Globally, soils contain approximately twice as much carbon as the atmosphere. Therefore, any change in carbon dynamics within soils could substantially alter atmospheric CO2 concentrations. Recovery of soil carbon following abandonment of agricultural land, increasingly common, could be significant at either global or regional scales. The long-term pattern of soil carbon change as well as carbon sequestered in different physical soil fractions is largely unknown. We tested the hypothesis that soil carbon increases at a steady rate for about 100 years following agricultural abandonment with similar patterns for total and labile carbon pools. We compared carbon dynamics across three common but distinct soil types: a sandy-loam, loam, and a clay soil, in southeastern Ontario, Canada. We developed chronosequences containing sites spanning 15 to 95 years since agricultural abandonment, agricultural fields and mature forests. Carbon content increased significantly and at a steady rate in all soil types between 0 and 5 cm, but increased faster in the clay soil. There was no change in carbon storage at depth. Light fraction carbon content, determined by density fractionation, did not recover following abandonment. The average rate of carbon sequestration was relatively low in the marginal soils we studied (10 gCm-2yr-1), but that rate is similar for all land abandoned in the past 100 years. Our results also suggest that most carbon is not sequestered in a labile pool, but is associated with mineral particles making it more resistant to decomposition in the future.
