Soil texture influences salinity-associated changes in soil organic matter: evidence from FTIR analysis in Mesopotamian soils
Raghad Mouhamad 
1 Scientific Research Commission, Baghdad, Iraq.
2 Al-Siraj University: Falluja, Al-Anbar, Iraq. E-mail addresses: ahmadki@umkc.edu
3 Department of Geology, College of Science, University of Baghdad, Baghdad, Iraq. Email: hind_fadhil84@yahoo.com
*Corresponding author: raghad.s.mouhamad@src.edu.iq; ahmadki@umkc.edu
ARTICLE
ABSTRACT
Soil salinity is a major constraint in arid and semi-arid agroecosystems, yet its interaction with soil texture in shaping soil organic matter (SOM) composition remains insufficiently understood. This study investigated texture-dependent SOM responses to salinity using a combined physicochemical and Fourier transform infrared (FTIR) spectroscopy approach in Mesopotamian soils. A total of 17 topsoil samples representing four texture classes (clay, silty clay, clay loam, and silty clay loam) were analyzed across a wide salinity gradient (electrical conductivity, EC: 7.95–105.5 dS m⁻¹). SOM characteristics were assessed using organic carbon (OC) and FTIR-derived functional groups, including hydroxyl (O–H), carbonyl (C=O), and mineral-associated bands (C–O/Si–O), as well as the Organic Strength Index (OSI). No significant relationship was observed between EC and OC (r = −0.35, p = 0.17). In contrast, FTIR-derived indices showed clearer responses to salinity, with OSI negatively correlated with EC (r = −0.55, p < 0.05). Increasing salinity was associated with reduced intensities of organic functional groups (O–H and C=O) and increased contributions of mineral-associated bands. Principal component analysis (PCA) revealed a dominant gradient separating mineral-associated and organic-related variables along PC1, with EC aligned with mineral components and opposed to OC and OSI. Texture-dependent patterns were observed: clay soils exhibited a wider range of salinity values, whereas other soil textures had narrower ranges. FTIR-derived indices showed stronger associations with salinity-related changes in SOM composition than bulk OC measurements and highlighted texture-dependent variability in SOM responses across the investigated soils.
Keywords: Mesopotamian soils; Soil texture; OSI; Salinity gradient; FTIR spectroscopy
Graphical abstract. Surface soils (0–30 cm) from four texture classes (clay, silty clay, clay loam, silty clay loam) were sampled along a wide salinity gradient (EC: 7.95–105.5 dS m⁻¹) in Mesopotamian soils. Bulk SOC showed no significant correlation with EC, whereas FTIR-derived indices – particularly OSI – revealed significant salinity-driven compositional shifts. PCA separated mineral-associated bands (EC, C–O/Si–O) from organic components (OC, OSI, O–H, C=O). Texture modulated the response, with clay soils showing the widest EC range. FTIR indices were more sensitive to salinity-induced SOM changes than bulk OC.
INTRODUCTION
Soil organic matter (SOM) is crucial for regulating nutrient cycles, forming soil structure, and storing carbon in land ecosystems. The stabilization of SOM is mainly due to the physical, chemical, and biological interactions between organic matter and minerals. This is especially important in fine-textured soils, which have larger specific surface areas¹˒²⁵. However, the stability of SOM is affected by various external stresses, including soil salinity, a major limiting factor in arid and semi-arid environments³˒⁴.
Salinization induces several changes in the soil physicochemical state, namely the development of ionic imbalance, which causes soil structural disintegration, changes in microbial activity, and alterations in carbon transformation pathways⁴˒¹⁶. Hence, salinity may alter SOM composition rather than in its quantity, potentially increasing the relative contribution of mineral-associated SOM fractions³˒⁴˒¹⁰.
It is especially pronounced in the Mesopotamian agricultural zone, where sodic soils dominate, containing high carbonate content, high salinity, and a high sodium adsorption ratio²⁶. Therefore, soil degradation and erosion are intensified here, and SOM dynamics are regulated by two main factors: carbon supply and ion-induced physicochemical constraints that govern SOM stabilization and turnover²⁶.
Another important factor affecting SOM dynamics is the soil texture. Fine-textured soils Favor SOM stabilization due to better organo–mineral interactions, whereas intermediate soils are expected to have some variability in SOM stabilization and mineralization under stress conditions²˒⁹˒¹⁰. However, in saline soils, there may be a decline in the stability of aggregates even in fine-textured soils, although the role of the mineral fraction in the regulation of SOM stability should be recognized¹⁰˒¹².
Despite numerous studies reporting SOM transformation under salinity stress, most rely on estimates of soil organic carbon, which do not capture compositional changes. To analyze the SOM, FTIR spectroscopy is a powerful technique that allows characterization of the functional groups responsible for interactions between organic and mineral SOM¹³˒¹⁴. Combining FTIR spectroscopy with quantitative indices enables the identification of the effects of soil stressors on the SOM structure.
At present, insufficient data on the interaction between salinity and soil texture in relation to FTIR-derived characteristics of SOM are available under field conditions, particularly for the highly heterogeneous soils of Mesopotamia. This study presents exploratory field-based evidence from Mesopotamian soils on the associations among salinity, soil texture, and FTIR-derived SOM characteristics.
Therefore, this study was undertaken to: (i) quantify changes in soil organic carbon content across different textures along a salinity gradient, (ii) analyze changes in the composition of SOM using FTIR spectroscopy, and (iii) find texture-dependent patterns of SOM transformation using FTIR-derived SOM indices and multivariate analysis.
MATERIAL AND METHODS
Study Area
This study was conducted in the alluvial plains of central Iraq within the Mesopotamian agricultural region influenced by the Tigris River system. The region is characterized by arid to semi-arid climatic conditions, low annual precipitation, high evaporation rates, and widespread soil salinization¹⁵˒²². Mean annual rainfall ranges between 150 and 200 mm, with high summer temperatures and mild winter conditions. The investigated soils are predominantly calcareous and alkaline, with salinity as a major environmental constraint affecting soil productivity and SOM dynamics¹⁶˒¹⁷.
Soil Sampling and Texture Classification
A total of seventeen topsoil samples (0–30 cm) were collected from agricultural locations representing naturally occurring salinity gradients across the Mesopotamian plain. Electrical conductivity (EC) values ranged from 7.95 to 105.5 dS m⁻¹. At each sampling location, composite soil samples were obtained by combining several subsamples collected at the same site to reduce local heterogeneity.
Soil texture was determined using particle-size distribution analysis according to the USDA classification system²˒¹². The investigated soils were classified into four texture groups: clay (n = 6), silty clay (n = 6), clay loam (n = 3), and silty clay loam (n = 2). Soil samples were air-dried, gently crushed, homogenized, and passed through a 2 mm sieve prior to laboratory analyses. The investigated dataset represented naturally occurring field systems rather than experimentally imposed salinity gradients. In Mesopotamian alluvial soils, obtaining broad salinity gradients simultaneously associated with distinct texture classes under comparable environmental conditions is particularly challenging because salinity distribution is strongly influenced by depositional heterogeneity, irrigation history, fluctuating groundwater conditions, and localized agricultural management practices. Consequently, the investigated soils reflected naturally occurring pedological variability under real field conditions.
Physicochemical Analysis
Electrical conductivity (EC) and pH were measured in a 1:1 soil-to-water suspension (w/v) using calibrated conductivity and pH meters¹⁶. Organic carbon (OC) content was determined using the Walkley–Black wet oxidation method, whereas total nitrogen (N) was measured using the Kjeldahl digestion procedure. Available phosphorus (P) was determined colorimetrically in accordance with standard soil analytical protocols.
All measurements were performed in triplicate, and mean values were used for subsequent statistical analyses.
FTIR Spectroscopy and Spectral Analysis
Before FTIR analysis, soil samples were finely ground (<75 µm) and thoroughly homogenized. Soil organic matter composition was characterized using mid-infrared Fourier Transform Infrared (FTIR) spectroscopy. Spectra were recorded using a Bruker Tensor FTIR spectrometer (Bruker Optics, Germany) equipped with an attenuated total reflectance (ATR) diamond crystal.
Spectra were collected over the 4000–400 cm⁻¹ range at a spectral resolution of 4 cm⁻¹, with 32 scans averaged per sample. A background spectrum was recorded prior to each measurement to minimize atmospheric interference, and contact pressure was kept constant throughout all analyses. Each sample was analyzed in triplicate, and average spectra were used for subsequent analyses.
Spectral preprocessing included baseline correction, normalization, and smoothing prior to peak assignment²³˒²⁷. Band interpretation followed previously established assignments for SOM-associated functional groups²³˒²⁷. The selected spectral regions included 3200–3400 cm⁻¹ corresponding mainly to hydroxyl groups (O–H), 1600–1700 cm⁻¹ corresponding to carbonyl-associated bands (C=O), and 1000–1100 cm⁻¹ associated with mineral and carbohydrate-related vibrations (C–O/Si–O). Relative band intensities were extracted from processed spectra for subsequent comparative analyses.
FTIR-Derived Indices and Organic Strength Index (OSI)
Relative FTIR band intensities were used to calculate several spectral indices, including the aliphatic index (AI), aromatic index (ArI), hydroxyl index (HI), carbonyl index (CI), and silicate index (SI). These indices were used to compare relative changes in SOM-associated and mineral-associated spectral components across the investigated soils²³˒²⁷.
The Organic Strength Index (OSI) was calculated according to the following equation:
OSI = (AI + ArI + HI) / (CI + SI)
The OSI was used in this study as a relative composite spectral metric to explore SOM compositional variation among soils with differing salinity and texture. The OSI should be interpreted as a relative exploratory spectral metric rather than an absolute quantitative indicator of SOM quality.
Statistical Analysis
Descriptive statistical analyses, including means, standard deviations, minimum values, and maximum values, were calculated for all measured variables. Prior to statistical analysis, the normality of the data was evaluated. Pearson correlation analysis was performed to assess relationships among salinity (EC), organic carbon (OC), and FTIR-derived spectral indices using a significance threshold of p < 0.05.
Principal component analysis (PCA) was conducted to explore multivariate relationships among physicochemical and FTIR-derived variables. Prior to PCA, variables were standardized to a zero mean and unit variance. Considering the relatively limited sample size (n = 17), PCA results were interpreted as exploratory rather than confirmatory in nature.
All statistical analyses were performed using SPSS version 26 (IBM Corp., Armonk, NY, USA).
RESULTS
Differences in Soil Physicochemical Properties
The soil physicochemical properties showed variability across the investigated texture classes, as shown in Table 1. Electrical conductivity (EC) ranged from 7.95 to 105.50 dS m⁻¹, indicating a wide range of salinity. Soil pH values were relatively stable, ranging from 7.10 to 8.63, indicating alkaline conditions across all samples.
Table 1. Physicochemical properties of soils across texture classes, considering different salinity levels, are presented as means and standard deviations.
Soil organic carbon (OC) ranged from 0.05 to 0.94%, with overlapping values across the different texture classes. Clay soils showed the greatest range in electrical conductivity (EC), while clay loam soils had a narrower range of OC. A negative association between EC and OC was observed (r = −0.35); however, this relationship was not statistically significant (p = 0.17) (Figure 1). The substantial variability in organic carbon (OC) values observed at intermediate electrical conductivity (EC) levels suggests a lack of a consistent linear correlation.
Figure 1. Relationship between electrical conductivity (EC) and organic carbon (OC) across the investigated Mesopotamian soils. Symbols indicate soil texture classes. No significant relationship was observed between EC and OC (p = 0.17), despite a weak negative trend.
Effect of Soil Texture on FTIR-based Indices
Variations in FTIR-based functional group indices were observed among soil textures (Table 2; Figures 2–5).
Table 2. FTIR-based functional group indices and the Organic Strength Index (OSI) were calculated for various soil textures.
In clay soils, the O-H and C=O peaks declined with increasing EC, whereas the C-O/Si-O peaks increased in the same soils. The OSI values ranged between 1.19 and 1.96. In the silty clay soils, the FTIR-based indices were variable, and the OSI values ranged from 1.73 to 3.00.
Figure 2. Representative FTIR spectra of clay soils across the investigated salinity gradient (EC = 7.95–105.5 dS m⁻¹). Relative decreases in organic-associated bands and increased contribution of mineral-associated spectral signals were observed with increasing salinity.
Figure 3. Representative FTIR spectra of silty clay soils across the investigated salinity gradient (EC = 14.83–48.6 dS m⁻¹).
Figure 4. Representative FTIR spectra of clay loam soils across the investigated salinity gradient (EC = 20.1–63.2 dS m⁻¹).
Figure 5. Representative FTIR spectra of silty clay loam soils across contrasting salinity conditions (EC = 20.6–64.09 dS m⁻¹).
Pearson correlation test The Pearson correlation coefficient between EC and OC was weak and negative (r = −0.35) but not statistically significant (p = 0.17).
Table 3. Pearson correlation matrix (r) among salinity (EC), organic carbon (OC), and FTIR-derived indices, p-values in parentheses.
In contrast, stronger relationships were observed between EC and FTIR-derived indices. EC showed:
- a negative correlation with O–H (r = −0.42, p = 0.09),
- a negative correlation with C=O (r = −0.48, p = 0.06),
- a significant positive correlation with mineral-associated bands (C–O/Si–O; r = 0.52, p = 0.03),
- a significant negative correlation with OSI (r = −0.55, p = 0.02).
· OC significantly correlated positively with O-H (r = 0.61, p = 0.01), C=O (r = 0.58, p = 0.02), and OSI (r = 0.64, p = 0.01); and negatively correlated with mineral-associated bands (r = −0.47, p = 0.05)
These results indicate stronger statistical associations between salinity and FTIR-derived indices compared to bulk OC.
Principal component analysis (PCA)
The first two principal components explained 63.1% of the total variance (PC1: 37.6%, PC2: 25.5%) (Table 4).
Table 4. Eigenvalues and explained variance (%) of principal components (PC1–PC3).
PC1 was positively associated with EC (0.61) and mineral-associated bands (C–O/Si–O; 0.63), and negatively associated with OC (−0.58), O–H (−0.62), C=O (−0.59), and OSI (−0.65) (Table 5).
Table 5. PCA loadings of physicochemical properties and FTIR-derived variables for principal components (PC1–PC3).
PC2 correlated with phosphorus (P; 0.58) and OC (0.49), but EC had a negative correlation (-0.42). The PCA biplot (Figure 6) revealed that samples differed based on PC1 due to opposing factors for mineral and organic variables. EC-higher samples were plotted at the positive end of PC1, whereas OC and OSI-higher samples were located at the negative end of PC1.
Figure 6. PCA biplot showing multivariate relationships among salinity, OC, P, and FTIR-derived variables across the investigated soils. Coloured symbols represent soil texture classes.
Overall, the PCA structure supported the correlation analysis by separating EC and mineral-associated spectral bands from OC and organic-associated variables along PC1. This pattern suggests salinity-associated compositional shifts in SOM across the investigated soils, although these relationships should be interpreted as exploratory due to the limited dataset.
DISCUSSION
The present results indicate that salinity-related variation in these Mesopotamian soils was expressed more clearly through FTIR-derived compositional indices than through bulk OC concentration alone. Although EC exhibited a weak negative trend with OC, this relationship remained statistically non-significant, suggesting that salinity effects within the investigated soils were not consistently reflected by total carbon content. Similar inconsistencies between salinity intensity and SOC persistence have been reported in heterogeneous dryland systems¹⁴.
In contrast, FTIR-derived indices showed clearer compositional responses along the salinity gradient. The progressive decline in O–H and aliphatic-associated signals, together with the increasing C–O/Si–O intensity, suggests a shift in the relative contribution of organic- and mineral-associated spectral components. Previous studies have shown that salinity and sodicity can influence SOM stabilization processes, aggregate structure, and organo–mineral interactions under salt-affected conditions²¹. The observed spectral redistribution, therefore, likely reflects compositional adjustment within SOM-associated spectral components rather than uniform carbon depletion.
The decrease in OSI under highly saline conditions further supports this interpretation. Unlike OC measurements, which quantify only bulk carbon concentration, OSI integrates both organic- and mineral-associated FTIR responses. Within the present dataset, this approach appeared more responsive to salinity-associated compositional variability. FTIR-based SOM characterization has previously been used to evaluate structural and functional-group variation that is not always captured through bulk carbon measurements alone²³.
Texture-related variability was also evident across the studied soils. Clay soils displayed the broadest EC distribution and contained the most saline samples. Fine-textured soils are known to promote SOM stabilization through greater mineral surface area and stronger organo–mineral associations². However, salinity and sodicity may simultaneously alter aggregation dynamics, microbial activity, and SOM transformation pathways²⁰. Consequently, the texture-related differences observed here should be interpreted cautiously because subgroup replication remained limited.
The PCA results supported the same general compositional pattern. PC1 separated EC and mineral-associated spectral signals from OC, O–H, AI, ArI, and OSI, indicating an organic–mineral compositional pattern associated with salinity variation. Similar PCA-based approaches have been used to evaluate SOM transformation patterns under saline conditions¹. Nevertheless, because the present analysis was derived from a relatively small field dataset, the multivariate structure should be considered exploratory rather than predictive.
Interpretation of FTIR spectra in calcareous alkaline soils also requires caution. The C–O/Si–O region may include overlapping contributions from silicate minerals, carbonate phases, and mineral-bound organic compounds. Therefore, the increase in intensity within this spectral region should not be attributed exclusively to SOM transformation. This limitation is particularly relevant in Mesopotamian soils, where salinity, alkalinity, and carbonate accumulation frequently coexist within the same degradation system¹⁶.
Overall, the results suggest that salinity in these soils was more strongly associated with SOM compositional modification than with a consistent depletion of bulk OC stocks. FTIR-derived indices, particularly OSI, provided useful exploratory evidence of these compositional shifts across saline calcareous soils.
However, the study remains constrained by a limited sample size, an unequal representation of texture classes, single-season sampling, and potential mineral interference in FTIR bands. Additional studies integrating larger datasets, seasonal sampling, SOM fractionation, and complementary spectroscopic approaches would help further evaluate the compositional patterns observed within these saline calcareous soils.
Study limitations
The present study should be interpreted in light of the limitations inherent to field-based investigations of heterogeneous Mesopotamian soils. Although the number of samples was relatively limited, the investigated dataset represented naturally occurring salinity–texture combinations collected under real agricultural field conditions rather than experimentally imposed laboratory gradients. In Iraqi alluvial soils, obtaining broad salinity gradients simultaneously associated with distinct texture classes under comparable environmental conditions is particularly challenging because salinity distribution is strongly influenced by depositional heterogeneity, irrigation history, fluctuating groundwater levels, and localized land-management practices. Additional limitations include the relatively small representation of subgroups within certain texture classes and the reliance on a single sampling campaign focused solely on surface soils. Furthermore, interpreting FTIR spectra in calcareous alkaline soils requires caution because mineral phases, carbonate-associated compounds, and silicates may overlap with spectral regions typically attributed to SOM functional groups. Therefore, the observed FTIR patterns should be interpreted as compositional tendencies within this field dataset rather than definitive mechanistic evidence applicable to all saline soils.
CONCLUSIONS
Salinity-related variation in the investigated Mesopotamian soils was more clearly expressed by FTIR-derived compositional indices than by bulk organic carbon measurements alone. Although OC showed only a weak and non-significant relationship with salinity, FTIR-derived variables revealed consistent shifts in SOM composition along the salinity gradient. Increasing EC was generally associated with lower organic-associated spectral signals and greater relative contribution of mineral-associated spectral bands. Among the evaluated indices, OSI provided the clearest compositional response to salinity, suggesting that integrated FTIR-based approaches may be useful for detecting salinity-associated variation in SOM composition in saline calcareous soils, where bulk OC changes remain limited. Exploratory PCA further supported the separation between salinity-related mineral-associated variables and organic-associated SOM components. Texture-related variability was also evident, particularly in clay soils, although these patterns should be interpreted with caution, given the limited sample size. Overall, the findings suggest that salinity in these Mesopotamian soils was associated primarily with modifications in SOM composition rather than with uniform depletion of total organic carbon. This study highlights the potential of FTIR-derived compositional indices as complementary tools for evaluating SOM dynamics in salt-affected soils under arid and semi-arid conditions.
Funding
This research did not receive external financial support. Field activities and laboratory analyses were conducted with logistical support from the Scientific Research Commission in Baghdad, Iraq.
This research did not receive external financial support. Field activities and laboratory analyses were conducted with logistical support from the Scientific Research Commission in Baghdad, Iraq.
Conflict of Interest
The authors declare no conflicts of interest related to this study. The supporting institution was not involved in the study design, data collection, data analysis, interpretation of results, manuscript preparation, or the decision to publish the work.
The authors declare no conflicts of interest related to this study. The supporting institution was not involved in the study design, data collection, data analysis, interpretation of results, manuscript preparation, or the decision to publish the work.
Author Contributions (CRediT) Conceptualization: R.M.; Methodology: R.M., Kh.A.; Investigation: R.M., H.F.; Formal Analysis: R.M., Kh.A.; Data Curation: R.M.; Visualization: R.M.; Writing – Original Draft: R.M.; Writing – Review & Editing: R.M., H.F.; Supervision: R.M.; Funding Acquisition (internal support): R.M. All authors have read and approved the final version of the manuscript. Data Availability: The datasets generated and analyzed during the current study are available from the corresponding author upon reasonable request.
Ethical Approval
This study did not involve human participants or experimental animals; therefore, ethical approval was not required.
This study did not involve human participants or experimental animals; therefore, ethical approval was not required.
Informed Consent Not applicable.
Acknowledgments: The authors express their gratitude to the Scientific Research Commission in Baghdad, Iraq, for its logistical assistance throughout the fieldwork and laboratory analyses. Furthermore, the University of Baghdad is recognized for providing research facilities and analytical resources. The authors further thank colleagues who provided technical advice and engaged in methodological discussions during this work.
AI Use Disclosure. All scientific data presented in this study, including FTIR spectra and statistical analyses, were generated and interpreted by the authors using established scientific methods. No artificial intelligence system was used to produce or manipulate research data. Language polishing and formatting adjustments were performed with the assistance of generative AI tools under full author supervision. These tools were used solely for linguistic refinement and did not contribute to the study's scientific design, interpretation, or conclusions, in accordance with the editorial guidelines of BioNatura Journal.
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Received: April 4, 2026 / Accepted: May 28, 2026 / Published (Online First): May 29, 2026 / Issue Date: June 15, 2026 (Europe/Madrid)
Citation: Mouhamad R, Ahmad K, Al-Gburi HF. Soil texture influences salinity-associated changes in soil organic matter: evidence from FTIR analysis in Mesopotamian soils. BioNatura Journal: Ibero-American Journal of Biotechnology and Life Sciences. 2026;3(2):8. https://doi.org/10.70099/BJ/2026.03.02.8
Correspondence should be addressed to: raghad.s.mouhamad@src.edu.iq; ahmadki@umkc.edu
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