Greenhouse gas emissions and carbon sequestration potential in restored freshwater marshes in the Sacramento San-Joaquin Delta, California

Background/Question/Methods

Wetlands have the ability to accumulate significant amounts of carbon (C), and thus wetland restoration has been widely proposed as a means to sequester atmospheric C to help mitigate climate change.  However, wetlands are also the largest natural source of methane (CH₄), a potent greenhouse gas (GHG) that can offset wetland C sequestration and feedback to climate change.  Few studies have examined the balance between C uptake by wetland vegetation and ecosystem CH₄ dynamics during wetland development or the controls on CH₄ emissions in these ecosystems.  In this study, we examined to what degree CH₄ emissions offset C sequestration during wetland restoration, and how this changes during ecosystem development.  To address these questions, fluxes of CO₂ and CH₄ were measured continuously for multiple years using the eddy covariance method at two restored freshwater marshes of differing ages (a younger wetland restored in 2010 and an older wetland restored in 1997) in the Sacramento San-Joaquin Delta, California.  Net annual C and GHG budgets were calculated by integrating these measurements over a one-year period (April-April) and accounting for the higher global warming potential of CH₄.

Results/Conclusions

Both restored wetlands were net sinks of atmospheric CO₂, with the younger wetland sequestering 141 and 1409 g CO₂ m^-2 yr^-1 in 2011-2012 and 2012-2013, respectively, and the older restored wetland sequestering 2394 and 1466 g CO₂ m^-2 yr^-1 in 2011-2012 and 2012-2013, respectively.   However, both wetlands were large sources of CH₄, with higher CH₄ emissions from the younger wetland (up to 68 g CH₄ m^-2 yr^-1) than the older wetland (up to 51 g CH₄ m^-2 yr^-1).  Both the wetlands were always C sinks with the younger wetland sequestering less C than the older wetland (up to 334 and 622 g C m^-2 yr^-1, respectively).  In terms of the GHG budgets, the younger wetland was a net GHG source, emitting on average 954 g CO₂ eq m^-2 yr^-1, while the older wetland was consistently a GHG sink, sequestering on average 755 g CO₂ eq m^-2 yr^-1.  This study suggests that restored wetlands have the potential to act as net C and GHG sinks and may depend on the time since restoration. However, as continuous CO₂ and CH₄ measurements from restored wetlands are still rare, more long-term flux measurements are needed to better understand how C sequestration changes over time.