Soil carbon dynamics in the upper Mersey estuarine floodplain, northwest England : implications for soil carbon sequestration

There have been several studies on soil carbon dynamics in estuarine floodplains. However, little attempt has been made to examine the effects of heavy metal contamination on these processes. This represents a knowledge gap that needs to be filled to understand better soil carbon decomposition in contaminated estuarine floodplains and the implications for carbon sequestration. The current work aims to close this knowledge gap. Field and laboratory investigations were conducted to collect data. These include (a) a soil survey to characterise heavy metal contamination in the study area, (b) seasonal monitoring of key parameters (soil organic carbon, bulk density, plant biomass etc.) at three selected sites with different land uses/land covers, and (c) a laboratory experiment to evaluate the impacts of heavy metals on soil carbon content, characterisation of humic substances, adsorption capacity, and microbial activities. The historically contaminated Upper Mersey Estuarine floodplain was selected as the study site. The results indicated that the study area has elevated concentration of heavy metals with arsenic, chromium, copper, lead, and zinc being the major contaminants. Seasonal variations in organic carbon content and bulk density showed that, grazing locations stored more organic carbon and recorded high bulk density compared to non-grazing locations. The presence of heavy metals inhibited the activities of soil microbes, impeded decomposition of organic matter, resulting in incomplete carbon mineralisation and enhanced soil carbon storage. The functional group composition of the soil humic substances was also affected. The findings obtained from this study have implications for understanding the role of soil carbon in limiting heavy metal mobilisation and the importance of microbial activity in soil carbon budgets, the management of saltmarsh under grazing regimes, national carbon budgets, and the design of future studies