Soil microbial ecosystems are complex, and depending on environmental conditions, soil type, and land use, the soil may be a net source or net sink for atmospheric methane, leading to net decreases or increases in emitted methane.
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 from any source, for example other elevated natural emissions elsewhere in the ecosystem. Decreasing methanogenesis helps to reduce methane emissions from soils, and could be an emissions avoidance approach, but isn’t an atmospheric methane removal approach.
Various soil amendments are known to increase methanotrophy in some conditions, including additions of copper and trace inorganic minerals such as lanthanum and cerium, introduced via silicate dust or other means, and adding 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. Net methane emissions over the lifetime of any soil amendment should be factored into assessing overall climate impacts, as well as other impacts, including changes to soil nitrous oxide emissions and soil carbon dynamics, which are condition-dependent.
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 to atmospheric removal is at the early stage of conceptualization.
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.
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.
Since the effectiveness of soil amendments is not yet known, their scalability cannot yet be determined. It may depend on the suitability of the land for methane uptake enhancement and raw material availability.
A net methane uptake of 10 Mt/yr would require enhancing the current methane soil sink by 20% to 100%. This requirement might be somewhat less if methanogenesis suppression co-benefits are factored in.
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. That presents an opportunity to enhance methane uptake on agricultural land without directly affecting natural ecosystems.
Not all land is suitable for net methane uptake. High soil moisture conditions favor methanogenesis over methanotrophy, resulting in net methane production. Nitrogen-rich soils, including fertilized soils, often exhibit lower rates of methanotrophy as well as high rates of nitrous oxide emissions.
The scale of soil amendment deployment may be limited by 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. We need to ensure that whatever we add to an agricultural or natural ecosystems doesn’t have negative impacts on waters from runoff, human health, the productivity of agricultural lands, or the stability of the environment. Measuring and testing of possible impacts, first at laboratory scale and and then in small-scale field experiments is a crucial prerequisite for deployment.
On the other hand, soil amendments may also have positive impacts, such as improving agricultural productivity, sequestering carbon, reducing nitrous oxide emissions, or decreasing chlorinated hydrocarbon pollutants. These may help incentivize adoption, and make solutions more scalable.