Measuring Plant-Available Mg, Ca, and K Pools in the Soil—An Isotopic Dilution Assay

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 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.

Conventional analysis methods underestimate the plant-available pools of calcium, magnesium and potassium in forest soils

“…The majority of samples showed an E X :Exch X ratio greater than 1 for Ca, Mg and K. For a substantial proportion of the dataset (43%, 27% and 47% respectively for Ca, Mg and K), E x pools were more than 50% larger than the Exch X pools. This trend was confirmed by a Wilcoxon-Mann-Whitney comparison test that showed that isotopically exchangeable pools were significantly larger than their respective exchangeable pool, though homoscedasticity between the compared pools was not met for Mg. Our results are consistent with previous studies for Ca and K. Blum and Smith 41 showed, in an isotopic dilution assay using the 45 Ca radiogenic isotope over 16 different type of soils, that E Ca was significantly greater than Exch Ca for 6 soil samples, lower for 2 samples and not significantly different for 8 samples. Reeve et al 43 showed that 5 soil samples out of 7 showed an E Ca pool greater than the Exch Ca pool.…”

“…Using radio-isotopes, previous studies found that the K and Ca isotopically exchangeable pool was larger than the exchangeable pool (over 45 agricultural soil studied) [38][39][40][41][42][43] . Two recent studies 44,45 , focused on soil samples from the Breuil-Chenue experimental forest and using a stable isotopic dilution approach, (i) showed that soil pools of Mg, Ca and K in addition to the exchangeable pool significantly contributed to chemical equilibrium reactions between the liquid and solid phases of the soil, and (ii) suggested that these pools were supported by secondary non-crystalline mineral phases such as Al and Fe (hydr)oxides, and amorphous aluminosilicates.…”

Weathering rates in Swedish forest soils

“…The weathering submodel in PROFILE was later built in to the dynamic version SAFE (Alveteg et al, 1995), which was mainly used for acidification assessments, but has also been used for studying the dynamics of weathering rates . Later, the SAFE model was coupled with the tree growth model PnET (Aber and Federer, 1992), the decomposition model DECOMP (Walse et al, 1998;Wallman et al, 2004) and the hydrological model PULSE (Lindström and , resulting in the forest ecosystem model ForSAFE (Wallman et al, 2005;Belyazid et al, 2006). The ForSAFE model simulates the integrated biogeochemical processes of a forest ecosystem.…”

“…There are several different pools of base cations in soil and also several different flows, e.g. decomposition, uptake, ion exchange and weathering, and it is difficult to distinguish between these different sources and sinks (Rosenstock et al, 2019) and to define the pools accurately (van der Heijden et al, 2018). Therefore, a number of indirect methods to quantify weathering rates have been developed: process-based modelling (Sverdrup and Warfvinge, 1993); soil measurements where the depletion of weathering products in different soil layers is determined in order to assess average weathering rates since soil formation (Olsson et al, 1993); and budget calculations where the flows in the mass balance, except weathering, are measured (Lundström, 1990;Jacks and Åberg, 1987;Wickman and Jacks, 1991;Sverdrup et al, 1998).…”

“…The input data were also examined for uncertainties relating to the generalisations made when estimating normative mineralogy based on total chemical analyses (Casetou-Gustafson et al, 2018, 2019a. Two other potential sources of uncertainties that have been explored, but that are still widely discussed, were revisited: (1) the role of biological weathering that might generate weathering not included in the current generation of biogeochemical models (Banfield et al, 1999;Finlay et al, 2009Finlay et al, , 2019 and (2) model simplifications related to base cation exchange and aluminium complexation (Gustafsson et al, 2018;van der Heijden et al, 2018). Furthermore, the weathering kinetics used in models were revisited (Sverdrup et al, 2019), and weathering rates representing not only the rooting zone but the full catchment scale were studied to assess the export of weathering products to surface waters (Ameli et al, 2017;Erlandsson Lampa et al, 2019).…”