Complex Carbon Picture Clearer

A new study looks at a poorly understood process with potentially critical consequences for climate change. Emma Sayer, postdoctoral fellow at the Smithsonian Tropical Research Institute, Jennifer Powers, an assistant professor in the University of Minnesota’s Department of Ecology, Evolution and Behavior, and Edmund Tanner, researcher at Cambridge University, published the findings of their long-term study on the effects of increased plant litter on soil carbon and nutrient cycling in the December 12 edition of PLoS ONE.

As CO2 concentrations in the atmosphere continue to rise, increases in plant productivity – and litterfall – are likely. The study considers the impact of an increase in organic matter on the ground on processes belowground. Results suggest that the balance of carbon stored in the soils (thought to be a long-term sink for carbon) can be changed with the addition of fresh leaf litter. The capacity of soils to store carbon might then diminish if global environmental changes such as CO2 increases and nitrogen deposition boost plant productivity.

Over the course of the 5-year experiment, the fluxes of carbon dioxide from the soil surface to the atmosphere in a tropical forest in Panama were measured. These CO2 fluxes (also called soil respiration) come from two main sources: the respiration of roots and the decomposition of litter and soil organic matter by fungi, bacteria, and other microorganisms.

“To our surprise, the litter addition plots showed substantially higher amounts of soil respiration than would be predicted by the increase in leaf litter,” said Powers. “We suspect that this extra CO2 in the litter addition plots was coming from the decomposition of ‘old soil organic matter’, which was stimulated by adding large quantities of fresh leaf litter.” This effect, the stimulation of the decomposition of old, ‘stored’ organic carbon by the addition of fresh organic matter is known as the ‘priming effect.’ “There are important links between above-and belowground processes and we need to understand these links in order to assess the impact of global change and human disturbance on natural ecosystems” said Sayer.

The study has implications for policy makers considering new approaches to capping carbon emissions such as carbon sequestration. “Our results suggest unanticipated feedbacks to the carbon cycle that must be taken into account when estimating the potential for carbon sequestration in the soil,” Powers said.

Emma Sayer of the Smithsonian Tropical Research Institute and Cambridge University is the lead author of the study. Edmund Tanner, also of Cambridge University, and Jennifer Powers of the University of Minnesota are co-authors.

Citation: Sayer EJ, Powers JS, Tanner EVJ (2007) Increased Litterfall in Tropical Forests Boosts the Transfer of Soil CO2 to the Atmosphere. PLoS ONE 2(12): e1299. doi:10.1371/journal.pone.0001299