Plant-Microbe Feedbacks Drive Coastal Wetland Responses to Global Change
Genevieve Noyce1* (email@example.com), Nicholas Bruns2, Alex Smith2, Matthew Kirwan2, Pat Megonigal1
1Smithsonian Environmental Research Center, Edgewater, MD; 2Virginia Institute of Marine Science, Gloucester Point, VA
Warming temperatures and elevated carbon dioxide (CO2) may increase wetland productivity and organic matter accumulation, but unknown plant-microbe feedbacks make it difficult to model how wetlands will respond to global change. The Salt Marsh Accretion Response to Temperature eXperiment (SMARTX) was established in the Smithsonian’s Global Change Research Wetland in 2016 to advance model representation of coastal wetland responses to global climate change. In SMARTX, researchers actively manipulate whole-ecosystem temperature through feedback-controlled heating from the plant canopy to 1.5 m soil depth, as well as atmospheric CO2 concentration.
Throughout the experiment, a moderate amount of warming (+1.7 °C) has consistently maximized marsh elevation gain and belowground carbon (C) accumulation, consistent with previously observed nonlinear effects on belowground net primary productivity (NPP), which researchers hypothesized was driven by asynchrony between the microbially mediated supply and plant demand of nitrogen (N). Using a classic “functional balance” model depicting root vs. shoot allocation, researchers have recently been able to accurately predict annual belowground NPP as a function of plant N demand and soil N supply. This work provides the first model of marsh C accumulation that treats root-to-shoot ratios as dynamic, capturing the coupling of N-cycling and C storage.
At higher temperatures, marsh elevation loss increased and was associated with increased C mineralization and microtopographic heterogeneity, a potential early warning sign of marsh drowning. Under warming conditions, the balance between microbial decomposition and NPP is largely negative, with high rates of decomposition (inferred from methane emissions) reducing organic matter storage. This likely interacts with plant effects, where high root growth during the summer brings in oxygen and organic C, increasing rates of decomposition. Elevated CO2 also led to a decline in C accumulation, especially when combined with warming, despite increased inputs of belowground NPP. This indicates that enhanced root production under future climate conditions may not increase marsh resilience, due to plant-microbe feedbacks resulting in high rates of aerobic decomposition.