CARBO-Extreme teams have conducted field experiments that simulated drought in different climates and vegetation types, from Atlantic pine forests to alpine meadows. Unpublished results show that in grasslands, drought markedly slowed photosynthesis, which stores carbon in leaves, roots and soil. It had a smaller effect on soil respiration, which releases carbon, so the net result was a decline in carbon uptake.
The experiments also showed that plants and soils keep a ‘memory’ of disturbances, says Michael Bahn, an ecologist at the University of Innsbruck in Austria who oversees a grassland experiment. He simulated a series of droughts, and found that later ones had a larger effect on net carbon release. Existing biosphere models do not capture such effects, which Bahn thinks might be due to changes in soil microbes.
Such omissions could lead to a large bias in the models. The world’s soils contain almost 100 gigatons of carbon — twice as much as the entire atmosphere. Just a 10% increase in soil-respiration rates, says Bahn, would release more CO2 in a year than humans pump out.