Getting to the Root of Plant‐Mediated Methane Emissions and Oxidation in a Thermokarst Bog

Abstract

Vascular plants are important in the wetland methane cycle, but their effect on production, oxidation, and transport has high uncertainty, limiting our ability to predict emissions. In a permafrost-thaw bog in Interior Alaska, we used plant manipulation treatments, field-deployed planar optical oxygen sensors, direct measurements of methane oxidation, and microbial DNA analyses to disentangle mechanisms by which vascular vegetation affect methane emissions. Vegetation operated on top of baseline methane emissions, which varied with proximity to the thawing permafrost margin. Emissions from vegetated plots increased over the season, resulting in cumulative seasonal methane emissions that were 4.1-5.2 g m −2 season −1 greater than unvegetated plots. Mass balance calculations signify these greater emissions were due to increased methane production (3.0-3.5 g m −2 season −1) and decreased methane oxidation (1.1-1.6 g m −2 season −1). Minimal oxidation occurred along the plant-transport pathway, and oxidation was suppressed outside the plant pathway. Our data indicate suppression of methane oxidation was stimulated by root exudates fueling competition among microbes for electron acceptors. This contention is supported by the fact that methane oxidation and relative abundance of methanotrophs decreased over the season in the presence of vegetation, but methane oxidation remained steady in unvegetated treatments; oxygen was not detected around plant roots but was detected around silicone tubes mimicking aerenchyma; and oxygen injection experiments suggested that oxygen consumption was faster in the presence of vascular vegetation. Root exudates are known to fuel methane production, and our work provides evidence they also decrease methane oxidation. Plain Language Summary Methane is a greenhouse gas with a greater ability to warm the earth than carbon dioxide. Wetlands are the largest natural source of methane to the atmosphere. To understand future climate change, scientists need to predict the amount of methane released from wetlands. Many factors affect the amount of methane generated by soil microbes (called methane production) and how much methane is released into the atmosphere (called methane emission). Methane traveling through soils can also get converted to carbon dioxide through methane oxidation. Wetland plants influence production, transport, and oxidation of methane, but studies disagree on their overall effect on emissions. In this study, we used multiple methods to identify how plants affect the methane cycle. Plants appeared to increase methane production and, to our surprise, decrease methane oxidation. We created a theory for why plants increased methane emissions, advancing understanding of plant-soil interactions that contribute to wetland methane emissions.

Plant mediated methane efflux from a boreal peatland complex

“…1). When CH 4 bypasses the oxic peat and live Sphagnum layers via aerenchymous plants, a larger proportion of CH 4 produced in the anoxic peat layer may avoid oxidation by the methanotrophic microbes in the aerobic peat layer and be released to the atmosphere (Schimel 1995;Whalen 2005;Turner et al 2020; Fig. 1).…”

“…The estimated proportion of plant transport of the total site-level CH 4 efflux varies widely from 27 to 98% Vegetation in rainwater-fed, nutrient-poor bogs with a surface topography that varies from shrub dominated drier hummocks to sedge dominated wet hollows provides less suitable litter for methanogenesis than sedge-dominated minerotrophic fens. Sedges and other aerenchymous plant species act as conduits of CH 4 to the atmosphere, bypassing oxidation by methanotrophic microbes in the oxic peat and living moss layer, with only negligible oxidation along this bypass route (Turner et al 2020). Simultaneously, they transport oxygen into the anoxic peat layer (light color around the sedge roots) (Fritz et al 2011;Münchberger et al 2019).…”

Supplementary material to "Simulated methane emissions from Arctic ponds are highly sensitive to warming"

“…Since the plant stems also transport oxygen into the rooting zone, we assume a fixed fraction of the plant-mediated methane to be oxidized (f ox = 0.2) reducing the methane flux from plants F plant . The value 0.2 is an conservative estimate based on the work of Turner et al (2020) and Ström et al (2005), who measured the oxidation rates of the plant species dominating our study region. The plant-mediated flux then is defines as…”

Hot spots of methane emission in West Siberian middle taiga wetlands disturbed by petroleum extraction activities

“…В других исследованных экосистемах эти растения-индикаторы найдены не были. Такое увеличение эмиссии метана для парцелл, покрытых растениями с мощной корневой системой, можно связать с тем, что под влиянием легко разлагаемой органики корневых экссудатов увеличивается продукция метана [Valentine et al, 1994;Chasar et al, 2000;Chanton et al, 2008] и/или снижается удельная скорость метанотрофии [Turner et al, 2020], а также с ускорением транспорта метана из анаэробных слоѐв через эти растения и их корневую экосистему [Laanbroek, 2010]. Если условно считать «горячими» величины эмиссии выше третьего квартиля для всей полученной выборки (что соответствует удельному потоку 5.9 мг•м -2 •час -1 ), то четыре из восьми таких значений эмиссии метана были получены именно в тех элементах экосистем, в растительном покрове которых присутствовали пушица влагалищная и пухонос.…”