Carbon sequestration potential of tropical pasture compared with afforestation in Panama

Abstract

Tropical forest ecosystems play an important role in regulating the global climate, yet deforestation and land-use change mean that the tropical carbon sink is increasingly influenced by agroecosystems and pastures. Despite this, it is not yet fully understood how carbon cycling in the tropics responds to land-use change, particularly for pasture and afforestation. Thus, the objectives of our study were: (1) to elucidate the environmental controls and the impact of management on gross primary production (GPP), total ecosystem respiration (TER) and net ecosystem CO(2) exchange (NEE); (2) to estimate the carbon sequestration potential of tropical pasture compared with afforestation; and (3) to compare eddy covariance-derived carbon budgets with biomass and soil inventory data. We performed comparative measurements of NEE in a tropical C(4) pasture and an adjacent afforestation with native tree species in Sardinilla (Panama) from 2007 to 2009. Pronounced seasonal variation in GPP, TER and NEE were closely related to radiation, soil moisture, and C(3) vs. C(4) plant physiology. The shallow rooting depth of grasses compared with trees resulted in a higher sensitivity of the pasture ecosystem to water limitation and seasonal drought. During 2008, substantial amounts of carbon were sequestered by the afforestation (-442 gCm(-2), negative values denote ecosystem carbon uptake), which was in agreement with biometric observations (-450 gCm(-2)). In contrast, the pasture ecosystem was a strong carbon source in 2008 and 2009 (261 gCm(-2)), associated with seasonal drought and overgrazing. In addition, soil carbon isotope data indicated rapid carbon turnover after conversion from C(4) pasture to C(3) afforestation. Our results clearly show the potential for considerable carbon sequestration of tropical afforestation and highlight the risk of carbon losses from pasture ecosystems in a seasonal tropical climate.