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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 C-3 to C-4 and then back to C-3. Conversion of a plant community dominated by one photosynthetic pathway to another changes the C-13/C-12 ratio of inputs to SOC. Using the time since abandonment and the shift in belowground delta C-13 of SOC relative to the aboveground delta C-13 plant community, we estimated C-3-C and C-4-C changes during secondary succession.

SOC content (g kg(-1)) increased linearly (R-2=0.89 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. delta C-13 increased in the bulk soil and its three aggregate fractions (> 250,250-25, and < 25 mu m) during succession, but the effect of soil depth and its interaction with succession age were significant only for soil aggregate fractions. Polynomial curves described the change in delta C-13 over the chronosequence for both depths. delta C-13 in the bulk soil had increased from -28 parts per thousand. to -24 parts per thousand. by 35 years after abandonment for both depths but then decreased to -26 parts per thousand. at 60 years 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. (C) 2012 Elsevier B.V. All rights reserved.