Classic models describe what happens when the temperature increases as follows: Due to the effect of warming, the soil inhabitants eat up fresh biomass faster, but their appetite for the old material remains the same. As a result, soil respiration would increase, but the soil would ultimately remain a carbon sink. “This is precisely what we question. Soil microbes very much lead an independent existence. How they behave may not be quite as easy to predict as was thought,” says Markus Reichstein.
The researcher and his colleagues assume that the priming effect kicks in: “Whenever fresh new biomass is available to the microbes, their appetite for the old humus material increases.” The decomposition process accelerates and the soil emits more carbon dioxide. In addition, the microbes thrive and proliferate. “The warmer the soil becomes, the further the system is intensified,” explains Reichstein. Fresh biomass acts almost as an aperitif that stimulates the microorganisms’ appetite for older food.
However, some soil processes counteract the decomposition: chemical interactions between minerals hold the carbon in the soil at a constant level. Iron and aluminum hydroxides, for example, often accumulate carbon on their surfaces, from which it initially does not detach.
The researchers devised a number of possible mathematical formulas that can be programmed in a computer to calculate as simply as possible and with sufficient accuracy how these processes interact. They want to use laboratory and field experiments to determine which of these formulas is most suitable, so they must now shift their focus below ground.
Reichstein and his colleagues succeeded in attracting millions in funding for their project from the European Research Council (ERC). As part of the QUASOM project (quantifying and modeling pathways of soil organic matter as affected by abiotic factors, microbial dynamics and transport processes), they aim to merge data from new field experiments with data from other European research projects in a soil simulation model.
Marion Schrumpf from the biogeochemical processes department brought valuable experience with field data to the team, having already collected field data as part of the Carbo-Europe project (Assessment of the European Terrestrial Carbon Balance). This project explored the question of how the activity in the soil changes over time throughout the continent, and how forests and arable areas can be managed so that their soils bind as much carbon dioxide as possible. Sixty-one research institutes from 17 European countries participated in this project, which was headed by the now retired Founding Director of the Max Planck Institute in Jena, Ernst-Detlef Schulze.
The researchers involved frequently broke new methodological ground. “A comprehensive soil inventory had simply never been carried out before,” reports Marion Schrumpf. The data available on forest soils, for example, is extremely sparse. “The forestry sector was interested in its tree population, but not in the soil on which they grow,” says the researcher. The little data available stems from agriculture. Farmers and agricultural scientists have been observing how arable soils react to different management practices for more than 100 years. These long time series of soil analyses can now be used to learn something about the effect of climate change on soil carbon. However, the agricultural data is often far from complete. “Besides, this work was carried out from an entirely different perspective,” says Schrumpf. Agriculture is interested primarily in information about soil fertility. The scientists working on Carbo-Europe and QUASOM, on the other hand, look at the soil from the perspective of climate research: high carbon content in the soil not only increases fertility and yields, but also means that carbon is being removed from the air.
As part of Carbo-Europe, Marion Schrumpf worked on core samples from 12 locations. The samples were taken in places that also had towers for atmospheric measurements. “As a result, we had a good supply not only of soil data, but of other environmental parameters as well,” says the researcher. She examined a total of more than 9,000 samples – an enormous undertaking.