Study reveals impact of rising CO2 on tropical forest soils

As CO2 emissions rise, the soils of tropical forests are likely to release more of their stored carbon into the atmosphere, EU-funded research has revealed, at the heart of this positive
feedback loop is plant litter, the amount of plant litter on the tropical forest floor is predicted to increase in the future as a result of higher CO2 levels, rising temperatures and changing
rainfall patterns.

‘As litterfall represents a major flux of carbon from vegetation to soil, changes in litter inputs are likely to have wide-reaching consequences for soil carbon dynamics,’ the researchers
explain in their article in the journal PLoS One.

‘Such disturbances to the carbon balance may be particularly important in the tropics because tropical forests store almost 30% of the global soil carbon, making them a critical component of
the global carbon cycle,’ they add.

For five years, the scientists monitored the fluxes of carbon dioxide from the soil surface to the atmosphere at a number of plots of land in a lowland tropical forest in Panama. Some of the
plots of land had litter material added to them on a regular basis, while others had litter removed. These CO2 fluxes, which are also known as soil respiration, are produced by the respiration
of roots and the decomposition of litter and soil organic matter by fungi, bacteria and other microorganisms.

‘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,’ commented lead author Emma Sayer of the University of Cambridge in the UK and the Smithsonian Tropical Research Institute in Panama.

The study revealed that increased levels of litter led to disproportionately large increases in the amount of CO2 released into the atmosphere from the soil. The scientists had predicted an
increase in soil respiration of 9% in the plots where extra litter had been added. In fact soil respiration went up by 43%.

‘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 Jennifer Powers of the
University of Minnesota in the US. ‘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.’

The results have implications for policy makers considering carbon sequestration as a partial solution to reducing levels of CO2 in the atmosphere. ‘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,’ said Dr Powers.

EU funding for the study came from a Marie Curie Outgoing International Fellowship to Emma Sayer.