Afforestation and subsequent expansion of trees on former grasslands may significantly impact the structure and activity of the soil microbial community, altering soil aggregation and affect its potential to store and cycle organic matter (OM). We investigated OM dynamics in aggregate-size topsoil samples collected along a Mediterranean alpine ecotone consisting of three vegetation types (grassland/shrubland, mixed shrubland-pine, and pine forest) in central Spain. Analytical determinations of soil organic carbon (SOC), total nitrogen (TN), particulate OM (POM), mineral-associated OM (MaOM), and the stable isotopic composition of carbon were conducted in each of the four aggregate-size fractions considered. Additionally, the structure of the microbial community (assessed as PLFA abundance), and the β-glucosidase and β-glucosaminidase activities were determined in bulk soil samples. More than half of the soil mass was contained within small macroaggregates regardless of vegetation type. SOC and TN values increased with decreasing aggregate-size classes across all vegetation types. The stability of microaggregates was negatively affected by the expansion of woody vegetation, which resulted in tree-dominated stands showing comparatively lower SOC and TN values in the smaller aggregate-size classes. On the other hand, these vegetation dynamics promoted soil macro-aggregation. While SOC contents did not show significant differences between land covers, vegetation shifts induced changes in the soil microbial community. Soil δ¹³C values, the abundance of gram-positive bacteria and β-glucosidase activity were significantly higher in grasslands/shrublands than in forests, while significantly higher fungi/bacteria ratio was observed in forests. Small macroaggregates appear to play a key role in the stabilisation of relatively unprocessed OM across all vegetation types, as suggested by their significantly higher concentrations of POM. However, this fraction represents the most labile pool of OM, and as such, it is the most exposed to mineralisation. We conclude that the afforestation and potential vegetation shifts experienced in Mediterranean alpine grasslands lead to distinct changes in soil microbial communities, aggregation and soil OM dynamics, which given the strong temperature sensitivity to decomposition commonly reported in cold environments, suggests that soil OM in these high-elevation ecosystems may become highly vulnerable to environmental change.

在原草地上植树造林和随后的树木扩张可能会显着影响土壤微生物群落的结构和活动，改变土壤聚集并影响其储存和循环有机物质 (OM) 的潜力。我们调查了沿地中海高山交错带收集的聚合大小表土样品中的 OM 动态，该交叉带由西班牙中部的三种植被类型（草地/灌木地、混合灌木地松和松林）组成。土壤有机碳 (SOC)、总氮 (TN)、颗粒 OM (POM)、矿物相关 OM (MaOM) 和碳的稳定同位素组成的分析测定在所考虑的四种聚集体大小部分中进行。此外，微生物群落的结构（评估为 PLFA 丰度），β-葡萄糖苷酶和β-氨基葡萄糖苷酶活性在大块土壤样品中测定。无论植被类型如何，超过一半的土壤质量都包含在小的大团聚体中。SOC 和 TN 值随着所有植被类型的聚集体尺寸类别的减少而增加。微团聚体的稳定性受到木本植被扩张的负面影响，这导致以树木为主的林分在较小的团聚体级别中显示出相对较低的 SOC 和 TN 值。另一方面，这些植被动态促进了土壤宏观聚集。虽然 SOC 含量在土地覆盖之间没有显示出显着差异，但植被变化引起了土壤微生物群落的变化。土壤δ 无论植被类型如何，超过一半的土壤质量都包含在小的大团聚体中。SOC 和 TN 值随着所有植被类型的聚集体尺寸类别的减少而增加。微团聚体的稳定性受到木本植被扩张的负面影响，这导致以树木为主的林分在较小的团聚体级别中显示出相对较低的 SOC 和 TN 值。另一方面，这些植被动态促进了土壤宏观聚集。虽然 SOC 含量在土地覆盖之间没有显示出显着差异，但植被变化引起了土壤微生物群落的变化。土壤δ 无论植被类型如何，超过一半的土壤质量都包含在小的大团聚体中。SOC 和 TN 值随着所有植被类型的聚集体尺寸类别的减少而增加。微团聚体的稳定性受到木本植被扩张的负面影响，这导致以树木为主的林分在较小的团聚体级别中显示出相对较低的 SOC 和 TN 值。另一方面，这些植被动态促进了土壤宏观聚集。虽然 SOC 含量在土地覆盖之间没有表现出显着差异，但植被变化引起了土壤微生物群落的变化。土壤δ 微团聚体的稳定性受到木本植被扩张的负面影响，这导致以树木为主的林分在较小的团聚体级别中显示出相对较低的 SOC 和 TN 值。另一方面，这些植被动态促进了土壤宏观聚集。虽然 SOC 含量在土地覆盖之间没有显示出显着差异，但植被变化引起了土壤微生物群落的变化。土壤δ 微团聚体的稳定性受到木本植被扩张的负面影响，这导致以树木为主的林分在较小的团聚体级别中显示出相对较低的 SOC 和 TN 值。另一方面，这些植被动态促进了土壤宏观聚集。虽然 SOC 含量在土地覆盖之间没有显示出显着差异，但植被变化引起了土壤微生物群落的变化。土壤δ¹³C 值、革兰氏阳性菌的丰度和β-葡萄糖苷酶活性在草地/灌木林中显着高于在森林中，而在森林中观察到的真菌/细菌比率显着更高。正如其显着较高的 POM 浓度所表明的那样，小的大团聚体似乎在稳定所有植被类型中相对未加工的 OM 方面发挥着关键作用。然而，这一部分代表了最不稳定的 OM 库，因此，它最容易受到矿化的影响。我们得出结论，地中海高山草原的植树造林和潜在的植被变化导致土壤微生物群落、聚集和土壤 OM 动态的明显变化，这给出了寒冷环境中常见的对分解的强烈温度敏感性，