How forest trees 'feed' soil organisms

In a large-scale experiment reminiscent of wrapping artist Christo, researchers from the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) administered labelled carbon dioxide (CO2) to mature pine trees. This enabled them to show for the first time how quickly the sugars produced during photosynthesis reach the organisms living in the soil and how drought hinders these processes.

Forest trees release some of the sugars they produce in their leaves through photosynthesis into the soil. They feed the organisms that live in the soil, and in turn benefit from them as, among other things, the vast network of root fungi (mycorrhiza) helps them to absorb more water and nutrients than they could using their roots alone. However, little is yet known about how changing environmental conditions influence and disrupt this balanced interplay.

To remedy this, a team led by soil researcher Frank Hagedorn took part in a large-scale WSL experiment in 2017 designed to trace the paths taken by absorbed carbon in the tree and its surroundings. Carbon is not only the most important building block for all living matter but also the 'C' in CO2, the gas that is both the staple food of trees and a major cause of climate change. Forests are therefore key sinks of climate-affecting CO2.

For the experiment – a world first – in the Pfynwald forest (canton of Valais), the researchers wrapped ten 100-year-old pine trees, around 12 metres high, in huge plastic bags and then 'fed' them with a variant of CO2, known as an isotope, which is heavier than the CO2 found in the air. This is completely harmless, but means that the origin of the carbon can be traced even years later. This method has only been usable for the past decade or so.

Just four days to reach root zone

After only four days, Hagedorn and his fellow researchers were able to detect the first 'heavy' sugars in the soil, as they report in the journal Global Change Biology. The area of soil supplied with tree sugars was three times larger than the area covered by the tree's crown, which serves to indicate the size of the underground network. The heavy carbon variants could still be found in the soil for a whole year afterwards.

"We were thus able to prove for the first time that the trees temporarily store the sugars that are formed by the leaves during photosynthesis and only use them over time to build wood cells or release them into the soil through the roots," says Hagedorn. "That is important to know because much of the soil life is fed by the trees with these sugars." In total, around a third of the CO2 absorbed by the tree is displaced into the soil and consumed by the organisms there, with part of it being released again as CO2.

Soil organisms in 'drought-induced torpor'

Drought significantly reduced the amount and distribution of sugars released by trees into the soil. Half of the pines gassed with CO2 have been irrigated since 2003, while the other half are exposed to the naturally dry conditions found in Valais. Thanks to the experiment, the researchers also established that pines under drought conditions built up a 50% smaller underground network than those with an optimal water supply. This means that drought-stressed pines can absorb less water and nutrients from the soil even in good times, which further restricts their growth.

In addition, it was shown that the subterranean community is inactive during severe drought, entering a kind of drought-induced sleep. As a result, the trees have less water and nutrients at their disposal, which inhibits their growth. "Conditions below ground control what happens above ground," is how Hagedorn summarises it. This could mean that the forest ecosystem is able to store less CO2 as drought conditions increase – something that should be factored in when forecasting the effects of climate change, he concludes.