Accurately quantifying soil base cation pool sizes is essential to interpreting the sustainability of forest harvests from element mass-balance studies. The soil-exchangeable pool is classically viewed as the bank of “available” base cations in the soil, withdrawn upon by plant uptake and leaching and refilled by litter decomposition, atmospheric deposition and mineral weathering. The operational definition of this soil bank as the exchangeable (salt-extractable) pools ignores the potential role of “other” soil nutrient pools, including microbial biomass, clay interlayer absorbed elements, and calcium oxalate. These pools can be large relative to “exchangeable” pools. Thus neglecting these other pools in studies examining the sustainability of biomass extractions, or need for nutrient return, limits our ability to gauge the threat or risk of unsustainable biomass removals. We examine a set of chemical extraction data from a mature Norway spruce forest in central Sweden and compare this dataset to ecosystem flux data gathered from the site in previous research. The 0.2 M HCl extraction released large pools of Ca, K, Mg, and Na, considerably larger than the exchangeable pools. Where net losses of base cations are predicted from biomass harvest, exchangeable pools may not be sufficient to support more than a single 65-year forest rotation, but acid-extractable pools are sufficient to support many rotations of net-ecosystem losses. We examine elemental ratios, soil clay and carbon contents, and pool depth trends to identify the likely origin of the HCl-extractable pool. No single candidate compound class emerges, as very strongly supported by the data, as being the major constituent of the HCl-extractable fraction. A combination of microbial biomass, fine grain, potentially shielded, easily weatherable minerals, and non-structural clay interlayer bound potassium may explain the size and distribution of the acid-extractable base cation pool. Sequential extraction techniques and isotope-exchange measurements should be further developed and, if possible, complemented with spectroscopic techniques to illuminate the identity of and flux rates through these important, and commonly overlooked, nutrient pools.
In an attempt to decrease net
The nutrient mass-balance (i.e., input–output budget) approach is commonly used to estimate the net gain or loss of nutrient elements in the soil or ecosystems under different management and climate scenarios (Akselsson et al., 2007a; Knust et al., 2016; de Oliveira Garcia et al., 2018). This mass-balance approach typically accounts for inputs via atmospheric deposition and the weathering of primary minerals as well as secondary clay minerals, and exports occur via leaching and biomass removals (Nilsson et al., 1982; Van Breemen et al., 1984). Under this definition, the mass-balance equation estimates the change in nutrient content in a “source and sink” reservoir of the soil. For base cations, this source and sink reservoir is assumed to be the pool of Mg, Ca, Na, and K stored in the soil as exchangeable cations adsorbed on the cation-exchange complex. This quantity is conventionally measured by ion-exchange soil extractions using concentrated salts (salt-extractable exchangeable pools; Akselsson et al., 2007a; Vangansbeke et al., 2015; Knust et al., 2016).
Accurate estimates of soil nutrient stocks are important for policies on sustainable levels of biomass extraction levels or critical loads, i.e., the maximum level of atmospheric pollutant deposition that will not damage sensitive aspects of the ecosystem, as well as for determining the maximum level of forest biomass harvest, the “critical biomass harvest” that will not result in unacceptable soil and stream acidification (Akselsson and Belyazid, 2018). The net balance of nutrients between influx and efflux in an ecosystem or soil can only be meaningfully interpreted in the context of the magnitude of soil nutrient reserves. If, for example, whole-tree harvesting results in a net decrease of soil Ca, it is equally important to know if this loss is likely to result in significant reductions in the availability of Ca to plants in 1, 10 or 100 forest rotations, as it is to know whether that balance is positive or negative.
Base cation nutrient reserves in soil are commonly estimated by measuring salt-extractable quantities, commonly with barium salts or ammonium salts as extractants, to represent what is available to plants (Skinner et al., 2001; McLaughlin and Philips, 2006; Brandtberg and Olsson, 2012; Zetterberg et al., 2016). However, on timescales of years to decades, salt-extractable base cation reserves in the soil do not appear to accurately predict what is available for leaching or biological uptake (Mengel and Rahmatullah, 1994; Bailey et al., 2003; Hamburg et al., 2003; Lucash et al., 2012). This may be due to the presence of other significant pools of base cations in the soil, which are not salt extractable but which contribute to the plant-available pools of base cations. Calcium is known to form strong complexes with organic compounds such as the oxalate ion, which are poorly soluble and not salt extractable (Dauer and Perakis, 2014), but these complexes may be available to plants and microbes over relatively short timescales. The microbial biomass of the soil is quite large, has considerably higher nutrient concentrations than bulk organic matter, turns over very rapidly, and may thus represent a significant pool of available base cations that is missed in salt extractions (Yamashita et al., 2014; Lorenz et al., 2010; van der Heijden et al., 2014). Aluminum and iron oxides and hydroxides may also directly or indirectly (via organic matter or phosphate bridges) adsorb base cations (Kinniburgh et al., 1976; Grove et al., 1981). For example, in a highly weathered tropical soil, Hall and Huang (2017) showed that a significant amount of Mg, Ca and K were sequestered in Fe (hydr)oxide secondary mineral phases, and the microbial dissolution of these phases increased the concentrations of weak-acid-extractable cations. Significant stocks of soil potassium may be stored in the interlayers of clay minerals (Sparks, 1987). This pool of interlayer K is not accounted for by salt-extractable exchangeable cations but may be quite significant for plant nutrition: Falk Øgaard and Krogstad (2005) showed that interlayer K accounted for 26 % to 43 % of K uptake in grassland ecosystems. Not accounting for these additional pools of base cations in the soil may explain why mass-balance models often fail to reproduce the empirically measured change in salt-extractable exchangeable pools (Löfgren et al., 2017; van der Heijden et al., 2014).
Measurements of salt-extractable base cations may be complemented by soil
extractions using strong acids. Aqua regia, HF, or lithium metaborate fusion
extract the total or near total reserves of elements, which may then be used
to (i) estimate the relative distribution of minerals in the soil (Posch and
Kurz, 2007) and (ii) estimate the mineral weathering flux based on
assumptions of weathering kinetics derived from laboratory dissolution
experiments (Warfvinge and Sverdrup, 1992). Moderately concentrated (defined
here as 0.1 M–1 M) strong acid extractions (primarily HCl, but also
In order to examine the potential importance of non-exchangeable sources of
base cations in relation to net losses of base cations from forest
harvesting we utilized published data from a particularly well-studied
forest site at Kindla, Sweden, to form a mass-balance budget of base cations
under different harvest scenarios, and we compared annual losses, where
predicted, to base cation pools in the soil as defined by different
extractants. The potential for stem-only or whole-tree harvesting to result
in net losses of base cations was examined, then these estimated losses were
compared to extraction-defined (distilled water,
Kindla is a catchment within the Swedish national integrated monitoring
program (Löfgren et al., 2011) and a core site in Long-Term Ecosystem Research (LTER) Europe
(
The forest examined at Kindla is a naturally regenerated, uneven-aged Norway spruce (
Sample description for nine samples from three plots, three depths at each plot, along the hydrology gradient at Kindla.
For each plot and harvest scenario the mass balance of each base cation, Ca,
K, Mg, and Na, was calculated as follows:
As part of the EU FP7 funded SoilTrEC project (Banwart et al., 2011) a
series of non-sequential chemical extraction were performed on nine composite
mineral soil samples, representing the E, B, and C horizons at the three
soil sampling sites. Extractions were performed with distilled water, 0.1 M
For the soils, elements, and harvest scenarios that have a net elemental loss over time, we examined the potential for different soil base cation pools to sustain those losses. We assumed that ecosystem fluxes (deposition, weathering, leaching, biomass uptake) would remain constant, and calculated the number of 65-year harvest rotations that each extractant-defined base cation pool could offset net ecosystem losses for each harvest scenario–element-soil combination under which a net ecosystem loss was predicted.
In the mineral soil, concentrations of Ca and Mg in
Extractable contents of calcium
Humus layer 0.1 M
Inputs of Ca, K, and Na via weathering were 1.8-, 1.5-, and 1.3-fold greater, respectively, than inputs via atmospheric deposition, while deposition inputs of Mg were 2-fold greater than weathering inputs. Leaching losses of Ca and Mg were highest in the downslope Regosol from the groundwater discharge area, and leaching losses were less than the combined inputs of weathering and deposition for Ca, K, and Na, but not for Mg (Table 2). Whole-tree harvesting resulted in considerably greater elemental losses than stem-only harvest (2.3–2.6 fold greater). Whole-tree harvesting resulted in net loss of Ca, K, and Mg on all three plots, while stem-only harvesting resulted in net losses of only Mg on all three plots and net losses of Ca on the downslope Regosol (Table 2).
Mass balances for the three soils collected along the hydrological
gradient at Kindla based on annual fluxes of Ca, Mg, K, and Na (mg m
Mass balances in bold are negative, indicating a net loss of base cations from the system.
For each harvest scenario–element-soil combination under which a net
ecosystem loss was predicted, we calculated the number of 65-year harvest
rotations that each extractant-defined base cation pool could offset. Based
solely on the mineral soils, the exchangeable (
Number of forest harvest rotations (65 years) each base cation pool
may offset net ecosystem losses for each of the three soils. All extractions
were performed on the mineral soil. For
“
Given the large size and potential to buffer against leaching and many forest harvest rotations of net base cation loss, understanding the chemical nature and availability of the HCl-extractable pool is important to forming management recommendations. Due to lack of data from the organic horizon, this chapter is primarily focused on the results from the mineral soils. Extractions with strong acids at moderate (0.05 M–1 M) concentrations, as was performed in this study, have been used to selectively extract calcium oxalate (Dauer and Perakis, 2014; Cromack et al., 1979), apatite (Blum et al., 2002; Nezat et al., 2007), non-exchangeable clay interlayer K (Simonsson et al., 2016; Li et al., 2015), and Fe and Mn oxides (Krasnodebska-Ostrega et al., 2001) from soils.
Despite the prevalence of their use, the use of HCl and
Microbial biomass has been suggested to be an important reservoir of N and
P, and potentially of base cations. We estimated the potential size of the
mineral soil microbial biomass base cation pools as a function of soil
carbon and microbial biomass carbon to nutrient ratios (for more detail see
Supplement methods). Microbial biomass was estimated to constitute a small but
potentially significant fraction of Ca, Mg, and K pools, particularly K.
Excluding the humus layer, which is dominated (88 %–100 %) by the
Base cation pool sizes of calcium
Mineral soil base cation concentrations from different soil fractions as defined by extractant and relative molar fraction of each base cation within each extraction.
If we assume that both the 0.2 and 0.5 M HCl extractions extract all, or
nearly all, of the
The pool of aqua-regia-only Ca is far larger (4–5 fold) than either of the HCl-only-extractable Ca pools (Table 4; Fig. 2). This stands in contrast to K, Mg, and
Na, for which the 0.2 M HCl-only pools are nearly as large or larger than
the aqua-regia-only fraction. Calcium also stands out in that the 0.5 and 0.2 M HCl-only digestible fractions were similar in size while,
in contrast, for K, Mg, and Na the stronger acid yielded relatively less
additional cations (less than half as much as the 0.2 M HCl only). Calcium
is the dominant cation (comprising 50 % or more of total base cations, on
a molar basis) in both the exchangeable and aqua regia base cation pools,
but in the HCl-extractable pools it comprised a smaller portion (Table 4).
In contrast we see for Mg that the two HCl-only pools combined are larger
than aqua-regia-only pools of Mg, and we also see that the mole fraction of
base cations is heavily enriched for Mg in the HCl-extractable fraction.
HCl-extractable Mg also displays a strong depth trend in each of the three soils, being 5–70 times as abundant in the C-horizon HCl-only extracts as
compared to E-horizon extracts. While the amounts of both Mg and Ca increase
with depth in the HCl extractions, HCl-extractable K does not display the
same behavior; in the upslope and midslope soils its abundance is
relatively stable with depth, and the relative abundance of K
(
Base cations may be bound to organic compounds in the soil and
organically bound base cations may represent a significant pool of
exchangeable cations (Duchesne and Houle, 2008; Richardson et al., 2017).
The humus layer 0.1 M
Soil organic matter (SOM) may also form protective coatings on soil minerals reducing their solubility (Drever and Stillings, 1997). It is possible that a significant portion of mineral surfaces were shielded with SOM, and the HCl extractions may have removed these protective layers. This could potentially explain why, particularly for K and Mg, we see considerably greater BC yields in the 0.2 M HCl-only fraction than we see in the 0.5 M HCl-only fraction because 0.2 M HCl is sufficient to remove these protective coatings, and the resultant burst of mineral surface dissolution is not a function of acid strength but a function of the removal of these coatings. To examine the potential for organic matter coatings on minerals to shield them from weathering but be readily removed by 0.2 M HCl, chemical treatments utilizing hydrogen peroxide or surfactants (Chao, 1984) could be used to remove or reduce such coatings without the use of a strong acid.
Non-structural K, strongly bound in clay interlayers has been observed to be
present in large quantities in a variety of soils (Moritsuka et al., 2003;
Li et al., 2015). Clay interlayer K is typically extracted with dilute or
concentrated
HCl is commonly used, both as a single extractant and in sequential
extractions to dissolve Fe and Mn oxides in soils (Chao and Sanzolone, 1992;
Krasnodebska-Ostrega et al., 2001). Soil Fe and Mn oxides can be important
reservoirs for base cations not extractable with typical salt-exchange
assays (Krasnodebska-Ostrega et al., 2001). Indeed we extracted large
amounts of Fe in the 0.2 M HCl-only pool (> 50 % total Fe,
0.01 %–0.1 % total soil dry mass), though relatively little Mn (
While more aggressive digestions (aqua regia, HF, LiBO
Our data indicate that the large pools of 0.2 M HCl-extractable base cation pools may be comprised of a combination of microbial biomass, the dissolution of fine grain, potentially OM-shielded, easily weatherable minerals (such as biotite, apatite, chlorite, and hornblende), as well as by the presence of significant stores of non-structural clay interlayer bound K. Determining the physicochemical nature of these base cation pools is central to incorporating them into mass-balance and geochemical models widely used today to inform management and policy. If these base cation pools derive from primary mineral weathering, then weathering models may already account for these pools or may need to be adjusted to account for these large and potentially labile pools; if they are a result of organic complexation or secondary minerals, then understanding what controls their formation and availability depends on a better understanding of their chemical nature. Combining sequential extraction approaches with X-ray diffraction methods on small soil samples of different particle size classes may help elucidate the potential importance of primary minerals to the HCl-extractable pools, while other spectroscopic methods (e.g., micro, nanoSIMS, STXM) may be employed to determine the role of organic or secondary mineral coatings or identify secondary mineral or non-crystalline base cation pools.
Among the many studies which have predicted net ecosystem losses of base
cations with biomass (stem only or whole tree) extractions, exchangeable
base cation pools, when measured, are commonly not sufficient to sustain
more than a single harvest rotation, assuming productivity, growth, and base
cation uptake are not reduced by reductions in base cation availability
(Akselsson et al., 2007a, b; Duchesne and Houle, 2008;
Knust et al., 2016). We are not aware of any other studies of ecosystem
mass balance of base cations that have attempted to explore the sizes of
non-exchangeable nutrient pools in relation to net ecosystem losses over
time. Those that have compared HCl- or
The relevance of the size of these large pools of acid-extractable base cations to forest nutrient relations is a function of their flux rates or bioavailability. If the base cations released from 0.2 M HCl-only pools are highly stable over centuries and not available to refill depleted exchange sites, from which plant uptake may occur, then they are of little relevance to policy decisions on sustainable harvest levels. If, on the other hand, they are in equilibrium with the exchangeable pools and available for plant uptake in significant amounts on decadal or century timescales then these pools and their size relative to the exchangeable pool are of immediate relevance to forest management recommendations in light of the immediate need to reduce fossil fuel emissions.
A number of studies have examined the bioavailability and flux into
exchangeable pools of acid-extractable non-exchangeable pools of base
cations across a variety of soils. Callesen et al. (2004) examined 19 Danish
forest soils comparing exchangeable base cation amounts to base cations
pools which were extractable with 0.01 M
Simonsson et al. (2016) used
Our mass-balance estimates indicate that stem-only harvesting would
moderately deplete and whole-tree harvesting would markedly deplete
exchangeable cation pools, such that current 0.1 M
Many soils appear to have very large, relative to exchangeable pools, base cation reserves in the mineral soil, which are not extractable with conventional exchangeable cation assays but which appear to be available for plant uptake and interact with the exchangeable pool on the scale of years to decades. These large pools should be addressed in ecosystem mass-balance research and accounted for and considered in nutrient management recommendations. To develop forest management policy based on these putatively available pools, a better understanding of their physicochemical nature, bioavailability and flux rates is needed.
Data are stored at the Department of Aquatic Sciences and Assessment at the Swedish University of Agricultural Sciences and are made available on request by stefan.lofgren@slu.se or lars.lundin@slu.se.
The supplement related to this article is available online at:
NPR, KB, and SL conceived the idea for the paper. LL, SL, and EM contributed data. NPR analyzed the data and wrote the paper. All authors contributed ideas, participated in discussions about the paper, and were involved with writing and interpretation.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Quantifying weathering rates for sustainable forestry (BG/SOIL inter-journal SI)”. It is not associated with a conference.
The authors would like to thank the following funding agencies responsible for this work. The Swedish Research council for funding the research consortium: Quantifying weathering Rates for Sustainable Forestry; the Swedish Energy Agency; the European Commission; and the International Cooperative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) for providing support for data collection and for the SoilTrEC project.
This research has been supported by the Svenska Forskningsrådet Formas (grant no. 212-2011-1691).
This paper was edited by Boris Jansen and reviewed by two anonymous referees.