Chronosequence of two forest disturbances recorded in the soils of the Tatra Mountains (Carpathians)

Abstract. Over recent decades, the frequency of forest disturbances such as windthrow events and bark beetle outbreaks has increased in mountainous regions of Europe. These disturbances often co-occur; however, they alter soil conditions in different ways and consequently affect soil microorganisms through distinct mechanisms. Understanding their effects on soil microorganisms is crucial, as soil biological processes play a fundamental role in carbon sequestration and nutrient cycling. This study investigates the response of soil microbial communities to forest disturbances under active forest management, where tree trunks were removed, in the Tatra Mountains. We assessed soils affected by windthrow and bark beetle outbreaks across a chronosequence spanning 9 to 57 months after disturbance.

The analyses included soil chemical properties, soil respiration, SIR induced microbial biomass, the activity of hydrolytic enzymes (β-D-cellobiosidase, β-glucosidase, β-xylosidase, N-acetyl β-glucosaminidase, phosphatase, arylsulfatase), and the catabolic profiles of bacteria and fungi (Biolog® ECO and FF). Microbial responses varied depending on disturbance type and time since disturbance. Indicators of microbial activity did not change linearly over time, suggesting a rapid microbial response during the initial post disturbance period. However, soils affected by bark beetle outbreaks exhibited less abrupt changes than windthrow affected soils, likely due to the continuous input of organic matter, the absence of an initial lag phase in decomposition, and more gradual shifts in environmental conditions.

Substrate utilisation patterns (catabolic profiles) differed over time between windthrow and bark beetle affected soils, indicating distinct trajectories of microbial succession. Microbial responses also varied between soil horizons. The litter horizon was more sensitive to changes in enzymatic activity and substrate utilisation than the humus horizon, likely because it depends strongly on inputs of fresh organic matter. In contrast, variability in the humus horizon was more closely associated with soil chemical properties, particularly nitrogen availability and soil pH.