Soil carbon dynamics in a chronosequence of secondary forests in northeastern Costa Rica

Secondary forests are becoming an increasingly important tropical landscape component with the potential to provide environmental services such as soil carbon storage. Substantial losses of soil carbon can occur with tropical forest conversion to pasture, but stocks can sometimes be restored with the development of secondary forest. Few studies have taken advantage of shifts in vegetation from C4 to C3 communities to determine soil carbon turnover following secondary forest development on pasture. Because trees quickly colonize abandoned pastures in northeastern Costa Rica, we expected to find evidence of increased soil carbon storage and gradual soil carbon turnover following pasture abandonment. Three early successional and nine late successional secondary sites ranging in age from 2.6 to 33 years, as well as four pastures were used in this study. At each site, mineral soil samples up to 30 cm depth were collected from three plots to determine bulk density, percent soil carbon, and stable carbon isotope values (δ13C). The δ13C of soil respired CO2 was also determined at each site. Contrary to expectations, soil carbon storage did not increase with secondary forest age and was unrelated to increases in aboveground carbon storage. However, pastures stored 19% more carbon than early and late successional sites in the top 10 cm of mineral soil, and successional sites stored 14–18% more carbon than pastures between 10 and 30 cm. δ13C data indicated that most pasture-derived soil carbon in the top 30 cm of soil turned over within 10 years of pasture abandonment and subsequent colonization by trees. Overall, these data indicate that total soil carbon storage remains relatively unchanged following land use transitions from pasture to secondary forest. This is likely due to the presence of large passive pools of mineral-stabilized soil carbon in this region of Costa Rica. The contribution of these forests to increased carbon storage on the landscape is primarily confined to aboveground carbon stocks, though other environmental services may be derived from these forests. In the context of global carbon accounting, it appears that future carbon credits may be best applied to aboveground carbon storage in secondary forests regrowing on soils with large mineral-stabilized soil carbon pools.