Hydrogen soil dynamics in northern boreal and subarctic Sweden

Wetland ecosystems store a large amount of organic carbon in the form of peat and are the largest natural source of CH4. Thawing of northern wetland permafrost results in an increase in the pool of soil carbon that is made available for decomposition processes and CH4 production. Some subarctic mire sites are also getting wetter as the climate warms. An increase in inundated areas in conjunction with increased amounts of organic matter could give rise to potential feedbacks to warming temperatures via increased emissions of reduced trace gases, such as CH4 and H2, to the atmosphere. H2 soil dynamics in peatlands and forests are complex because of the many microbial-mediated reactions driving H2 production and consumption. H2 couples oxidative and reductive processes in anaerobic environments. The aim of this project was to determine if high-latitude boreal and subarctic soils can change from a sink to a source of H2 to the atmosphere by identifying the microbial processes controlling the production and consumption of H2. Does H2 production and emissions to the atmosphere occur under temporary anoxia in organic -rich soils and soil horizons and do episodic weather events, particularly rain and freeze-thaw cycles, drive H2 production and release from natural soils due to the release of labile organic material and anaerobic conditions. Porewater soil gas profiles from different sub-habitats were determined in Stordalen mire in subarctic Sweden using buried ePTFE tubing and samples manually obtained using a stainless steel sipper. Trends in H2 concentration between the microporous tubing and sipper samples generally agree. The H2 concentration is higher in the tubing possibly due to preferential diffusion into the air-filled tubing by H2, which has a low solubility in water. The wettest site dominated by Eriophorum had the highest concentration of H2 with a maximum of 39.3 ppmv H2 at a depth of 30 cm. A mesic site dominated by Sphagnum had the next highest H2 concentration with 37.6 ppmv H2 at 20 cm below the ground surface. A Carex-dominated site also had increasing H2 concentration with depth. The concentrations of soil H2 in the dry palsa site were lower than ambient air indicating consumption at this site. Soil H2 was also measured in boreal forest soils, which typically act as a sink of atmospheric H2. Manual field sampling revealed that H2 concentrations were higher above the surface of the ground than at the base of the O horizon suggesting H2 deposition. An incubation experiment designed to test the interactions between soil moisture, temperature, and substrate addition indicated that warm, dry forest soils with added glucose are the highest consumers of H2 while warm, dry forest soils with no substrate addition produce the most H2. With the exception of the soil with the greatest glucose addition, the soils incubated at 20°C produced more H2 than soils at 12°C and 4°C.