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.

中文翻译：

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欧洲温带森林严重酸化下土壤细菌多样性的进一步减少


尽管近几十年来工业氮和硫沉积有所减少，但土壤酸化仍然是欧洲森林健康面临的持续挑战，尤其是在集约化农业和城市化地区。使用来自荷兰和德国的 49 个样地（每个 30 × 30 m）样本的表层土壤 eDNA 宏条形码和功能注释，我们研究了严重酸化对不同森林类型下细菌分类多样性的影响，并探讨了对养分循环的潜在功能影响。此外，我们评估了哪些已知影响土壤细菌群落的土壤参数会影响这些嗜酸性群落。在这里，我们首次在自然条件下证明，随着 pH 值在人类活动的加剧下进一步降低，极端酸性土壤 （pH <4.5） 中的土壤细菌多样性在森林类型中继续以类似的方式下降。我们的结果证实 pH 值是土壤细菌群落的关键驱动因素，即使在极酸性土壤中也是如此。持续的严重酸化继续减少细菌群落，有利于适应极端酸度的分类群，主要参与顽固的碳降解化合物（例如纤维素分解电位 = 0.78%–9.99%），同时减少与氮循环相关的分类群（例如固定电位 = 6.72%–0.00%）。总而言之，我们的研究结果表明，在已经极度酸性的土壤中，细菌多样性进一步下降，可能会通过固定和矿化过程的变化来破坏养分循环。我们的研究强调了欧洲温带森林持续酸化至极低的 pH 值，进一步破坏了森林生态系统的功能。 正如这里所展示的，在如此严重的酸化梯度下，细菌多样性的显著减少，强调了将严重酸化的森林纳入保护计划和监测的必要性，以防止欧洲土壤进一步退化而无法修复。