Methane is a potent greenhouse gas, second only to carbon dioxide in its contribution to atmospheric warming. Understanding how this gas interacts with terrestrial ecosystems is important for predicting future climate trends. While the question of whether plants absorb methane seems straightforward, the mechanisms involved are complex and indirect. Plants do not directly absorb methane through their leaves or stems like they do carbon dioxide. Instead, methane consumption is a biological process mediated entirely by specialized microorganisms living in the soil and around plant roots, positioning the plant as a facilitator rather than an absorber.
The Microbial Mechanism of Methane Consumption
The direct consumption of methane is performed by prokaryotes known as methanotrophs, or methane-eating bacteria. These microorganisms are unique because they use methane as their sole source of carbon and energy. This process, called methane oxidation, is the primary way methane is naturally removed from the environment. The chemical conversion is initiated by the enzyme methane monooxygenase (MMO), which transforms the stable methane molecule into methanol. The methanol is then further oxidized to produce carbon dioxide and cellular biomass. Aerobic methanotrophs require oxygen to perform this conversion and are classified into two groups based on their carbon assimilation pathways.
How Plant Roots Influence Methane Uptake
The plant root system is an active moderator of soil conditions that significantly supports methanotrophic activity. The immediate soil layer surrounding the roots, known as the rhizosphere, is a hotspot for methane consumption. Roots facilitate this process by creating an oxygen-rich microenvironment and providing a food source for the microbes.
Plant roots, especially in waterlogged soils, transport oxygen from the leaves down to the root tips, a process called radial oxygen loss. This aeration creates the essential aerobic-anaerobic interface where oxygen-dependent methanotrophs can thrive. Studies show that methane oxidation rates can be more than double in soil containing intact roots compared to root-free soil, demonstrating the direct influence of this oxygen supply.
Roots also release a variety of organic compounds, called exudates, into the soil, which can account for up to 40% of the carbon fixed during photosynthesis. These exudates, which include sugars, amino acids, and organic acids, stimulate microbial growth and activity within the rhizosphere. However, the influence of these exudates is complex, as they can also fuel other microbial communities, leading to competition for resources and space.
The Paradox of Plant-Dominated Methane Sources
The role of plants as methane-consuming facilitators is complicated because certain plant-dominated ecosystems are major global methane sources. Wetlands, marshes, and rice paddies are naturally waterlogged, creating anoxic (oxygen-depleted) conditions in the deeper soil layers. In this anaerobic environment, methanogens (methane-producing archaea) thrive by breaking down organic matter. These methanogens produce large quantities of methane gas that attempt to diffuse upward toward the atmosphere.
The specialized anatomy of many aquatic plants, such as rice and sedges, provides a preferential escape route. These plants possess aerenchyma, which are internal gas-filled channels that transport oxygen from the leaves down to the submerged roots for survival. The aerenchyma acts as a low-resistance “chimney,” allowing methane produced deep in the soil to be vented directly to the atmosphere through the plant stem. This plant-mediated transport is the dominant emission pathway in many wetlands, allowing the gas to bypass the thin aerobic surface layer where methanotrophs would normally consume between 60% and 90% of the methane.
Terrestrial Ecosystems and the Global Methane Budget
The activity of soil methanotrophs in terrestrial ecosystems represents a significant natural sink for atmospheric methane globally. Well-aerated soils, such as those found in upland forests, grasslands, and drylands, consistently draw methane out of the atmosphere. Estimates suggest that global soils consume approximately 32 to 36 Teragrams (Tg) of methane annually, accounting for about 10% of the total global methane sink.
The capacity of this sink is highly sensitive to environmental conditions, particularly soil moisture and temperature. High soil moisture content can quickly turn a methane sink into a source by reducing the air-filled pore space necessary for aerobic methanotroph activity. Conversely, rising soil temperatures, particularly in high-latitude soils, can stimulate methanotrophic activity, potentially increasing the sink capacity in those regions. Land use practices and the deposition of nitrogen from pollution can also inhibit the methane oxidation rate in agricultural and forest soils.