Abstract
Vegetated coastal habitats (VCHs), such as mangrove forests, salt marshes and seagrass meadows, have the ability to capture and store carbon in the sediment for millennia, and thus have high potential for mitigating global carbon emissions. Carbon sequestration and storage is inherently linked to the geochemical conditions created by a variety of microbial metabolisms, where physical disturbance of sediments may expose previously anoxic sediment layers to oxygen (O2), which could turn them into carbon sources instead of carbon sinks. Here, we used O2, hydrogen sulfide (H2S) and pH microsensors to determine how biogeochemical conditions, and thus aerobic and anaerobic metabolic pathways, vary across mangrove, salt marsh and seagrass sediments (case study from the Sydney area, Australia). We measured the biogeochemical conditions in the top 2.5 cm of surface (0–10 cm depth) and experimentally exposed deep sediments (>50 cm depth) to simulate undisturbed and physically exposed sediments, respectively, and how these conditions may affect carbon cycling processes. Mangrove surface sediment exhibited the highest rates of O2 consumption and sulfate (SO42-) reduction based on detailed microsensor measurements, with a diffusive O2 uptake rate of 102 mmol O2 m-2 d-1 and estimated sulfate reduction rate of 57 mmol Stot2- m-2 d-1. Surface sediments (0–10 cm) across all the VCHs generally had higher O2 consumption and estimated sulfate reduction rates than deeper layers (>50 cm depth). O2 penetration was <4 mm for most sediments and only down to ∼1 mm depth in mangrove surface sediments, which correlated with a significantly higher percent organic carbon content (%Corg) within sediments originating from mangrove forests as compared to those from seagrass and salt marsh ecosystems. Additionally, pH dropped from 8.2 at the sediment/water interface to <7–7.5 within the first 20 mm of sediment within all ecosystems. Prevailing anoxic conditions, especially in mangrove and seagrass sediments, as well as sediment acidification with depth, likely decreased microbial remineralisation rates of sedimentary carbon. However, physical disturbance of sediments and thereby exposure of deeper sediments to O2 seemed to stimulate aerobic metabolism in the exposed surface layers, likely reducing carbon stocks in VCHs.
Highlights
Vegetated coastal habitats (VCHs) such as seagrass meadows, salt marshes and mangrove forests are increasingly recognized as high-value ecosystems (Costanza et al, 1997; Harborne et al, 2006; Barbier et al, 2011) due to their extraordinary high carbon capture and storage capacity in sediments (Duarte et al, 2005; McLeod et al, 2011; Fourqurean et al, 2012; Ricart et al, 2015; Atwood et al, 2017), frequently termed “blue carbon”
The percent organic carbon content (%Corg) of the mangrove, salt marsh and seagrass surface sediments were 4.50 ± 0.96, 1.61 ± 0.52, and 1.25 ± 0.26%, which was significantly higher than in the deep sediments where the %Corg amounted to 1.91 ± 0.76, 0.34 ± 0.05, and 1.56 ± 0.09%, respectively (p < 0.05)
Our results showed that in the absence of biological O2 introduction mangrove and seagrass surface sediments had higher rates of O2 consumption and estimated sulfate reduction than salt marsh surface sediments in the Sydney area (NSW, Australia), with highest rates measured in the sediment surface of mangroves
Summary
Vegetated coastal habitats (VCHs) such as seagrass meadows, salt marshes and mangrove forests are increasingly recognized as high-value ecosystems (Costanza et al, 1997; Harborne et al, 2006; Barbier et al, 2011) due to their extraordinary high carbon capture (sequestration) and storage capacity in sediments (Duarte et al, 2005; McLeod et al, 2011; Fourqurean et al, 2012; Ricart et al, 2015; Atwood et al, 2017), frequently termed “blue carbon”. Sediments of VCHs are generally considered anoxic due to high organic carbon input and microbial activity (Blaabjerg et al, 1998; Hansen et al, 2000; Nielsen et al, 2001; Holmer and Laursen, 2002) and slow diffusion of O2 from seawater into the porewater. The high microbial activity is driven by large pools of organic material originating from the plants themselves (Miyajima et al, 1998) or as a result of increased sedimentation capturing organic material from the water column and adjacent land catchments (Burdige, 2005). In the absence of O2 introduction at depth by plant roots (termed radial O2 loss) or burrowing animals, transport of gasses and solutes across the surface of cohesive sediments occurs through the diffusive boundary layer (DBL), a thin water layer in which molecular diffusion is the only means of transport (Jørgensen and Revsbech, 1985). Respiratory products of anaerobic metabolism in the anoxic part of the sediment include sulfide (S2−), pyrite (FeS2) or diagenetically produced organic sulfur, dinitrogen (N2), ammonium (NH4+), methane (CH4), and other products (Burdige, 2011)
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