Pedogenic carbonates and carbon pools in gypsiferous soils of a semiarid Mediterranean environment in south Italy

Soil carbonates are key features in soils of arid and semiarid environment, playing an important role from pedogenetic, landscape history, paleoclimatic and environmental points of view. The objectives of this work were (i) to study pathways of pedogenic carbonate (PC) formation, (ii) to distinguish between lithogenic and pedogenic inorganic C by using the natural C isotope abundance, and (iii) to estimate the soil C pools in a gypsiferous semiarid Mediterranean environment (Sicily, Italy). Five soil pedons developed on calcareous and non-calcareous parent materials from Holocene (10,000 years BP) to Upper Tortonian (7.2–5.3 Ma BP) in age were surveyed. During field soil description, the highest stage of carbonate morphology was found in soils developed on non-calcareous Holocene colluvial deposits (youngest deposits in age) which also showed the highest amount of PC. The great amount of PC in soils developed on youngest deposits was ascribed to a soil–landscape relationships. Being located in a doline overhung by gypsum outcrops, precipitation of Ca2+ from gypsum dissolved by rainfall and biogenic CO2 is reliable. The significant positive relationship between soil organic C and pedogenic carbonates δ13C values confirms that PC was formed from biogenic CO2. Organic C pool in the first cubic meter of soil ranged from 17 to 42 kg, whilst pedogenic inorganic C pool from 2.8 to 30.7 kg. The estimated rate of inorganic C accumulation in soils developed on youngest deposits was 2.5 g m−3 y−1, whereas the rate was negligible on older parent material. The hypothesized pathways of PC formation were ex-novo precipitation of gypsum–Ca2+ and biogenic CO2 and dissolution of lithogenic CaCO3 and re-precipitation of Ca2+ with biogenic CO2. From an environmental prospective, investigated soils may act as a sink of C when Ca2+ from gypsum is available for the formation of pedogenic carbonates.