Wetlands Stop Emitting Methane When Water Level Is Too High – But Is That a Good Thing?

High water levels result in higher wetland methane emissions for a couple reasons. First, more water on the wetland surface creates the ideal oxygen-deprived conditions for microbial growth down in the soil. Methanogenic microbes ramp up the process of methanogenesis, producing methane to send up through the water as diffuse gas. Second, wetland plants continue to photosynthesize and enrich soil with byproducts called exudates, encouraging bacterial growth even further. Evolutionary adaptation allows plants to continue business as normal despite the extreme, sometimes acidic conditions of waterlogged wetland landscapes. Scientists have observed this cause and effect of water level on wetland methane emissions around the world, many times. But research shows that there could be an ideal water table depth (WTD) for wetlands, past which this relationship that scientists have come to know and love, no longer applies.

Observations at forested wetland US-ALQ, and the more open but nearby wetland US-Los, in 2019 and 2020.

At first, lower wetland methane emissions when the water level is too high sounds like a good thing. Extreme precipitation leading to higher water levels could become more common with a changing climate. A lower amount of wetland-produced greenhouse gases in the atmosphere as a result of this could help slow climate warming. Environmental managers and beavers alike would swoon knowing that wetlands could be dammed with the bonus of lower-than-normal methane emissions. Controlling the water table would no doubt be an easy way to limit wetland gas emissions, and on a broader scale, manipulate the climate. It would be like playing God! But alas, there is a caveat to being God, at least when you ignore the way that biology works. Let me explain.

Research recently published in JGR Biogeosciences shows that wetland methane emissions could actually be lower during extremely wet years because plants aren’t able to supply necessary carbon substrate (i.e., food) to soil microbes hanging out near their roots. This can be determined using a statistical method called lag analysis. By delaying one variable and comparing it to a nonmoving variable, then finding the strength of the correlation, it is possible to determine lagged wetland responses to environmental factors. Think of it like finding the time between when a bee stings a person and the exact moment they start to feel the pain. And then correlating the toxicity of the bee with the pain level.

In the article, authors delved into why wetland methane emissions were lower in a wet year when scientists would have otherwise expected them to be higher. Prolonged periods of high water table levels can deprive roots of oxygen, therefore causing plant stress and limiting plant productivity (i.e., Gross Primary Productivity, or GPP). Controlling water table to limit wetland gas emissions is simple and has climate benefits. On the other hand, the outcome for microbes and plants in the long run is potentially detrimental.

Root exudates and emissions, adapted from Trends in Plant Science (Preece & Peñuelas, 2020).

There could be more to controlling wetland methane emissions than simply keeping water at the “Goldilocks” level, where methanogens are wet enough to perform methanogenesis, but plants aren’t drowning in too much water to perform photosynthesis. For instance, warmer stream temperatures can increase wetland methane emissions in the summer months, but will lower them in the winter, even when accounting for the separate impact of air temperature. Furthermore, soil methanogens might only be able to sustain themselves on preexisting soil organic matter (i.e., old food) for a certain amount of time. What happens when microbes run out of food, and too-high water levels prevent plants from allocating more of it through roots to nearby soil? Wetland managers aiming to control methane emissions may need to closely watch and regulate water temperatures, and alter water levels between high and low, to allow for recovery of soil, plants, and microbes.

Wetland management and climate control doesn’t end here. Some of the relationships observed in the study previously mentioned, such as that of stream temperature and methane emissions, were significant but weak. Others, such as the relationship between stream velocity and methane emissions, were inconsistent across years. This means more studies need to be conducted to make sure the relationships are observed at multiple sites around the world. Furthermore, the Goldilocks water level could vary site to site, or perhaps within a site from year to year. Studying this relationship in a wider array of wetlands could verify that the original observations were correct, and help determine exactly what about extremely high water levels limits methane emissions. Closer examination of wetland water chemistry, water velocity, and microbial composition might help scientists and the broader public get closer to understanding, and eventually controlling, the climate.

For a free full-text, read-only version of the article in JGR Biogeosciences click here.

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