Invasion History and Soil Depth Shape Microbial Functional Groups and Methanogen Communities in Phragmites australisCoastal Wetlands

Coastal wetlands are among the most productive places on Earth, acting as massive sponges that soak up carbon from the atmosphere and store it in the soil. However, these waterlogged environments are also natural factories for greenhouse gases. Because the soil is often saturated with water and lacks oxygen, specific communities of microscopic organisms take over the job of breaking down dead plant matter. In doing so, they release methane, a potent gas that traps heat in the atmosphere. The balance between storing carbon and releasing gas depends heavily on the plants growing above the soil. Different plants drop different kinds of leaves and roots, and they pump different amounts of oxygen into the ground through their stems. These subtle changes in the underground environment dictate which microbes thrive and what gases they produce. When a new plant species moves in and displaces the native ones, it can rewrite the rules for the entire microbial community living beneath the surface.

In the brackish marshes along the Rhode River in Maryland, a tall, invasive grass known as Phragmites australis has been aggressively replacing native marsh grasses for decades. Researchers set out to understand how this takeover changes the invisible world of soil microbes, specifically looking at how the invasion alters the breakdown of organic matter and the production of methane. They focused on three key factors: the type of plant growing above, how deep they dug into the soil, and how long the invasive grass had been living there. By comparing marshes that had been invaded for a long time, those invaded more recently, and areas still dominated by native grasses, the team could see how the underground microbial world shifts over time and space.

The scientists collected soil cores from these marshes, slicing them into layers to examine the topsoil, the middle layer, and the deeper soil down to fifty centimeters. They measured the activity of enzymes, which are the tools microbes use to chew up complex plant material, and counted the genes of specific microbial groups responsible for breaking down carbon and nitrogen. They also identified the different types of methane-producing microbes, known as methanogens, to see which ones were present. The results showed that the invasive grass fundamentally changed the microbial landscape, but the nature of that change depended on how long the invasion had been happening and how deep the soil was.

In the soils dominated by the invasive Phragmites, the microbes showed a reduced ability to break down simple, easy-to-digest carbon. This was measured by a drop in the activity of an enzyme called beta-glucosidase, which is responsible for digesting fresh plant sugars. This effect was most pronounced in the deeper soil layers and in marshes that had been invaded recently. The researchers suggest that the invasive grass provides a different quality of food for the microbes, perhaps one that is harder to digest or that changes the chemistry of the soil in a way that slows down the decomposition of fresh carbon. Interestingly, the ability to break down tougher, more complex materials like lignin did not change in a consistent pattern, indicating that the invasion does not simply stop all microbial work but rather shifts the type of work the microbes are doing.

The story of methane production told a different tale. The invasive grass was associated with a higher abundance of methane-producing microbes, especially in the deeper layers of the soil. While native marsh soils showed a fairly even distribution of these microbes from top to bottom, the soils under the invasive grass saw their numbers increase significantly as the researchers dug deeper. This suggests that the extensive root systems of the invasive grass are delivering carbon substrates to the deep soil, fueling a larger population of methane producers far below the surface. However, the community of these methane producers changed over time. In marshes that had been invaded for a long time, the dominant microbes were those that produce methane using hydrogen. In contrast, in marshes invaded more recently, the dominant group was a different type that produces methane from acetate, a simple organic acid. This shift implies that as the invasion matures, the chemical environment of the soil evolves, favoring different microbial strategies for generating gas.

The invasion also altered the microbes involved in nitrogen cycling, specifically those that remove nitrogen from the soil. In the recently invaded marshes, the native grass soils contained significantly higher numbers of microbes carrying genes for denitrification, the process that converts nitrogen compounds into gases. The invasive grass soils had fewer of these microbes. Yet, in the marshes that had been invaded for decades, this difference largely disappeared, with both native and invasive sites showing similar levels of these nitrogen-processing microbes. This indicates that the initial shock of the invasion disrupts the nitrogen cycle, but over time, the microbial community adjusts and stabilizes, even as the plant cover remains different.

These findings highlight that the impact of an invasive plant is not a single event but a process that unfolds over years and extends deep into the ground. The invasive grass does not just change what grows above; it restructures the microbial engine room below, altering how carbon is broken down, how methane is produced, and how nitrogen is cycled. The depth of the soil and the duration of the invasion are critical factors in determining the outcome. By understanding these underground shifts, scientists can better predict how coastal wetlands will respond to future invasions and how these changes might influence the global climate. The study confirms that the history of an invasion and the vertical structure of the soil are essential pieces of the puzzle in understanding the long-term consequences of plant invasions on coastal ecosystems.