A novel belowground in-situ gas labeling approach: CH4 oxidation in deep peat using passive diffusion chambers and C-13 excess

In-vitro incubation of environmental samples is a common approach to estimate CH4 oxidation potential. Here we developed and verified an in-situ method utilizing passive diffusion chambers (PDC, silicone tubes) to deliver C-13-labeled CH4 into peat for the determination of the CH4 oxidation potential based on C-13 excess of CO2. To target CH4 oxidation under semi-aerobic and anaerobic conditions, we installed 20 PDCs (30 ml volume) below the water table in profiles from 35-cm to 2-m depths of a peatland in north-eastern Sweden in July 2017 using a peat auger. C-13-labeled CH4 was injected into PDCs through tubing twice during 12 days (day 0 and 6) and samples were collected at days 1, 3, 6, 8 and 11. Background (non-labeled) delta C-13 of CO2 ranged from -7.3 (35 cm) to +5.7 parts per thousand (200 cm) with depth. These delta C-13 values rose to +110 and +204 parts per thousand after the CH4 injection. The estimated CH4-derived C in CO2 was the lowest at the bottom of the profile (0.3 mu mol L-1), whereas the maximum was at 100 cm (6.1 mu mol L-1) at five days after the second labeling. This corresponded to 1.5-7.2% of the total CH4 pool to be oxidized, depending on depth. This novel approach with belowground in-situ C-13 labeling of gases demonstrated the suitability of tracing the transformations of these gases in soil depth by PDCs and for the first time verified the in-situ occurrence of a deep-peat CH4 oxidation. (C) 2021 Elsevier B.V. All rights reserved.