Effects of mangrove plants on heavy metal risk in sediment based on SEM-AVS analysis

•The physicochemical properties in mangrove sediments were different from that of mudflat.•The levels of AVS and SEM in mangrove forest sediments were lower than that of mudflat.•pH acted as an important role in distributions of AVS and SEM in mudflat.•There were no ecological risks of heavy metals due to metal-binding function of TOC.

Sediment cores in mangrove forests (Kandelia obovata, Avicennia marina, and Sonneratia caseolaris) and adjacent mudflat were collected to quantify the distribution of acid-volatile sulfide (AVS) and simultaneously extracted metals (SEM) in Futian mangrove forest, South China. The influence of mangrove species on the ecological risk due to heavy metals was also evaluated based on SEM-AVS analysis. The results showed that the presence of mangrove plants improved the moisture content (MC), total organic carbon (TOC), electrical conductivity (EC), and reduced pH value compared to the mudflat. The levels of AVS in mangrove forest sediments were lower than in the mudflat, which may be related to their marine origin or the negative effects of root-secreted oxygen on AVS formation. In the mudflat, the peak level of AVS appeared deeper in the sediment profile compared to K. obovata and S. caseolaris sediment, which may be related to that the oxygen released from mangrove root could affect the reducing condition and affect AVS formation. Zn contributed to the main components of SEM in all sediments. Of all the factors investigated, pH had important influence on the distribution of AVS and SEM in the mudflat. The values of SEM/AVS and [SEM]-[AVS] in the mudflat were lower than in the mangrove forest sediment at 10-40 cm depth, with no ecological risk in all sediments. Taking into account the TOC concentration, there were no adverse effects due to heavy metals in the sediments of the mangrove forest or the mudflat.