Modeled CO2 Emissions from Coastal Wetland Transitions to Other Land Uses: Tidal Marshes, Mangrove Forests, and Seagrass Beds

Abstract

The sediments of coastal wetlands contain large stores of carbon which are vulnerable to oxidation once disturbed, resulting in high levels of CO 2 emissions that may be avoided if coastal ecosystems are conserved or restored. We used a simple model to estimate CO 2 emissions from mangrove forests, seagrass beds, and tidal marshes based on known decomposition rates for organic matter in these ecosystems under either oxic or anoxic conditions combined with assumptions of the proportion of sediment carbon being deposited in either oxic or anoxic environments following a disturbance of the habitat. Our model found that over 40 years after disturbance the cumulative CO 2 emitted from tidal marshes, mangrove forests, and seagrass beds were ∼70-80% of the initial carbon stocks in the top meter of the sediment. Comparison of our estimates of CO 2 emissions with empirical studies suggests that (1) assuming 50% of organic material moves to an oxic environment after disturbance gives rise to estimates that are similar to CO 2 emissions reported for tidal marshes; (2) field measurements of CO 2 emissions in disturbed mangrove forests were generally higher than our modeled emissions that assumed 50% of organic matter was deposited in oxic conditions, suggesting higher proportions of organic matter may be exposed to oxic conditions after disturbance in mangrove ecosystems; and (3) the generally low observed rates of CO 2 emissions from disturbed seagrasses compared to our estimates, assuming removal of 50% of the organic matter to oxic environments, suggests that lower proportions may be exposed to oxic conditions in seagrass ecosystems. There are significant gaps in our knowledge of the fate of wetland sediment carbon in the marine environment after disturbance. Greater knowledge of the distribution, form, decomposition, and emission rates of wetland sediment carbon after disturbance would help to improve models.

The undervalued contribution of mangrove protection in Mexico to carbon emission targets

“…Losses of carbon were considered to be 80% of the total ecosystem carbon stock when mangroves were converted to shrimp ponds (Kauffman, Heider, Norfolk, & Payton, ); and 66% when mangroves were converted to cattle pasture (Kauffman et al., , ). These values were based on in situ measurements of carbon loss in the region and by models of organic carbon decomposition after mangrove disturbance (Lovelock et al., ). Emissions from degradation were determined similarly to deforestation, except losses were capped at 20% of the total carbon stock (Lovelock, Ruess, & Feller, ).…”

“…The losses of carbon from mangrove forests will be mostly through CO 2 emissions directly from the forest floor and from dissolved inorganic carbon that is released to adjacent creeks (Borges et al., ; Chen et al., ; Lovelock et al., ) Emissions of CO 2 stabilize after 5 years and are expected to continue for at least 20 years (Lovelock et al. 2011, )…”

“…In forested wetlands such as mangroves, up to 95% of the carbon is stored in deep soils, not in the vegetation (Donato et al., ). Mangrove deforestation and degradation causes the release of large amounts of CO 2 (Lovelock, Fourqurean, & Morris, ) that has not been accounted in national emission programs. In this study, we compare historical and current emissions from mangrove deforestation in Mexico to its INDC committed in the Paris Agreement.…”

Reduced water motion enhances organic carbon stocks in temperate eelgrass meadows

“…These same coastal ecosystems, however, are in accelerating decline worldwide, with anthropogenic conversion rates ranging from 0.4% to 3% annually (Waycott et al ; Howard et al ). The degradation and loss of blue carbon habitats not only limits capacity for ongoing carbon sequestration and storage, but can also rerelease carbon that has accumulated over decades, centuries, or millennia (Mateo et al ; Pendleton et al ; Lovelock et al ). Thus, there is a policy push to include mangroves, tidal marshes, and seagrass meadows in national greenhouse gas inventories and carbon financing projects (IPCC ; Hejnowicz et al ; Sutton‐Grier and Moore ; Needelman et al ).…”

Modeling Organic Carbon Accumulation Rates and Residence Times in Coastal Vegetated Ecosystems

“…Although plant litter quality is the primary controller over litter decay rates, with slower decay associated with low-quality, complex tissues (Hemminga & Buth, 1991;Zhang et al, 2008), the rate of litter decay is not linked to the rate of the litter-derived OC decay once it is part of the soil (Gentile et al, 2011), where other protection mechanisms dominate Keil & Mayer, 2014;von Lützow et al, 2008). This is illustrated by the close correspondence between our fast-pool k 1 estimates (0.028 ± 0.014 yr −1 ) and modeled soil OC decay under (2003), Harrison (1989), Nicastro et al (2012), Chiu et al (2013), and Mateo and Romero (1996); (e) Fourqurean and Schrlau (2003), Kenworthy and Thayer (1984), and Romero et al (1992); (f) Kirwan and Blum (2011), Hemminga and Buth (1991), and Christian (1984); (g) Hemminga and Buth (1991) the mixed-oxic conditions (0.042 yr −1 ; Lovelock et al, 2017) typically found within the seagrass rhizosphere (Brodersen et al, 2017), and the deviation of our k 1 estimates from decay rates of above-and below-ground litter from seagrass and other blue carbon ecosystems (Table 4). The slow decay of soil OC reported here helps to explain the high OC stocks found in P. oceanica soils Fourqurean et al, 2012) and calls for further research quantifying OC decay rates under in situ soil conditions.…”