In many forest ecosystems, plant-available pools of Mg, Ca, and K are assumed to be stored in the soil as exchangeable cations adsorbed on the cation exchange complex (exchangeable pools). However, between soil minerals and exchangeable cations exists a gradient of Mg, Ca, and K storage forms that have not been fully characterized and may play an important role in plant nutrition and biogeochemical cycles. We hypothesize that sources of Mg, Ca, and K in the soil other than the conventionally measured exchangeable pools are plant-available on very short time scales (<1 day). In the present study, we developed and applied an isotopic dilution technique using the stable isotopes ²⁶Mg, ⁴⁴Ca, and ⁴¹K to trace and quantify the pools of Mg, Ca, and K (isotopically exchangeable pools) in the soil of a hardwood forest that contribute directly to equilibrium processes between the soil water and the soil. We characterize the equilibrium between the soil and soil solution using both a batch approach and a flow-through approach in order (i) to develop and determine the best routine method to measure the isotopically exchangeable pools and (ii) to further the characterization of the forms of storage of Mg, Ca, and K in the isotopically exchangeable pools. We first show that the flow-through reactor approach (equilibrium in unsaturated soil columns) is the most adequate to measure the isotopically exchangeable pools with the fewest equilibrium disturbances. We then show that isotopically exchangeable pools of Mg, Ca, and K are greater than traditionally measured exchangeable pools. The isotopically exchangeable pools of Mg, Ca, and K are mainly composed of traditionally measured exchangeable pools (88.8–98.5% for Mg, 74.7–97.7% for Ca, and 68.7–77.1% for K) but are also composed of pools extracted with the Tamm reagent (oxalic acid, pH 3) and nitric acid (1 mol·L^–1): 1.5–11.2% for Mg, 2.3–25.3% for Ca, and 22.9–31.3% for K. Storage forms of Mg, Ca, and K in the isotopically exchangeable pool could include chelation with soil organic matter, retention on soil aluminum and iron oxides and hydroxides through phosphate and/or organic acid bridges and site-specific adsorption. The isotopic dilution method is a relevant tool to quantify the plant-available pools of Mg, Ca, and K on short time scales (source and sink pools) and is a very promising approach to characterize and quantify the processes responsible for the depletion and/or replenishment of these pools over longer time scales.

中文翻译：

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测量土壤中植物可用的镁，钙和钾库—同位素稀释法

在许多森林生态系统中，假定有植物可用的Mg，Ca和K库作为吸附在阳离子交换复合体上的可交换阳离子存储在土壤中（可交换池）。但是，在土壤矿物质和可交换阳离子之间存在Mg，Ca和K储存形式的梯度，尚未完全表征，并且可能在植物营养和生物地球化学循环中起重要作用。我们假设除常规测得的可交换池外，土壤中的Mg，Ca和K的来源在非常短的时间内（<1天）即可被植物利用。在本研究中，我们使用稳定的同位素²⁶ Mg，⁴⁴ Ca和⁴¹开发并应用了同位素稀释技术K用于追踪和量化硬木林土壤中的Mg，Ca和K的池（同位素可交换池），这些池直接有助于土壤水与土壤之间的平衡过程。我们使用分批方法和流通方法来表征土壤和土壤溶液之间的平衡，以便（i）开发和确定测量同位素可交换池的最佳常规方法，以及（ii）进一步表征同位素可交换池中Mg，Ca和K的储存形式 我们首先表明，流通式反应器方法（非饱和土柱中的平衡）最适合测量平衡扰动最少的同位素可交换池。然后，我们表明Mg，Ca，和K大于传统测量的可交换池。Mg，Ca和K的同位素可交换池主要由传统测量的可交换池组成（Mg为88.8–98.5％，Ca为74.7–97.7％，K为68.7–77.1％），但也包括用Tamm试剂（草酸，pH 3）和硝酸（1mol·L）^–1）：Mg的1.5–11.2％，Ca的2.3–25.3％和K的22.9–31.3％。同位素，可交换池中Mg，Ca和K的存储形式可能包括与土壤有机质的螯合，保留在土壤中。通过磷酸盐和/或有机酸桥以及针对特定地点的吸附作用，将土壤铝，铁的氧化物和氢氧化物污染土壤。同位素稀释法是一种在短时间范围内量化植物中镁，钙和钾的库（源库和库库）的相关工具，并且是一种非常有前途的方法，用于表征和量化造成消耗和/或消耗的过程或在更长的时间范围内补充这些池。