Soil amendments involve modifying the composition of soils to increase methanotrophy (the consumption of methane by bacteria and archaea). This method mimics a natural sink of methane through soil uptake by methanotrophs.
Various soil amendments are known to increase methanotrophy in some conditions, including silicate dust which may contribute copper and trace inorganic minerals such as lanthanum and cerium, and organic materials such as biochar, cover crop residues, compost, and sewage. While experiments have shown measurable effects on methane fluxes in plot studies and laboratory incubations, none have yet been tested at scale. Other soil amendments may be discovered as well. As soil amendments may affect soil nitrous oxide emissions and soil carbon dynamics depending on the environmental conditions, assessment of their net climate impacts must include assessment of these factors.
These approaches may warrant additional exploration in natural systems and/or agricultural settings. This overview focuses on the subset of soil amendments that have potential for a net uptake of atmospheric methane. Applying any of these approaches for atmospheric methane removal is at the early stage of conceptualization. Little is yet known about the potential effectiveness and side effects.
There is insufficient peer-reviewed literature on soil amendments for methane removal to assess feasibility.
Little is also known about the cost per unit of methane uptake using soil amendments. Cost will depend on the methane uptake rate and capacity and may vary over time or with varying environmental conditions. Raw materials, transport, application, and ongoing monitoring are also factors in the cost.
There is insufficient literature on soil amendments for methane removal to assess scalability. The potential scale may depend on the suitability of the land for methane uptake enhancement and raw material availability.
As a benchmark, a net annual uptake of 10 million metric tons of methane (830 Mt CO2e using GWP20) would require enhancing the current global methane soil sink by 20% to 100%. This requirement might be somewhat less if methanogenesis suppression co-benefits are factored in.
The scale of soil amendment deployment may be limited by the availability of suitable land and availability of materials, like compost, biochar, silicate dust, or basaltic rock, though these limitations cannot be assessed before the impact of each soil amendment on net methane flux is assessed.
Interventions in natural or agricultural systems must be approached very carefully. Whatever is added to an agricultural or natural ecosystems must have socially-acceptable impacts on waters from runoff, human health, the productivity of agricultural lands, and the environment. Measuring and testing of possible impacts, first at laboratory scale and then in small-scale field experiments, is a crucial prerequisite for deployment.
Soil amendments may also have positive impacts, such as improving agricultural productivity, sequestering carbon, reducing nitrous oxide emissions, or decreasing chlorinated hydrocarbon pollutants. These impacts may help incentivize adoption and make solutions more scalable.
Over 40% of ice-free land has been modified by humans, primarily for agriculture. Agricultural soils generally have lower rates of methane uptake (by as much as a factor of 7) relative to native soils, which may present an opportunity to enhance methane uptake on agricultural land without directly affecting natural ecosystems.
Methanogens produce methane, and methanotrophs and some ammonia-oxidizing bacteria consume methane. The addition of soil amendments can alter the dynamics of these soil microbial systems, for example by providing them more micronutrients. Increasing methanotrophic activity would increase the rate of drawdown of atmospheric methane or suppression of natural methane emissions. Decreasing methanogenesis helps to reduce methane emissions from soils, and could be an emissions avoidance approach, but isn’t an atmospheric methane removal approach considered here.
Methanotrophs can achieve net uptake of methane at atmospheric concentrations of methane (2 ppm), but their methane consumption rate is extremely slow. The extent of natural global net methane uptake from soils is poorly understood, estimated to be somewhere between 11 and 49 Mt/yr. For soil amendments to be a feasible atmospheric methane removal approach, the consumption rate would need to increase significantly. Whether that’s possible is not yet known.
Several potential pathways have been proposed for how soil amendments can enhance methanotrophy to create a net atmospheric methane sink:
The methane uptake and side effects of soil amendments are affected by temperature, moisture, pH, salinity, oxygen availability, soil type, the existing soil microbiological community, and nutrient profile, especially nitrogen, and may vary over time. The full range of behaviors has not been well characterized in agricultural or natural environments. While methane uptake in upland soils has been studied widely in a variety of ecosystems, most research regarding mitigation interventions affecting the production or consumption of methane in soils has focused on rice paddies. Interventions to enhance atmospheric removal by soils or vegetation are at the early stage of investigation.
Key questions for this approach include: