The extent to which the increasingly frequent episodes of drought-induced tree decline and mortality could alter key soil biogeochemical cycles is unclear. Understanding this connection between tree decline and mortality and soils is important because forested ecosystems serve as important long-term sinks for carbon (C) and essential nutrients (e.g., nitrogen and phosphorus). In order to fill in this knowledge gap, we conducted a study on 13 sites distributed across the Spanish Iberian Peninsula where the dominant tree species was the Mediterranean evergreen Holm oak (Quercus ilex L. subsp. ballota [Desf.] Samp), a species that has shown important drought-induced crown defoliation and mortality rates in recent decades. Our study covered different climatic, soil, land-use type (forests, dehesas, and open woodlands), and crown defoliation (healthy, affected, and dead Holm oaks) gradients that characterize this species distribution within the Spanish Iberian Peninsula. Specifically, the soil C and nutrient content (nitrogen, N; phosphorus, P; magnesium, Mg), several functional parameters (heterotrophic respiration (R_H); N mineralization (i.e., N ammonification, R_amm; and N nitrification, R_nit)), and relative abundances of key microbial soil functional groups (nitrifiers and ectomycorrhizal fungi (ECM)) were studied. Our results showed that aside from the potential effects associated with the climatic gradient, Holm oak decline and mortality resulted in soil stoichiometric imbalances triggered by net losses of essential oligonutrients (e.g., Mg) and the accumulation of very mobile forms of nitrogen (NO₃⁻ - N) and available phosphorus (Av P). Changes in the abundance of key microbial soil functional groups (nitrifiers and ECM) co-occurred with observed nitrate and available P accumulation. Therefore, we conclude that the potential vulnerability of soil C and nutrient stocks to ongoing changes in climate may strongly depend on tree vulnerability to climate change, its effect on soil-plant relationships, and how this may impact the ecology and functioning of key soil functional groups and key metabolic pathways.

目前尚不清楚干旱引起的树木衰落和死亡日益频繁的程度是否会改变关键的土壤生物地球化学循环。了解树木砍伐与死亡率和土壤之间的这种联系非常重要，因为森林生态系统是碳（C）和重要营养素（例如氮和磷）的重要长期汇。为了填补这一知识空白，我们对分布在西班牙伊比利亚半岛上的13个站点进行了研究，其中主要的树种是地中海常绿Holm橡树（Quercus ilex L. subsp。ballota）[Desf。Samp]，该物种在最近几十年中显示出重要的干旱导致冠叶脱落和死亡率。我们的研究涵盖了不同的气候，土壤，土地利用类型（森林，地势和开阔的林地）和树冠的落叶（健康的，受影响的和死掉的霍尔姆橡树）梯度，这些梯度表征了西班牙伊比利亚半岛内该物种的分布。具体而言，土壤碳和养分含量（氮，氮，磷，磷，镁，镁），几个功能参数（异养呼吸（R _H）；氮矿化（即，N氨化，R _amm；以及氮硝化，R _nit））），并研究了关键微生物土壤功能组（硝化剂和外生菌根真菌（ECM））的相对丰度。我们的研究结果表明，除了与气候梯度相关的潜在影响，Holm的橡木下降和死亡导致由必需oligonutrients（例如，Mg）和氮的非常移动形式积累的净损失触发土壤的化学计量不平衡（NO ₃ ^--N）和有效磷（Av P）。关键的微生物土壤功能组（硝化剂和ECM）的丰度变化与观察到的硝酸盐和有效磷积累共同发生。因此，我们得出结论，土壤C和养分储量对气候持续变化的潜在脆弱性可能在很大程度上取决于树木对气候变化的脆弱性，其对土壤与植物关系的影响以及这如何影响关键土壤功能的生态和功能人群和关键的代谢途径。