Mercury migration during leaf litter decomposition in a coastal artificial mangrove wetland

We implemented a 10-month in-situ decomposition experiment on Laguncularia racemosa litter within an urban coastal mangrove in China. By integrating total mercury (THg) measurements with stable isotope tracing, we examined mercury migration and transformation patterns in the intertidal zone. Mercury dynamics showed a distinct two-stage evolution. During the early decomposition phase (March-June), the THg concentrations significantly decreased from 84.0 ± 5.1 to 58.6 ± 2.4 ng·g^-1, indicating a net release, with a loss rate of 8.5 ± 3.1 ng·g^-1·month^-1. The significant negative correlation between THg and δ²⁰²Hg (r = -0.980, p < 0.05) suggests the preferential migration of light mercury isotopes. Conversely, during the late decomposition stage (July-December), we observed mercury net enrichment at an average rate of 5.8 ± 3.2 ng·g^-1·month^-1. Calculations using a Δ¹⁹⁹Hg-based isotope mixing model indicated that this transition from a "source" to a "sink" was driven by exogenous inputs. Specifically, external mercury contributions from surrounding sediments and tidal waters increased from 36.6 ± 7.3% initially to 64.5 ± 12.9% in the later phase. Furthermore, the litter exhibited a distinct Δ¹⁹⁹Hg/Δ²⁰¹Hg ratio (1.21 ± 0.14), differing from that of the surrounding environmental media, implying specific photochemical or microbially mediated mercury transformation processes within the decomposition microenvironment. With an estimated annual mangrove litterfall mercury flux of ≥73.5 μg·m^-2·yr^-1, litter serves not only as a downward vector for atmospheric mercury but also acts as an active late-stage sink, playing a pivotal role in mercury retention and regional biogeochemical cycling in coastal wetlands.