We investigated the release of mercury from soil to soil solution and the formation of methylmercury in polluted agricultural floodplain fields during a flooding incubation experiment.

Abstract. Floodplain soils polluted with high levels of mercury (Hg) are potential point sources to downstream ecosystems. Repeated flooding (e.g., redox cycling) and agricultural activities (e.g., organic matter addition) may influence the fate and speciation of Hg in these soil systems. The formation and aggregation of colloids and particles influence both Hg mobility and its bioavailability to microbes that form methylmercury (MeHg). In this study, we conducted a microcosm flooding–draining experiment on Hg-polluted floodplain soils originating from an agriculturally used area situated in the Rhone Valley (Valais, Switzerland). The experiment comprised two 14 d flooding periods separated by one 14 d draining period. The effect of freshly added natural organic matter on Hg dynamics was assessed by adding liquid cow manure (+MNR) to two soils characterized by different Hg (47.3±0.5 or 2.38±0.01 mg kg−1) and organic carbon (OC: 1.92 wt % or 3.45 wt %) contents. During the experiment, the release, colloid formation of Hg in soil solution and net MeHg production in the soil were monitored. Upon manure addition in the highly polluted soil (lower OC), an accelerated release of Hg to the soil solution could be linked to a fast reductive dissolution of Mn oxides. The manure treatments showed a fast sequestration of Hg and a higher percentage of Hg bound by particulate (0.02–10 µm). Also, analyses of soil solutions by asymmetrical flow field-flow fractionation coupled with inductively coupled plasma mass spectrometry (AF4–ICP–MS) revealed a relative increase in colloidal Hg bound to dissolved organic matter (Hg–DOM) and inorganic colloidal Hg (70 %–100 %) upon manure addition. Our experiment shows a net MeHg production the first flooding and draining period and a subsequent decrease in absolute MeHg concentrations after the second flooding period. Manure addition did not change net MeHg production significantly in the incubated soils. The results of this study suggest that manure addition may promote Hg sequestration by Hg complexation on large organic matter components and the formation and aggregation of inorganic HgS(s) colloids in Hg-polluted Fluvisols with low levels of natural organic matter.

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<p>Soils in legacy sites of chlor-alkali and acetaldehyde production are point sources of mercury (Hg) to downstream eco-systems. Flooding and agricultural activities may influence the fate of Hg by altering redox conditions, microbial activity and carbon budgets. However, the complex interplay between these parameters is still not well understood. The aim of this work was to better understand the effect of flooding and fertilisation on the release/sequestration of Hg in a polluted  floodplain soil.</p><p>We conducted a flooding-draining incubation experiment on two Hg polluted fluvisols (2.4 ± 0.1 and 44.8 ± 0.5 mg.kg<sup>-1</sup> Hg). The soils originated from an agriculturally used floodplain situated in the Rhone Valley (Valais, Switzerland) and were exposed to Hg pollution by an acetaldehyde producing plant until the 1970’s. They were incubated in triplicates for each treatment. During 56 days the soils were alternately flooded and drained in intervals of 14 days. For flooding, we used an artificial rain water and a 1:1.5 soil:water ratio. The influence of agricultural activites was studied by adding 0.6% (w/w) of liquid manure in a separate treatment. We monitored pore water Hg<sub>total</sub>, Eh, pH, DOC and relevant metals in daily time intervals. Further, the sampled pore water was filtered in distinct intervals (10µm / 5µm / 0.45µm / 0.020µm) at specific time points and analyzed for Hg<sub>total</sub>. Additionally, the 0.45µm fraction was sampled to study the evolution of colloidal Hg with AF4-ICP-MS.</p><p>We observed differences between soil treated with or without manure. In the microcosms (MCs) treated with manure, we observed a Hg<sub>total</sub> release along with reductive disolution of Mn-oxides peaking (Hg<sub>total</sub>: 20.8 µg.L<sup>-1</sup>) after 5 days of flooding. Subsequently, pore water Hg<sub>total</sub> decreased with a simultanous decrease in pore water SO<sub>4</sub><sup>2-</sup>. This is likely due to the onset of sulfate reduction. Additionally, we observed the increase of inorganic colloidal Hg in the range of 10nm hydro dynamic diameter in manure treated MCs with higher contaminated soil during the first 2 and 10 days of incubation.</p><p>In the MCs without manure addition, the onset of reductive dissolution of Mn oxides was 2 days later. Pore water Hg<sub>total</sub>  peaked only after 7 days of flooding (19.76 µg.L<sup>-1 </sup>Hg) and remained at the same levels until the end of the first flooding period. This is likely due to a lower microbial activity and a lower labile carbon pool in the untreated compared to the treated soils.</p><p>Flooding of our polluted fluvisol releases Hg after few days. The additional manuring accelerates this process. However, it as well accelerates the subsequent decrease of Hg<sub>total</sub> in the pore water. This is among others due to the formation of Hg nanoparticles. We plan to use electron microscopy in order to draw conclusions about the nature of these Hg nanoparticles.</p>