Data from: Tropical tree species traits drive soil cation dynamics via effects on pH: a proposed conceptual framework

Humid tropical forests are major players in the global carbon cycle, despite evidence that cations (rock-derived, positively charged ions) can limit or co-limit net primary productivity (NPP). In mature forests, tight cation cycling, i.e., without leaching losses, could maintain cation stocks on site. That mechanism does not explain how regenerating tropical secondary forest trees start from seeds and accrue large cation stocks in biomass, when growing on soils depleted in available cations. We propose a new mechanism that links tree species’ traits to soil cation availability via impacts on soil pH, a ‘master’ biogeochemical driver. We tested plot-level effects of tree species on soil pH, soil extractable cations, and cation accrual in biomass in a unique, 25-yr-old, randomized-complete-block experiment in which climate, soil, and previous land-use history were similar across four native tree species grown in mono-dominant plantations in Costa Rica. Surface-soil pH in this Oxisol, initially 4.52 (±0.02), declined to 4.14 (±0.02) under <i>Pentaclethra macroloba</i>, a nodulated legume, and increased to 4.71 (±0.08) under <i>Vochysia guatemalensis</i>, an aluminum (Al) accumulator. The range in pH corresponds to a five-fold difference in proton concentrations, which is sufficient to alter dispersion of organo-mineral colloids. Cation stocks in biomass differed across species by 1.7-, 1.9-, 2.8-, 2.9-, 3.1-, 3.5-, and 17.2-fold for iron (Fe), calcium (Ca), potassium (K), manganese (Mn), strontium (Sr), magnesium (Mg), and Al, respectively. Differential acquisition of available soil cations was an unlikely explanation for measured differences among species because changes in extractable soil cation stocks were unrelated to cation accrual in biomass. Soil pH and biomass cation stocks were highly correlated, however. By our proposed conceptual framework, species traits that strongly increase proton concentrations and decrease pH in soil, e.g., support of N fixation, increase colloid aggregation, reducing cation availability. Traits that reduce soil protons and increase pH, e.g., Al⁺³ accumulation, disperse colloids, thereby releasing cations occluded during pedogenesis. This highlights a novel biogeochemical role for the Al-accumulation trait, i.e., liberation of occluded soil cations. Further studies would clarify effects of soil pH on cation supply via colloid dispersion, and its importance for nutrient acquisition in cation-depleted soils.湿润热带森林是全球碳循环的核心参与者，尽管已有研究证据表明阳离子（cations，岩石来源的带正电离子）能够限制或共同限制净初级生产力（NPP，net primary productivity）。在成熟森林中，闭合的阳离子循环——即无淋溶损失——可维持原位阳离子库。但该机制无法解释：当恢复中的热带次生林木由种子萌发开始生长，并在有效阳离子匮乏的土壤中发育时，如何在生物量中积累大量阳离子库。 我们提出了一种全新机制，该机制通过树木物种性状对土壤pH值这一"关键生物地球化学驱动因子"的影响，将其与土壤阳离子有效性关联起来。我们在哥斯达黎加开展了一项独特的25年生随机完全区组试验（randomized-complete-block experiment），试验中四种以单优人工林方式种植的本土树种的对应样地，其气候、土壤条件及既往土地利用历史均保持一致，借此检验了树木物种对土壤pH、土壤可提取阳离子以及生物量中阳离子积累的样地水平效应。 该氧化土（Oxisol）的表层土壤初始pH值为4.52（±0.02），在结瘤豆科植物*Pentaclethra macroloba*的种植下降至4.14（±0.02），而在铝（Al）富集植物*Vochysia guatemalensis*的种植下升至4.71（±0.08）。pH值的变化范围对应质子浓度相差五倍，这足以改变有机矿物胶体的分散状态。 不同物种的生物量阳离子库差异显著，其中铁（Fe）、钙（Ca）、钾（K）、锰（Mn）、锶（Sr）、镁（Mg）及铝（Al）的差异分别达到1.7倍、1.9倍、2.8倍、2.9倍、3.1倍、3.5倍及17.2倍。由于土壤可提取阳离子库的变化与生物量中的阳离子积累并无关联，因此仅通过有效土壤阳离子获取量的差异来解释物种间的实测差异，并不具备合理性。然而，土壤pH值与生物量阳离子库之间存在高度相关性。 基于我们提出的概念框架，能够显著提升土壤质子浓度、降低土壤pH值的物种性状（例如支持固氮作用），可增强胶体聚集，进而降低阳离子有效性；而能够减少土壤质子、提升土壤pH值的性状（例如铝离子（Al³+）富集），则会促进胶体分散，从而释放成土过程中被固定的阳离子。这一发现凸显了铝富集性状的全新生物地球化学作用，即释放被固定的土壤阳离子。 未来的研究可进一步阐明土壤pH值通过胶体分散对阳离子供应的影响，以及其在阳离子匮乏土壤中对养分获取的重要性。