Human Influence on the Carbon Cycle in Secondary Tropical Forests

Recently it has been proposed that reforestation of tropical pasture lands may be an effective way to sequester CO2, but the effects of re-forestation on the soil C pool are not well known. In an attempt to gain a better understanding of the soil C dynamics during the process of tropical forest re-growth, soil samples were collected from active pasture and 10, 30, and 80 year-old secondary forest, in the Cayey region of Puerto Rico. We are conducting a long term incubation experiment to determine the effects of future climate change and atmospheric N deposition on soil CO2 effluxes and the relative importance of pasture or forest source C in soil C losses. Pasture soils exhibited the highest rates of soil respiration, followed by the secondary forests, which tracked closely under all treatments except N addition. There were few effects of added N on CO2 effluxes; the 80 year-old forest showed the lowest rates of soil CO2 efflux under N additions. In contrast, CO2 fluxes were consistently higher in soils at elevated temperature relative to the controls. In the re-growing forests, C-13 isotope data reveal that warming also increased the proportion of C3 derived C as opposed to C4 (i.e., old pasture) derived-C in respiration. After two months of incubations, no effect of warming on 13-CO2 was seen in pasture sites. Our results indicate that soil C pools in re-growing tropical forests are highly sensitive to temperature, but soil C losses do not appear to be responsive to added N.