Ce 1100

Sulphide in groundwater was fixed with a zinc acetate solution (final concentration 1%). The zinc sulphide precipitate was removed by centrifugation at 4,500 rcf for 10 min and the pellet was discarded. The supernatant was used for sulphur isotopic measurement of sulphate. The supernatant was acidified to ∼pH 3 with 1 M HCl and heated to 90°C in a water bath. Sulphate was precipitated as BaSO₄ by the addition of BaCl₂ (10% solution). Samples were left to cool overnight then centrifuged at 4,500 rcf for 10 min. The supernatant was discarded and the BaSO₄ pellet air-dried. The ³²S/³⁴S isotope ratio of sulphate (δ³⁴S_SO4 ‰ VCDT) was measured at the Stable Isotope Laboratory (Faculty of Geosciences and Environment, Université de Lausanne, Lausanne, Switzerland). The S isotope composition was measured with a He carrier gas and a Carlo Erba (CE 1100) elemental analyser linked to a Thermo Fisher Delta V mass spectrometer. Samples were reacted at 1,050°C in a stream of He-carrier gas spiked with oxygen gas. External reproducibility of standards was <0.15‰ and samples were calibrated against IAEA standards S1 and S3 (Ag₂S) and NBS-127 (BaSO₄) with accepted values of −0.3, −32.1, and 20.3‰, respectively.

Soil samples were collected at the end of the experiment after 539 days (May 31^st, 2010). Two mineral soil cores (Ø 5 cm) from the middle of each mesocosm were transferred to the lab and stored at 4°C until further use. The cores were divided into 1 cm soil sections (0–1, 1–2, 2–3, 3–4 and 4–5 cm) and the two corresponding sections from each mesocosm were homogenized to get a composite sample. Soil was sieved (Ø 2 mm) and dried at 105°C for 24 h. Subsamples were ground in a ball mill (Retsch MM200, Haan, Germany). Organic C concentration was measured with an elemental analyzer (EA; vario Max, Elementar Analysesysteme GmbH, Hanau, Germany). Inorganic C was below the level of detection (LOD = 0.027% C) in all samples along the whole soil profiles (0–5 cm). To obtain isotope signatures (δ¹³C) of soil organic C, ground subsamples were weighed into tin capsules and measured using an EA-IRMS system [26] . Here, an EA (CE 1100) was coupled on-line via a Con Flo III interface with a Delta plus isotope ratio mass spectrometer (all supplied by Thermo Fisher, Bremen, Germany).