Soil carbon dynamics during secondary succession in a semi-arid Mediterranean environment

Clarifying which factors cause an increase or decrease in soil organic carbon (SOC) after agricultural abandonment requires integration of data on the temporal dynamics of the plant community and SOC. A chronosequence of abandoned vineyards was studied on a volcanic island (Pantelleria, Italy). Vegetation in the abandoned fields was initially dominated by annual and perennial herbs, then by Hyparrhenia hirta (L.) Stapf, and finally by woody communities. As a consequence, the dominant photosynthetic pathway changed from C3 to C4 and then back to C3. Conversion of a plant community dominated by one photosynthetic pathway to another changes the 13C/12C ratio of inputs to soil organic carbon (SOC). Using the time since abandonment and the shift in belowground δ13C of SOC relative to the aboveground δ13C plant community, we estimated SOC turnover rate. SOC content (g kg−1) increased linearly (R2 = 0.79 and 0.73 for 0–15 and 15–30 cm soil depth) with the age of abandonment, increasing from 12 g kg−1 in cultivated vineyards to as high as 26 g kg−1 in the last stage of the succession. δ13C increased in the bulk soil and its three fractions during succession, but only for soil fractions the effects of soil depth and its interaction with succession age were significant. Polynomial curves described the change in δ13C over the chronosequence for both depths. δ13C in the bulk soil had increased from −28 to −24‰ by 30 yr after abandonment for both depths but then decreased to −26‰ at 60 yr after abandonment (corresponding with maturity of the woody plant community). Overall, the results indicate that abandoned vineyards on volcanic soil in a semi-arid environment are C sinks and that C storage in these soils is closely related to plant succession.