Interactive influences of salinity and sodicity levels on depth-wise soil organic matter and micronutrient elements in Thailand

Soil salinity and sodicity are the major environmental issues that lead to the deterioration of soil properties, nutrient cycling, and soil ecosystems around the globe. Nevertheless, the reciprocal effects of salinity and sodicity levels on depth-wise soil organic matter (SOM) and micronutrients remain elusive, particularly in Thailand. For a better understanding of such an issue, soil samples were collected from 38 sites at depths of 0–20, 30–50, 60–80, and 80–120 cm where they were affected by salts with variable levels of salinity and sodicity, having electrical conductivity (ECe), and exchangeable sodium percentage (ESP) from 0.20–74.70 dS m–1, and 2.74%–113.23%, respectively. Soil physicochemical properties, including distribution of sand, silt, and clay, pH, soil organic carbon (SOC), and micronutrients (Fe, Zn, Mn, Cu, and B) were determined. The results exhibited that SOC content, ranging from 3.36–14.74 g kg–1, was higher in topsoil (0–20 cm) compared to the other three soil depths and it correlated negatively with ECe (0–20 and 80–120 cm) and ESP (80–120 cm), suggesting the declines in SOC amount due to high salinity and sodicity levels. Topsoil Mn concentration (0.06–182.06 mg kg–1) also tended to be greater than the other soil depths while Fe concentration in that soil depth (0.02–33.99 mg kg–1) tended to be smaller. The ECe correlated negatively with the concentrations of Fe, Cu (all soil depths), and Zn (30–50 and 60–80 cm), and positively with Mn concentration (60–80 and 80–120 cm), suggesting that the availability of Fe Cu and Zn is vulnerable to high salinity and sodicity levels. Overall, our findings highlight that high salinity and sodicity levels brought about a reduction in SOC content and low concentrations of micronutrients in soils, irrespective of Mn concentration.


Introduction
Salinization and sodification are natural processes in the soils of arid and semi-arid regions that bring about a crucial problem for agricultural development and sustainable use of soil and water resources (Furquim et al 2010, Abdel Aal and Ibrahim 2013, Jafarpoor et al 2021).As such, soil salinity, determined by the electrical conductivity (EC e ) value, and sodicity, determined by the exchangeable sodium percentage (ESP) value, are the two major environmental issues causing land degradation around the globe, with an area of ca. 1 × 10 9 ha, especially in the arid and semi-arid regions (Rengasamy 2006, Fan et al 2016, Zhang et al 2020).Moreover, as a result of climate change, with predicted increasing temperatures and growing frequency and severity of droughts in moister climate regions, as well as the aggravating drying and globalization of drylands, soil salinization and sodification have become global concerns (Cook et al 2014, Huang et al 2017, Stavi et al 2021).
In Thailand, it is estimated that an area of ca.2.3 × 10 6 ha is being affected by salinity and sodicity.Inland salt-affected soils in Thailand are formed through geochemical processes and distributed mainly in the northeastern part of about 1.84 × 10 6 ha, while coastal salt-affected soils originate from seawater scattered along the coast of 0.43 × 10 6 ha (Arunin and Pongwichian 2015).Salinization and solonization in Thailand have several unique characteristics and have been accelerated by human activities, for instance, improper land management practices, deforestation, irrigation, salt-making, and construction of roads and reservoirs.All of these human activities, irrespective of salt-making, are believed to promote salinization and solonization by causing saline groundwater levels to rise toward the surface.Concerns with regard to the potential future expansion of areas of salt-affected soils and their effects on land and water resources are one of the most important issues for farmers who live in the affected areas (Iwai et al 2012).
Salt accumulation and sodification affect soil quality and health, which is reflected in specific soil properties, comprising biological, physical, and chemical characteristics (Abdel Aal and Ibrahim 2013).Such soil properties are negatively impacted by salts considerably, especially soluble salts (Yu et al 2014), for example, poor soil structure due to soil colloid dispersion and excess sodium concentration causing toxicity in plants (Li et al 2012, Zhang et al 2021).However, the interactive effects of salinity and sodicity levels on the amount and concentration of soil organic matter (SOM) and micronutrient elements are still elusive, particularly in the deeper layer soils.
SOM is a pivotal pedospheric variable that plays an imperative role in agricultural practices and environmental functions (Ondrasek andRengel 2012, Sonsri et al 2024).In the global carbon (C) cycle, SOM is the major source and sink of atmospheric C (Wattel-Koekkoek et al 2003).It contains ca.1500 Pg of C within a depth of 1 m, which exceeds the amount of total C present in vegetation and atmospheric reserves (Oelkers and Cole 2008, Sollins et al 2009, Ontl and Schulte 2012).Since the large C share of SOM, even a small fluctuation in SOM content may cause a dramatic increase in atmospheric carbon dioxide, methane, and nitrous oxide concentrations, which would henceforth impact global warming (Lal 2004, Eglin et al 2010, Mustafa et al 2020).Although SOM dynamics are still largely elusive, environmental and biological controls on its formation and persistence predominate (Schmidt et al 2011) in which favorable soil environmental conditions may secure higher SOM levels and soil quality.In contrast, in the arid and semi-arid regions, soils may be highly affected by drought, leading to higher severity of salinity and sodicity presumably further impacting SOM content (Emran et al 2020).
Micronutrients, including iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), and boron (B), play a vital role in the growth and production of crops via their involvement in the synthesis of chlorophyll, proteins, lipids, nucleic acids, cell wall, as well as structural integration, carbohydrate metabolism, and tolerance to stress (Barker andPilbeam 2015, Shireen et al 2018).It has been shown that the variability of micronutrients in soils depends on several factors.Anand et al (2019) exhibit that poor soil organic C (SOC), low clay content, intensive cropping systems, the application of micronutrient-free chemical fertilizers, and the exclusion of organic manures from cropping systems are the determinants responsible for micronutrient deficiency in soils.Availability of micronutrients to plants is also controlled by soil pH, salinity level and salt composition, suggesting that micronutrient availability in salt-affected soils is immensely complex (Fageria et al 2011).
To date, there is a dearth of information on the amount and concentration of SOM and micronutrients in soil profiles, as well as their interactions with salinity and sodicity levels in Thailand.Understanding such an issue may be beneficial for agricultural and land management to accomplish sustainable goals.In this study, we thus aimed to explore the interactive effects of salinity and sodicity levels on the depth-wise amount of SOM and concentration of micronutrients.We hypothesized that high levels of salinity and sodicity might bring about SOM reduction by enhancing the flocculation of clay particles into aggregates and restricting substrate availability, leading to SOM decomposition.We further hypothesized that the concentration of micronutrients might be decreased due to high salinity and sodicity levels occurring via the formation of micronutrient complexes with dominant ions in salt-affected soils, such as sodium and bicarbonate ions.

Description of study area
The study was carried out in Kamphaeng Saen district, Nakhon Pathom province, in the central part of Thailand.This location covers a total area of 405 km 2 and includes irrigation and drainage canals, fields, roads, and bridges.The geographic coordinates (UTM) of the study area lie between 47 594639-607124 E and 1548585-1561252 N (figure 1).The regional climate is a tropical savanna with a mean annual temperature of 27 °C-30 °C.Generally, the rainy season is between mid-August and mid-October with average annual precipitation of 903 mm year -1 .The ratio of precipitation to evaporation is mainly 0.75 longer than six months.The soils examined in the present study had been formed on a semi-recent terrace or semi-recent terrace on an alluvial fan.The parent materials of those soils were derived from the alluvium complex mixed with old marine sediments (Phankamolsil et al 2021a).The land use types in the study locations were chiefly sugarcane plantations, as well as pasture and abandoned lands at some sites.The soil in this study was classified as Typic Haplustalfs (Soil Survey Staff 2014a).

Soil collection and preparation
An orthographic map was used as a base map to identify the spatial distribution of salt-affected soils that commonly presented as bare areas on that map in which 355 sites were initially observed (Supplementary Information figure S1).After the field observation, soil samples from representative sites (a total of 38 sites) were collected for this study (figure 1), where those soils were affected by salinity and/or sodicity, having the ranges of the EC e , and ESP values from 0.20-74.70dS m -1 (figure S2), and 2.74%-113.23%(figure S3), respectively, regardless of soil depths (Phankamolsil et al 2021a).The soil samples at each location were collected using a hand auger at four depths, i.e., 0-20, 30-50, 60-80, and 80-120 cm, respectively.Soil samples were air-dried, gently disaggregated and passed through the 2-and 0.5-mm stainless steel sieve to eliminate gravel, plant debris, and plant litter as much as possible prior to soil analysis.

Soil analyses
Particle size distribution was determined by a combination of sieve and pipette analysis described by Gee and Bauder (1986).Soil pH (H 2 O) was measured in water using a 1:1 soil to solution ratio (Soil Survey Staff 2014b).
The EC e of a saturation extract at 25 °C was measured with an electrical conductivity meter (United States Salinity Laboratory Staff 1954).The ESP was computed as the exchangeable sodium (Na + ) divided by the cation exchange capacity (CEC), determined by saturating the exchange sites with an index cation (NH 4+ ) using 1 M ammonium acetate at pH 7.0 (Thomas 1982), and multiplied by 100 (United States Salinity Laboratory Staff 1954).SOC was determined by oxidation with 1 M potassium dichromate and back titration with 1 M ferrous ammonium according to Nelson and Sommers (1996).The aqua regia (3:1, v/v, HCl to HNO 3 ) digestion procedure (ISO standard 11466) is considered suitable for the analysis of trace elements in soils and is used to estimate the maximum element availability to plants (Vercoutere et al 1995, Chen andMa 2001), with less chemical consumption, less harmful to the environment, and less time and cost-efficient than the other digestion method, e.g., the tri-acid digestion method (Moursy et al 2020).Thus, total concentrations of micronutrients (Fe, Zn, Mn, Cu, and B) in bulk soil samples of each soil depth at each study site were determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 7300 DV, PerkinElmer, Waltham, MA, USA) on the aqua regia digestion (Wongpokhom et al 2008).

Statistical analysis
The relationships between the salinity (EC e value) and sodicity (ESP value) of soil samples and the investigated soil physicochemical properties (particle size distribution, soil pH, SOC, and concentration of Fe, Zn, Mn, Cu, and B) at each depth were examined using correlation analysis in SPSS 18.0 (SPSS Inc., Chicago, IL, USA).

Results
3.1.General physicochemical properties of soils 3.1.1.Distribution of sand, silt, and clay in soils The distribution of sand, silt, and clay varied largely among soil depths and study sites, ranging from 2.5%-78.4%,4.1%-57.3%,and 15.8%-69.1%,respectively (figure 2).Across all sites, the clay content tended to increase with increasing soil depth, to a large extent in 80-120 cm of the S7 (69.1%; figure 2(C)).On the other hand, sand and silt contents generally showed erratic patterns of accumulation among the soil depths (figures 2(A) and (B)).The sand content was largest in 80-120 cm of the S38 (78.4%), whereas the silt content was greatest in 80-120 cm of the S23 (57.3%).

Soil pH
The pH of the soil varied considerably among the soil depths and study sites, having values between 4.24 and 8.26 (figure 3).In general, the soil pH of most study locations tended to be greater in the lower soil layer (80-120 cm) relative to the other soil depths.Among the study sites, the higher soil pH values were observed in the S9, S12, S13, S14, and S30 (7.19-8.24),regardless of soil depths, while the lowest value was observed in the topsoil (0-20 cm) of the S2 (figure 3).

Concentration of micronutrients in soils
The concentration of Fe, Zn, Mn, Cu, and B in soils at different depths is presented in table 1.The Fe concentration varied substantially, 0.01-549.86mg kg -1 , irrespective of study sites and soil depths.The concentration of Fe in the topsoil (0.02-33.99 mg kg -1 ) tended to be smaller than the other three soil depths.Among the study sites, Fe concentration in the subsoil was markedly greater in S7, S13, and S33.The concentration of Zn was generally low, 0.01-1.37mg kg -1 , for all sites and soil depths (table 1).Zn concentration in 30-50 and 60-80 cm (0.01-1.31 and 0.01-1.06mg kg -1 , respectively) tended to be slightly higher than in 0-20 and 80-120 cm.The concentration of Mn ranged from 0.04-182.06mg kg -1 (table 1).In most sites, Mn concentration tended to be greater in topsoil (0.06-182.06 mg kg -1 ) than in the other three soil depths.Among the locations, a higher concentration of Mn in topsoil was observed in S2, S29, S37, and S38, amounting to 182.06, 32.79, 51.15, and 34.86 mg kg -1 , respectively.The Cu concentration was meager, 0.01-0.35mg kg -1 , for all sites and soil depths (table 1).The highest value was observed in 80-120 cm at S7.The concentration of B in these soils was also small, 0.01-1.61mg kg -1 , for all sites and soil depths (table 1).The greatest concentration was observed in the topsoil of S30.

Relationships between the salinity and sodicity of soil and the physicochemical properties of soil
Figure 5 shows the relationships between the salinity and sodicity of soil and the examined soil physicochemical properties at different soil depths.In the 0-20 cm, the EC e exhibited a negative correlation with the SOC content, Fe and Cu concentrations (P < 0.05).The ESP was correlated positively with the sand particle size and Fe concentration (P < 0.01), whereas negatively with silt (P < 0.05) and clay (P < 0.01) particle sizes (figure 5).The negative correlation between the EC e and the concentrations of Fe, Zn, and Cu was observed at a depth of 30-50 cm (P < 0.01), while the ESP at this depth was correlated positively with sand particle size (P < 0.01) and negatively with silt (P < 0.05) and clay (P < 0.01) particle sizes (figure 5).In the 60-80 cm, the EC e was correlated negatively with the concentrations of Fe (P < 0.01), Zn (P < 0.05), and Cu (P < 0.01), but it was positively correlated with the Mn concentration (P < 0.05).The ESP in the 60-80 cm was correlated positively with sand particle size (P < 0.01) and negatively with silt (P < 0.05) and clay (P < 0.01) particle sizes (figure 5).The EC e measured at 80-120 cm was correlated negatively with clay particle size (P < 0.01), SOC content, and Fe and Cu concentrations (P < 0.05), meanwhile positively with sand particle size and Mn concentration (P < 0.05).The ESP at a depth of 80-120 cm showed a negative correlation with the SOC content, as well as silt and clay particle sizes (P < 0.01), but it demonstrated a positive correlation with the sand particle size (P < 0.01), and soil pH (P < 0.05).

Reciprocal effects of salinity and sodicity levels on depth-wise soil organic matter content
Soil salinity and sodicity are subjected to modified hydrologic processes, which may impact soil carbon cycling and organic matter decomposition (Wong et al 2010).The results observed in the present study suggested that high levels of water-soluble salt (high salinity level; figure S2) and high levels of exchangeable sodium (high sodicity level; figure S3) in Kamphaeng Saen soil brought about a reduction in SOM accumulation in the topsoil, 0-20 cm, and/or subsoil, 80-120 cm (figure 4).The negative correlations between the EC e value and SOC content at the depths of 0-20 cm and 80-120 cm (P < 0.05), as well as that between the ESP value and SOC content at a depth of 80-120 cm (P < 0.01; figure 5) support it.
It has been suggested that several processes affecting SOC stocks and fluxes occur during salinization and sodification.For instance, the formation of soil aggregates can physically protect SOC, while any process that disrupts the aggregates is likely to increase C mineralization, leading to the loss of C (Pulleman and    4.3.Reciprocal effects of salinity and sodicity levels on depth-wise soil micronutrients Salinity and sodicity levels, as well as salt composition, are likely the factors associated with the availability of micronutrients in salt-affected soils (Fageria et al 2011, Bhaduri et al 2022).The present research revealed that the EC e of Kamphaeng Saen soil interplayed with the concentration of soil micronutrients.The finding suggested that a high level of EC e (figure S2) possibly resulted in a low concentration of Fe (table 1), as shown by the negative correlation between the EC e and the Fe concentration at the depths of 0-20 cm (P < 0.05), 30-50 cm (P < 0.01), 60-80 cm (P < 0.01), and 80-120 cm (P < 0.05), respectively (figure 5).The factor responsible for the decrease in Fe solubility in salt-affected soils (high EC e value) can be both sodium and bicarbonate ions (Rabhi et al 2007), which might cause the precipitation of Fe in the forms of hydroxides and oxides (Nikoosefat et al 2023).High salinity levels also brought about the low concentration of Zn.This was suggested by the negative correlation between the EC e and the Zn concentration at the depths of 30-50 cm (P < 0.01), and 60-80 cm (P < 0.05) respectively (figure 5).It has been suggested that in most saline soils, the low solubility of Zn is due to a high calcium concentration, a dominant characteristic of such soils (Jamalomidi et al 2006).The inhibiting mechanisms of Zn solubility can be adsorption, precipitation, and fixation mechanisms, creating many forms of Zn, e.g., in the form of complexes or fixed within the mineral composition of calcium carbonate (Akrawi et al 2021, Aljumaily et al 2022).The interactive effects of salinity levels on low Cu concentration were also observed based on the negative correlation between the EC e and the Cu concentration at the depths of 0-20 cm (P < 0.05), 30-50 cm (P < 0.01), 60-80 cm (P < 0.01), and 80-120 cm (P < 0.05), respectively (figure 5).The low solubility of Cu in saline soils might be a result of its higher residual and carbonate forms and lower exchangeable and water-soluble forms under salinity conditions (Mohiuddin et al 2022).A strong association of Cu with carbonate was also demonstrated by Ponizovsky et al (2007).The precipitation and complexation processes may be the responsible factors contributing to this since Cu can predominantly form precipitates and complex with carbonates (Pakuła andKalembasa 2013, Angel et al 2021).
In contrast to the concentrations of Fe, Zn, and Cu, the concentration of Mn was likely increased with increasing salinity levels.This was suggested based on the positive correlation between the EC e and the Mn concentration at the depths of 60-80 cm (P < 0.05), and 80-120 cm (P < 0.05), respectively (figure 5).Previous work indicated that saline soils containing Na, Ca, Cl, ClO 4 , and NO 3 caused a significant increase in Mn concentration in soils.In addition, greater concentrations of the Na-and Ca-salt solutions are presumably responsible for increasing soluble Mn, partly by displacing the exchangeable Mn via Ca-Mn and Na-Mn exchange reactions and partly via the formation of soluble Mn-ligand complexes (Khattak et al 1989).
Among the different soil depths, Fe concentration in the topsoil tended to be smaller than the other three soil depths (table 1).The lower Fe concentration in the topsoil than in subsoil may be due to the interaction between SOC, which was higher in topsoil (figure 4), and Fe since Fe can form complexes with C (Wu et al 2019, Button et al 2022).Moreover, many compounds within SOC carry a negative charge, while most metal oxides, such as Fe hydroxide, have a positive charge and a high capacity to sorb SOC at relevant pH values (Kaiser and Guggenberger 2007).The Mn concentration, in contrast, tended to be greater in topsoil than in the other three soil depths (table 1).The higher concentration of Mn in topsoil than in subsoil was also observed by Jiang et al (2009), and its vertical distribution in the soil is presumably influenced by anthropogenic and natural inputs, weathering, leaching, and biological cycling (Jobbágy andJackson 2001, Keskinen et al 2019)

Conclusions
The present findings allow us to gain a better understanding of the interactive effects of salinity and sodicity levels on the depth-wise amount and concentration of SOM and micronutrient elements in salt-affected soils.The high levels of salinity and sodicity in the soil led to a decrease in the amount of SOM in the topsoil, 0-20 cm, and/or subsoil, 80-120 cm.High soil salinity levels also affected the concentration of micronutrients in the soil.In detail, its high value resulted in low concentrations of Fe and Cu for all soil depths, as well as Zn at depths of 30-50 and 60-80 cm.Conversely, the concentration of Mn in the subsoil (60-80 and 80-120 cm) increased with the rising levels of soil salinity.Based on these findings, it is suggested that the enhancements of SOC content and micronutrient availability in the areas faced with high levels of salinity and sodicity are requisite.The application of organic amendments is presumably among the agricultural practices to achieve that.Utilizing the organic amendments might reduce salinity and sodicity levels to deliver: (i) improvements in production yields from land currently in agricultural use, and (ii) recovery of contaminated/abandoned land for agricultural use.Hence, the role of organic amendments in promoting SOC accumulation and micronutrient availability in high salinity and sodicity areas, including their responsible mechanisms, merits further studies.

Figure 1 .
Figure 1.Study location and sampling sites.Adapted with permission from Phankamolsil et al (2021a).
4.1.Factors contributing to the variability in soil physicochemical properties It has been observed that sand and silt contents revealed erratic patterns of accumulation among the soil depths (figures 2(A) and (B)).This finding may be due to the soils in the present study that had been formed on a semirecent terrace or semi-recent terrace on an alluvial fan (Phankamolsil et al 2021a).An alluvial fan is a large-scale
Marinissen 2004, Wong et al 2010,Bailey et al 2019, Sonsri andWatanabe 2023).In soils with high levels of salinity and sodicity, high EC e (i.e., 4-8 dS m -1 , yields of many crops are restricted; 8-16 dS m -1 , only tolerant crops yield satisfactorily; and >16 dS m -1 , only a few very tolerant crops yield satisfactorily due to toxicity;Shirokova et al 2000) in the soil solution results in the flocculation of clay particles into aggregates, which may restrict substrate availability and thus bring about the decomposition of SOM.On the other hand, the dispersion of aggregates on wetting of soils under sodicity conditions can increase the accessibility and availability of previously protected SOM, and accelerate C loss(Oades 1984, Wong et al 2008, Chibowski 2011).The negative correlation between the clay particle size and the EC e , 80-120 cm, and ESP for all soil depths (P < 0.01; figure5) agrees with this.Across the different soil depths, the SOC was greater in the topsoil while smaller in the lower layer soil (80-120 cm; figure4).Higher SOC content in topsoil compared to subsoil was also observed byAngst et al (2018) andInagaki et al (2023).This supports the knowledge that the majority of carbon inputs to soils occur in the surface horizons(Dorji et al 2014, Tautges et al 2019, Antony et al 2022).Additionally, the microbial community undergoes substantial changes with increasing soil depth, revealing both a strongly decreased biomass and activity in response to lower substrate availability and accessibility(Herre et al 2022).

Figure 5 .
Figure 5. Color map on correlations between the electrical conductivity (EC e ) and exchangeable sodium percentage (ESP) values of soil samples and the investigated soil physicochemical properties at each soil depth.* and ** denote the presence of significant correlation at P < 0.05 and <0.01, respectively.

Table 1 .
Concentrations of micronutrient elements in Kamphaeng Saen soil at different depths.