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 Pentaclethra macroloba, a nodulated legume, and increased to 4.71 (±0.08) under Vochysia guatemalensis, 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关键参与者，尽管已有证据表明阳离子（cations）可限制或共同限制净初级生产力（NPP）。在成熟森林中，紧密的阳离子循环（即无淋溶损失）可维持林内原位的阳离子储量。但该机制无法解释热带次生林更新树木在有效阳离子耗竭的土壤中生长时，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：通过影响土壤pH这一核心生物地球化学驱动因子，将树木物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年生随机完全区组试验，该试验中四种原生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，借此探究了树种对样地尺度土壤pH、土壤可提取阳离子及生物量中阳离子积累的影响。该氧化土（Oxisol）的初始表层土壤pH为4.52（±0.02），在具根瘤的豆科树种Pentaclethra macroloba林下降至4.14（±0.02），而在铝（Al）积累树种Vochysia guatemalensis林下升至4.71（±0.08）。pH的变化对应质子浓度相差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可达：铁（Fe）1.7倍、钙（Ca）1.9倍、钾（K）2.8倍、锰（Mn）2.9倍、锶（Sr）3.1倍、镁（Mg）3.5倍，以及铝（Al）17.2倍。物种间的实测差异难以通过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高度相关。基于本研究提出的概念框架，能够显著提升土壤质子浓度、降低土壤pH的树种性状（如促进固氮作用）会增强胶体团聚，从而降低阳离子有效性；而能够减少土壤质子、提升土壤pH的性状（如三价铝（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。这揭示了铝积累性状的全新生物地球化学作用，即释放土壤中被固定的阳离子。未来研究可进一步明确土壤pH通过胶体分散作用对阳离子供给的影响，及其在阳离子耗竭土壤中对养分获取的重要性。