Unraveling long-term soil organic matter dynamics under drought in forest soils and their link to ecosystem properties

Project Information

DRYSOM

Grant agreement ID: 846134

DOI

10.3030/846134

Project closed

EC signature date 29 March 2019

Start date 1 September 2019

End date 24 March 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 237 007,68

EU contribution

€ 237 007,68

Coordinated by

EIDGENOSSISCHE FORSCHUNGSANSTALT WSL
Switzerland

Periodic Reporting for period 1 - DRYSOM (Unraveling long-term soil organic matter dynamics under drought in forest soils and their link to ecosystem properties)

Reporting period: 2019-09-01 to 2021-12-31

Summary of the context and overall objectives of the project

Severe drought periods are predicted to further increase in Europe. The increasing frequency of drought will affect not only the tree growth but also the capacity of forests to store the CO2 from the atmosphere. While drought effects have intensively been studied for plants, much less is known about their impact on soils. Soils represent a large reservoir of CO2 – larger than the atmosphere and vegetation together – and thus have a crucial role in the CO2 removal and in the climate change mitigation.
The proposed project aimed to estimate how reoccurring summer droughts affect changes in the quantity of carbon stored in the soils of forests, where the CO2 can be stored for long periods of time as soil organic matter. Research activities were carried out in a unique long-term irrigation experiment in a dry pine forest (Valais, Switzerland) ongoing since 2003:
https://www.wsl.ch/en/about-wsl/instrumented-field-sites-and-laboratories/experimental-sites-in-forests/pfynwald.html#tabelement1-tab2.
Specifically, the project objectives were to: (1) quantify the amount of new carbon transferred from the roots and from the fungi associated with the plant roots (i.e. mycorrhizal fungi) into the soils and the losses of ‘old’ organic matter from the soils, by burying small soil bags with different mesh sizes, and relating these changes to the communities of bacteria and fungi living in the soils, (2) determine drought impacts on the long-term storage of organic matter in the soil, how this is affected by soil fauna, and what are the effects on the main sources of organic matter (e.g. foliage, roots and fungi), (3) link the knowledge of soil processes to ecosystem properties measured by collaborating research groups at the Swiss Federal Institute for Forest, Snow and Landscape Research WSL. The use of a long-term experiment, together with novel approaches combining isotopic and DNA analysis, offered a unique opportunity to identify the effects of repeated droughts on the amount and the stability of organic matter in the soils, where CO2 can be stored at long time scales. The obtained results are highly relevant for facing current climate change impacts on forests, representing essential ecosystems both for the environment and for the society.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The researchers showed that if drought events become more frequent due to climate change, litter-degrading organisms living in forest soil, such as earthworms and smaller organisms such as springtails and woodlice, will suffer, thus impacting carbon sequestration in soils. Thus, forests may absorb less carbon in the long term as organic matter in the soil. The study demonstrates the vital role of soil fauna in the carbon cycle of forests. Such organisms react most sensitively to drought, even more than fungi or microorganisms.
These results (objective 2 and 3 of DRYSOM) are published in the scientific journal Global Change Biology: https://onlinelibrary.wiley.com/doi/10.1111/gcb.16122.
Results of the research project are also published in the scientific journal Ecological Monographs (objective 3): https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecm.1507. This research work (Bose et al., 2022) provides a quantitative understanding on how the different functions of individual trees and the whole forest ecosystem responded to increased water availability, and how these responses developed over time.
Further research articles covering results obtained in the project (objective 1 and 2) are currently in preparation or in review in scientific journals.
Outcomes of the project DRYSOM were communicated to the scientific community at leading international conferences (EGU General Assembly 2023, 2022, 2021, WORLD BIODIVERSITY FORUM 2022, Forest Ecosystem Monitoring Conference FORECOMON 2021, EUROSOIL 2021).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The project demonstrated that the soil fauna is highly sensitive to drought events occurring in forests. If forest soils become too dry, this inhibits the activity and the quantity of soil organisms and forests may absorb less carbon in the long term. This finding is significant in a climate change perspective, because through dead-litter feeding, soil organisms ultimately bind the carbon in the CO2 from the atmosphere into the soil humus, where it is stored over long periods of time.
The researchers additionally developed a novel approach to trace the origin of organic matter in soils. By analysis of hydrogen composition in the organic matter, researchers were able to disentangle the contribution from foliage, roots, and fungi to the build-up of organic matter in soils. Thus, this method has the potential to provide a faster and cost-effective way to analyze the sources of organic matter in soils, which still represent one of the key unknowns for the understanding of soil organic matter responses to climate change.
The results obtained in this project will improve the prediction of soil organic matter and soil carbon changes in response to increasing frequency of drought events, and thus will be highly relevant not only for scientists but also stakeholders and policy-makers to face pressing climate change challenges.

Earthworms in the Pfynwald forest, Valais, Switzerland (Photo: Claudia Guidi, WSL)