Increased leaf-litter input may boost carbon release from tropical forest soils, partially offsetting predicted gains in forest carbon storage, according to research published online this week in Nature Climate Change.

Enhanced tropical forest productivity stimulated by increasing carbon dioxide concentrations could act as a substantial carbon sink, potentially slowing down the rate of global warming. However, feedbacks between increased productivity and soil carbon dynamics remain unexplored, limiting our ability to predict future changes in carbon storage.

Emma Sayer and colleagues added leaf litter to forest soils in Panama and monitored them for six years to investigate the effects of enhanced litterfall on carbon storage in the soil. Using isotope measurements to distinguish between carbon sources, they found that litter addition significantly increases the release of carbon dioxide from soil organic carbon through a process known as ‘priming’, where soil microbes are stimulated by the addition of easily decomposable organic matter.

The researchers estimate that a 30% increase in litterfall could release about 0.6 tonnes of carbon per hectare from lowland tropical forest soils each year. This amount of carbon is greater than estimates of the climate-induced increase in forest biomass in Amazonia over recent decades.