Trace Metal Content and Availability of Essential Metals in Agricultural Soils of Alicante (Spain)

: This article analysed the presence of trace metals (essential elements and pollutants) and soil properties in rural abandoned soils of a Mediterranean region. The soil properties determined were texture, pH, equivalent calcium carbonate, soil organic matter, availability of micronutrients (Fe, Mn, Cu, and Zn), and acid microwave digestion extraction to measure the trace metals considered as main pollutants (Cu, Zn, Cd, Cr, Ni, and Pb). Descriptive statistics and correlations were used to determine the relations among these parameters. pH, soil organic carbon (SOC), and clay were the main properties that controlled the availability of essential metals. pH was the main factor related to these metals in these calcareous soils. However, SOC, which can be incremented by adding organic fertilizers for soil rehabilitation as a sustainable practice, played an important role. Mean values of the metal composition in soils (Cu, Zn, Cd, Cr, Ni, and Pb) obtained in this study were similar to values reported for other areas in the Mediterranean basin.


Introduction
Agriculture has been practiced for a long time in Mediterranean regions. Intensive agriculture and irrigation have been developed in the inner valleys between ranges and coastal areas. In mountainous areas and piedmonts, dry farming was the main type of cultivation for centuries, and soils were fertilized using organic amendments (i.e., manure). Nowadays, these regions are suffering from the abandonment of these traditional activities and the consequent soil degradation [1].
European policy has the aim of improving and maintaining sustainable agriculture and recovering the landscapes of mountain regions. Therefore, checking the soils to provide information about the soil quality status and developing correct environmental policy is important [2].
It is well known that soil quality is usually associated to the organic matter content in the soils. Concerning the physical properties and the soil organic carbon, organic matter amendments (an indicator that belongs to the environmental and economic pillar of sustainability) can increase water holding capacity, soil porosity, water infiltration, and percolation while decreasing soil crusting and bulk density [3][4][5]. Nowadays, the use of organic wastes as a source of soil organic matter has increased in Europe towards a circular economy [6]. A major assessable effect of the repeated application of

Materials and Methods
In this work, one hundred agricultural soil samples were taken from different abandoned cultivation plots in the northwest of the province of Alicante (Spain) and analysed to check their quality based on trace metal composition (potential harmful elements), micronutrient availability, and soil properties (texture, pH, organic matter, and calcium carbonate content). The parcels were located in an area that is 24 km 2 (Figure 1), corresponding to fluvial terraces, and one sample was taken from each plot, approximately equally distributed in the area (based on a regular grid of the parcels), equivalent to four/five samples taken per square kilometre. The climate is Mediterranean (Csa, Köppen-Geiger climate classification) with a mean annual temperature of 15 • C and average rainfall of 500 mm, and the altitude is about 600 m above sea level. This area is in the upper valley of the Vinalopó River, and limestone neogenic sediments, consisting of sands, gravels, ridges, silts, and clays, determine the lithology. Soils present in the area are predominantly calcaric Fluvisols [27].
Soil samples (arable layer, first 20 cm) were taken and analysed, determining soil characteristics: texture by the Bouyoucos method [28] indicating the proportion of size particles (sand, silt, clay); pH 1:2.5 w/v water extract [29]; equivalent calcium carbonate content (CO 3 ) using the Bernard calcimeter [30]; and soil organic carbon (SOC) by the Walkley-Black method [31]. The availability of essential metals (Cu, Fe, Mn, and Zn) were determined by extracting with DTPA, as it is a commonly used method to determine this property [32][33][34], and measured in the extract by atomic absorption spectrometry. The content of trace elements (Metal-T) that can be considered as environmental pollutants (Cd, Cr, Cu, Ni, Pb, and Zn) was measured in soil samples after microwave acid digestion with HNO 3 -H 2 O 2 [35,36], quantifying the concentration of metals by ICP-OES analysis. Statistical analyses were performed using SPSS v. 24.0© software. Descriptive statistical parameters (mean, standard deviation) for soil data were calculated. Moreover, bivariate correlation (Pearson) analysis was applied among all the variables analysed; linear multiple regression analysis was used to learn the influence of soil properties on extractable and metal content in soils [37]. Table 1 shows the descriptive statistics of soil properties and trace elements of the one hundred soil samples.

Results
The texture of the soils varies between sandy-loam and clay-loam, which may depend on the relative position of the soils close to the Vinalopó River, which crosses through the centre of the area from NW to SE. These soils are calcareous soils, with an important content of equivalent calcium carbonate (33%) and the pH of all of the samples was basic (mean value, 8.6).
The abandoned agricultural soils have low SOC contents and extractable micronutrients, which can be associated to degradation processes (e.g., reduction of the input or organic matter and oxidation of SOC, precipitation of metal-carbonates, losses of nutrients by run-off or leaching, among others). Fe was the major micronutrient extracted, followed by Mn, Cu, and Zn. The relative contents of the available microelements, expressed as individual percentages of all of them extracted with DTPA, were Fe (37.6%), Mn (35.1%), Cu (16.9%), and Zn (10.4%). The mean values of the metal composition determined by acid microwave digestion (Cd, Cr, Cu, Ni, Pb, and Zn) of the soils followed this decreasing order: Zn > Cr > Ni > Pb ≈ Cu > Cd ( Table 1). The relative content of each element, considering a pool of all of them, was Zn (30.5%), Cr (22.5%), Ni (18.3%), Pb and Cu (14.2%), and Cd (0.3%). These soils had a very low concentration of Cd, which is positive considering its high potential toxicity [38]. The concentrations of these metals for the area studied are in accordance with the values given for total concentration in topsoil in the GEMAS (Geochemical Mapping of Agricultural and Grazing Land Soil, http://www.eurogeosurveys.org/ projects/gemas/) project, although the scale and detail of sampling of GEMAS and this study were different. We compared the values obtained in this study with the limits given in GEMAS for the area where the soils are located ( Table 2). Table 2. Comparison between mean values of M-T in topsoil of this study and those given in GEMAS (interval of the geochemical baseline of maps in: http://www.eurogeosurveys.org/projects/gemas/). The main differences can be observed for Cr and Pb, where the values in this area are under the values given in GEMAS. The extraction method employed to determine the composition (nitric acid digestion in microwave) might be responsible in part. However, the scarcity of sources of pollution (industry and transport) for both trace metals in the area may be another important factor to consider.

Zn-T (mg/kg)
In general, for all the parameters considered, low standard deviation values were obtained, especially for total composition of trace metals. This may indicate that the metal concentrations are closely related to the geochemical composition of the parent material of the soils (quaternary sediments) [36]. For instance, the high Ni, Cd, and Cr mean values in agricultural soils in Thiva have been ascribed to local parent materials enriched with these specific metals [39]. Mean levels of the total metal concentrations obtained in this study are similar to the corresponding values reported by other authors in Mediterranean environments, as indicated in Table 3 [39][40][41][42][43][44].
In comparison with the Canadian, Dutch, and Spanish guideline values for these pollutants in agricultural soils (Table 4), concentrations of all metals analysed are always lower than the threshold value established.
The bivariate correlations (Pearson's R) among all the parameters were determined among the soil characteristics (Appendix A). Negative correlations between pH and Fe-DTPA, Mn-DTPA, and Cu-DTPA were observed. Consequently, an increase of the pH of these soils, i.e., applying a basic amendment, could diminish the availability of these micronutrients for plants. There is also a positive correlation between Cu-T and Cu-DTPA. The concentration of pollutants of these soils, for most of the metals analysed, was related positively or negatively with clay, silt, pH, CaCO 3 equivalent (CO 3 ), and SOC.   After analysing the results of bivariate correlations, a multiple correlation analysis was carried out between each of the metals (micronutrients extracted with DTPA and metal concentration after acid microwave digestion Metal-T) and the characteristics of the soils. The results of the Pearson test give significant differences, especially for soil pH.

Cd-T Ni-T Pb-T Cr-T Cu-T Zn-T Reference
Most of the trace elements analysed, available essential metals or concentration of pollutants, are correlated with the soil pH. The sorption/desorption reactions of metal(loid)s on/from soil sorbents are influenced by pH, the nature of soil components, and the presence and concentrations of cations and inorganic anions. In general, low pH can favour metal mobility, affecting variable charge of the surfaces, whereas metal cations are fixed more easily at high pH [46]. Moreover, a high pH favours the precipitation of metal-carbonates [47]. Table 5 shows the equations obtained, and the coefficients R and F. pH, SOC, clay, and CO 3 were closely related to the content of pollutants (extracted by acid microwave digestion). The most significant values (R and F) were obtained for the multiple regressions obtained for Ni-T and Zn-T. However, Ni-T was associated to Clay and Zn-T was to SOC as secondary important factors in these multiple regression tests, after which the pH was the most important for both metals.

Discussion
In these calcareous soils, it is important to notice that the pH seemed to be the most important factor controlling micronutrient availability, although the chemical form of the metal in the soil is also important for understanding the metal uptake by plants [48]. Negative correlation coefficients with the pH were found for Fe, Mn, and Cu (DTPA extraction), but it was positive for Zn. However, as several researchers report, it is important to consider the influences of other soil properties, such as the case of Zn [49,50]. In this work, clay and silt were related, as multiple regression analysis showed, along with Zn.
The scarcity of organic matter may be, in part, responsible for the great pH influence, because its content seems to be low enough to have poor influence (associated to the abandonment of traditional agricultural practices without the addition of organic inputs).
The SOC of these soils can be incremented, adding organic amendments, which is a sustainable practice based on traditional agriculture. The addition of organic amendments may improve the physical, chemical, and biological properties of soil [51]. However, it should be taken into account that soil organic carbon can control the metal content and availability, and this strategy can move pollutants with environmental risk.
High levels of metals in soils increase their uptake by plants, which depends not only on metal content in soils but also is determined by the soil pH value, organic matter, and clay contents, and is influenced by fertilization and the use of pesticides [21,52]. Micó et al. (2006) [41] found, using 54 samples of agricultural soils throughout the province of Alicante (surface of 5816 km 2 ), a significant correlation between lithogenic metals and some soil properties, such as soil organic matter, clay content, and carbonates. This previous work is very relevant and is centred on finding the baseline of soil metal concentration. Starting from this point, the present study paid attention to the next step, i.e. what would happen if we considered soils for restoration and change the soil properties.
Although the pH is in fact more important in this case, as statistical results indicated, the variation of pH and a possible decrease of this by applying organic amendments, may favour the mobilization of pollutants in these calcareous soils. The choice of a particular amendment is often problematic and the soil characteristics may be considered before its application [53].
It is important to check the availability of micronutrients and the effects on trace metal composition to ensure soil quality. On the one hand, it was observed that pH is the factor that most affects the availability of micronutrients analysed. On the other hand, trace metals are related to clay and SOC, and less with the equivalent calcium carbonate (CO 3 ). However, great retention of Cr, Cu, Ni, Zn, and Cd in calcaric Fluvisols has been reported [54]. In this sense, the effect of pH may be associated to the relation between soil pH and the presence of carbonates.
In general, the correlations obtained were higher between the soil properties and the metal concentration extracted by acid microwave digestion than those obtained for the essential metals extracted with DTPA. This result should be considered for agriculture or soil rehabilitation purposes. In fact, pollutants present in the soils can be mobilized and be available for plants, but with environmental consequences if inadequate fertilization is applied. These are the two faces of the same situation: availability of micronutrients versus mobility and environmental pollution.
Determining which is the main factors controlling the bioavailability of metals is important for future soil rehabilitation and landscape recovery, whether they come from bedrock or from anthropogenic inputs. Nunes et al. (2014) [55], in a work done with Mediterranean soils located in the west of Spain, suggested that the enhanced mobility of heavy metals is related to anthropic activities. Agriculture has a great influence due to the inputs of micronutrients/pollutants with fertilization and the effect on soil properties. These effects could be the same when restoring soils. At policy level, managing heavy metal balances in the environmental framework requires well-defined soil protection policies with a specific focus on agricultural soils [21].
In some cases, a combination of both factors, bedrock composition and the influence of anthropic activities, should be considered. Mainly, the presence of metals needs to be determined in the soils before adding organic amendments. Along these lines, the GEMAS project can be considered a good tool before soil rehabilitation.

Conclusions
Soil is a key factor of sustainability and landscape, and soil quality is in fact one of the major considerations that should be considered before land restoration. In this case, the results of this study provide data about trace metal levels in calcareous soils of a Mediterranean area that can help to improve the strategies for land rehabilitation. The most important soil properties related to metal content should be considered. Many land restoration techniques involve the addition of organic matter to the soils and this can pose a risk if this modifies soil conditions (i.e., pH) and, as a consequence, the availability and mobility of trace metals.
Our results indicate that soil pH influences the available forms of essential metals (DTPA extraction). Moreover, trace metals were positively correlated with SOC and clay, and negatively correlated with the pH values of soils.
The mean values of the metal composition in soils (Cd, Cr, Cu, Ni, Pb, and Zn) obtained in this study were similar to values reported for other areas in the Mediterranean basin [36], and these results may be extended to other calcareous soils. Their total content in soil seems to be determined by the bedrock composition.
The soil quality of this area is associated to a low metal content, and this facilitates the possibility of adding manure or other organic fertilizers to improve soil quality without increasing the mobility of pollutants (i.e., by leaching to underground waters). The amendments can increment the soil organic matter content, which may positively affect physical properties, carbon sequestration by soils, help develop sustainable agriculture or forestry activity, recover the landscape, and promote compliance with the strategic objectives of the European Union's environmental policy. Rain-fed agriculture could be recovered, improving soil quality.
These actions are focused on the strategies for sustainable development. However, the type of organic matter and the effects on trace elemental composition should be considered before to avoid the mobilization of metals, for food safety [24], and to maintain micronutrient availability. Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.