Measurement of dissolved H2 concentrations in methanogenic environments with a gas diffusion probe
A probe for the measurement of dissolved H2 in anoxic methanogenic environment was developed. The probe was based on the diffusion of dissolved H2 through a silicone membrane into a gas space at the end of the probe. This gas space was flushed with N2 and analyzed gaschromatographically for H2. The probe was calibrated in aqueous solutions of the known gas space was flushed The detection limit was about 7 nM H2, the depth resolution of vertical profiles was about 5–10 min. The probe could also be used to measure dissolved CH4 and CO. The probe allowed the measurement of in situ concentrations of dissolved H2 that were unbiased by eventual effects of an extraction procedure on the steady state between production and consumption of H2. However, it could not be implanted into the environment for periods longer than hours because of dramatically increased H2 signals due to microbial growth on the silicon membrane. The probe was used to measure vertical profiles of dissolved H2 and CH4 in cores of the deep sediment of Lake Constance and of anoxic paddy soil. The vertical CH4 profiles measured with the probe compared well with those measured by extraction. In the lake sediment, concentrations of H2 increased with depth and reached values in the range 10–60 nM H2 within the methanogenic zone, below about 2–5 cm depth. In the anoxic paddy soil, H2 concentrations also increased with depth reaching values of 40–100 nM H2. Dissolved CO was about 5–20 nM. The turnover of H2 was very dynamic as revealed when starch powder was added to the flooding water. Within 1 day, H2 concentrations increased to 80 μM H2 in the upper 2 cm of the soil core, but after 5–7 days decreased again to the lower values measured previously.