As the world is increasingly impacted by climate change, people are turning to nature-based climate solutions to increase carbon storage and limit emissions from the world’s ecosystems. Restoring degraded wetlands is an especially popular natural climate solution in the state of California, where land is literally sinking due to the removal of groundwater. By measuring the amount of carbon uptake with specialized equipment, scientists and project managers can estimate how much carbon is taken out of the atmosphere and processed by plants and microbes. Measuring soil accumulation over the years can also indicate how much of that carbon is stored in soils, versus exported through streams or tides. A study recently published in the Journal of Geophysical Research: Biogeosciences revealed new information on just how efficient restoring wetlands as a natural climate solution can be. It focused on three restored wetlands in the San Francisco Bay Delta, where the South Bay Salt Pond Restoration Project (pictured above) is located.
Methane emissions from a wetland with no tidal influence were much higher than from two nearby tidal wetlands in the study. This comes as no surprise because sulfate-reducing bacteria (sulfate is abundant in seawater) outcompete methanogenic bacteria in coastal wetlands, and instead of producing methane, they oxidize it. By modeling the impact of methane emissions and carbon uptake on the future climate, the authors were able to show that restored nontidal wetlands had a positive instantaneous radiative forcing effect, meaning they actually contributed to climate warming in the first fifty years after restoration. When looking far into the future (300+ years), the nontidal wetlands will begin to have a much greater cooling effect on the climate than the tidal wetland.
The three wetlands had differences in previous land use and restoration strategies, as well as wetland characteristics such as salinity, tide, and surface elevation. Despite unique features, the three wetlands were all carbon sinks and had no statistical differences in whole-ecosystem carbon flux over multiple years of measurement, at least according to the eddy covariance flux (see previous posts What the Flux, Allequash Creek Wetland, and Sapelo Island Week One for flux tower photos!).
The nontidal wetlands stored essentially all of their net atmospheric carbon uptake in soil, which accumulated on the wetland surface over the years. In comparison, the tidal wetland stored only a fraction (13-23%) of its carbon uptake in soils. The authors predicted that the “missing” carbon was exported from the ecosystem by the tide. Overall, nontidal wetlands were better at carbon burial but were worse at immediately cooling the climate.
Perhaps the answer here is to restore more tidal wetlands, but to improve their carbon uptake with ecosystem engineering. Careful control of wetland hydrology (e.g., installing culverts with control valves) can actually make restored wetlands larger CO2 sinks than natural wetlands in some cases. Adding more vegetation cover in the tidal wetland, which was 70% mudflat at the time of study, might help speed up sediment accumulation as well.
Study authors stress that despite the positive implications of soil carbon storage, it should not be used as an immediate sign that climate mitigation has been achieved. Sediment accumulation does not always equate to soil carbon accumulation, as a study of South Florida mangroves found algal mats contributed to increased surface elevation in dead zones, but did not store any significant amount of carbon or nitrogen. Modeling the impact of wetland restoration into the future demonstrated that climate implications can change over time. Other studies have also shown that natural climate solutions implemented the wrong way can have null or negative effects. For instance, replanted mangroves can experience die-off shortly after restoration begins. Continuous monitoring of multiple pathways for carbon and methane to enter and exit ecosystems is essential to achieve a better understanding of natural climate solutions.