature Climate Change * View all journals * Search * Log in * Explore content * About the journal * Publish with us * Sign up for alerts * RSS feed 1. nature 2. nature climate change 3. brief communications 4. article Biomethane produced from maize grown on peat emits more CO[2] than natural gas Download PDF Download PDF * Brief Communication * Open access * Published: 09 September 2024 Biomethane produced from maize grown on peat emits more CO[2] than natural gas * Chris D. Evans ORCID: orcid.org/0000-0002-7052-354X^1, * Rebecca L. Rowe ORCID: orcid.org/0000-0002-7554-821X^2, * Benjamin W. J. Freeman^1, * Jennifer M. Rhymes ORCID: orcid.org/0000-0001-9347-9863^1, * Alex Cumming^3, * Isobel L. Lloyd ORCID: orcid.org/0000-0003-2518-6916^4, * Daniel Morton^2, * Jennifer L. Williamson^1 & * ... * Ross Morrison^3 Show authors Nature Climate Change (2024)Cite this article * 1215 Accesses * 101 Altmetric * Metrics details Subjects * Agriculture * Biogeochemistry * Climate-change mitigation * Energy supply and demand * Environmental impact Abstract Cultivation of maize for biomethane production has expanded rapidly, including on drained peat soils. The resulting soil CO[2] emissions at the point of feedstock production are largely overlooked when assessing biogas climate mitigation potential. On the basis of field-scale flux measurements, we calculate that soil CO[2] emissions from biomethane feedstock production on drained peat exceed embodied emissions for an equivalent amount of natural gas by up to a factor of three. Similar content being viewed by others [41598_2020] Soil CO[2] emission in response to organic amendments, temperature, and rainfall Article Open access 03 April 2020 [43247_2024] Underestimation of carbon dioxide emissions from organic-rich agricultural soils Article Open access 30 May 2024 [41561_2021] Biochar in climate change mitigation Article 02 December 2021 Main Biomethane is the main fuel component of biogas, a mixture of methane (CH[4]) and carbon dioxide (CO[2]), produced by means of anaerobic digestion of organic matter. Production of biomethane as fuel has increased fourfold since 2000^1. A principal driver of this increase has been the climate mitigation benefits of generating energy from materials such as food and livestock waste or recently photosynthesized crop biomass, such that the net emission of CO[2] to the atmosphere is close to zero. The bioenergy industry estimates that biomethane production via anaerobic digestion has the potential to reduce GHG emissions by 10-13% and meet 6-9% of global primary energy demand^2. To achieve these amounts, however, it will be necessary to greatly expand the cultivation of feedstocks such as maize (Zea mays) grown specifically for biomethane production to occupy ~7% of the present global agricultural land area^2. The assumption of low emissions from crop-based biomethane depends critically on the carbon balance of the land on which the crop is grown. On a mineral soil, it is reasonable to assume an approximately neutral carbon balance, with the export of recently assimilated carbon in harvested biomass having little impact on the long-term soil carbon balance^3. Where crops are grown on peat, however, this assumption does not hold. All forms of conventional agriculture on peat require drainage, exposing peat to oxidation and driving rapid and sustained soil CO[2] emissions. Cultivated peatlands are estimated to have the highest GHG emission intensity of any agricultural land globally^4, generating 2-3% of all anthropogenic GHG emissions^5,6. In addition to food production, drained peatlands are increasingly used to produce biomass for bioenergy. Biodiesel derived from palm oil produced on tropical peat may result in 3-40 times more GHG emissions than fossil diesel^7. This finding led the US Environmental Protection Agency to exclude biodiesel derived from palm oil as a renewable fuel in 2011^8 and the European Union to recently announce a phase-out of palm oil in biofuels by 2030^9. So far, however, production of biomethane feedstock crops on peat, notably in Europe, has not received such critical attention. Taking the United Kingdom as a case study, the area of maize cultivation on drained peat (>40 cm) and peaty soils (soils with <40 cm of peat remaining as a result of long-term wastage) has risen from ~6,000 ha in 2015 (the first year for which national activity data are available) to >11,000 ha in 2020-2021^10. Over the same period, the proportion of UK maize grown for biomethane production increased from 20% to 34%^11. Assuming that the fraction of maize grown on peat being used for this purpose corresponds with the UK national average, this represents a threefold increase in maize cultivation for biogas on peat soils. Contributory factors in this growth have been government financial support for biogas production through the Renewable Heat Incentive (2011-2021) and Green Gas Support Scheme (2021-2025), policies that are intended to support energy sector decarbonization. UK Government conversion factors for embodied GHG emissions in biogas (0.001 kgCO[2]e m^-3 of gas with a 65% CH[4] content) compare highly favourably to those associated with natural gas (2.05 kgCO[2]e m^-3) (ref. ^12). The biomethane figure is based on a mixed waste and crop feedstock but excludes any soil CO[2] emissions from crop cultivation. Our data, obtained from field-scale measurements of drained peat under rotational cropping^6,13 show that soil CO[2] emissions under maize crops are consistently high (19.2-28.1 tCO[2] ha^-1 yr^-1) and similar to those for food crops grown in the same rotation (wheat and lettuce, 22.2-27.6 tCO[2] ha^-1 yr^-1) (Fig. 1). We therefore conclude that maize (and similar bioenergy crops) grown on peat probably generate similar soil CO[2] emissions to other food crops grown on the same soil and with the same drainage. On this basis, we applied the UK country-specific (Tier 2) emission factors (EFs) for cropland on peat of 27.1 (95% confidence interval 9.6-44.5) tCO[2] ha^-1 yr^-1 for cropland on deeper peat (>40 cm) and 16.0 (10.2-21.8) tCO[2] ha^-1 yr^-1 for cropland on thinner peaty soils^14 . The Intergovernmental Panel on Climate Change (IPCC) 'Tier 1' CO[2] EF for cropland on drained temperate peat soils 29.0 (23.8-34.5) tCO [2] ha^-1 yr^-1 (ref. ^15) is very similar to the UK Tier 2 figure for peat >40 cm, indicating that our calculations are applicable to temperate regions in general. Fig. 1: Annual CO[2] emissions to the atmosphere for a drained agricultural peatland under rotational cropping. figure 1 Data are derived from eddy covariance CO[2] flux data combined with carbon imports and exports^6,13. Error bars are calculated as the sum of the 95% confidence intervals on cumulative net ecosystem CO[2] exchange, calculated by accumulating standard deviations for each 30 min flux density using standard error propagation techniques^21 and a conservative estimate of 20% uncertainty on lateral carbon losses in harvested biomass. Full size image To calculate avoided emissions from substituting maize-derived biomethane for natural gas, we applied an average dry matter (DM) yield for UK-grown maize of 13.5 tDM yr^-1, along with published data on biogas yield and methane content from maize feedstocks^3, giving an estimated biomethane yield of 4,484 m^3 CH[4] ha^-1 yr^-1. We assumed that maize yields do not vary systematically by soil type. Combined with the UK Government's conversion factor for natural gas of 2.05 kgCO[2]e m^-3, this produces a maximum 'avoidable' fossil CO [2] emission, per unit area of maize cultivation, of 9.2 tCO[2] ha^ -1 yr^-1. This can be considered an effective break-even point for soil-derived CO[2] emissions; for maize-derived biomethane to have a lower warming impact than natural gas, soil emissions must be lower than this value. This is not the case for maize grown on peat soils: based on the Tier 2 EFs above we calculate that CO[2] emissions from thin peaty soils used to grow maize for anaerobic digestion are (depending on yields) ~1.7 times higher than the avoided fossil CO[2] emission from the resulting biomethane (Fig. 2). For thicker peat soils, this figure rises to 2.9. If we take the IPCC Tier 1 EF for cropland on drained organic soils, biogas derived from maize grown on peat is estimated to emit 3.2 times more CO[2] than natural gas. Expressing the UK figures in terms of the biomethane produced, we calculate embodied soil CO[2] emissions of 3.6 kgCO[2] m^-3 where maize feedstock is grown on thin peaty soils and 6.0 kgCO[2] m^-3 where it is grown on thicker peat, relative to the avoided natural gas emission of 2.05 kgCO[2] m^-3. Fig. 2: Annual net CO[2] emissions from maize-derived biomethane relative to a natural gas counterfactual, expressed per unit area of crop production, relative to the soil-derived CO[2] emissions from that land area. figure 2 The central black line is based on average biomethane yields per hectare of UK maize, while the upper and lower grey lines show the impact of an (arbitrary) 50% lower or higher biomethane yield, respectively. The dashed blue line shows the break-even point beyond which soil CO[2] emissions exceed the avoided emissions from substitution of natural gas. The solid yellow line shows the level of soil CO[2] emissions and net biogas emissions, resulting from bioenergy crop cultivation on drained thin peaty soils (UK Tier 2 EF); solid red and dashed black lines show the equivalent figures for thicker peat soils based on the UK Tier 2 EF and IPCC Tier 1 EF for temperate cropland on peat, respectively. Full size image Our analysis is not a full life-cycle analysis of biomethane production relative to that of natural gas. However, previous life-cycle analyses for maize-based biomethane production systems give estimates of 1.1-2.6 kg CO[2] m^-3 CH[4] (refs. ^16,17). These figures, incorporating emissions from fertilizer production, field nitrous oxide (N[2]O) emissions, fuel use during production and transport, fugitive CH[4] emissions from anaerobic digestion plants and disposal of digestate, are thus smaller in total than our single soil CO[2] emission value for maize grown on peat; the missing piece of the jigsaw is larger than the rest of the jigsaw combined. Including elevated non-CO[2] GHG emissions from peatlands under crop cultivation, including high nitrous oxide (N[2]O) emissions from peat oxidation and CH[4] emissions from drainage ditches, would further increase emissions associated with maize-production on peat^15. On the other hand, it could be argued that the counterfactual for drainage-based cultivation of biogas maize on peat is drainage-based cultivation of food crops, with the same field-scale GHG emissions. Where maize cultivation is integrated into the food production system, for example as the break crop in a vegetable rotation, biogas production could even be considered to offset some of the emissions from that system. However, many countries (including the United Kingdom^18) are seeking to reduce the area of peatland under cultivation and to reduce drainage intensity within remaining cultivated, in pursuit of net-zero emission targets. These targets are likely to place increasing pressure on domestic food production and risk simply displacing the GHG emissions and other environmental costs of food production if they drive an increased reliance on food imports^10. In this context, it does not seem wise to use land primarily for bioenergy generation, in circumstances where the bioenergy has a higher embodied CO[2] emission than the fossil fuel it replaces. Furthermore, estimated GHG emissions reductions from rewetting formerly drained cropland (32 and 21 tCO[2]e ha^-1 yr^-1 for peat and peaty soils, respectively, UK Tier 2 EFs^14) far exceed the emissions reductions that could be achieved by using the same land for maize-based biogas production and can therefore be expected to generate greater long-term climate mitigation benefit^19. Although our analysis focuses on maize and on the UK as a case study, our conclusions apply to all bioenergy crops grown on drained peat and to all countries in which they are or could be grown. Our results do not imply that all forms of bioenergy production on peat will lead to increased emissions; indeed, the production of dedicated biomass crops on agricultural peatlands managed with higher water levels (often termed 'paludiculture') holds promise for effective climate mitigation^20 and is potentially compatible with bioenergy production. However, we recommend that all bioenergy projects on peat should account for GHG emissions from peat drainage and particularly the break-even point at which soil GHG emissions exceed the avoided emissions of the bioenergy. This will vary according to soil type, crop yield and energy yield, as illustrated by Fig. 2, as well as the type of fossil fuel substituted and may be mitigated through changes in land management such as raised water levels. This is urgent: our analysis suggests that renewable energy incentives, seeking to expand biogas production to meet net-zero goals, have led to the expansion of feedstock production onto drained peat soils and a resulting increase in energy system GHG emissions. Recent initiatives that combine crop-based biogas with carbon capture and storage are highly unlikely to produce 'carbon negative' energy if implemented on peat. Data availability All data analysed in the paper have been derived from previously published materials, which are included in the listed references. References 1. Global Bioenergy Statistics 2022 (World Bioenergy Association, 2022); www.worldbioenergy.org/uploads/ 221223%20WBA%20GBS%202022.pdf 2. Global Potential of Biogas (World Biogas Association, 2019); www.worldbiogasassociation.org/wp-content/uploads/2019/09/ WBA-execsummary-4ppa4_digital-Sept-2019.pdf 3. Styles, D. et al. Consequential life cycle assessment of biogas, biofuel and biomass energy options within an arable crop rotation. Glob. Change Biol. Bioenergy 7, 1305-1320 (2015). Article CAS Google Scholar 4. Carlson, K. M. et al. Greenhouse gas emissions intensity of global croplands. Nat. Clim. Change 7, 63-68 (2017). Article CAS Google Scholar 5. Leifeld, J. & Menichetti, L. The underappreciated potential of peatlands in global climate change mitigation strategies. Nat. Commun. 9, 1071 (2018). Article CAS Google Scholar 6. 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Area of Crops Grown for Bioenergy in England and the UK 2008-2020, Section 3: Anaerobic Digestion (UK Government, 2021); www.gov.uk/government/statistics/ area-of-crops-grown-for-bioenergy-in-england-and-the-uk-2008-2020 12. Greenhouse Gas Reporting: Conversion Factors 2023 (UK Government, 2023); www.gov.uk/government/publications/ greenhouse-gas-reporting-conversion-factors-2023 13. Evans, C. et al. Managing Agricultural Systems on Lowland Peat for Decreased Greenhouse Gas Emissions Whilst Maintaining Agricultural Productivity--SP1218 (DEFRA, 2023). 14. Evans, C. et al. Aligning the UK Peatland Code with the UK Peatland Inventory--SP0822 (DEFRA, 2023); https:// sciencesearch.defra.gov.uk/ProjectDetails?ProjectID=21088& FromSearch=Y&Publisher=1&SearchText=peatland%20code&SortString= ProjectCode&SortOrder=Asc&Paging=10#Description 15. 2013 Supplement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories: Wetlands (IPCC, 2014). 16. Adams, P. W. R. & McManus, M. Characterisation and variability of greenhouse gas emissions from biomethane production via anaerobic digestion of maize. J. Clean. Prod. 218, 529-542 (2019). Article CAS Google Scholar 17. Biograce-II Excel tool. Version 4b (Biograce, accessed 12 June 2024); https://biograce.net/biograce2/ 18. Build Back Greener (UK Government, 2021); www.gov.uk/government/ publications/net-zero-strategy 19. Gunther, A. et al. Prompt rewetting of drained peatlands reduces climate warming despite methane emissions. Nat. Commun. 11, 1644 (2020). Article Google Scholar 20. Tanneberger, F. et al. Saving soil carbon, greenhouse gas emissions, biodiversity and the economy: paludiculture as sustainable land use option in German fen peatlands. Reg. Environ. Change 22, 69 (2021). Article Google Scholar 21. Levy, P. E. & Gray, A. Greenhouse gas balance of a semi-natural peatbog in northern Scotland. Environ. Res. Lett. 10, 094019 (2015). Article Google Scholar Download references Acknowledgements This work was supported by the Peat Greenhouse Gas Removal Demonstrator project, funded by UK Research and Innovation (grant reference BB/V011561/1, award to C.D.E.). The flux data presented in Fig. 1 were collected as part of the UK Department for Environment, Food and Rural Affairs Lowland Peat 2 project (SP1218, award to C.D.E. and R.M.). The contribution of I.L.L. was supported by the Natural Environment Research Council Panorama Doctoral Training Partnership (grant reference NE/S007458/1). We are grateful to the many farmers and other land managers whose support makes our work possible. Author information Authors and Affiliations 1. UK Centre for Ecology and Hydrology, Bangor, UK Chris D. Evans, Benjamin W. J. Freeman, Jennifer M. Rhymes & Jennifer L. Williamson 2. UK Centre for Ecology and Hydrology, Lancaster, UK Rebecca L. Rowe & Daniel Morton 3. UK Centre for Ecology and Hydrology, Wallingford, UK Alex Cumming & Ross Morrison 4. School of Geography, University of Leeds, Leeds, UK Isobel L. Lloyd Authors 1. Chris D. Evans View author publications You can also search for this author in PubMed Google Scholar 2. Rebecca L. Rowe View author publications You can also search for this author in PubMed Google Scholar 3. Benjamin W. J. Freeman View author publications You can also search for this author in PubMed Google Scholar 4. Jennifer M. Rhymes View author publications You can also search for this author in PubMed Google Scholar 5. Alex Cumming View author publications You can also search for this author in PubMed Google Scholar 6. Isobel L. Lloyd View author publications You can also search for this author in PubMed Google Scholar 7. Daniel Morton View author publications You can also search for this author in PubMed Google Scholar 8. Jennifer L. Williamson View author publications You can also search for this author in PubMed Google Scholar 9. Ross Morrison View author publications You can also search for this author in PubMed Google Scholar Contributions The study was conceived by C.D.E. and R.M. R.M. and A.C. were responsible for the collection and analysis of flux tower data described in the paper, with additional analysis by I.L.L., J.M.R. and C.D.E. R.L.R. and B.W.J.F. led the analysis of emissions associated with biomethane production from maize versus a natural gas counterfactual. D.M. and J.L.W. provided spatial data on maize extent. C.D.E. led the paper with contributions from all authors. All authors read and approved the final manuscript. Corresponding author Correspondence to Chris D. Evans. Ethics declarations Competing interests The authors declare no competing interests. Peer review Peer review information Nature Climate Change thanks Klaus Butterbach-Bahl and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Additional information Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Rights and permissions Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Reprints and permissions About this article Check for updates. Verify currency and authenticity via CrossMark Cite this article Evans, C.D., Rowe, R.L., Freeman, B.W.J. et al. Biomethane produced from maize grown on peat emits more CO[2] than natural gas. Nat. Clim. Chang. (2024). https://doi.org/10.1038/s41558-024-02111-1 Download citation * Received: 13 April 2024 * Accepted: 01 August 2024 * Published: 09 September 2024 * DOI: https://doi.org/10.1038/s41558-024-02111-1 Share this article Anyone you share the following link with will be able to read this content: Get shareable link Sorry, a shareable link is not currently available for this article. 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