Rhizospheric organic matter drives mercury accumulation and reduction in deep subtropical forest soils: mercury isotope insights

The rhizosphere plays a pivotal role in the biogeochemistry of toxic heavy metals in soil-plant systems. Yet, our understanding of the rhizosphere processes driving mercury (Hg) cycling remains limited. The unique characteristics of the rhizosphere microenvironment, particularly the elevated binding capacity and redox activity of microbial-derived carbon, support the hypothesis that localized organic matter drives Hg enrichment and speciation shifts, ultimately governing Hg biogeochemical cycling at the soil-vegetation interface. Herein, we report that in a subtropical forest, the rhizosphere soil (Rs) has significantly higher Hg concentration than that in non-rhizosphere soil (NRs), specifically in the deeper soil horizon (41.2 ± 42 for Rs <i>versus</i> 14.3 ± 22.8 ng g^-1 for NRs). Similarly, the Δ¹⁹⁹Hg values in deeper Rs were significantly more negative than those in deeper NRs, with an average shift of -0.21 ± 0.14‰. Further correlations between Δ¹⁹⁹Hg and indexes of rhizospheric microbial carbon indicated that the abiotic reduction of Hg by microbial organic matter contributed to this shift of Δ¹⁹⁹Hg between Rs and NRs. Finally, the Rayleigh model estimated an average 32-72% Hg reduction in deeper soil horizons, highlighting the complex interactions between plants, microorganisms, and organic matter within the rhizosphere regarding Hg accumulation and sequestration.