Measuring Plant-Available Mg, Ca, and K Pools in the SoilAn Isotopic Dilution Assay Gregory van der Heijden,* ,† Je ́ re ́ mie Bel, † Carol-Ann Craig, ‡ Andrew J. Midwood, ‡,⊥ Louis Mareschal, § Jacques Ranger, † Etienne Dambrine, ∥ and Arnaud Legout † † INRA UR 1138 Bioge ́ ochimie des Ecosyste ̀ mes Forestiers, FR-54280 Champenoux, France ‡ The James Hutton Institute, Aberdeen AB15 8QH, United Kingdom § CIRAD UMR Eco&Sol, FR-64060 Montpellier, France ∥ INRA − UMR 042 CARRTEL, Universite ́ de Savoie, FR-73376 Le Bourget-du-Lac Cedex, France *S Supporting Information ABSTRACT: 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 min- erals 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 26 Mg, 44 Ca, and 41 K to trace and quantify the pools of Mg, Ca, and K (isotopically exchangeable pools) in the soil of a hardwood forest that contrib- ute 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. KEYWORDS: forest ecosystem, biogeochemistry, soil fertility, plant nutrition, stable isotope tracing, soil solution equilibrium 1. INTRODUCTION Magnesium, calcium, and potassium are three essential and major nutrients for plants. They contribute to the chemical, physical, and biological components of soil fertility. 1 Forest ecosystems are generally developed on acidic and nutrient-poor soils, and the plant-available pools of Mg, Ca, and K in the soil are often very low. 2−4 Plant-available Ca, Mg, and K are assumed to be mainly stored as exchangeable cations adsorbed on the surface of min- eral and organic particles. 5,6 Exchangeable cations are in equi- librium with dissolved cations (Mg 2+ , Ca 2+ , and K + ) in the soil solution from which they are absorbed by plant roots. Exchange- able Mg, Ca, and K pools are traditionally measured by cation- exchange soil extractions, which use dissolved concentrated cations to displace the cations initially sorbed on the soil cationic exchange capacity with a solid:solution ratio between 1:5 and 1:10. 7,8 The extraction reagent (NH 4 OAc, cobaltihexamine, BaCl 2 , SrCl 2 , KCl, NH 4 Cl, etc.) varies among studies. However, many studies have shown that the reagent used does not influ- ence the measurement of exchangeable Mg and Ca pools in noncalcareous soils. 9−11 NH 4 + is often favored because of its greater K + displacement power compared with Ba or Sr. 9−12 In the long term, the exchangeable pools vary as a function of the inputs of Mg, Ca, and K to the forest ecosystem (chemical and Received: December 18, 2017 Revised: February 8, 2018 Accepted: February 8, 2018 Published: February 8, 2018 Article Cite This: ACS Earth Space Chem. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acsearthspacechem.7b00150 ACS Earth Space Chem. XXXX, XXX, XXX−XXX