Subsoil CO2 and CH4 and their advective transfer from faulted grassland to the atmosphere

Abstract

Measurements of CO2 and CH4 in 3 m deep groundwater, soil‐gas, and soil‐atmosphere fluxes were completed in two grasslands in central Italy having the same soil conditions but different subsoil fault‐linked secondary permeability. Unfaulted grassland displays gas‐phase equilibrium between soil‐air and groundwater, typical soil‐gas diffusion profiles, and diffusive soil to atmosphere gas transfer; on the basis of oxygen depletion assessment, assuming a ratio of 1:1 between biogenic O2 consumption and CO2 production, the measured soil CO2 concentrations are consistent with a normal production in the soil by biologic activity. The faulted grassland, instead, has higher CO2 and CH4 concentrations (up to 6% and 10 ppmv in soil‐air) and flux (1.2 mL m−2 s−1 and 1.3 μL m−2 s−1) resulting from a combination of soil biologic and endogenous components, with evidence of gas transfer from the saturated to the unsaturated zone, and advective gas transfer from soil to the atmosphere. The extra‐soil source in the faulted zone, which is about 0.3–4 times the background soil biologic production, is likely related to migrating crustal gas. Whatever the C gas origin and depth may be (biogenic or abiogenic, shallow or deep), the present results demonstrate that the subsoil‐derived component occurring in the soil in areas with active tectonics cannot be ignored a priori in the assessment of the C terrestrial sources. In particular, the assumption that dryland soils are sinks for methane, owing to methanotrophic consumption, may be not true in areas affected by active and gas‐bearing faults. Accordingly, it would be very important to assess at global scale the actual role in the carbon dioxide and methane cycle of soils within the active tectonic bounds.