Elevated methylmercury production in mercury-contaminated paddy soil resulted from the favorable dissolved organic matter variation created by algal decomposition

Algae-derived organic matter (AOM) may considerably regulate methylmercury (MeHg) production and accu-mulation in the paddy fields by changing the soil-dissolved OM (SDOM) properties. In this study, a 25-day microcosm experiment was performed to compare the responding mechanisms of MeHg production in the Hg-contaminated paddy soil-water system to the input of algae-, rice-, and rape-derived OMs. Results showed that algal decomposition could release much more cysteine and sulfate than crop straws. Compared with crop straw-derived OMs, AOM input greatly increased the dissolved organic carbon concentrations in soil but resulted in a greater decrease in tryptophan-like fractions while accelerated the formation of high-molecular-weight fractions in soil DOM. Moreover, AOM input significantly increased MeHg concentrations in the pore water by 19.43%-3427.66% and 52.81%-5846.57% compared to rape-and rice-derived OMs, respectively (P < 0.05). And, a similar MeHg changing pattern was also observed in the overlying water (10-25 d) and the soil solid-phase particles (15-25 d) (P < 0.05). Correlation analysis revealed that MeHg concentrations in the AOM-added soil-water system had significantly negative and positive relationships with the tryptophan-like C4 frac-tion and molecular weight (E2/E3 ratio) of soil DOM, respectively (P < 0.01). These findings suggest that AOM has a higher capacity than crop straw-derived OMs to promote MeHg production and accumulation in the Hg-contaminated paddy soils by creating a favorable soil DOM variation and providing more microbial electron donors and receptors.

Automated summary

Algae-derived organic matter (AOM) can significantly regulate the production and accumulation of methylmercury (MeHg) in paddy fields by changing the properties of soil-dissolved organic matter (SDOM). In this study, a 25-day microcosm experiment was conducted to compare the effects of AOM, rice straw, and rapeseed straw on MeHg production in a Hg-contaminated paddy soil-water system. Results showed that AOM input increased dissolved organic carbon concentrations in soil, while decreasing tryptophan-like fractions and promoting the formation of high-molecular-weight fractions in soil DOM. AOM input also significantly increased MeHg concentrations in pore water, overlying water, and soil solid-phase particles compared to rice and rapeseed-derived OMs. Correlation analysis revealed negative and positive relationships between MeHg concentrations and tryptophan-like C4 fraction and molecular weight of soil DOM, respectively. These findings highlight the importance of AOM in promoting MeHg production in Hg-contaminated paddy soils.