Tropical soils linked to carbon emissions

Scientists continue to unravel the feedback loops caused by global warming, a major one being the soil’s release of carbon dioxide (CO₂).

Now, a study published in the journal Nature reports that soils in tropical forests are far more sensitive to global warming than previously thought. 

Researchers warmed tropical soil by four degrees Celsius – an increase projected for tropical latitudes by the turn of the century – and found its CO₂ emissions increased by 55% compared to soils at normal temperatures.

The extra emissions, they say, equate to more than eight tonnes of carbon per hectare per year released to the atmosphere, suggesting tropical soils create “a potentially substantial positive feedback to climate change”.

“Soils form a living skin around the Earth,” says first author Andrew Nottingham from the University of Edinburgh, UK. “They contain vast amounts of organic matter – more carbon is held in soils than is held in plants and the atmosphere put together.”

Soil “breathes”, releasing CO₂, as microbes use up carbohydrates and break down organic matter, a process that is accelerating with warmer temperatures along with microbial biomass. Theory says this would be more pronounced at higher latitudes, but the new study suggests otherwise.

It stems from the Soil Warming Experiment in a Lowland Tropical Forest (SWELTR), conducted by Nottingham and collaborators from Panama and Australia.

They set up five pairs of experimental and control plots in tropical forest soil on Barro Colorado Island in Panama, warming the test areas – more than 120 square metres – using underground electric cables over two years. 

The “surprisingly large loss of soil carbon” was 34% to 37% higher than that found in a temperate forest using a similar study design, and greater than a meta-analysis of studies at higher latitudes.

Unlike higher latitudes, the study didn’t find any compensation in enzyme or microbial activities, which shows, the team writes, “that soil carbon in tropical forests is highly sensitive to warming”.

They note that while their results contradict current theoretical models, they align with recent measurements that show highly sensitive ecosystem-wide carbon cycling in tropical regions with seasonal temperature fluxes. 

“This sensitivity adds to our growing understanding of how important tropical forests are to our global environment,” says study co-author Patrick Meir from Australian National University, adding that current climate predictions will need to be revised.

It delivers a double whammy, as trees in tropical forests will also release carbon more rapidly once temperatures pass a critical threshold – and chopping down trees will only make it worse, says Nottingham.

“Deforestation has a huge impact on carbon storage, [causing] a 43% decline in topsoil carbon after conversion to agriculture” he says. “Furthermore, deforestation exposes soil to incident radiation, amplifying any effect of climate warming.”

“Tropical forests also harbour the vast proportion of the world’s biodiversity, a vital resource for the planet and for humanity … they are essential for our survival.”