Hypoxic and acidic — Soils on mofette fields

• Mofette fields are natural laboratories to study effects of enhanced CO2 concentrations.
• Accelerated weathering of silicates at low pH, decelerated formation of pedogenic oxides
• Accumulation of soil organic matter and reduced alteration of plant material in soil
• CO2 may be designated a soil-forming factor.
• Considering CO2 in soil classification is demanded.

Mofette fields are areas characterized by CO2 that ascends from the earth mantle to the surface owing to volcanic activities. Geogenic CO2 may almost completely make up the soil atmosphere on mofette fields, inducing fundamental effects on both the vegetation and soils. Mofette fields are thus a natural laboratory to study the effects of increased CO2 concentrations in the soil atmosphere. Plants adapt physiologically or anatomically to the specific gas budget on mofette fields, forming a typical, often azonal, mofette vegetation. Soil formation and development are similarly controlled by the gas budget. The acidifying effect of CO2 induces low soil pH, accelerated silicate weathering and leaching of base cations. Formation of secondary Fe and Mn oxides is decreased under partial to total O2 exclusion, and the oxides remain poorly crystalline. Reduced Fe minerals may be stable in a CO2 atmosphere. Soil organic matter accumulates on mofette fields and stays in unaltered and less decomposed state, as bioturbation is diminished and microbial communities shift to anaerobic and acidophilic ones, utilizing geogenic CO2 instead of plant-derived C. Several soil properties and functions are controlled by CO2 that acts as a soil-forming factor. Thus, we suggest to emphasize its effects more distinctly in soil classification by a ‘mofettic’ qualifier applicable to all soil reference groups of the WRB classification.