Environmental drivers and microbial dynamics in mangrove carbon sequestration : a case study from Hong Kong

Abstract

Globally, mangroves habitats store immense amounts of carbon despite their small global surface cover, making them efficient carbon sinks. Mangrove soils are the primary carbon sink, storing up to two third of the total carbon. Soil carbon sequestration in mangrove habitats is carried out through the carbon cycle which is linked to other biogeochemical processes such as nitrogen or sulfur cycles. These processes are driven by the activity of microbial communities and interlinked to environmental parameters. One of the main challenges to our understanding of mangrove carbon stock is determining what drives the discrepancies in their carbon stock capacity and how microbial communities are interlinked to these discrepancies. This thesis aimed to fill some of these key gaps by examining how environmental settings drive the balance between carbon accumulation and retention, microbial community dynamics, and the local versus external microbial communities. We used the mangrove habitats around Hong Kong as a case study, pertaining to the heterogenous environmental settings, that is marine-influenced eastern side and freshwater-influenced western side. Firstly, we determined the quantity of carbon buried to be on average 115 ± 8 Mg C ha-1, placing Hong Kong mangroves in the lower end of the global spectrum of soil carbon stock. The spatial variation in mangrove stands across Hong Kong (two eastern and two westers) suggested that carbon stock is heavily driven by the total organic matter inputs, soil fluxes and porosity, and hydrodynamics. For example, while Mai Po had the highest mass-specific soil organic carbon contents, its stock was the lowest at 77 ± 3 Mg C ha-1, mainly because of low sediment density and high tidal pumping leading to I decreased in carbon retention. Secondly, to determine how microbial communities interacted with fluctuating environments and carbon dynamics, we used metabarcoding of total DNA to investigate microbial community structure. We found less than 30% dissimilarity across microbial communities between sites, even if the environmental setting differed, suggesting that the microbial communities in Hong Kong mangrove are broadly resistant to environmental variations. Nonetheless, slight taxonomic adjustments driven by variation in salinity, carbon, nitrogen, and sulfur content, sediment content, and hydrodynamics suggested varied biogeochemical processing capabilities across spatial and temporal scales. Finally, we analysed iDNA and exDNA to investigate which part of the microbial community was linked to local activity in the soil, thus most likely responsible for local biogeochemical processes, as opposed to microbes imported through hydrological movements. We found no overlap in microbes identified through iDNA and exDNA and that most of the local active microbial community was composed of terrestrial and mixed habitat taxa. Overall, these findings enhance our understanding of the variability in carbon stock capabilities of mangroves, and highlight the importance of standardized methodology to assess stock. In addition, we highlight the resistance of microbial communities and their ability to maintain their function despite fluctuating environments. Finally, we highlight the importance of properly differentiating between local and external microbes in a habitat to understand their role in biogeochemical processes.