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Chemical and molecular scale speciation of copper, zinc, and boron in agricultural soils of the Canadian prairies

Publication: Canadian Journal of Soil Science13 May 2021https://doi.org/10.1139/cjss-2020-0162

Abstract

The general incidence of copper (Cu), zinc (Zn), and boron (B) deficiencies in soils of the Canadian prairies may be related to identifiable, highly variable, inherent soil attributes. The objective of this study was to investigate the variability of selected properties and their relationship with the bioavailability, forms, and distribution of Cu, Zn, and B in a range of prairie soils. The nature of these micronutrient distributions were evaluated by measuring extractable concentrations, supply rates, and by separation into various chemical pools through sequential extraction and spectroscopic speciation analyses. Soil pH was found to be the least variable property [coefficient of variation (CV) < 13%], whereas carbonate content was the most variable (CV > 130%). The Cu and B availability showed strong negative correlation with the sand content in all soils. Path coefficient results indicated that organic carbon had the highest positive direct effect on availability and supply of Cu and B in Grey soils. Extractable Zn was positively correlated with organic carbon content of Brown and Dark Brown soils. Overall, high sand content and low organic matter were identified as important soil properties contributing to the deficiency of Cu, Zn, and B. The major proportion of Cu, Zn, and B was found in the recalcitrant residual fraction (59%–88%), with the smallest proportions in labile soluble, exchangeable forms (2%–8%). The X-ray absorption near edge structure revealed that Cu and Zn associated with carbonate minerals were dominant forms of these micronutrients present in all soils. Chemisorption is likely a major process regulating the bioavailability of Cu and Zn in prairie soils.

Résumé

On pourrait associer la carence générale en cuivre (Cu), en zinc (Zn) et en bore (B) dans le sol des Prairies canadiennes à la grande variabilité de certains attributs, identifiables, du sol. Les auteurs voulaient préciser la variabilité de propriétés précises du sol et leurs liens avec la biodisponibilité, la forme et la distribution de ces trois éléments dans divers sols des Prairies. La distribution des micro-oligoéléments a été évaluée d’après la concentration de l’élément extractible, son taux d’approvisionnement et sa séparation en divers bassins chimiques par extraction séquentielle et analyse de la spéciation au spectroscope. Le pH du sol est la propriété qui varie le moins (CV < 13 %), la teneur en carbonate se trouvant à l’autre extrémité de la fourchette (CV > 130 %). La disponibilité du Cu et du B présente une forte corrélation négative avec la concentration de sable dans le sol. Les résultats obtenus pour le coefficient de dépendance indiquent que le carbone organique exerce le plus fort impact positif sur la disponibilité et l’approvisionnement du Cu et du B dans les sols gris. La quantité de Zn extractible est positivement corrélée à la concentration de carbone organique dans les sols bruns et brun foncé. En règle générale, une grande quantité de sable et une faible teneur en matière organique jouent un rôle important dans la carence en Cu, Zn et B. La proportion la plus élevée de Cu, de Zn et de B se retrouve dans la fraction des résidus récalcitrants (59–88 %), et la plus faible sous la forme d’ions échangeables, solubles et labiles (2–8 %). La spectroscopie de structure près du front d’absorption des rayons X (XANES) révèle que le Cu et le Zn associés aux minéraux carbonatés constituent la forme dominante de ces micro-oligoéléments, peu importe la nature du sol. La chimisorption est sans doute le principal mécanisme qui régule la biodisponibilité du Cu et du Zn dans les sols des Prairies. [Traduit par la Rédaction]

Introduction

Sustainable agriculture focuses on profitable crop production with effective nutrient management practices to maintain soil quality and productivity. Continuous cultivation of high-yielding crop varieties without replenishing plant nutrients is one of the major causes of depletion of natural fertility, and most often results in soil degradation over the years (Scherr 1999; Cakmak 2002). Furthermore, the field scale variability of soil properties and nutrient content in the Canadian prairies deserves proper nutrient management plan to optimize farming efficiencies. Field sites that have impaired plant nutrient supply produce low yields and diminished farm returns (Hilliard and Reedyk 2015). Monitoring plant nutrient availability and detecting nutrient deficiencies are, therefore, needed as part of nutrient management for maximizing crop yield and quality.
Plant availability of nutrient elements required in low amounts by plants, often termed “micronutrients”, vary widely with soil and environmental conditions (Havlin et al. 2013). In the Canadian prairies, micronutrients such as copper (Cu), zinc (Zn), and boron (B) have a “patchy” nature of distribution in total and available amounts across landscapes and fields, such that site and crop-specific management is required (Karamanos 2000; Malhi and Karamanos 2006). The Grey Luvisol and Grey Black transitional mineral soils with coarse texture and low organic matter content and calcareous soils with high CaCO3 and pH are known to have the greatest incidence of micronutrient deficiencies for crop production (Singh et al. 1985; Raza et al. 2002; Karamanos et al. 2003a, 2003b; Malhi and Karamanos 2006; Rahman et al. 2020; Rahman and Schoenau 2020). Organic and peaty soils, although less common in the agricultural region, are also susceptible to micronutrient like Cu deficiency due to their low mineral content (Karamanos et al. 2003a).
In the diagnosis of micronutrient deficiency, the simple soil test extractions used for plant-available micronutrient assessment in prairie soils have often been reported as ineffective and inconclusive as the sole basis for assessing soil micronutrient status and making micronutrient recommendations (Liang et al. 1991a; Karamanos and Goh 2001; Karamanos et al. 2003a). In previous research, significant crop yield responses to micronutrient fertilization were often not observed, even though the soils were deemed critically deficient according to a soil test (Liang et al., 1991a, 1991b; Karamanos et al. 2003a, 2003b). This may be partly a consequence of the failure of the test to remove all or a portion of the fraction that is biologically available to the plant. As well, applied micronutrient fertilizer may be rapidly transformed into and subsequently distributed among different chemical fractions that are not available for plants (Liang et al. 1991a; Alloway 2009). For example, high pH and CaCO3 content of calcareous soils can promote surface adsorption of Zn and Cu, thereby reducing plant availability (Lafuente et al. 2008; Alloway 2008, 2009). Inner-sphere complexation of micronutrients into soil organic and mineral surfaces is also known to reduce plant availability (Roberts et al. 2005). Considering the high degree of spatial variability of soil properties that typically exists within prairie landscapes, it can be postulated that specific soil properties are likely to have a significant control on the plant availability of micronutrients through the process of chemical complexation and adsorption–desorption mechanisms. For example, soils with high clay, organic matter, and other characteristics like pH-dependent variable charge can provide additional exchange sites to increase Cu and Zn retention in soils (Havlin et al. 2013). Similarly, B availability is associated with organic matter. Increased adsorption of B to clay minerals under high pH and low organic matter content can reduce B availability (Havlin et al. 2013). As such, the soil properties may have considerable predictive power in diagnosis of micronutrient deficiency, along with soil and plant analysis. However, it is not well known what specific soil conditions contribute to low availability of Cu, Zn, and B in the soil, and to what extent deficiencies conclusively exist across the prairies. Furthermore, it is not revealed that why the soil-applied fertilizer are not consistently effective in increasing crop yield. Understanding the nature of Cu, Zn, and B distribution in different pools and their physicochemical forms obtained from sequential extraction and spectroscopic speciation may generate advanced knowledge to evaluate the fate of applied fertilizer.
Sequential extraction is effectively used for more detailed insight into form and functioning of a particular nutrient by partitioning the nutrient contained in the soil into different chemical pools based on specific solubility ranges produced by different chemical extractants (Viets 1962; Shuman 1991). In prairie soils, major proportions of Cu, Zn, and B were found in the recalcitrant residual fraction (Liang et al. 1991a, 1991b; Raza et al. 2002; Rahman et al. 2020; Rahman and Schoenau 2020), which are not easily accessed by crop plants. The labile soil solution and exchangeable fractions were the smallest (Rahman et al. 2020; Rahman and Schoenau 2020) and often influenced by soil chemistry that favors adsorption–desorption mechanisms (Havlin et al. 2013). Therefore, synchrotron-based approaches to speciation like X-ray absorption near edge structure (XANES) spectroscopy are advanced techniques that can be used alongside chemical extractions to identify mineral nutrient species (physicochemical forms) at a molecular level. Previously, XANES has proven successful in speciation of phosphorus in fertilizer- and manure-amended agricultural soils in the prairies (e.g., Kar et al. 2011, 2012). Moreover, the spectroscopic speciation of contaminated agricultural and calcareous soils revealed that Cu and Zn were mostly speciated onto carbonate and (hydr)oxide minerals (Ponizovsky et al. 2007; Strawn and Baker 2008). However, Cu was found to be preferentially speciated onto organic matter than carbonate minerals (Strawn and Baker 2008). As an element-specific technique, XANES is used to detect specific chemical forms of plant nutrients that are present or produced in the soil environment due to changes in oxidation state, isomorphous substitution, and chemical complexation as related to soil conditions and management practices (Kar et al. 2012). A combination of chemical and spectroscopic speciation that can provide quantitative and qualitative information on form and behavior of Cu and Zn in soil was suggested over a decade ago as a potentially useful investigative tool (White and Zasoski 1999), but has not been utilized to a wide extent. The goal of the research work was to examine how Cu, Zn, and B distribution varies among labile and stable forms in prairie soils and its relationship with soil properties and conditions. Identification of soil factors that contribute to deficiency and their influence on the distribution among identifiable fractions is emphasized. The knowledge will contribute to the development of improved site-specific micronutrient recommendations for agricultural production across Canadian prairie soils.

Materials and Methods

Study locations and soil sampling

This study was performed using 44 soil samples collected from annually cropped fields located across the soil-climatic zones of the Canadian prairies (Saskatchewan, Alberta, and Manitoba). The soils of different soil-climatic zones [Brown and Dark Brown (14 soils), Black (22 soils), and Grey (8 soils)] greatly differ based on the nature and amount of the organic matter due to the influence of climate and vegetation. The sites sampled were selected to provide contrast in basic soil properties like soil organic matter content, texture, and pH. Moreover, major consideration was given for the sites that are suspected to have, or have been confirmed to have, a Cu, Zn, B deficiency problem. In the fall of 2014 after harvest, soil samples were collected from farm fields at the 0–15 cm depth. Basic information on sampling site including landscape position, previous crop grown, elevation, and soil associations were recorded during sampling, along with the GPS coordinates (Supplementary Table 11). Collected soils were air-dried at room temperature; subsamples were ground with a wooden roller, passed through a 2 mm sieve, and then stored in plastic vials for wet chemical and synchrotron analysis.

Analytical techniques

All of the chemicals used in the study were of analytical reagent grade. Double-deionised water (18 MΩ) was used for preparing the solutions and dilutions. Soil samples were analyzed for basic soil properties including organic matter content, sand, silt, and clay percentage, pH, electrical conductivity (EC), and carbon content. Soil pH and EC were measured in water using a soil to water ratio of 1:2 with a Beckman 50 pH meter (Beckman Coulter, Fullerton, CA, USA) and an Accumet AP85 pH/EC meter (Accumet, Hudson, MA, USA) (Hendershot et al. 2007; Miller and Curtin 2007). The particle-size distribution was determined using a modified pipette method (Indorante et al. 1990). Total carbon (TC) and organic carbon (OC) content of soils were determined by dry combustion at 1100 and 813 °C, respectively, with the LECO-C632 carbon analyzer (LECO© Corporation, St. Joseph, MI, USA) (Skjemstad and Baldock 2007). Prior to measuring OC, soil samples were treated with HCl to remove carbonate from the samples (Harris et al. 2001). The difference between TC and OC was reported as inorganic carbon or carbonate content in soil samples (Bisutti et al. 2004)
Soil “available” micronutrient extraction was performed using 0.005 mol·L−1 diethylene–triamine–pentacetic acid (DTPA) extraction for Cu and Zn (Lindsay and Norvell 1978), and hot water extraction for B (Raza et al. 2002). Plant-available micronutrient supply rate was measured by a sandwich method using ion-exchange resin membrane strips for Cu and Zn and in situ burials of PRS™-probes for B (Qian et al. 2007). Sequential extraction techniques were used to chemically separate Cu, Zn, and B in the soil into different fractions. The modified Community Bureau of Reference (BCR) procedure (Zemberyova et al. 2006) was employed for sequential extraction of Cu and Zn in the soil. In the BCR procedure, acetic acid (0.11 mol·L−1) was used to extract soil solution-carbonate-exchangeable fraction (fraction 1). The oxyhydroxide fraction (fraction 2) was then extracted using 0.5 mol·L−1 hydroxylamine hydrochloride. The organic-bound fraction (fraction 3) was removed using concentrated hydrogen peroxide (8.8 mol·L−1) followed by 1.0 mol·L−1 ammonium acetate adjusted to pH 2. Soil B fractions were separated into five pools based on the procedure of Raza et al. (2002). According to Raza et al. (2002), hot water extraction provided better results for Saskatchewan soils and, as such, was used in this study for available B extraction. Distribution of B into other fractions such as specifically adsorbed (fraction 1), oxide bound (fraction 2), and organically bound (fraction 3) were then determined sequentially on same soils after hot water extraction using 0.05 mol·L−1 KH2PO4, 0.2 mol·L−1 acidic NH4-oxalate, and 0.02 mol·L−1 HNO3 + 30% H2O2, respectively. The residual fraction (fraction 4) was calculated by subtracting the above four fractions from the total concentration determined from HNO3 + H2O2 digestions (USEPA method 3051a; USEPA 2007). The concentrations of Cu and Zn in extracting solutions were measured by atomic absorption spectrometry (Varian Spectra 220 Atomic Absorption Spectrometer; Varian Inc., Palo Alto, CA, USA), whereas B was measured using the 4100 MP-AES Microwave Plasma-Atomic Emission Spectrometer (Agilent Technologies). Several reference materials such as BCR-701, SRM 5709a, and internal soil standards were used to validate the extraction and digestion procedures.

XANES measurement and data processing

Molecular speciation of Cu and Zn was performed using the hard X-ray micro analysis beamline (06ID-1), whereas B speciation used the variable line spacing planar grating monochromator beamline (11ID-2) at Canadian Light Source, Saskatoon, Canada. The spectra collection modes include fluorescence mode for Cu and Zn using Si (III) double-crystal monochromator, whereas simultaneous total electron yield and total fluorescence yield modes were used for B. The K-edge XANES spectra of Cu (∼8950–9050 eV), Zn (∼9600–9750 eV), and B (∼180–220 eV) were collected in solid state at room temperature. Samples were prepared independently to establish reproducibility of results. Multiple scans were made on each sample, and the average was reported. The collected spectra were processed, and linear combination fitting (LCF) analysis performed using the ATHENA software (Ravel and Newville 2005).

Statistical analysis

Measurements in the laboratory were conducted on samples in triplicate, and the mean reported. Descriptive statistical analysis was performed for all variables using PROC UNIVARIATE in SAS® version 9.4 (SAS Institute Inc. 2013). Principle component analysis (PCA) was performed using JMP 13 to assess the relation between soil properties and plant-available micronutrients.
Soil properties, micronutrient concentrations, supply rates, fractionation, and the correlations among the parameters are grouped and presented according to soil-climatic zone (Brown and Dark Brown, Black, Grey), as this is a large-scale management zone partitioning commonly used in western Canada.

Results and Discussion

Variability in soil properties, soil extractable, and supply rate of Cu, Zn, and B

The descriptive statistics of basic soil characteristics, soil extractable, and supply rate of Cu, Zn, and B are summarized according to soil zone (Table 1 and Supplementary Table 21). The average pH revealed the slightly acidic nature of the Black (pH = 6.74) and Grey soils (pH = 6.40) as well as the more alkaline nature of the Brown and Dark Brown soils (pH = 7.11) used in this study. The EC range indicated that none of the Brown, Dark Brown, and Black soils were considered saline. There were several saline Grey soils as reflected in the EC range (0.14–6.87 mS·cm−1). Further, the Brown, Dark Brown, and Black soils had a wide range of sand content, ranging from very fine (clay) to coarse (sandy) textured in nature, whereas the Grey soils tended overall to be sandier, ranging from medium to coarse textured. The variation in deposition of glacial tills such as very coarse-grained, coarse-grained, and fine-grained parent materials can results in soil texture variability within the sites of a field. The OC content of Black soils ranged from 1.24% to 13.0%, Grey soils from 0.86% to 12.9%, and Brown and Dark Brown from 1.03% to 3.80% (Table 1). Highest OC values in the Black and Grey soil zones were reflective of organic soils. Deposition of moss-like organic materials, vegetation cover, and precipitation pattern significantly contributes in soil organic matter buildup as such organic or peat soil formation, which is considered for partitioning different soil-climatic zones. Among the measured soil properties, the inorganic carbon content showed the higher coefficient of variation (CV = >130%), whereas pH had the lowest variability (CV = <13%). As well as soil-climatic zone that reflects long-term differences in precipitation effectiveness for plant growth, organic matter addition and weathering during soil formation, differences in parent material, past management, and slope position within the zones also contribute to variations in properties observed among the soils. For example, in typical hummocky landscapes of the prairies, both pH and inorganic carbon content are higher in eroded upper slope positions compared with lower slope and footslope complexes (Landi et al. 2004; Papiernik et al. 2005; Farenhorst et al. 2009). Some studied soils are collected from the eroded knolls, which are typically low in OC content. Subsurface inorganic carbon is often exposed in those sites due to tillage and erosion. The amount of OC was found to be increased moving from knoll to foot slope positions due to erosion along with greater organic matter inputs in more moist depressions (Pennock et al. 1994; Landi et al. 2004).
Table 1.
Table 1. Descriptive statistics of basic soil properties, soil extractable [diethylene–triamine–pentacetic acid for copper (Cu) and zinc (Zn), hot water for boron (B)], and supply rate (ion-exchange membrane for Cu, Zn, and B) of micronutrients in 44 surface soils (0–15 cm) collected from agricultural fields across the soil zones in Saskatchewan, Alberta, and Manitoba.

Note: EC, electrical conductivity (mS·cm−1); FC, moisture content at field capacity (%); OC, organic carbon (%); IC, inorganic carbon (%). Sand is reported as a percent (%). Values in parentheses are the coefficients of variation (%).

According to the critical deficiency criteria of soil-extractable Cu (DTPA-Cu <0.4 mg·kg−1 as reported in Karamanos et al. 2003a), Zn (DTPA-Zn < 0.5 mg·kg−1 as reported in Goh and Karamanos 2004), and B [hot-water-soluble B (HWSB) = 0.35 mg·kg−1 as reported in Karamanos et al. 2003b], all the tested Brown and Dark Brown soils contained marginal to an adequate amount of micronutrients. The range of extractable Cu in Black (0.25–1.65 mg·kg−1) and Grey (0.20–2.00 mg·kg−1) soils indicated that approximately 20% of these soils were critically deficient in Cu for crop production according to soil test. About 10% of the studied soils were rated deficient in available Zn, whereas some marginally B-deficient soils were also identified. The amount of Cu, Zn, and B in the studied soils is often dependent on the soil-forming parent materials and mineralogical composition (Havlin et al. 2013). Moreover, several soil characteristics such as pH, organic matter content, clay content, and free lime are considered as the key factors for determining their availability in soils (Maqsood et al. 2016; Rahman et al. 2020; Rahman and Schoenau 2020). For example, Cu retention is higher than Zn in soils with high amount of clay and organic matter due to greater affinity for adsorption sites (Havlin et al. 2013). Overall results showed that the status of extractable available Cu, Zn, and B varied greatly with soil properties, irrespective of different types of soils. Similar spatial variations in supply rates of micronutrients were observed. However, the highest variability in supply rate of Cu and Zn was observed in Grey (CV = 141%), and Brown and Dark Brown (CV = 106%) soils, respectively. Large spatial variations in nutrient availability and supply are often considered to be a leading cause of variability in crop yields (Papiernik et al. 2005).

Nature and distribution of micronutrient among different fractions in soil

Sequential extractions conducted on the experimental soils indicated strong spatial dependency for relative distribution of Cu, Zn, and B among different chemical forms representing varying degrees of mobility and bioavailability (Table 2). Usually, the speciation of micronutrients is markedly controlled by several soil parameters such as pH, texture, carbon content, and cation-exchange capacity (Raza et al. 2002; Qian et al. 2003; Maqsood et al. 2016). Overall, low concentrations of Cu and Zn were found in the labile fraction (F1) as per expectation in agricultural soils. The mean concentrations of Cu and Zn in the soil solution-carbonate-exchangeable fraction were less than 0.57 and 1.83 mg·kg−1, respectively (Table 2). However, this fraction maintains equilibrium with moderately labile forms depending on soil conditions. A recent study showed that the soil-applied Cu and Zn were mostly resided in the solution-carbonate-exchangeable fraction and oxyhydroxide fraction (Rahman and Schoenau 2020). Boron concentration in the labile specifically adsorbed fraction (F1) varied between 0.13 and 8.28 mg·kg−1 in Brown and Dark Brown soils, and 0.01 and 10.2 mg·kg−1 in Black soils (Table 2). The Grey soils had a lower range of the labile fraction: from 0.01 to 1.52, indicating generally lower B availability in Grey soil zone compared with the others. The organically bound fraction was dominant over the non-specifically or specifically adsorbed and oxyhydroxide fractions. The B fractionation results are identical with the fractions of Brown and Grey Luvisol soils from Saskatchewan (Raza et al. 2002). Rahman and Schoenau (2020) reported that soil-applied boric acid was speciated onto specifically adsorbed fraction and oxyhydroxide fraction. It is well understood that water-soluble forms are readily bioavailable and maintain equilibrium with exchangeable and moderately labile forms depending on soil conditions. However, micronutrients incorporated into crystalline lattices of clay minerals contained in the residual form (F4) are considered as biologically inactive or inert. The greater proportion of these micronutrients, in general, were associated with the occluded or residual form. Several researchers (Liang et al. 1991a, 1991b; Raza et al. 2002; Qian et al. 2003; Maqsood et al. 2016; Anderson et al. 2018) reported a similar trend of micronutrient distribution in agricultural soils of Canadian prairies.
Table 2.
Table 2. Descriptive statistics for chemically separable fractions (mg·kg−1) of copper (Cu), zinc (Zn), and boron (B) in 44 surface (0–15 cm) soils sampled from across the soil zones of the prairies.

Note: F1, soil solution-carbonate-exchangeable fraction (Cu and Zn) or specifically adsorbed fraction (B); F2, oxyhydroxide fraction; F3, organic-bound fraction; F4, residual fraction; F5, total concentration in soil. Values in parentheses are the coefficients of variation (%).

Relationship between soil properties and predicted bioavailability of micronutrient

The PCA was conducted to clarify the relationship between soil properties and availability of Cu, Zn, and B as predicted by soil test extraction and supply rate measurement in the soils (Fig. 1). The influence of soil properties on available concentration and supply of micronutrients was evident with several significant correlation coefficients and direct path coefficients (Tables 36). A significant negative correlation between DTPA-Cu and sand content was observed in the Brown and Dark Brown (r = −0.69; p < 0.01), and Black soils (r = −0.83; p < 0.01), indicating that lower available Cu concentrations tended to be associated with coarser-textured soils in comparison to clay soils (Fig. 1, Table 3). The supply rate of Cu was also negatively correlated with sand content in Brown and Dark Brown soils (r = −0.78; p < 0.01) but not in the Black soils (Table 4). Typically, the Cu retention is higher in clays and organic matter due to the high adsorption affinity. Sharma et al. (2004) found significant positive correlation between DTPA-Cu and clay content in soils of northwestern India. Generally, Cu-containing minerals that supply available Cu through weathering dominate in the clay size fraction (Liang et al. 1991a).
Fig. 1.
Fig. 1. Principle component analysis depicting the correlation among basic soil properties: pH, sand, organic carbon (% OC), and inorganic carbon (% CO3) with predicted availability (EX, extractable; HWS, hot water soluble) and supply (SR, supply rate) of copper (Cu), zinc (Zn), and boron (B) in contrasting soils collected from Brown and Dark Brown (14 soils), Black (22 soils), and Grey (8 soils) soil zones of the Canadian prairies. For Cu and Zn, diethylene–triamine–pentacetic acid extraction method was used for measurement of available concentration (mg·kg−1), and the resin membrane sandwich method was used for supply rate (μg·cm−2·24 h−1) measurements. Hot water extraction and in situ burial of PRS™-probes were used for available concentration and supply rate of B. [Colour online.]
Table 3.
Table 3. Pearson’s correlation coefficients for soil attributes as well as available soil extractable concentrations and supply rates of micronutrients assessed in prairie soils from different soil zones.

Note: EC, electrical conductivity (mS·cm−1); FC, moisture content at field capacity (%); OC, organic carbon (%); IC, inorganic carbon (%). Sand is reported as a percent (%). *, p < 0.05; and **, p < 0.01.

Table 4.
Table 4. Path coefficient analysis showing direct and indirect effects of soil properties on available soil extractable concentrations and supply rates of copper in prairie soils from different soil zones.

Note: OC, organic carbon; IC, inorganic carbon; Corr., correlation coefficient; RE, residual effect of path analysis. The values for RE apply to each set of variables under the “Extractable” and “Supply rate” headings, respectively. *, p < 0.05; **, p < 0.01. Underlined values indicate direct effect.

Table 5.
Table 5. Path coefficient analysis showing direct and indirect effects of soil properties on available soil extractable concentrations and supply rates of zinc in prairie soils from different soil zones.

Note: OC, organic carbon; IC, inorganic carbon; Corr., correlation coefficient; RE, residual effect of path analysis. The values for RE apply to each set of variables under the “Extractable” and “Supply rate” headings, respectively. *, p < 0.05; **, p < 0.01. Underlined values indicate direct effect.

Table 6.
Table 6. Path coefficient analysis showing direct and indirect effects of soil properties on available soil extractable concentrations and supply rates of boron in prairie soils from different soil zones.

Note: OC, organic carbon; IC, inorganic carbon; Corr., correlation coefficient; RE, residual effect of path analysis. The values for RE apply to each set of variables under the “Extractable” and “Supply rate” headings, respectively. *, p < 0.05; **, p < 0.01. Underlined values indicate direct effect.

There was a significant positive correlation between soil pH and supply rate of Cu in soils from the Brown and Dark Brown soil zone. A similar positive correlation between pH and DTPA-Cu for the agricultural soils of northern China was reported by Zang et al. (2015). Usually, the availability of Cu is higher at lower soil pH. However, the narrow range of pH values (6.05–7.33) in the current study may not allow the relationship between soil pH and available Cu to be most clearly revealed. The path analytic model did confirm that soil pH had a direct negative effect on extractable Cu in all the studied soils (Table 4). Although higher pH is associated with more negative charge and presumably greater Cu adsorption, the higher pH and negative charge may also promote retention of Cu in the system. Similarly, the sand content had a profound direct negative influence on available extractable concentration and supply rate of Cu in Brown and Dark Brown soils. In Grey soils, the extractable Cu was positively correlated with OC content and EC, but the supply rate did not show significant correlation with any of the soil properties. In the Grey soils located in more moist environments with greater leaching and higher sand content, the lower Cu availability could be related to fewer adsorption sites. High organic matter content is more important in providing exchange sites for adsorption and retention of Cu. Organic carbon had the highest positive direct effect on the supply rate of Cu in Grey soils. However, the indirect negative path coefficients of inorganic carbon had nullified the positive influence of OC on Cu supply rate in Grey soils. It could be related to adsorption of Cu by carbonate minerals. Positive correlation with EC may be a consequence of higher EC being reflective of less leaching and lower losses of micronutrient cations such as Cu.
The DTPA-extractable soil Zn was negatively correlated with soil pH (r = −0.54; p < 0.05) and positively correlated with OC content (r = 0.70; p < 0.01) in the Brown and Dark Brown soils (Table 3). Soil pH is an important determinant of how much Zn is available for plant uptake. Zinc is more readily available for plant uptake under acidic soil conditions. It is well understood that high soil pH results in increased adsorption of Zn by hydroxides and carbonate minerals, thereby decreasing mobility and bioavailability in calcareous soils (Alloway 2008; Lafuente et al. 2008). Our results showed the trend of negative relationship; however, it was insignificant due to slightly acidic to neutral pH range. The highest positive path coefficient indicated that OC also directly controlled the availability and supply of Zn in the Brown and Dark Brown soils. Zeng et al. (2011) found that DTPA-Zn was positively correlated with organic matter, and subsequently contributed in enhancing Zn bioavailability in paddy soils. In Grey soils, the extractable Zn showed negative correlation with sand content, which may be evidence of particular importance of cation release and retention by clay minerals. Moreover, higher leaching loss of Zn in sandy and low organic matter content soils is often associated with the wetter environment typical of the Grey soil zone. In addition, sand content exhibited the highest negative direct effect on availability and supply rate of Zn.
Hot-water-extractable B was positively correlated with pH, EC, and carbon content of Black and Grey soils (Table 3). A significant positive correlation between soil pH and hot-water-soluble B in Saskatchewan soils was also reported by Raza et al. (2002). Typically, B availability decreases with increasing pH due to favorable surface complexation reactions with organic and inorganic ligands (Lehto 1995). Earlier research (Bingham et al. 1971; Schalscha et al. 1973; Keren and Mezuman 1981; Keren et al. 1985; Goldberg and Glaubig 1986) reported that B adsorption increased as a function of soil solution pH in the range between pH 3 and 9 and then decreased in the pH range of 10–11.5. This does not explain the effect observed in the current study, as the pH range of soils was within the pH range where adsorption would be expected to increase, and availability decrease as pH increased. Our path coefficient results revealed that although soil pH showed significant positive correlation with HWSB, the greater indirect positive influence of soil carbon was mainly responsible for this outcome (Table 6). A significant negative correlation between extractable B and sand content was observed in the Black soils. Arora and Chahal (2014) also found a significant negative correlation between HWSB and sand content of alluvial soils in India. Similar to available Cu and Zn, sandy soil texture appears to be a characteristic associated with predicted low availability of B according to soil test. Again, the path analysis confirmed a positive direct effect of soil OC on extractable B of all of the soils, whereas pH had a higher negative effect on supply rate of B in the Black soil.

Correlation between soil properties and sequentially extracted micronutrient fractions

The coefficients for correlations between soil properties and different sequentially extracted fractions of Cu, Zn, and B are provided in Table 7. Among the soil attributes, soil carbon and sand content appeared to be most important in controlling the distribution of micronutrients among the different chemically separable pools. Total concentrations of Cu, Zn, and B were correlated positively with OC content of the Black soils. A positive significant relationship between soil moisture content at field capacity and total concentrations of these micronutrients was found in Brown, Dark Brown, and Black soils. This is likely due to higher organic matter and clay content, which tends to increase water-holding capacity of soil. Moreover, soil physical properties like soil structure could be a considerable factor for increased water-holding capacity of soils. Sand percentage was negatively correlated with residual and total fractions in the Brown, Dark Brown, and Black soils, pointing to the importance of the clay size fraction and mineralogical compositions in contributing relatively stable micronutrient bearing minerals that may only act as very long-term sources of micronutrient through weathering. In addition, oxyhydroxide- and organic-bound fractions of Zn showed significant positive correlation with OC content of Brown and Dark Brown, and Black soils. Earlier research (Cavallaro and McBride 1984; Davis 1984) reported that Zn can be more readily associated with the oxide minerals, whereas Cu interacts more strongly with the organic component of the soil. A significant negative relation between Cu concentration in soil solution-carbonate-exchangeable fraction and sand percentage was found in all studied soils. Therefore, the likelihood of Cu deficiency can be predicted to be increased in coarse-textured mineral soils, especially those that are low in organic matter. In prairie soils, clay percentage was reported to have a significant positive correlation with oxide bound and residual fractions of Cu (Liang et al. 1991a). The B fractionation results of the current study agree well with Raza et al. (2002) who did not observe significant correlations between OC content and B fractions, including organically bound fraction of Saskatchewan soils. Sandy texture may be equally or more important than low organic matter content as an attribute related to low B availability and fertility.
Table 7.
Table 7. Pearson’s correlation coefficient for relationships between soil attributes and different fractions (sequentially extracted) of copper (Cu), zinc (Zn), and boron (B) in prairie soils from different soil zones.

Note: F1, soil solution-carbonate-exchangeable fraction (Cu and Zn) or specifically adsorbed fraction (B); F2, oxyhydroxide fraction; F3, organic-bound fraction; F4, residual fraction; F5, total concentration in soil. *, p < 0.05; **, p < 0.01.

Spectroscopic speciation

The XANES spectra of Cu and Zn for selected soils and calculated fit results are presented in Figs. 2 and 3. The LCF results showed that Cu-carbonate and Cu-acetate were common Cu species in all studied soils. Copper methoxide was identified in five of eight soil samples. The best fit for the Zn K-edge XANES spectra was obtained by combining the ZnCO3, and Zn-sorbed montmorillonite spectra. These species were not consistent among soils from different soil zones and likely to be varied with soil physicochemical properties and total Zn concentration. However, ZnCO3 was dominant species in all studied soils. Several studies (e.g., McBride and Bouldin 1984; Rodriguez-Rubio et al. 2003; Ponizovsky et al. 2007) reported that carbonate and (hydr)oxide minerals can adsorb or immobilize micronutrient metals by providing a large reactive surface. For B, the K-edge XANES spectra of contrasting soils did not show distinguishing spectral features in the pre- and post-edge regions, possibly due to inherent low content in soil samples obtained from farm fields. However, the qualitative analysis identified that trigonal species [B(OH)3] predominate in all soils (data not shown). The bulk XANES analysis suggests that B speciation in agricultural soils depends more on total B concentration than soil chemical properties, or may reflect a lack of resolution power at the low levels found in these soils.
Fig. 2.
Fig. 2. (A) Normalized copper (Cu) X-ray absorption near edge structure K-edge spectra of selected agricultural soils (0–15 cm) from different soil-climatic zones of the Canadian prairies. (B) Results of linear combination fit showing the relative proportion of Cu species in soil samples. LCF, linear combination fitting. [Colour online.]
Fig. 3.
Fig. 3. (A) Normalized zinc (Zn) X-ray absorption near edge structure K-edge spectra of selected agricultural soils (0–15 cm) from different soil-climatic zones of the Canadian prairies. (B) Results of linear combination fit showing the relative proportion of Zn species in soil samples. LCF, linear combination fitting. [Colour online.]
X-ray absorption spectroscopy can provide detailed information about the structure and chemical forms of micronutrients that are present in soils under specific soil conditions. Evidence from the extended X-ray absorption fine structure spectroscopic model of Cu2+ and Zn2+ adsorption to calcite surfaces has suggested that mononuclear inner-sphere complexes were formed through substitution of these metals into Ca sites (Elzinga and Reeder 2002). They also reported that Zn adsorption complexes were in tetrahedral coordination, whereas Cu complexes were a Jahn Teller distorted octahedral coordination. In agricultural soils, Cu was preferentially speciated onto organic matter through bidentate inner-sphere coordination with carboxyl or amine groups (Strawn and Baker 2008). The Cu coordination at the calcite surfaces was very similar in presence or absence of dissolved humic acid, but the amount of Cu2+ adsorption decreased with increasing amount of humic acid (Lee et al. 2005). This could be due to the coating of humic substances on mineral surfaces that can block the adsorption sites (Lai et al. 2002). Furthermore, Cu interactions with organic matter may include formation of chelate ring structures with amino, carboxyl, or carbonyl functional groups (Karlsson et al. 2006). Lee et al. (2004) found that local coordination of Zn2+ at montmorillonite surfaces was mostly octahedral. Zinc was also found to be precipitated as Zn–Al layered double hydroxide or Zn-phyllosilicate with increased surface loadings in contaminated soils (Ford and Sparks 2000). Micronutrient metal adsorption onto carbonate and aluminosilicate clay minerals is governed by soil pH and redox conditions, and changes in these conditions as related to crop production practices can significantly alter speciation and solubility. The spectroscopic speciation work conducted in this study supports the importance of Cu association with organic matter and the interaction of Zn with clay minerals, and carbonate forms significant for both. Boron was more difficult for spectroscopic identification of dominant species.
Overall, the sequential extractions revealed that the smallest proportion of Cu and Zn were speciated onto soil solution-carbonate-exchangeable fraction, which are often related to the active carbonate content of soils. Crop management practices including fertilization regimes play a vital role in generating active carbonate in soils. While the spectroscopic speciation mostly identifies the pedogenic carbonate related to parent materials and carbonate minerals such as calcite, aragonite, and dolomite. The combination of sequential and speciation can better explain the fate of applied fertilizer. For example, if the added Cu and Zn are adsorbed by the active carbonates, it can be reversible for plant uptake based on the changing soil equilibrium and moisture conditions. However, over the longer period of time, if the Cu and Zn are speciated onto pedogenic carbonate through inner-sphere complexations, it may not be available for plant utilization.

Conclusion

This study revealed that according to the soil test extraction critical deficiency criteria used for prairie soils, among the soils evaluated, Cu deficiency was most prevalent, with 20% of the Black and Grey soils identified as deficient in Cu, 10% of all soils deficient in Zn, and none of the soils deficient in B. Lower availability of Cu, Zn, and B is associated with sandy soil texture and low OC content of soils. Sequential extraction revealed that labile fractions occupied a relatively small proportion of total micronutrient, with the residual fraction dominant, followed by organically bound. XANES speciation identified carbonate associated forms of Cu and Zn to be predominant. These forms likely reside mainly in the recalcitrant residual fraction. The knowledge generated from this study will provide a better understanding of the fate of soil-applied fertilizer, helping to reveal how the applied nutrients will be speciated onto different pools over a growing season or a longer period of time.

Competing Interests Statement

There is no conflict of interest.

Contributors’ Statement

Noabur Rahman and Ryan Hangs conducted sample analysis and wrote the manuscript. Derek Peak helped in XANES data collection and analyses. Jeff Schoenau supervised the project and reviewed the manuscript.

Funding

This work was financially supported by the Western Grains Research Foundation and the Agriculture and Agri-Food Canada Agri-Innovation Program.

Acknowledgements

We gratefully acknowledge the financial support of the Western Grains Research Foundation and the Agriculture and Agri-Food Canada Agri-Innovation Program. We appreciate the help of Ning Chen, Weifeng Chen, Lucia Zuin, and Dongniu Wang for technical assistance with the hard X-ray micro analysis and variable line spacing planar grating monochromator beamlines during data collection at Canadian Light Source. We wish to sincerely acknowledge the valuable contribution of the reviewers regarding the improvement of this manuscript.

Footnote

1
Supplementary data are available with the article at https://doi.org/10.1139/cjss-2020-0162.

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Canadian Journal of Soil Science cover image
Canadian Journal of Soil Science
Volume 101Number 4December 2021
Pages: 581 - 595
Editor: M. Anne Naeth

History

Received: 31 December 2020
Accepted: 12 April 2021
Published online: 13 May 2021

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Key Words

  1. soil properties
  2. bioavailability of micronutrients
  3. sequential extraction
  4. correlation
  5. XANES

Mots-clés

  1. propriété du sol
  2. biodisponibilité des micro-oligoéléments
  3. extraction séquentielle
  4. corrélation
  5. XANES

Authors

Affiliations

Noabur Rahman mdr422@usask.ca
Department of Soil Science, University of Saskatchewan, Saskatoon, SK S7N 5A8, Canada
Ryan Hangs
Department of Soil Science, University of Saskatchewan, Saskatoon, SK S7N 5A8, Canada
Derek Peak
Department of Soil Science, University of Saskatchewan, Saskatoon, SK S7N 5A8, Canada
Jeff Schoenau
Department of Soil Science, University of Saskatchewan, Saskatoon, SK S7N 5A8, Canada

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