Characterising blue carbon: An assessment of soil carbon in temperate coastal wetlands

Abstract

Vegetated coastal wetlands (mangrove forests, tidal marshes and seagrass meadows) provide a suite of ecosystem services and are some of the most productive habitats in the world. As part of their ecosystem services, vegetated coastal wetlands store a significant amount of carbon in their above and below ground biomass and soils; and it is known as blue carbon. These ecosystems, despite their small global footprint of just 2% of the Earth’s surface, store significant amounts of carbon and act as significant carbon sinks. A majority (>50%) of the carbon stored by these environments is accounted for as soil organic carbon (SOC). The saline and saturated nature of blue carbon soils results in slower turnover rates of organic matter, promoting long-term storage of carbon. The capture and retention of carbon in these natural environments is, in part, contributing to climate change mitigation. The work presented in this thesis focuses on the quality of blue carbon, specifically the chemical composition of carbon, to better understand some of the mechanisms that underpin the long-term stability of carbon in mangrove and tidal marsh soils of temperate coastal wetlands. The focus of current blue carbon research has been on understanding the drivers of SOC accumulation and the quantifying SOC stocks within and across blue carbon habitats at both a regional and global extent but does not address its quality. The quality of SOC, for example its chemical composition, is one of the controlling factors of its long-term stability in the environment. However, in the same way the quantity of carbon should not be the sole focus, the quality of SOC in any ecosystem should not be assessed without first quantifying current carbon storage. In this thesis I explore both aspects of blue carbon systems, as follows: In Chapter Two mangrove and tidal marsh surface soil (top 10 cm) carbon and nitrogen stocks and their (within site) spatial variability were quantified and compared, across nine selected case study sites in temperate coastal wetlands. This led investigations to Chapter Three and Four (Part 2), that assessed if combined infrared resonance spectroscopy and partial least squared regression analyses (IR/PLSR) could successfully predict carbon and nitrogen stocks and the allocation of SOC to size fractions, that were quantified in Chapter Four (Part 1), in blue carbon soils. Then in Chapter Five, the chemical composition of temperate blue carbon in temperate coastal wetland soils were investigated. Overall, it was found that differences in surface soil (top 10 cm) carbon and nitrogen stocks in temperate coastal wetlands were driven by the characteristics of a site and its inherent environmental conditions rather than the vegetation. Although, vegetation did effect surface soil (top 10 cm) carbon and nitrogen stocks at some sites and sampling highlighted significant within site spatial variability of the stocks. The highest proportion of OC was allocated to the humus pool (58 % and 53 % for mangrove and tidal marsh samples, respectively), supporting the longevity notion of blue carbon. However, a dominance of labile carbon forms (O-alkyl) in the surface soils (top 10 cm), irrespective of vegetation type, suggests SOC in the blue carbon environment is vulnerable to rapid decomposition should environmental conditions of the soil change. Overall, the summation of this work provides a comprehensive assessment of SOC chemistry in temperate blue carbon ecosystems. Additionally, the application of robust IR/PLSR predictive algorithms developed in this thesis can provide rapid and cost-effective estimates of carbon and nitrogen stocks that will improve future estimates and can account for the variability of stocks in blue carbon soils.