Rapid alteration of organic matter cycling in a boreal peatland in response to rising temperatures

Global warming and increasing air temperatures also result in rising soil temperatures. Although acceleration of soil organic carbon cycling can be expected, the order of magnitude and speed of adaptation of carbon cycling to warming still remains largely unknown. This is especially crucial in boreal peatlands, where large reserves of terrestrial carbon are stored and these systems are known for their vulnerability to environmental changes.

We investigated the organic matter composition in the SPRUCE (Spruce and Peatland Responses Under Changing Environments) experiment, where a boreal peatland was exposed to temperatures of up to +9°C and increased CO₂ concentration compared to control conditions in open top chambers. A broad set of molecular markers (e.g., free extractable and bound lipids, lignin, benzene polycarboxylic acids) was used to trace incorporation and cycling of organic matter in the peat profile down to three meters depth four years after the start of the experiment.

A strong response to increasing temperature was observed in the plant, microbial and peat chemical composition, the latter mainly in the acrotelm (0-30 cm) and partially also in the mesotelm (30-70cm). The response of the plant chemical composition was species-specific with the exception of nitrogen concentrations that increased for all plants. This is related to the stronger degradation of peat organic matter and thus increasing availability of nitrogen with rising temperature. All investigated molecular markers indicated a very fast response of carbon cycling in the whole acrotelm of the peat profile. This resulted from a dropping water table and thus more oxic conditions in the peat, which further enabled increasing shrub and tree root growth and increasing microbial abundance and activity. As a consequence of the more aerobic conditions, not only the comparatively easily degradable free extractable lipids, but also slow cycling polymeric substances such as suberin/cutin, lignin, and benzene polycarboxylic acids rapidly degraded and reflect an unexpectedly fast cycling of organic matter in the boreal peatland with increasing temperature. The acceleration of carbon cycling within the peatland with rising temperature is also reflected by the partial uptake of respired CO₂ by the plants as indicated by the bulk and compound-specific d¹³C composition of the plants. Overall, our results illustrate the fast alteration of organic matter cycling in a boreal peatland when exposed to increasing temperature.