Arsenic (As) occurs in soils mostly in inorganic forms, whereas the organic forms usually occur only in trace amounts. Peatlands are waterlogged, generally anoxic, organic soils representing the first step in coal formation; the contribution of organic vs. inorganic As species in this environment has received little research attention. Here, 57 peat samples collected throughout a 4-m deep, free-floating mire were analysed for total As and for its organic species, including dimethylarsinic acid (DMA), methylarsonic acid (MA), trimethylarsine oxide (TMAO) and arsenobetaine (AB), by HPLC-ICPMS. Aqueous trifluoroacetic acid was used as extractant, resulting in an average extraction efficiency of almost 80%. Total As concentration throughout the profile ranged between 0.2 and 9.8 mg/kgpeat (mean: 1.4 ± 1.2 mg/kgpeat). Organic As species (DMA + MA + TMAO + AB) accounted, on average, for 28 ± 10% of total As (range: 6–51%), and for 37 ± 13% of the extracted As (range: 7–64%). The relative abundance of organoarsenicals generally followed the order DMA > TMAO ~ MA ≫ AB. A positive correlation (p < 0.001) was found among all organic As compounds, whereas their concentrations were negatively correlated with total sulfur content. The submerged zone (bottom 300 cm) showed average and maximum concentrations of organoarsenic compounds that were almost twice those found in the top 100 cm. This study shows that significant proportions of methylated As species occur even in peat samples characterized by low total As concentration (mostly < 2 mg/kg). Finally, this work provides the first evidence of organoarsenic species in free-floating mires, i.e., a globally distributed but scarcely investigated ecosystem.
