Controlling heavy metals pollution is vital for the restoration of carbon and nitrogen transformation function of mangrove ecosystems in the Greater Bay Area, China

Abstract

Pollutants, especially for heavy metals (HMs), discharged from anthropogenic activities are the potential risks to newly planted mangrove forests in the restoration process. Thus, it is crucial to investigate how HMs affect the biological processes that take place in these mangrove habitats, e.g., the transformation of carbon and nitrogen. In this study, correlations between HMs pollution, carbon storage, and nitrogen biotransformation in the Great Bay Area (GBA) were examined at five typical sites with different mangrove restoration ages. The results indicated that there were distinct differences in the concentration distribution of HMs (ranges: 0.2–369.7 mg/kg) at five sample sites, with Cd (0.2–1.6 mg/kg) being the heavily polluted element in sediments. The fluctuation of SOC (16.0–25.7 mg/kg), δ13C (−27.5 to −20.2), and NO3−-N (1.2–3.3 mg/kg) was positively correlated with the content of HMs in sediments, indicating that these components in mangrove ecosystems might have similar emission source and pose a potential threat to mangrove carbon storage. Notably, sediments with high HMs concentration decelerated carbon decomposition rate of mangrove ecosystems, especially for sites with newly planted mangrove forest. Moreover, a large abundance of Euryarchaeota was found in the HO site, suggesting that HMs may promote the conversion of carbon to CH4. The inflow of contaminants also induced the significant increasing proportion of microorganisms related to nitrogen fixing, denitrification, and dissimilarity nitrogen reduction processes, resulting a decline in the stability of co-occurrence network of nitrogen transformation. Fortunately, mangrove site with the establishment of natural reserves and controlling pollutant discharge could contribute to the rapid restoration of its ecological function, e.g., nutrient cycling and co-occurrence stability, in SZ site. Therefore, future management should focus on the decrease of pollutants discharge and pay more attention to the ecological function restoration of mangrove wetlands in the GBA.