Fig. 3. CH₄ oxidation activities of sediments in the stable-isotope-probing (SIP) microcosm.

The average CH₄ concentration in the headspace (A), average CH₄ oxidation potential at each time point (B) and average CH₄ oxidation potential of each treatment during the whole SIP microcosm incubation (240 d) (C), and the δ¹³C and δ¹⁸O of CO₂ in the headspace of the serum vials without the addition of inhibitors or electron acceptors (D) (n = 3). Five treatments were constructed by adding 2-bromoethanesulfonic acid (BES, a methanogen-specific inhibitor) and/or sodium molybadate (SM, a specific inhibitor for sulfate reduction), or Na₂SO₄ (EA, as a supplementary electron acceptor) into the vials to adjust the final concentrations of BES, SM and Na₂SO₄ to 10, 5,and 3 mM, respectively. The five treatments were as follows: (1) the addition of BES and SM (BES+SM); (2) the addition of BES (BES); (3) the addition of SM (SM); (4) the addition of EA (EA); (5) unamended (none). The gas composition was 1–2% of H₂, 5% of CH₄ and 93–94% of N₂ in the headspace of the serum vials. The arrows indicate timepoints when ¹²CH₄/¹³CH₄ was injected to reinstate the initial headspace CH₄ concentration. Error bars indicate standard deviation. Error bars are not shown in A and B for visual clarity.