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，尽管已有研究表明阳离子（源自岩石的带正电离子）会限制或共同限制净初级生产力（NPP）。成熟森林中，紧密的阳离子循环（即无淋溶损失）可维持原位的阳离子储量。但该机制无法解释：当生长在有效阳离子耗竭的土壤中时，热带次生林的再生幼树如何从种子萌发开始，并在生物量中积累大量阳离子储量。本研究提出一种新机制：通过影响作为核心生物地球化学驱动因子的土壤pH值，将树种性状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年定位随机完全区组实验，该实验中四种乡土树种的单优人工林的气候、土壤及既往土地利用历史均一致，以此检验树种对样地尺度土壤pH、土壤可提取阳离子以及生物量中阳离子积累的影响。该氧化土的表层土壤初始pH为4.52（±0.02），在具根瘤的豆科树种*Pentaclethra macroloba*种植下降至4.14（±0.02），而在铝（Al）积累树种*Vochysia guatemalensis*种植下升至4.71（±0.08）。pH的变化范围对应质子浓度相差5倍，这足以改变有机矿物胶体的分散状态。不同树种的生物量阳离子储量差异显著：铁（Fe）、钙（Ca）、钾（K）、锰（Mn）、锶（Sr）、镁（Mg）及铝（Al）分别相差1.7、1.9、2.8、2.9、3.1、3.5及17.2倍。研究观测到的树种间差异无法用有效土壤阳离子的差异化获取来解释，因为土壤可提取阳离子储量的变化与生物量中的阳离子积累并无关联。但土壤pH与生物量阳离子储量却呈现高度相关性。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pH的树种性状（如促进固氮作用）会增强胶体聚合，从而降低阳离子有效性。而能够减少土壤质子、提升pH的性状（如三价铝离子（Al³+）积累）则会分散胶体，进而释放成土过程中被固定的阳离子。这一发现揭示了铝积累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。未来研究可进一步阐明土壤pH通过胶体分散对阳离子供应的调控作用，及其在阳离子耗竭土壤中对养分获取的重要意义。