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. Floodplain soils polluted with high levels of mercury (Hg) are potentialpoint sources to downstream ecosystems. Repeated flooding (e.g., redoxcycling) and agricultural activities (e.g., organic matter addition) mayinfluence the fate and speciation of Hg in these soil systems. The formationand aggregation of colloids and particles influence both Hg mobility andits bioavailability to microbes that form methylmercury (MeHg). In this study,we conducted a microcosm flooding–draining experiment on Hg-pollutedfloodplain soils originating from an agriculturally used area situated inthe Rhone Valley (Valais, Switzerland). The experiment comprised two 14 dflooding periods separated by one 14 d draining period. The effect offreshly added natural organic matter on Hg dynamics was assessed by addingliquid cow manure (+MNR) to two soils characterized by different Hg (47.3±0.5 or 2.38±0.01 mg kg−1) and organiccarbon (OC: 1.92 wt % or 3.45 wt %) contents. During the experiment,the release, colloid formation of Hg in soil solution and net MeHgproduction in the soil were monitored. Upon manure addition in the highlypolluted soil (lower OC), an accelerated release of Hg to the soil solutioncould be linked to a fast reductive dissolution of Mn oxides. The manuretreatments showed a fast sequestration of Hg and a higher percentage of Hg bound byparticulate (0.02–10 µm). Also, analyses of soilsolutions by asymmetrical flow field-flow fractionation coupled withinductively coupled plasma mass spectrometry (AF4–ICP–MS) revealed arelative increase in colloidal Hg bound to dissolved organic matter (Hg–DOM)and inorganic colloidal Hg (70 %–100 %) upon manure addition. Ourexperiment shows a net MeHg production the first flooding and drainingperiod and a subsequent decrease in absolute MeHg concentrations after thesecond flooding period. Manure addition did not change net MeHg productionsignificantly in the incubated soils. The results of this study suggest thatmanure addition may promote Hg sequestration by Hg complexation on largeorganic matter components and the formation and aggregation of inorganicHgS(s) colloids in Hg-polluted Fluvisols with low levels of naturalorganic matter.

Mercury mobility, colloid formation and methylation in a polluted Fluvisol as affected by manure application and flooding–draining cycle