A re-evaluation of wetland carbon sink mitigation concepts and measurements: A diagenetic solution

Abstract

Aquatic canopy ecosystems ability to mitigate greenhouse gases (GHG) is currently based on the rate of sedimentary organic carbon accumulation (CA) and the protection of vulnerable stocks from remineralisation. However, remineralisation of allochthonous inputs constrains CA as sequestration, assessments neglect remineralisation over climatic scales, and often fail to account for recalcitrant material. The article clarifies the meaning of stock and sequestration as mitigation services through their net ecosystem production (NEP) and addresses the concerns through a series of hypothetical evolving ecosystems. A diagenetic solution is proposed that accounts for continuous remineralisation of CA and the remineralised fraction of labile allochthonous inputs to estimate the NEP. The solution was applied and tested for a seagrass and mangrove ecosystem. Uncorrected and corrected average CA was greater than the cal. NEP values by a factor of two for the seagrass and 30 for the mangrove. Nevertheless, the NEP values fell within reported ranges i.e., 27.6 g C m-2 yr-1 (mangrove) and 7.2 g C m-2 yr-1 (seagrass). The overestimate was largely maintained after including vulnerable stocks in the total carbon accreditation calculus. However, with the inclusion of CA, the total average carbon mitigation rates converged to 1 124 (seagrass) and 1 783 g C m-2 yr-1 (mangroves), when argued, in some circumstances, as a vulnerable stock concept after hindcasting to their original time of annual deposition. Mitigation concepts and measurements require re-evaluation and will assure that carbon credits are not overvalued, which would otherwise permit GHG emissions above the capacity of the ecosystem.