Overview
Gfeller and colleagues tested how liquid cow manure and flooding-draining cycles changed mercury mobility, colloid formation, and methylmercury production in Hg-polluted agricultural floodplain soils. This is upstream supply-chain pathway evidence: the paper does not measure a finished food, but it directly addresses how manure application and redox cycling in agricultural soil can mobilize total mercury (Hg) and promote or limit methylmercury (MeHg) formation.
Key numbers
Table 1 reports that the cornfield high-mercury low-carbon Fluvisol (HMLC) contained HgT 47.3 ± 0.5 mg kg−1 dry weight and MeHg 26.9 ± 0.2 µg kg−1 dry weight, with MeHg / HgT 0.06%. The pasture low-mercury high-carbon Fluvisol (LMHC) contained HgT 2.4 ± 0.3 mg kg−1 dry weight and MeHg 6.4 ± 0.2 µg kg−1 dry weight, with MeHg / HgT 0.28%. The liquid cow manure contained HgT 0.045 ± 0.001 mg kg−1 dry weight and MeHg < 0.02 µg kg−1 dry weight.
Table 1 also reports associated soil and manure element concentrations on a dry-weight basis. HMLC soil contained Mn 493 ± 21 mg kg−1, chromium (Cr) 56 ± 4 mg kg−1, Co 10.75 ± 0.06 mg kg−1, nickel (Ni) 81.7 ± 0.8 mg kg−1, Cu 40.1 ± 1.2 mg kg−1, Zn 61.8 ± 0.5 mg kg−1, arsenic (As) 11.74 ± 0.07 mg kg−1, cadmium (Cd) 0.21 ± 0.04 mg kg−1, lead (Pb) 20.8 ± 0.5 mg kg−1, V 17.2 ± 0.4 mg kg−1, Ba 60.2 ± 1.1 mg kg−1, and uranium (U) 1.74 ± 0.08 mg kg−1. LMHC soil contained Mn 672 ± 38 mg kg−1, Cr 64 ± 5 mg kg−1, Co 11.22 ± 0.43 mg kg−1, Ni 78.3 ± 2.9 mg kg−1, Cu 28.0 ± 0.7 mg kg−1, Zn 47.3 ± 2.0 mg kg−1, As 16.04 ± 0.72 mg kg−1, Cd 0.17 ± 0.01 mg kg−1, Pb 18.34 ± 0.5 mg kg−1, V 20.99 ± 1.1 mg kg−1, Ba 76.9 ± 1.6 mg kg−1, and U 1.29 ± 0.01 mg kg−1.
For net methylmercury production, Table 3 reports HMLC MeHg means of 26.9 µg kg−1 at day 0, 30.14 ± 2.19 µg kg−1 at day 14, 52.04 ± 10.65 µg kg−1 at day 28, and 30.03 ± 5.05 µg kg−1 at day 42. The corresponding net MeHg production values were 12.0%, 73.1%, and −32.4% for days 14, 28, and 42.
With manure added to HMLC, Table 3 reports MeHg means of 26.9 µg kg−1 at day 0, 43.41 ± 1.99 µg kg−1 at day 14, 57.79 ± 13.79 µg kg−1 at day 28, and 30.94 ± 3.43 µg kg−1 at day 42. The corresponding net MeHg production values were 81.1%, 20.7%, and −45.9%.
For LMHC, Table 3 reports MeHg means of 6.4 µg kg−1 at day 0, 8.11 ± 1.09 µg kg−1 at day 14, 12.07 ± 1.1 µg kg−1 at day 28, and 7.95 ± 0.35 µg kg−1 at day 42. The corresponding net MeHg production values were 10.0%, 37.2%, and −16.7%. With manure added to LMHC, MeHg means were 6.4 µg kg−1 at day 0, 10.86 ± 1.86 µg kg−1 at day 14, 14.31 ± 0.17 µg kg−1 at day 28, and 8.4 ± 0.09 µg kg−1 at day 42, with net MeHg production values of 38.1%, 26.6%, and −22.0%.
The results text states that the highest net MeHg production occurred during the first flooding period for manure treatments, up to +81%, and during the draining phase for treatments without manure, up to +73.1%. Across all microcosms, MeHg / HgT increased by a factor of 1.18 to 1.36 over the incubation.
The discussion estimates that about 12.8 ± 4.2 µg kg−1 Hg, equal to 0.02% of HgTsoil, was evacuated by sampling during the experiment. The authors interpret Hg mobilization in the HMLC Fluvisol as mainly driven by reductive dissolution of Mn oxyhydroxides, while the conclusion states that manure addition diminished HgT mobility, facilitated Hg complexation with fresh natural organic matter and beta-HgS(s) formation, and had only a limited effect on net MeHg production.
Methods (brief)
The study used soils from agriculturally used fields in the Rhone Valley, Switzerland, affected by historical Hg pollution from an upstream chemical plant that used Hg in chlor-alkali electrolysis, acetaldehyde production, and vinyl chloride production. Composite 0-20 cm soil samples were collected from a cornfield and pasture field next to the Grossgrundkanal canal. Fresh soil was incubated in triplicate microcosms under two 14 d flooding periods separated by one 14 d draining period, with or without 0.6% (w/w) liquid cow manure. Total Hg and trace elements were measured after nitric acid microwave digestion by ICP-MS. Methylmercury was analyzed by an adapted Gygax et al. method, and Hg colloids in soil solution were characterized by AF4-ICP-MS.
Related evidence
- Irrigation water and soil amendments as anthropogenic inputs to cropland
- Soil-to-plant transfer of heavy metals
- Mercury
- Mercury, Total
- Methylmercury
- Manganese
- Iron
- Copper
- Arsenic
- Cadmium
- Lead
- Chromium
- Cobalt
- Nickel
- Zinc
- Vanadium
- Barium
- Uranium
Update history
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