Carbon sequestration in Mediterranean soils following afforestation of abandoned crops: Biases due to changes in soil compaction and carbonate stocks

Crop replacement by secondary forests increases soil organic carbon (OC) stocks. Quantifying this increase is subject to biases: the changes in soil compaction and, in calcareous zones, the changes in soil inorganic carbon (IC) content, which affect the carbon balance. How relevant are both biases, though? We studied this problem in NE Spain, subhumid Mediterranean, in a set of current crops, old forests, and new forests grown on croplands abandoned after 1956. We compared soil OC stocks on a fixed-depth basis (30 cm), and on an equivalent-mass basis: uppermost 425 kg m−2 of fine mineral matter (amount in crop soils). Down to 30 cm, OC stocks were lowest in crops (5.45 kg m−2) and highest in old forests (7.50 kg m−2), which gives a potential OC sequestration in the mineral soil of 2.05 kg m−2. On an equivalent-mass basis, the potential is 2.50 kg m−2, meaning that the fixed-depth criterion underestimates OC sequestration by more than 20%. These figures are surpassed by those of IC: crops store 17.73 kg IC m−2 down to 30 cm, old forests 12.28, suggesting a potential loss upon afforestation of 5.45 kg IC m−2. Nevertheless, the lack of negative relationships between IC losses and OC gains within a given land use suggests that the contrasting IC stocks result from past management, mainly tillage. The decarbonation is only apparent. Our results stress the need of accounting for changes in carbonate content and soil compaction in future studies about carbon sequestration in soils.