Mélody Rousseau

Deadwood plays a vital role in forest ecosystems, influencing soil biodiversity through nutrient enrichment and niche partitioning. While the effects of specific attributes of deadwood logs on soil biodiversity are well studied, it remains unclear whether and how the volume of deadwood affects soil biodiversity at the scale of forest stands. Additionally, the effects on soil biodiversity may differ between gradually accumulated deadwood and large volumes resulting from sudden stand-level disturbance events. In this study, we aim to assess such effects on alpha and beta diversity of soil microbes and microfauna. Soil samples were gathered from forest plots following a gradient of deadwood volumes in European temperate forests in Germany and The Netherlands. Using extracellular DNA metabarcoding, we analysed the soil diversity of bacteria, fungi and microarthropods. For the Bavarian Forest National Park, we also compared the diversity patterns of these biotas between areas affected by bark beetle outbreaks and unaffected areas of Norway spruce forest. Increased deadwood stock had a marginal effect on soil microbial and microarthropod community composition, with no impact on overall diversity. Bark beetle-affected areas had distinct soil communities, with a lower fungal and microarthropod diversity. Our study provides the first insights into soil diversity patterns associated with increased deadwood volume at the forest stand. While shifts in soil biodiversity composition were minimal, the retention of deadwood in European temperate forests can promote heterogeneity in soil communities. Furthermore, changes in soil biodiversity following bark beetle outbreaks may have long-term consequences on forest regeneration.

Despite a decrease in industrial nitrogen and sulfur deposition over recent decades, soil acidification remains a persistent challenge to European forest health, especially in regions of intense agriculture and urbanisation. Using topsoil eDNA metabarcoding and functional annotations from a sample of 49 plots (each 30 × 30 m) located in The Netherlands and Germany, we investigated the effect of severe acidification on bacterial taxonomic diversity under different forest types and explored potential functional implications for nutrient cycling. Furthermore, we assessed which soil parameters known to influence soil bacterial communities affect these acidophilic communities. Here, we are the first to demonstrate under natural conditions that soil bacterial diversity in extremely acidic soils (pH <4.5) continues to decline similarly across forest types as pH further decreases under intensifying human activity. Our results confirmed pH as the key driver of soil bacterial communities, even in extremely acidic soils. Ongoing severe acidification continues to reduce bacterial communities, favouring taxa adapted to extreme acidity and primarily involved in recalcitrant carbon-degradation compounds (e.g. cellulolysis potential = 0.78%–9.99%) while simultaneously diminishing taxa associated with nitrogen cycling (e.g. fixation potential = 6.72%–0.00%). Altogether, our findings indicate a further decline in bacterial diversity in already extremely acidic soils, likely disrupting nutrient cycling through changes in immobilisation and mineralisation processes. Our study highlights the continuous acidification of European temperate forests to extremely low pH levels, further disrupting forest ecosystem functioning. The significant reduction in bacterial diversity under such a severe acidification gradient, as demonstrated here, underscores the necessity to include severely acidified forests in conservation programmes and monitoring to prevent further degradation of European soils beyond repair.