Biomethane produced from maize grown on peat emits more CO2 than natural gas

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. Based on 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.


Main text
Biomethane is the main fuel component of biogas, a mixture of methane (CH4) and carbon dioxide (CO2), produced via anaerobic digestion (AD) of organic matter.Production of biomethane as fuel has increased four-fold since 2000 1 .A major driver of this increase has been the climate mitigation benefits of generating energy from materials such as food and livestock waste, or recently photosynthesised crop biomass, such that the net emission of CO2 to the atmosphere is close to zero.It has been estimated that biomethane production via AD has the potential to reduce global greenhouse gas (GHG) emissions by 10-13% and meet 6-9% of global primary energy demand 2 .To achieve these levels, however, it will be necessary to greatly expand the cultivation of feedstocks such as maize (Zea mays) grown specifically for AD to occupy ~7% of the current 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 CO2 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 .To date, however, production of biomethane feedstock crops on peat, notably in Europe, has not received such critical attention.
Taking the UK 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 ~6000 ha in 2015 (the first year for which national activity data are available) to > 11000 ha in 2020-2021 10 .Over the same period, the proportion of UK maize grown for AD increased from 20% to 34% 11 .Assuming that the fraction of maize grown on peat being utilised for AD corresponds with the national average, this represents a three-fold increase in maize cultivation for biogas on peat soils.Contributory factors in this growth have been government financial support for biogas production via the Renewable Heat Incentive (2011-2021) and Green Gas Support Scheme (2021-2025), policies which are intended to support energy sector decarbonisation.UK Government conversion factors for embodied GHG emissions in biogas (0.001 kg CO2e m -3 of gas with a 65% CH4 content) compare highly favourably to those associated with natural gas (2.05 kg CO2e m -3 ) 12 .The biomethane figure is based on a mixed waste and crop feedstock, but excludes any soil CO2 emissions from crop cultivation.Our data, obtained from field-scale measurements of drained peat under rotational cropping 6,13 show that soil CO2 emissions under maize crops are consistently high (19.2-28.1 t CO2 ha -1 yr -1 ), similar to those for food crops grown in the same rotation (wheat and lettuce, 22.2-27.6t CO2 ha -1 yr -1 ) (Figure 1) and to the Intergovernmental Panel on Climate Change (IPCC) 'Tier 1' CO2 emission factor (EF) for cropland on drained temperate peat soils of 29.0 (95% confidence interval 23.8-34.5)t CO2 ha -1 yr -1 14 .We therefore conclude that maize (and similar bioenergy crops) grown on peat likely generate similar soil CO2 emissions to other food crops grown on the same soil, and with the same drainage.On this basis, we applied the UK's country-specific ('Tier 2') EFs for cropland on peat of 27.1 (95% confidence interval 9.6-44.5)t CO2 ha -1 yr -1 for cropland on deeper peat (> 40 cm) and 16.0 t (10.2-21.8)CO2 ha -1 yr -1 for cropland on thinner peaty soils 15 .
Figure 1.Annual CO2 emissions to the atmosphere for a drained agricultural peatland under rotational cropping.Data are derived from eddy covariance CO2 flux data combined with carbon imports and exports 13 .
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 t DM yr -1 , along with published data on biogas yield and methane content from maize feedstocks 3 , giving a value of 4484 m 3 CH4 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 kg CO2e m -3 , this produces a maximum 'avoidable' fossil CO2 emission, per unit area of maize cultivation, of 9.2 t CO2 ha -1 yr -1 .This can be considered an effective break-even point for soil-derived CO2 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 CO2 emissions from thin peaty soils used to grow maize for AD are around 1.74 times higher than the avoided fossil CO2 emission from the resulting biogas (Figure 2).For thicker peat soils, this figure rises to 2.95.If we take the IPCC's Tier 1 EF for cropland on drained organic soils, biogas derived from maize grown on peat is estimated to emit 3.15 times more CO2 than natural gas.Expressing the UK figures in terms of the biomethane produced, we calculate embodied soil CO2 emissions of 3.55 kg CO2 m -3 where maize feedstock is grown on thin peaty soils, and 6.01 kg CO2 m -3 where it is grown on thicker peat, relative to the avoided natural gas emission of 2.05 kg CO2 m -3 .

Figure 2. Annual net CO2 emissions from maize-derived biomethane relative to a natural gas counterfactual, expressed per unit area of crop production, relative to the soil-derived CO2 emissions from that land area. 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 green line shows the 'break-even' point beyond which soil CO2 emissions exceed the avoided emissions from substitution of natural gas. The yellow and red lines show the level of soil CO2 emissions, and net biogas emissions, resulting from bioenergy crop cultivation on drained thin peaty soils and thicker peat soils.
Our analysis is not a full life-cycle analysis (LCA) of biomethane production relative to that of natural gas.A previous LCA for a maize-only AD system suggests additional production and transport emissions 16 , and fugitive CH4 emissions from crop-based AD plants may be high 17 .We also did not account for non-CO2 GHG emissions from peatlands under crop cultivation, including nitrous oxide (N2O) from peat oxidation and fertiliser use, or CH4 emissions from drainage ditches, which may both be considerable 14 .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 fieldscale GHG emissions.However, many countries (including the UK 18 ) are seeking to reduce the area of peatland under cultivation for food crops, and to reduce drainage intensity within remaining areas of cultivation, 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 take high-value land out of food production to generate bioenergy, if this bioenergy has a higher embodied CO2 emission than the fossil fuel it replaces.
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 19 , 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 'breakeven point' at which soil CO2 emissions exceed the avoided emissions of the bioenergy.This will vary according to soil type, crop yield and energy yield, as illustrated by Figure 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 when implemented on peat.