Enhanced decomposition offsets enhanced productivity and soil carbon accumulation in coastal wetlands responding to climate change

Abstract

Coastal wetlands are responsible for about half of all carbon burial in oceans, and their persistence as a valuable ecosystem depends largely on the ability to accumulate organic material at rates equivalent to relative sea level rise. Recent work suggests that elevated CO<sub>2</sub> and temperature warming will increase organic matter productivity and the ability of marshes to survive sea level rise. However, we find that organic decomposition rates increase by about 12% per degree of warming. Our measured temperature sensitivity is similar to studies from terrestrial systems, twice as high as the response of salt marsh productivity to temperature warming, and roughly equivalent to the productivity response associated with elevated CO<sub>2</sub> in C<sub>3</sub> marsh plants. Therefore, enhanced CO<sub>2</sub> and warmer temperatures may actually make marshes less resilient to sea level rise, and tend to promote a release of soil carbon. Simple projections indicate that elevated temperatures will increase rates of sea level rise more than any acceleration in organic matter accumulation, suggesting the possibility of a positive feedback between climate, sea level rise, and carbon emissions in coastal environments

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.…”

The Declining Role of Organic Matter in New England Salt Marshes

“…Our hypothesis that increased decomposition rates may explain the observed decline in organic matter deserves further study, as factors other than water temperature (e.g., organic matter quality, flooding regimes) play important roles in marsh organic matter decay rates (Charles and Dukes 2009;Kirwan and Blum 2011;Kirwan et al 2014). Likewise, the Valiela et al (1985) study was for a different New England estuary three decades ago, so more recent site-specific values are needed.…”

The role of changing climate in driving the shift from perennial grasses to annual succulents in a Mediterranean saltmarsh

“…For some coastal plant species, higher temperatures can result in enhanced growth, primary production, flowering and physiological processes of individuals, as well as range expansion of populations Coldren et al 2016). For other species, higher temperatures may negatively affect germination rates, physiological processes (Crosby et al 2017) and decomposition rates (Kirwan & Blum 2011) or result in plant death because of changes in soil chemistry, microbial communities and moisture content (Bertness, Gough & Shumway 1992; Lewis, Brown & Jimenez 2014). For many coastal ecosystems, the effects of heat waves on the key habitat-forming plants are not well understood (Coldren et al 2016).…”