A Re-evaluation of Wetland Carbon Sink Mitigation Concepts and Measurements: A Diagenetic Solution

Abstract

The capacity of wetlands to mitigate greenhouse gas (GHG) emissions is the sum of two services: the protection of vulnerable organic stocks from remineralisation, and the capacity to sequester GHGs relative to their anthropogenic replacements. Organic carbon accumulation (CA) down through the sediment column is often taken as the measure of sequestration because of its capacity to record long-term variability and trends. However, we demonstrate that: i) CA is not equivalent to sequestration as net ecosystem production (NEP) for open systems; it requires the subtraction of the initial deposition rate of labile allochthonous carbon sources; ii) CA also requires subtraction of intrinsically allochthonous recalcitrants down through the sediment column, and together with subtraction of autochthonous recalcitrants from organic stock services; iii) CA as a climatic mitigation service also requires a diagenetic correction, as the annual deposition of labile organic carbon continues to remineralise over the long-term; and iv) preserving of a wetland has a significantly greater mitigation potential than restoring one. To address the above concerns, a global diagenetic solution is proposed, applied and tested for a tropical seagrass and mangrove. As expected, traditional CA estimates were disproportionately larger than their respective diagenetically modelled NEPs, and together with stocks fell within the ranges reported in the literature, with a final carbon accreditation highly dependent on the choice of their anthropogenic replacements. The review demonstrates that mitigation concepts and measurements for natural carbon sequestration solutions require re-evaluation to avoid GHG emissions above their capacity or reduce the ability to fulfil emission targets.

An opinion on BlueCAM an Australian blue carbon method to estimate climate change mitigation benefits of coastal wetland restoration by Lovelock et al. (2022)

“…The sole use of sediment carbon accumulation as a measure of NEP as both a sequestration and mitigation service relative to baseline, however, does not account for the presence and impact of allochthonous organic carbon inputs, both recalcitrant and labile (Gallagher et al 2022). That is, the presence of allochthonous recalcitrants needs to be deducted from the accumulation of carbon stocks.…”

“…Allochthonous recalcitrants do not require the protection from remineralization that sedimentary burial can offset (Chew & Gallagher 2018). Thus, they play no role in sequestration and mitigation services, both for restoration and the avoidance of loss (Gallagher et al 2022). Whereas, the amount of labile allochthonous organic carbon that has been remineralized needs to be subtracted from the accumulation of carbon stocks, and not what remains in the sediment.…”

“…Whereas, the amount of labile allochthonous organic carbon that has been remineralized needs to be subtracted from the accumulation of carbon stocks, and not what remains in the sediment. The consumption and remineralization of labile allochthonous organic carbon will offset NEP atmospheric sequestration (Prairie et al 2018;Gallagher et al 2022).…”

“…That is to say, there was a clear loss of the surface sediment's dominant allochthonous organic signal with depth. Indeed, it has been demonstrated that ignoring allochthonous carbon remineralization from accumulation rates can result in a fourfold overestimation of sequestration in the Mangrove sediments of a Mexican lagoon (Gallagher et al 2022). Clearly, this autotrophic offset from the supply and remineralization of a labile allochthonous carbon needs to be addressed.…”

Differences in carbon sink capacity and carbon storage of several typical modified tidal wetlands in the Yangtze River Estuary

“…Natural wetlands are important carbon sinks owing to their high primary productivity, low temperature, and anaerobic environment created by perennial waterlogging period, resulting in low activity of heterotrophic microorganisms and low decomposition rate, which can lead to low soil respiration (Bernal & Mitsch, 2012;Gallagher et al, 2022). Estuary wetlands are located in the transitional zone between fresh water and salt water, and comprise a variety of habitats, including mud ats, tidal ats, salt marshes, sandbars, and islands (Wang et al, 2012).…”