Julia Schröder

On 26 October 2022, Julia Schröder defended and completed her doctoral thesis on "Microbial carbon cycling as affected by land use in subarctic soils under global warming" at the Institute of Geoecology at TU Braunschweig with distinction. She wrote the dissertation as part of the project "Breaking the Ice - Consequences of northwards extending agriculture for soil organic matter cycling under a changing climate" (funded by the German Research Foundation). The work was supervised by PD Dr. Christopher Poeplau and Prof. Dr. Christoph C. Tebbe.

Subarctic soils store large amounts of terrestrial carbon that is at risk of being lost through microbial decomposition under expanding agricultural activity and global warming. The aim of the work was to investigate how the conversion of boreal forests to cropland and grassland and warming affect the microbial carbon use efficiency (CUE) of soil microorganisms in subarctic soils. The CUE, a measure of metabolic efficiency, determines how much of the carbon taken up by soil microorganisms during the decomposition of soil organic matter is respired to CO₂.

The work was preceded by a methodological study in which it was investigated how the treatment of the soil samples prior to the actual analysis influences the results of the so-called ¹⁸O method for the determination of CUE. Although individual metabolic parameters were influenced by the type of pre-treatment, CUE was not. This led to the conclusion that CUE can be determined by the ¹⁸O method on frozen, dried as well as fresh soil samples after pre-incubation. In an additional chapter of her thesis, Julia Schröder presented further methodological problems and thus contributed to the development of the method.

In the main part of the thesis, Julia Schröder showed that CUE increased significantly after conversion to agricultural land. The changes in CUE were mainly caused by an increase in soil pH and the narrowing of the C:N ratio in the soil. Changes in the composition of the microbial community were not causal for the increase in CUE.

Beyond land use change, Julia Schröder also investigated how other global change factors will affect CUE in subarctic soils. In an additional experiment, she investigated how CUE behaves with 10°C warming or N fertiliser application and whether this response depends on the type of land use. In the short-term, CUE increased with warming regardless of land use type. As microbial growth was inhibited, CUE decreased after N addition.

Under changing environmental conditions, CUE is expected to increase in agricultural soils, which could partly mitigate CO₂ losses from increased soil organic matter decomposition.