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

Abstract:

Wetlands can be effective carbon sinks due to limited decomposition rates in anaerobic soil. As such there is a growing interest in the use of restored wetlands as biological carbon sequestration projects for greenhouse gas (GHG) emission reduction programs. However, using wetlands to offset emissions requires accurate accounting of both carbon dioxide (CO₂) and methane (CH₄) exchange since wetlands are also sources of CH₄. To date few studies have quantified CO₂ and CH₄ exchange from restored wetlands or assessed how these fluxes vary during ecosystem development. In this study, we report on multiple years of eddy covariance measurements of CO₂ and CH₄ fluxes from two restored freshwater marshes of differing ages (one restored in 1997 and the other in 2010) in the Sacramento-San Joaquin Delta, CA. Measurements at the younger restored wetland started in October 2010 and began in April 2011 at the older site.

The younger restored wetland showed considerable year-to-year variability in the first 4 years following restoration, with CO₂ uptake ranging from 12 to 420 g C-CO₂ m^-2 yr^-1. Net CO₂ uptake at the older wetland was overall greater than at the younger site, ranging from 292 to 585 g C-CO₂ m^-2 yr^-1. Methane emissions were on average higher at the younger wetland (46 g C-CH₄ m^-2 yr^-1) relative to the older one (33 g C-CH₄ m^-2 yr^-1). In terms of the GHG budgets (assuming a global warming potential of 34), the younger wetland was consistently a GHG source, emitting on average 1439 g CO₂ eq m^-2 yr^-1, while the older wetland was a GHG sink in two of the years of measurement (sequestering 651 and 780 g CO₂ eq m^-2 yr^-1 in 2012 and 2013, respectively) and a source of 750 g CO₂ eq m^-2 yr^-1 in 2014. This study highlights how dynamic CO₂ and CH₄ fluxes are in the first years following wetland restoration and suggests that restored wetlands have the potential to act as GHG sinks but this may depend on time since restoration.