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Introduction
Zn is essential for growth, development, and immune function maintenance (Read, Obeid, Ahlenstiel, and Ahlenstiel, 2019). Its deficiency can disrupt crucial biophysicochemical processes involved in normal plant function and detoxification under stress conditions (Sousa, Lopes, Fernandes, and Ramos, 2009; Hafeez, Khanif, and Saleem, 2013; Noman et al., 2019).
Zn deficiency afflicts around a quarter of the global population, specifically in Latin America, Africa, and South Asia, due to the impoverished fertility of soils in these regions (Bortolon and Gianello, 2009; Mossa et al., 2021). Understanding the complex behavior of Zn in soils, dictated by natural and human-influenced factors, poses challenges in grasping its retention, bioavailability, and mobilization (Alexakis, 2010).
Soil properties, like pH, texture, organic carbon (OC) content, calcium carbonate (CaCO3), total Zn (T-Zn) concentration, and the presence of competing cations, impact the distribution of Zn among various forms: E-Zn, carbonate-bound (Zn-CO3), organic matter-bound (OM-Zn), O-Zn, and R-Zn forms (Liu et al., 2020). The proportions of these forms are influenced by factors such as salinity level (EC), soluble salt type, ionic strength, and kinetic effects (Kabata-Pendias, 2001; Moreno-Lora and Delgado, 2020; Salinitro, Van Der Ent, Tognacchini, and Tassoni, 2020; Mohiuddin et al., 2022).
Bioavailable Zn includes soluble Zn (Zn2+) and E-Zn. Its concentration is generally low, ranging from 0.1 to 2 mg kg-1 in soil, representing a small percentage of T-Zn (Natasha et al., 2022). Zn deficiency in plants is mainly attributed to its low solubility and high retention in the solid phase of the soil, where a significant portion (30 to 60% of T-Zn) exists in an unattainable form, especially attached to mineral colloids (Alonso, Arias, Fernandez, Fernandez, and Serrano, 2006; Natasha et al., 2022). Factors contributing to Zn deficiency include excessive phosphorus, high pH, and concentrations of Na+, HCO3 -, and CO3 2- in the soil solution (Mousavi, Galavi, and Rezaei, 2012; Natasha et al., 2022).
Soils in arid regions, classified as Aridisols and Entisols, pose unique challenges, with alkaline pH, meager organic matter (OM) content, and accumulation of CaCO3, gypsum, and soluble salts. T-Zn concentrations in arid soils typically span between 41 to 130 mg kg-1, with E-Zn varying from 0.24 to 1.28 mg kg-1 (Praveen-Kumar, Tarafdar, Soni, and Mahesh, 2009). Zn speciation, influenced by factors like pH, displays variations like ZnCO3, ZnSO4, ZnPO4, and ZnCl+ at different pH ranges (Sadiq, 1991).
Extensive research has explored the field of Zn speciation in different soils and climates, encompassing calcareous soils (Sadiq, 1991), Vertisols (Dang, Tilled, Dalal, and Edwards, 1996), Regosols (Korchagin, Moterle, Escosteguy, and Bortoluzzi, 2020), Fluvisols (Ruzicic and Rako, 2017), paddy soils (Weiss et al., 2021), Chernozems (Bauer et al., 2019), Alfisols (Chakraborty, Chidanandappa, Dhananjaya, and Padhan, 2016), hydromorphic soils (Azouzi, Charef, and Hamzaoui, 2015), and saline soils (Mohiuddin et al., 2022). Nonetheless, the comprehension of Zn conduct in soils characterized by the simultaneous existence of gypsum, calcium carbonate, and soluble salts remains limited.
The research aims to investigate Zn speciation in soils from the Biskra region, located close to the Algerian desert and known for its advanced agriculture, particularly, date palm cultivation (Benmehaia, 2019). Focusing mainly on soils characterized by the accumulation of gypsum, CaCO3, and soluble salts, this study aims to explore the impact of these specific soil characteristics on Zn dynamics and bioavailability. The research involves an in-depth assessment of Zn distribution across various fractions, including exchangeable, carbonate-bound, organic matter-bound, oxide-bound, and residual forms. This analysis will provide valuable information on the factors governing the mobility, retention, and bioavailability of Zn to plants in arid soils. The results of this study will contribute significantly to the development of effective strategies to improve Zn availability to plants in arid regions, in order to effectively combat Zn deficiency in agricultural systems.
Materials and Methods
Main characteristics of the study area
Biskra is classified as a hyperarid climate with a mild winter in the bioclimatic stage (NATP, 20031; GFOA, 20192). Summer temperatures in this locale are scorching, averaging around 40 °C, while winter temperatures generally remain at approximately 28 °C. Precipitation in Biskra is notably scarce, measuring a mere 139.8 mm per year, and its infrequent occurrence is irregular (Faci, 20213).
In terms of pedoclimate, this region exhibits an arid humidity regime coupled with a hyperthermal temperature regime. The soils of Biskra manifest a diverse composition, predominantly comprising accumulations of CaCO3, gypsum, and saline deposits. Interestingly, in certain instances, two or three of these salts accumulate concurrently in the soil profile (Khechai, 20014; Bensaid, 1999).
Soil sampling
Ten profiles representative of the region (Figure 1) were selected based on various criteria, including the type and form of saline accumulations (CaCO3, gypsum, soluble salts), land use, natural vegetation, and the presence of eolian sand deposits on the surface. A total of 28 soil samples were taken from each horizon of the different profiles to a depth of 120 cm. Sampling took place in June 2021, with the aim of describing the main aspects of each profile.
Soil analysis
The soil samples were analyzed af ter being air-dried, crushed, and sieved to 2 milimeter.
We measured the pH of the soil solution with pH meter using a diluted 1: 2.5 extract, and the EC was determined by conductometry using a diluted 1: 5 extract. The total CaCO3 content was determined using the Bernard calcimeter method, and the gypsum content was determined gravimetrically (Mathieu and Pieltain, 2003; Bashour and Sayegh, 2007).
The Anne method was used to measure the quantity of OC; OM content was determined using the OC rate (Mathieu and Pieltain, 2003).
Af ter pretreating the samples to eliminate gypsum, we quantified the different particle size fractions using the universal Robinson pipette method (Mathieu and Pieltain, 2003). Sequential extraction was used to determine the different forms of Zn (Table 1), (Tessier, Campbell, and Bisson, 1979).
Table 1: Sequential extraction of Zn (Tessier, Campbell, and Bisson, 1979).
| Fractions extracted | Reagent(s) | Volume of extraction solution | Quantity of soil | Agitation time and temperature |
|---|---|---|---|---|
| E-Zn | MgCl2 (1 M), pH=7 | 8 mL | 1 g | 1 hour at room temperature |
| Zn-CO3 | CH3COONa (1M)/CH3COOH, pH=5 | 8 mL | Aliquot of the exchangeable fraction | 5 hours at room temperature |
| O-Zn | NH2OH•HCl (0.04 M) in 25% CH3COOH, pH = 2 | 20 mL | Aliquot of the carbonate-bound fraction | Occasional agitation for 6 hours at 96°C |
| OM-Zn | HNO3 (0,02M)/ H2O2 (30%) at pH =2 (3.2 M) NH4OAc dans 20% HNO3 | 3 mL HNO3, 8 mL H2O2, 5 mL NH4 COOH 20 mL dilution | Aliquot of the oxide-bound fraction | Occasional agitation for 5 hours at 85°C, followed by cooling and continuous agitation for 30 minutes at room temperature |
| R-Zn | HClO4-HF/ HCl | 4 mL-20 mL 25 mL dilution | Aliquot of the organic matter-bound fraction | Alternatively, drying in an oven at 30 ± 2°C for 48 hours |
The extraction process of different forms of Zn is conducted on the same aliquot in the order listed in Table 1. It is important to note that no pretreatment is performed on the soil samples prior to the extraction process, thereby ensuring the extraction of Zn quantities in the presence of all types of salts.
Results and Discussion
Characteristics of the studied samples
Table 2 shows the statistical characteristics of the studied samples, including alkaline pH values and high salinity, indicating their characteristic conditions. The samples exhibit moderate calcareousness with varying levels of gypsum content, low OC content, and predominantly sandy texture with some silt. The analyzed parameters display significant variability, reflecting local variations that can influence Zn speciation. The selected profiles accurately represent the soils in the study area, making them suitable for conducting this research.
Table 2: Statistical characteristics of the studied samples.
| Parameter | Minimum | Maximum | Mean | Standard Deviation | CV* |
|---|---|---|---|---|---|
| % | |||||
| pH | 7.48 | 8.56 | 7.80 | 0.31 | 4 |
| EC dS m-1 | 1.54 | 30 | 7.05 | 7.11 | 101 |
| CaCO3 % | Traces | 24.4 | 9.5 | 5.7 | 60 |
| CaSO4.2H2O % | 1.5 | 87.5 | 36.6 | 23 | 63 |
| Organic Carbon % | 0.06 | 1.17 | 0.52 | 0.34 | 70 |
| Clay % | Traces | 28 | 10 | 11.6 | 117 |
| Silt % | 0.5 | 37 | 16 | 8.8 | 55 |
| Sand % | 25.4 | 77.5 | 54.5 | 17.8 | 33 |
Zn Content in different forms
The studied soils show a notable Zn deficiency compared with Loué (1993) standards, with low T-Zn contents (0.41 mg kg-1 < T-Zn < 4.90 mg kg-1), which translates into low quantities of the other forms of Zn. Soil E-Zn content is extremely low (0.01 < E-Zn mg kg-1 < 0.11), around one-tenth of Loué’s (1993) standards. Similarly, concentrations of the other forms of Zn are very low, at around 0.03 mg kg-1 for Zn-CO3, 0.11 mg kg-1 for OM-Zn, 0.79 mg kg-1 for O-Zn, and 1.21 mg kg-1 for R-Zn. Despite their derisory quantities, the different forms of Zn show significant variations between samples (46% < CV < 230%), probably due to the variability of the soil parameters that govern Zn speciation. These results align with the findings of Sandstead (2015), Noulas, Tziouvalekas, and Karyotis (2018), Rahman, Hangs, Peak, and Schoenau (2021), and Tolay (2021), who showed that Zn deficiency is common in soils with high pH, alkalinity, abundant CaCO3 content, coarse texture, and low OM content.
The data in Table 3 highlight the glaring inequality of Zn distribution in the studied soils. R-Zn (54.81%) and O-Zn (35.9%) predominate, accounting for over 90% of T-Zn. The remaining forms are extremely weak, with only 5.26% for OM-Zn, 2.55% for E-Zn, and only 1.45% for ZnCO3. Consequently, the hierarchy of Zn form distribution in these soils is R-Zn > O-Zn > OM-Zn > E-Zn > ZnCO3. These results agree with those of Milivojević, Nikezic, Krstic, Jelic, and Dalovic (2010) in Vertisols, Mohiuddin et al. (2022) in saline soils, and Kabala and Singh (2001) in calcareous soils. Notably, Chlopecka, Bacon, Wilson, and Kay (1996) also highlighted the dominance of R-Zn and O-Zn in the soils they studied, while Hashemi and Baghernejad (2009) attribute the abundance of R-Zn in gypsum soils to its strong adsorption by palygorskite (Girija, Rattan, and Datta, 2013).
Table 3: Statistical characteristics of Zn forms content in soils.
| Forms of Zn | Minimum | Maximum | Mean | Standard Deviation | Content form/T-Zn content | CV |
|---|---|---|---|---|---|---|
| - - - - - - - - - - - - - mg kg-1 - - - - - - - - - - - - - | - - - - - - - % - - - - - - - | |||||
| E-Zn | 0.01 | 0.11 | 0.06 | 0.03 | 2.55 | 47 |
| ZnCO3 | 0.00 | 0.27 | 0.03 | 0.05 | 1.45 | 171 |
| OM-Zn | 0.00 | 0.83 | 0.12 | 0.27 | 5.26 | 230 |
| O-Zn | 0.05 | 3.24 | 0.79 | 0.74 | 35.9 | 94 |
| R-Zn | 0.24 | 2.82 | 1.21 | 0.63 | 54.81 | 52 |
| T-Zn | 0.41 | 4.90 | 2.21 | 1.03 | 100 | 47 |
The presence and limited proportions of OM-Zn can be attributed to extremely low OC contents, particularly in subsurface and deep horizons.
Effect of depth on Zn variation
The one-way ANOVA analysis used to assess the depth factor’s impact on variations in the concentrations of various forms of Zn showed non-significant results, indicating that these concentrations vary independently along the horizon (depth).
Relationship between EC, gypsum, and different forms of Zn
The soil samples that were studied were characterized by extreme variations in salinity (EC) (CV = 101%) and gypsum content (CV = 63%). This prompted research into the relationship between these two factors and the different forms of Zn. As a first step, simple regressions were used, the entire data set was analyzed and then the means of Zn forms were compared between homogeneous classes of CE and gypsum. The results of the simple regressions, examining the relationship between Zn forms with EC and gypsum rates, are reported in Table 4.
Table 4: Correlation coefficients (r) between EC, gypsum, and Zn forms.
| E-Zn | ZnCO3 | OM-Zn | O-Zn | R-Zn | |
|---|---|---|---|---|---|
| EC dS m-1 | - 0.254 | - 0.160 | - 0.363 | - 0.290 | 0.682** |
| Gypsum% | 0.179 | - 0.367 | - 0.130 | - 0.542** | - 0.190 |
**significance threshold α < 0.01
Table 4 shows that the correlations between EC on the one hand and E-Zn, ZnCO3, OM-Zn, and O-Zn on the other are weak, negative, and statistically insignificant (P > 0.05). However, these results show that although this correlation is not statistically significant, it does suggest that increasing EC is associated with a slight decrease in the levels of these Zn species. This result agrees with those of Keshavarz, Malakouti, Karimian, and Fotovat, (2006), who asserted that EC negatively influences the diversity and distribution of Zn in calcareous soils. Similarly, Khoshgof tar et al. (2004) demonstrated that T-Zn and E-Zn concentrations decrease with increasing EC. In contrast, Table 4 reveals that the correlation between EC and R-Zn is strong, positive, and statistically highly significant (r = 0.682; P < 0.01), indicating that R-Zn contents increase with EC. This result may be linked to the retention of Zn in mineral silicate networks and its low solubility due to soil alkalinity (Pierangeli, Guilherme, Oliveira, Curi, and Silva, 2003; Girija et al., 2013) and salinity. These results are exactly in line with those of Mohiduddin et al. (2022), who showed that the R-Zn is the dominant form of Zn under saline conditions.
Table 4 also reveals weak, statistically insignificant correlations between gypsum and E-Zn, ZnCO3, OM-Zn, and R-Zn (r < 0.3; P > 0.05). Except for the E-Zn form, which is positively correlated with gypsum (r = 0.179), all Zn forms decrease with increasing gypsum values. Lombnæs, Chang, and Singh (2008) reported that the increase in ionic strength caused by the abundance of exchangeable Ca from gypsum could be responsible for the decrease in Zn sorption, which explains the relationship between E-Zn and gypsum. Shukla and Mukhi (1980) have also shown that gypsum enriches the soil solution in Ca2+ ions, increasing the Ca:Na ratio and thus promoting Zn availability. On the other hand, Table 4 reveals that the correlation between gypsum and O-Zn is statistically highly significant and negative (r = -0.542; P < 0.01), describing the inverse relationship between gypsum abundance and O-Zn content. Ma et al. (2020) showed that Zn hydroxides and Zn sulfates represent a significant proportion of Zn-containing species in high pH gypsum soil samples, which is the case of the soils studied in this research.
Comparison of the means of Zn Forms according to EC and gypsum classes
To achieve this, a one-way analysis of variance was performed. The results of this analysis reveal three homogeneous classes for CE (dS m-1) and three homogeneous classes for gypsum (%), as shown in Table 5.
Table 5: EC and gypsum classes.
| Parameter | Classes | Intervals | Number |
|---|---|---|---|
| EC (dS m-1) | 1 | EC dS m-1 < 6 | 19 |
| 2 | 6 ≤ EC dS m-1 < 15 | 5 | |
| 3 | EC dS m-1 ≥ 15 | 4 | |
| Gypsum (%) | 1 | Gypsum % < 25 | 9 |
| 2 | 25 ≤ Gypsum % < 60 | 16 | |
| 3 | Gypsum % ≥ 60 | 3 |
It is noteworthy that group sizes exhibit significant variability for both CE and gypsum levels. Given this observation, a t-test was performed to compare the means of each Zn form within the three CE classes initially and subsequently between the three gypsum rate classes.
Comparison of mean values of Zn forms according to CE classes
According to the analysis presented in Table 6, there were no significant differences (P > 0.05) in the means of the E-Zn, ZnCO3, and OM-Zn forms between the three EC classes. This suggests that variation in EC levels has no significant influence on these forms of Zn in the studied samples. The low pH of the extraction solution used for extracting OM-Zn and the high ionic strength resulting from high Ca2+ concentrations could explain this result, by reducing carbonate solubility and subsequently decreasing ZnCO3 contents, as noted by Keshavarz et al. (2006).
Table 6: Comparison of means (p-values) of Zn forms according to CE classes.
| EC classes | E-Zn | ZnCO3 | OM-Zn | O-Zn | R-Zn |
|---|---|---|---|---|---|
| 1 × 2 | 0.344 | 0.536 | 0.934 | **0.004 | **0.001 |
| 2 × 3 | 1 | 0.948 | 0.590 | 0.604 | 0.240 |
| 1 × 3 | 0.392 | 0.817 | 0.298 | 0.103 | 0.183 |
As far as the O-Zn and R-Zn forms are concerned, there are significant differences (P < 0.05) only between classes 1 and 2. However, the differences in means between classes 2 and 3, as well as between classes 1 and 3 are not statistically significant (P > 0.05).
Overall, variations in EC classes have little impact on the levels of the various forms of Zn in the studied samples. However, it should be noted that EC class 2, corresponding to a salinity range of 6 dS m-1 to 15 dS m-1, appears to represent a critical threshold for variation in mean O-Zn and R-Zn contents. This finding is in line with the observations of Keshavarz et al. (2006), who stated that high salinity levels, around 15 dS m-1, affect O-Zn and R-Zn concentrations.
Comparison of means of Zn forms according to gypsum classes
According to Table 7, there are no statistically significant differences (P > 0.05) in the means of ZnCO3, OM-Zn, and R-Zn forms among the different classes of gypsum content. These findings suggest that the variation in gypsum content within the studied samples does not have a significant impact on the variation of these forms of Zn. However, elevated quantities of gypsum do exert a notable influence on both E-Zn and O-Zn. The difference in means of E-Zn between gypsum class 1 (gypsum < 25%) and class 2 (25 ≤ gypsum% < 60) is not statistically significant (P > 0.05). However, the differences in means of E-Zn between class 3 (gypsum > 60%) and classes 1 and 2 are significant (p < 0.05). Similarly, the difference in means of O-Zn between class 3 (gypsum > 60%) and class 1 is also significant (P < 0.05).
Table 7: Comparison of means (p-values) of Zn forms according to gypsum classes.
| Gypsum Classes | E-Zn | Zn-CO3 | OM-Zn | O-Zn | R-Zn |
|---|---|---|---|---|---|
| 1 × 2 | 0.996 | 0.908 | 0.493 | 0.495 | 0.899 |
| 2 × 3 | < 0.01** | 0.909 | 0.566 | 0.091 | 0.145 |
| 1 × 3 | < 0.01** | 0.990 | 0.966 | 0.027* | 0.107 |
These results suggest that the variation in gypsum content does not significantly impact the different forms of Zn in the studied samples, except for E-Zn and O-Zn when gypsum levels are excessive (gypsum > 60%). Elrashidi et al. (2010) observed that low concentrations of gypsum negatively affect the bioavailability of Zn in the soil. They found that gypsum quantities between 1% and 30% had no effect on Zn bioavailability, while quantities between 30% and 50% significantly increased the concentration of E-Zn. The authors attributed this result to the dissolution of Zn-containing minerals due to the high presence of sulfate and the acidity of the extraction solution. This observation aligns with the findings of Lindsay (1979) and previous studies.
Conclusions
Soil analysis conducted in the arid environment of the Biskra region reveals a conspicuous deficiency in T-Zn. The manifestation of Zn deficiency is evident in the low levels observed for the five studied forms. The distribution of Zn forms in the studied soils is predominantly governed by R-Zn and O-Zn, while the diminished levels of OM-Zn stem from the low proportions of OC in the subsurface and deep horizons. The hierarchical order of Zn form distribution in these soils is as follows: R-Zn (55%) > O-Zn (36%) > OM-Zn (5%) > E-Zn (3%) > ZnCO3 (1%).
The results indicate that EC and gypsum levels have no significant influence on the variation of Zn concentrations in the samples studied, except for R-Zn, which is influenced by EC (r = 0.682; P < 0.01), and O-Zn, which is influenced by gypsum levels (r = -0.542; P < 0.01). Overall, the comparison of the means of the different Zn forms between the three EC classes and the three gypsum content classes corresponds well with the previous results, confirming that, in general, the differences in means for Zn forms between EC and gypsum classes are statistically insignificant (P > 0.05), except O-Zn and R-Zn between EC classes 1 and 2, Zn-E between gypsum classes 1 and 3, and 2 and 3, and O-Zn between gypsum classes 1 and 3.
This study highlights that EC and gypsum rates have different impact on Zn forms in arid soils. Although the comparison of means for different Zn forms between EC and gypsum classes has revealed some minor differences, it overall confirms these findings. These results expand our knowledge of Zn speciation and availability in arid soils. Nevertheless, they emphasize the need for further research in order to acquire a deeper comprehension of this phenomenon and focus on its effect on Zn biogeochemical processes in such soils.
