A large portion of terrestrial carbon (C) is stored as soil organic matter (SOM) so the balance between C inputs to SOM and soil C loss through respiration has important implications for global climate change. Tree-mycorrhizal association is a promising predictor of SOM dynamics, with the effect of ectomycorrhizal (ECM) trees and associated fungi scaling with the percent basal area of ECM-associated trees in a forest stand. However, uncertainty remains in the generality of ECM effects across the different spatial scales at which soil properties are known to vary. To determine the spatial variability of ECM effects on SOM pools within a forest stand, we used likelihood modelling techniques to compare non-spatial models, which average out ECM effects at the stand scale, against spatial models, which account for the size and spatial distribution of ECM-associated trees within a stand (i.e., the tree neighborhood). To investigate how watershed-scale variability in soil fertility and pH mediate ECM effects, we quantified forest floor leaf litter and soil properties in forest stands along mycorrhizal gradients within four adjacent watersheds that varied in parent material and rainfall. We found that ECM effects on SOM pools scaled with stand-level ECM dominance rather than varying based on individual trees at the neighborhood scale. Although forest floor leaf litter and mineral C:N ratios consistently increased with ECM dominance across watersheds, ECM effects on O horizon formation and the composition of SOM at depth differed among watersheds. Specifically, watersheds with lower soil pH and fertility exhibited greater changes along mycorrhizal gradients in O horizon depth, leaf litter δ¹⁵N and in the difference between δ¹⁵N of leaf litter and mineral soil fractions. Overall, this study shows the potential for intrinsic soil properties to mediate the effects of ECM trees and associated fungi on SOM formation and persistence in the tropics.

大部分陆地碳 (C) 以土壤有机质 (SOM) 的形式储存，因此 SOM 的 C 输入与通过呼吸作用造成的土壤 C 损失之间的平衡对全球气候变化具有重要意义。树-菌根关联是 SOM 动态的一个有前景的预测因子，外生菌根 (ECM) 树木和相关真菌的影响与林分中 ECM 相关树木的基础面积百分比成比例。然而，在已知土壤特性变化的不同空间尺度上，ECM 效应的普遍性仍然存在不确定性。为了确定 ECM 对林分内 SOM 池的空间变异性，我们使用似然建模技术来比较非空间模型，该模型在林分尺度上平均 ECM 效应与空间模型，这说明了一个林分内与 ECM 相关的树木的大小和空间分布（即树木邻域）。为了研究土壤肥力和 pH 值的流域尺度变异如何介导 ECM 效应，我们量化了林分中林分落叶和土壤特性，沿着四个相邻流域内的菌根梯度，这些流域的母体材料和降雨量各不相同。我们发现 ECM 对 SOM 池的影响与站级 ECM 优势成比例，而不是根据邻域范围内的个别树木而变化。尽管森林地面落叶和矿物质 C:N 比率随着 ECM 在整个流域的优势而持续增加，但 ECM 对 O 层形成和深度 SOM 组成的影响在流域之间有所不同。具体来说，¹⁵ N 以及落叶和矿质土壤组分的δ ¹⁵ N之间的差异。总体而言，这项研究显示了土壤内在特性在调节 ECM 树和相关真菌对热带 SOM 形成和持久性影响方面的潜力。