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.

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The plant-available pools of calcium, magnesium and potassium are assumed to be stored in the soil as exchangeable cations adsorbed on the cation exchange complex. In numerous forest ecosystems, despite very low plant-available pools, elevated forest productivities are sustained. We hypothesize that trees access nutrient sources in the soil that are currently unaccounted by conventional soil analysis methods. We carried out an isotopic dilution assay to quantify the plant-available pools of calcium, magnesium and potassium and trace the soil phases that support these pools in 143 individual soil samples covering 3 climatic zones and 5 different soil types. For 81%, 87% and 90% of the soil samples (respectively for Ca, Mg and K), the plant-available pools measured by isotopic dilution were greater than the conventional exchangeable pool. This additional pool is most likely supported by secondary non-crystalline mineral phases in interaction with soil organic matter and represents in many cases (respectively 43%, 27% and 47% of the soil samples) a substantial amount of plant-available nutrient cations (50% greater than the conventional exchangeable pools) that is likely to play an essential role in the biogeochemical functioning of forest ecosystems, in particular when the resources of Ca, Mg and K are low.