Global dataset of soil organic carbon in tidal marshes.

Tidal marshes store large amounts of organic carbon in their soils. Field data quantifying soil organic carbon (SOC) stocks provide an important resource for researchers, natural resource managers, and policy-makers working towards the protection, restoration, and valuation of these ecosystems. We collated a global dataset of tidal marsh soil organic carbon (MarSOC) from 99 studies that includes location, soil depth, site name, dry bulk density, SOC, and/or soil organic matter (SOM). The MarSOC dataset includes 17,454 data points from 2,329 unique locations, and 29 countries. We generated a general transfer function for the conversion of SOM to SOC. Using this data we estimated a median (± median absolute deviation) value of 79.2 ± 38.1 Mg SOC ha-1 in the top 30 cm and 231 ± 134 Mg SOC ha-1 in the top 1 m of tidal marsh soils globally. This data can serve as a basis for future work, and may contribute to incorporation of tidal marsh ecosystems into climate change mitigation and adaptation strategies and policies.

Macroalgal Blooms Trigger the Breakdown of Seagrass Blue Carbon.

Intensive macroalgal blooms, a source of labile organic carbon (LOC) induced by coastal nutrient loading in some seagrass ecosystems, create ideal conditions for enhanced recalcitrant organic carbon (ROC) loss via the cometabolism effect. Here, we carried out a 62-day laboratory experiment to see if density-dependent addition of macroalgal biomass can influence the seagrass decomposition process, including seagrass detritus carbon chemistry, greenhouse emissions, and bacterial communities. We found that higher density macroalgal addition stimulated microbes to decompose ∼20% more of the seagrass biomass compared to other treatments, which was also reflected in enhanced (∼twofold) greenhouse gas emissions. Although the composition of the seagrass-associated microbiome communities was unaffected by the addition of macroalgae, we showed that high macroalgal addition caused a relative depletion in the ROC as lignin and lipid compounds, as well as δ13C depletion and δ15N enrichment of the seagrass detritus. These results suggest that macroalgal blooms may stimulate the remineralization of recalcitrant components of seagrass detritus via cometabolism, possibly through providing available energy or resources for the synthesis of ROC-degrading enzymes within the resident microbial population. This study provides evidence that cometabolism can be a mechanism for leading to reduced seagrass blue carbon sequestration and preservation.

Australian vegetated coastal ecosystems as global hotspots for climate change mitigation.

Policies aiming to preserve vegetated coastal ecosystems (VCE; tidal marshes, mangroves and seagrasses) to mitigate greenhouse gas emissions require national assessments of blue carbon resources. Here, we present organic carbon (C) storage in VCE across Australian climate regions and estimate potential annual CO2 emission benefits of VCE conservation and restoration. Australia contributes 5-11% of the C stored in VCE globally (70-185 Tg C in aboveground biomass, and 1,055-1,540 Tg C in the upper 1 m of soils). Potential CO2 emissions from current VCE losses are estimated at 2.1-3.1 Tg CO2-e yr-1, increasing annual CO2 emissions from land use change in Australia by 12-21%. This assessment, the most comprehensive for any nation to-date, demonstrates the potential of conservation and restoration of VCE to underpin national policy development for reducing greenhouse gas emissions.

Impacts of land reclamation on tidal marsh 'blue carbon' stocks.

Tidal marsh ecosystems are among earth's most efficient natural organic carbon (C) sinks and provide myriad ecosystem services. However, approximately half have been 'reclaimed' - i.e. converted to other land uses - potentially turning them into sources of greenhouse gas emissions. In this study, we applied C stock measurements and paleoanalytical techniques to sediments from reclaimed and intact tidal marshes in southeast Australia. We aimed to assess the impacts of reclamation on: 1) the magnitude of existing sediment C stocks; 2) ongoing C sequestration and storage; and 3) C quality. Differences in sediment horizon depths (indicated by Itrax-XRF scanning) and ages (indicated by lead-210 and radiocarbon dating) suggest a physical loss of sediments following reclamation, as well as slowing of sediment accumulation rates. Sediments at one meter depth were between ~2000 and ~5300 years older in reclaimed cores compared to intact marsh cores. We estimate a 70% loss of sediment C in reclaimed sites (equal to 73 Mg C ha-1), relative to stocks in intact tidal marshes during a comparable time period. Following reclamation, sediment C was characterized by coarse particulate organic matter with lower alkyl-o-alkyl ratios and higher amounts of aromatic C, suggesting a lower extent of decomposition and therefore lower likelihood of being incorporated into long-term C stocks compared to that of intact tidal marshes. We conclude that reclamation of tidal marshes can diminish C stocks that have accumulated over millennial time scales, and these losses may go undetected if additional analyses are not employed in conjunction with C stock estimates.

Beach-cast seagrass wrack contributes substantially to global greenhouse gas emissions.

Seagrass ecosystems have received a great deal of attention recently for their ability to capture and store carbon, thereby helping to mitigate climate change. However, their carbon-sink capacity could be offset somewhat if exported plant material - which accounts for ∼90% of total leaf production - undergoes microbial breakdown and is emitted into the atmosphere as a greenhouse gas. Here we measured emissions (CO2 and CH4) from the breakdown of exported seagrass plant material, focusing on beach-cast 'wrack'. We tested two seagrass species; Zostera nigricaulis and Amphibolis antarctica, which have contrasting morphologies and chemistries. We found that both species of wrack were substantial sources of CO2, but not CH4, during the decomposition process. Biomass loss and the coinciding CO2 emissions occurred over the 30-day experiment, and the pattern of CO2 emissions over this time followed a double exponential model (R2 > 0.92). The initial flux rate was relatively high, most likely due to rapid leaching of labile compounds, then decreased substantially within the 2-9 days, and stabilizing at < 3 µmol g-1 d-1 during the remaining decomposition period. Additionally, seagrass wrack cast high up on beaches that remained dry had 72% lower emissions than wrack that was subjected to repeated wetting in the intertidal zone. This implies that relocation of seagrass wrack by coastal resource managers (e.g. from water's edge to drier dune areas) could help to reduce atmospheric CO2 emissions. Scaling up, we estimate the annual CO2-C flux from seagrass wrack globally is between 1.31 and 19.04 Tg C yr-1, which is equivalent to annual emissions of 0.63-9.19 million Chinese citizens. With climate change and increasing coastal development expected to accelerate the rate of wrack accumulation on beaches, this study provides timely information for developing coastal carbon budgets.