Exploring Carbon Mineralisation and Burial in Coastal Wetlands

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Connolly, Roderick

Lee, Shing

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2017-08-15
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Abstract

Mangroves and saltmarshes are ‘blue carbon’ ecosystems and studies on their carbon (C) dynamics have become highly topical. In these coastal wetlands, the aboveground biomass sequesters C and allocates C to belowground roots. A significant portion of aboveground biomass is transferred to the sediment as litterfall. Belowground roots and their exudates are ultimately decomposed by microbial communities. As the product of root decomposition, CO2 is released from the sediment surface and organic C (OC) contributes to sediment C burial. In addition, the decomposition of litter and sediment organic material also contributes to CO2 efflux from the sediment surface. The patterns of belowground C dynamics in blue C ecosystems remain unclear, especially sediment C burial, root C decomposition, and the relative contributions of different sources to sediment CO2 efflux. Further, sediment CO2 efflux and C storage have typically been examined separately in past research. Improved understanding of these patterns will help consolidate the theory of C cycling in coastal wetlands and facilitate effective ‘blue C’ management. My thesis aims to fill these gaps via systematic quantitative reviews and synthesis of literature data, field surveys and laboratory microcosm experiments. Studies on C stock in saltmarsh sediments have increased since the previous major review (Chmura et al., 2003). However, uncertainties exist in estimating global carbon storage in these vulnerable coastal habitats, thus hindering the assessment of their importance. Combining direct data and indirect estimation, my thesis compiled studies involving 143 sites across the southern and northern hemispheres, and provides an updated estimate of the global average carbon accumulation rate (CAR) at 244.7 g C m-2 yr-1 in saltmarsh sediments. Based on region-specific CAR and estimates of saltmarsh area in various geographic regions between 40°S to 69.7°N, total CAR in global saltmarsh sediments is estimated at ~ 10.2 Tg C yr-1. Latitude, tidal range and elevation appear to be important drivers for CAR of saltmarsh sediments, with considerable variation among different biogeographic regions. The data indicate that while the capacity for carbon sequestration by saltmarsh sediments ranked the first amongst coastal wetland and forested terrestrial ecosystems, their carbon budget was the smallest due to their limited and declining global areal extent. However, some uncertainties remain for this global estimate owing to limited data availability. One of the aims of this thesis was to determine the drivers of root decomposition and its role in C budgets in mangroves and saltmarsh: the patterns of root decomposition, and its contribution to C budgets, in mangroves and saltmarsh. The impact of climatic (temperature and precipitation), geographic (latitude), temporal (decay period) and biotic (ecosystem type) drivers was explored using multiple regression models. Best-fit models explain 50% and 48% of the variance in mangrove and saltmarsh root decay rates, respectively. A combination of biotic, climatic, geographic and temporal drivers influence root decay rates. Rainfall and latitude have the strongest influence on root decomposition rates in saltmarsh. For mangroves, forest type is the most important; decomposition is faster in riverine mangroves than other types. Mangrove species Avicennia marina and saltmarsh species Spartina maritima and Phragmites australis have the highest root decomposition rates. Root decomposition rates of mangroves were slightly higher in the Indo-west Pacific region (average 0.16 % day-1) than in the Atlantic-east Pacific region (0.13 % day-1). Mangrove root decomposition rates also show a negative exponential relationship with porewater salinity. In mangroves, global root decomposition rates are 0.15 % day-1 based on the median value of rates in individual studies (and 0.14 % day-1 after adjusting for area of mangroves at different latitudes). In saltmarsh, global root decomposition rates average 0.12 % day-1 (no adjustment for area with latitude necessary). The global estimate of the amount of root decomposing is 10 Tg C yr-1 in mangroves (8 Tg C yr-1 adjusted for area by latitude) and 31 Tg C yr-1 in saltmarsh. Local root C burial rates reported herein are 51-54 g C m-2 yr-1 for mangroves (58-61 Tg C yr-1 adjusted for area by latitude) and 191 g C m-2 yr-1 for saltmarsh. These values account for 24.1-29.1% (mangroves) and 77.9% (saltmarsh) of the reported sediment C accumulation rates in these habitats. Globally, dead root C production is the significant source of stored sediment C in mangroves and saltmarsh. Mangroves are blue carbon ecosystems that sequester significant carbon but release CO2, and to a lesser extent CH4, from the sediment through oxidation of organic carbon or from overlying water when flooded. Previous studies, e.g. Leopold et al. (2015), have investigated sediment organic C (SOC) content and CO2 flux separately, but did not provide a holistic perspective for both components of blue carbon. Based on field data from a mangrove in southeast Queensland, Australia, this thesis used a structural equation model to elucidate (1) the biotic and abiotic drivers of surface SOC (10 cm) and sediment CO2 flux; (2) the effect of SOC on sediment CO2 flux; and (3) the covariation among the environmental drivers assessed. Sediment water content, the percentage of fine-grained sediment (<63μm), surface sediment chlorophyll and light condition collectively drive sediment CO2 flux, explaining 41% of their variation. Sediment water content, the percentage of fine sediment, season, landform setting, mangrove species, sediment salinity and chlorophyll collectively drive surface SOC, explaining 93% of its variance. Sediment water content and the percentage of fine sediment have a negative impact on sediment CO2 flux but a positive effect on surface SOC content, while sediment chlorophyll is a positive driver of both. Surface SOC was significantly higher in Avicennia marina (2994±186 g m-2, mean+ SD) than in Rhizophora stylosa (2383± 209 g m-2). SOC was significantly higher in winter (2771±192 g m-2) than in summer (2599±211 g m-2). SOC significantly increased from creek-side (865±89 g m-2) through mid (3298±137 g m-2) to landward (3933±138g m-2) locations. Sediment salinity was a positive driver of SOC. Sediment CO2 flux without the influence of biogenic structures (crab burrows, aerial roots) averaged 15.4 mmol m-2 d-1 in A. marina stands under dark conditions, lower than the global average dark flux (61 mmol m-2 d-1) for mangroves. CO2 flux is a critical component of the global C budget. While CO2 flux has been increasingly studied in mangroves, few studies partition components contributing to the flux. Partitioning CO2 flux sources helps constrain C budgets. We developed the combined 13C stable isotope labelling and closed chamber technique. The technique was used to partition CO2 efflux from the seedlings of mangrove Avicennia marina in laboratory microcosms, with a focus on sediment CO2 efflux. The result showed that (1) canopy was the chief component of ecosystem CO2 efflux, (2) the degradation of sediment organic matter was the major component of sediment CO2 efflux, followed by root respiration and litter decomposition via isotope mixing models. There was a significant relationship between δ13C values of CO2 released at the sediment-air interface and both root respiration and sediment organic matter decomposition. The findings provide the relative contribution of different components to ecosystem and sediment CO2 efflux, and thus can partition the sources of ecosystem respiration and sediment C mineralization in mangroves. My thesis demonstrates that saltmarshes play a disproportionately important role in sediment C accumulation (244.7 g C m-2 yr-1) although their global area extent is declining, and saltmarsh CAR varies with latitude, tidal range, elevation and biogeographic regions. Dead root production contributes a significant proportion to sediment C in both mangroves and saltmarsh, and the updated global estimate of root C decomposition (8 Tg C yr-1) in mangroves reflects an underestimate in the reported value (5 Tg C yr-1). In a sub-tropical mangrove forest, SOC content and CO2 flux are influenced by sediment physio-chemical properties, and/or species, light conditions. Sediment water content and the percentage of fine sediment have contrasting effect on both but sediment chlorophyll is a positive driver of both, and these factors may be highlighted in ‘blue C’ management of this mangrove. In establishing mangroves (Avicennia marina), the result of a laboratory microcosm experiment shows that canopy was the major source of ecosystem CO2 efflux. The decomposition of roots contributes more to sediment CO2 efflux in comparison with sediment organic matter, which is in contrast to the pattern in mature mangroves.

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Thesis (PhD Doctorate)

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Doctor of Philosophy (PhD)

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Griffith School of Environment

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Carbon mineralisation

Coastal wetlands

Ecosystems

Sediment C

Root decomposition

Chlorophyl

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