Soil heterogeneity refers to the uneven distribution of chemical, physical, and biological properties across space and time (Rezapour et al., 2022; Juleel et al., 2023). This variability is a common feature of natural soils but is especially pronounced in sodic and saline-sodic soils (Ondrasek and Rengel, 2021). The most visible feature is the irregular spatial distribution of salts, sodium, and alkalinity, forming patchy zones with contrasting soil conditions (Booltink et al., 2001). These patches appear as differences in soil color, surface crusting, or moisture content (Bazihizina et al., 2012). Horizontal variability develops through natural processes such as topographic gradients, hydrological flows, sediment deposition, and human agricultural practices (Figure 1) (Alharby et al., 2018; Delbari et al., 2019). Topography strongly shapes horizontal heterogeneity, as even slight elevation differences redistribute water, salts, and sediments, forming microzones with distinct properties (Kumawat et al., 2022). Nguyen et al. (2014) reported that higher-elevation areas receive more freshwater, while lower-elevation zones accumulate saline irrigation or natural seepage, creating variability within the same field. Similarly, in inland regions, shallow depressions often act as sink for salts transported through capillary rise or irrigation return flows while elevated positions remain comparatively less affected, resulting in a mosaic of saline and less saline patches (Dinneny, 2019). Recent field observations from irrigated marsh landscapes in Spain also demonstrated that irrigation return flows and variable irrigation water quality can produce marked salinity heterogeneity at the field scale, even under uniform cropping, highlighting the role of irrigation management in creating spatially variable salt patterns across agricultural fields (Egea et al., 2026).

Variability in soil texture also contributes to horizontal heterogeneity, as clay-rich zones retain sodium and drain poorly, while sandy soils promote faster salt leaching (Aldabaa et al., 2015). Such patterns reflect long-term geomorphic processes such as flooding, erosion, and sediment deposition (Zhou et al., 2024), indicating that present-day heterogeneity frequently reflects legacy soil-forming processes that predate current management. Human activities, including uneven tillage, ploughing, land levelling, and irregular irrigation combined with poor drainage or canal leakage, further intensify this variability by redistributing salts and organic matter across fields (Wang et al., 2023). The repeated use of brackish groundwater or low-quality irrigation water in certain parts of a field can exacerbate salt build-up locally, while zones closer to freshwater inlets may maintain comparatively lower salinity, reinforcing patchiness in salinity distributions and contributing to field-scale heterogeneity (Alharby et al., 2018). Fertilizer and amendments are also often applied unevenly, especially in smallholder systems, which further accentuates the patchy distribution of soil fertility and sodicity (Khan et al., 2023). In addition, historical land use practices such as grazing, manuring, crop rotations, and puddling leave persistent soil features including hardpan layers at different depths, which influence surface and subsurface salt accumulation patterns (Booltink et al., 2001). Long-term crop rotations that integrate rice with perennial pastures and livestock grazing further shape spatial soil properties by diversifying root architectures and organic matter inputs, which can buffer salt redistribution over time and promote more stable physical conditions compared with continuous rice monoculture (Lattimore, 1994; Vecchio et al., 2018). Biological heterogeneity also emerges as microbial communities, organic matter turnover, and root activity differ across micro zones due to variations in moisture, oxygen availability, and salt levels (Dinneny, 2019). For instance, zones with higher organic residue inputs often promote greater microbial activity and organic acid production, which locally modifies soil pH and sodium mobility (Rezapour et al., 2022). Recent advances in digital soil mapping, remote sensing, and geostatistical modelling now allow this horizontal heterogeneity to be quantified and visualized with increasing accuracy, proving valuable tools for understanding its formation and dynamics at the field scale and for designing spatially targeted management strategies (Gupta et al., 2020, Gupta et al., 2022).

Horizontal heterogeneity in saline-sodic soils strongly influences rice growth through the uneven distribution of salts, nutrients, and moisture across the field (Bazihizina et al., 2012; Liu et al., 2024; Yazhini et al., 2024). Chemical soil properties such as pH, EC, SAR, ESP, nutrient levels, and toxic ions (Na⁺, Cl^-, HCO₃^-) can change sharply over short distances (Mandal et al., 2008; Xu et al., 2020; Liu et al., 2024). These chemical imbalances disrupt nutrient availability and create localized zones of stress, resulting in patchy rice establishment and growth (Figure 2). In particular, areas enriched with Na⁺, HCO₃^- promote clay dispersion, poor aggregate stability, and reduced infiltration, which collectively create drought-like conditions in certain patches despite sufficient irrigation (Hafez et al., 2021). Such fine-scale chemical variability not only limits uniform rice establishment but also complicates field-level management practices.

Similarly, physical heterogeneity further modifies rice growth by altering soil bulk density, porosity, and texture, which regulate water movement, aeration, and root penetration (Mandal et al., 2008). The areas with clay-rich or compacted layers restrict infiltration and promote localized salt build-up, while sandy or well-structured areas enable leaching and maintain lower salinity and sodicity (Riaz et al., 2019). As a result, dynamic microenvironments develop within the root zone, where moisture, salinity, and oxygen levels fluctuate rapidly following irrigation or rainfall events (Preece and Peñuelas, 2020; Verma et al., 2024). Seasonal changes in bulk density and porosity, including surface crusting during dry periods and improved infiltration after wetting, further modify nutrient diffusion and availability to roots (Chuamnakthong et al., 2019). Also, variability in infiltration and water retention causes uneven salt leaching, intensifying spatial heterogeneity in salinity and sodicity (Singh et al., 2016; Hu et al., 2017). Consequently, patches with high EC and poor aeration restrict root density and nutrient uptake, while adjacent low-sodicity zones provide more favourable growth conditions (Li et al., 2017; Delgoda et al., 2023; Zhou et al., 2024).

Soil biological properties exhibit similar variability, as microbial diversity, enzymatic activity, and community composition respond to micro-scale shifts in salinity, sodicity, and moisture (Coonan et al., 2020). These uneven biological properties have strongly implications for rice growth because microbial processes regulate nutrient cycling, root colonization, and stress mitigation (Figure 2) (Yuan et al., 2007). High saline or compacted sodic patches often host reduced microbial biomass and enzymatic activity, forming “microbial deserts” that limit nutrient transformation and availability (Wijitkosum, 2020). In contrast, less-stressed areas support greater microbial richness, including beneficial groups such as arbuscular mycorrhizal fungi (AMF) and plant growth-promoting rhizobacteria (PGPR) (Sardinha et al., 2003). This patchy microbial distribution reinforces variability in nutrient supply, water use efficiency, and root development (Qin et al., 2016). Field studies show that bacterial and fungal communities can shift markedly over short distances (10–50 m) in sodic paddy soils, with some patches dominated by halotolerant taxa and others by plant growth-promoting microbes, leading to yield differences of up to 25% within the same field (Alharby et al., 2018; Feng et al., 2019; Khan et al., 2023; Wang et al., 2023). Such heterogeneity complicates fertilizers and water management because uniform applications fail to meet localized crop demands (Booltink et al., 2001). Currently, research on field-scale horizontal variability in rice paddies remains limited, with many studies still relying on spatially averaged measurements that obscure the true impacts of salinity and sodicity heterogeneity on rice performance (Dinneny, 2019).

Vertical soil variability refers to depth-dependent differences in physical, chemical, and biological properties along the soil profile (Corwin et al., 2003; Ashoka et al., 2023). It is most evident in gradients of salinity, sodicity, and alkalinity, which develop through the interaction of natural processes and land management practices (Bazihizina et al., 2012; Suska-Malawska et al., 2022). These gradients arise as salts move upward through evapotranspiration and capillary rise or downward through rainfall and irrigation-induced leaching, with soil properties regulate their redistribution and retention (Wang et al., 2017; Kashenge-Killenga et al., 2016). Seasonal fluctuations in water table and irrigation patterns further reinforce these gradients by altering the balance of salt and moisture within the profile (Qin et al., 2016). Multiple studies highlighted that soil salinity and sodicity levels can change markedly in response to rainfall variability, irrigation practices, and amendment applications, with climate factors often emerging as dominant controls on these shifts (Bazihizina et al., 2012; Li et al., 2022a). Furthermore, high salinity and sodicity are usually concentrated in the top 0-10cm layer due to evaporation, capillary rise, and insufficient surface water ponding, which lead to surface crust formation and surface sealing (Shahid et al., 2018; Liao et al., 2020). The 10–20 cm layer often retains higher levels of sodicity, while deeper layers (below 20 cm) generally exhibit lower salt concentrations, although compaction, poor drainage, and past waterlogging can still generate localized stress zones (Chi et al., 2012). The vertical distribution of salts is strongly influenced by soil texture, clay content, and cation exchange capacity, which regulate the transport and retention of soluble salts along the profile (Schubert and Qadir, 2024). In parallel, physical heterogeneity such as soil structure, bulk density, and porosity varies with depth and further shapes vertical heterogeneity (Yang et al., 2011). Dense or compacted subsoils limit water percolation and promote salt accumulation in upper horizons, and strengthen chemical gradients (Schubert and Qadir, 2024). Soil texture variations, such as clay-rich sublayers, further intensify vertical heterogeneity by trapping salts and modifying moisture retention (Wang et al., 2014; Osman, 2018). Furthermore, variation in root density and root exudation contributes to biological heterogeneity by altering microbial habitats and nutrient availability at different depths (Wang et al., 2017, Wang et al., 2024). The combined effects of chemical, physical, and biological stratification create a vertically layered environment that strongly influences root distribution, water uptake, and plant exposure to stress (Zhou et al., 2024). Despite its importance, most studies focus primarily on the upper 0–20 cm of the soil layer, with limited attention to deeper horizons (30-100), where critical constraints and processes persist (Liao et al., 2020). These subsoil layers can store significant amounts of salts, nutrients, and water, influencing long-term soil functioning, crop performance, and hydrological connectivity (Huang et al., 2022). Consequently, a comprehensive full-profile characterization is essential to understand salt accumulation patterns, drainage limitations, and nutrient cycling in saline-sodic systems (Wang et al., 2014).

Vertical heterogeneity in saline, sodic, and saline-sodic soils strongly affect soil functions and rice performance by creating depth-wise variation in chemical, physical, and biological properties (Qadir et al., 2007; Ahmadi and Souri, 2020). Chemical gradients are especially evident for soil pH, salinity, and sodicity indicators, nutrient levels, and harmful ion concentrations. Salts and alkaline ions commonly accumulate in the upper 0–20 cm due to limited leaching, capillary rise, and inadequate surface ponding, which elevates soil pH and reduces nutrient availability (Qadir et al., 2006; Hafez et al., 2021). These surface conditions restrict early root establishment, nutrient uptake, and photosynthetic efficiency, resulting in reduced tillering and biomass accumulation in rice (Figure 2) (Mandal et al., 2008; Roy et al., 2014). Elevated Na⁺ and carbonate/bicarbonate levels further disrupt ion homeostasis in plant tissue, affecting nutrient absorption and metabolism processes (Roy et al., 2014). In contrast, deeper soil horizons generally show lower salinity but are constrained by physical barriers such as compaction, poor drainage, and dense subsoil layers (Liu et al., 2023). These limitations restrict root penetration, water movement, and nutrient exploration, which collectively reduce rice growth potential and field-level productivity (Daliakopoulos et al., 2016). Vertical gradients in EC, pH, total carbon (TC), nitrogen content, C/N ratio, and exchangeable Ca²⁺, Mg²⁺, and Na⁺ have been reported even in short depth intervals, whereas available phosphorus and exchangeable potassium show high variability, even in relatively flat fields (Yana et al., 2000; Delgoda et al., 2023).

Furthermore, in sodic fields, the upper soil layer (0–20 cm) often exhibits severe sodicity and high bulk density, forming surface crusts that reduce porosity and infiltration (Jackson and Caldwell, 1996; Osman, 2018). These conditions exacerbate surface runoff, erosion, and localized waterlogging (Cemek et al., 2007). Beneath the compacted layer, poorly drained subsoil further restricts oxygen availability, although deeper layers may contain lower salt concentrations; they remain underutilized due to the barrier effect imposed by degraded surface layers (Speirs, 2010; Wang et al., 2017; Choudhary and Mavi, 2019). Moreover, the surface layers support higher microbial diversity, biomass carbon, and enzymatic activity, while subsoil layers accumulate salts, and organic matter depletion leads to reduced microbial abundance, altered community compositions, and suppressed enzyme activity (Yao et al., 2021). Kumawat et al. (2022) reported that microbial diversity and activity decline with depth due to lower organic matter availability, reduced oxygen diffusion, and greater salt accumulation. Consequently, surface layers function as hotspots of rhizosphere interactions, while subsurface horizons act as biological “deserts” characterized by microbial dormancy or mortality (Kumar et al., 2022). Li et al. (2020) highlighted that microbial biomass carbon and nitrogen declined by more than 40% in subsoil layers compared to surface layers of saline-sodic soils, while urease and phosphatase activities were reduced by nearly half. This vertical imbalance in microbial activity influences rhizosphere processes such as exudate decomposition, nitrogen transformation, and pathogen suppression, indirectly affecting rice growth and yield formation (Roy et al., 2014). Overall, the interaction among surface salt accumulation, soil compaction, and subsoil biological limitations generates complex challenges on root distribution, water uptake, and nutrient acquisition, posing major challenges for effective management of these systems. From a management perspective, horizontal and vertical soil heterogeneity require different intervention strategies. Horizontal heterogeneity is best addressed through spatially targeted, field-scale practices, such as localized amendment application, variable water management, and zoning of saline or sodic patches. In contrast, vertical heterogeneity requires depth-oriented interventions, including subsoil amelioration, improved drainage, deep leaching, and practices that enhance root penetration across restrictive layers. While horizontal heterogeneity mainly drives within-field variability in crop performance, vertical heterogeneity often limits the overall effectiveness of surface-applied treatments by restricting root access to deeper soil resources.Fageria, 20072.3 Temporal soil heterogeneity.

Temporal soil heterogeneity describes changes in soil properties over time that influence the distribution of salt and nutrients (Oliveira and Scheffers, 2019). These dynamics arise from the interaction among climatic factors, irrigation schedules, water management, crop cycles, and microbial activity (Figure 1) (Ali et al., 2014). For instance, rainfall events can temporarily dilute surface salts, whereas periods of high evaporation concentrate salts at the soil surface and in shallow horizons (Fageria et al., 2006; Fageria, 2007). Seasonal patterns such as during monsoon or wet seasons, surface leaching reduces salinity in the upper soil layers, while dry seasons promote salt accumulation through evapotranspiration and capillary rise (Wijayanto et al., 2023). Similarly, floods and droughts add further fluctuations, interacting with soil texture and microtopography to create both predictable seasonal trends and stochastic variability (Roy et al., 2014). Other, irrigation timing, method, and quality of water further shape these dynamics by controlling soil moisture and salt redistribution (Chhabra and Chhabra, 2021). For example, sodium-rich canal or groundwater irrigation can temporarily elevate surface sodicity, whereas subsequent freshwater irrigation may partially reverse these effects, resulting in dynamic chemical gradients (Singh et al., 2016). Repeated wetting and drying cycles alter soil physical properties through clay swelling and shrinkage, crack formation, and changes in porosity, affecting water and solute movement over time (Anil et al., 2005). These changes are especially pronounced in paddy soils due to the alternating flooded and drained conditions, which modify redox potential, nutrient availability, and seasonal salt distribution (Qadir et al., 2007; Morales et al., 2011; Chhabra and Chhabra, 2021). Hydrological processes such as capillary rise transport salts upward during dry periods, while rainfall-driven subsurface flow redistributes salts both horizontally and vertically (Bazihizina et al., 2012; Wijayanto et al., 2023). Biologically, organic inputs such as crop residues or manure decompose at different rates, causing fluctuations in nutrient availability and pH (Siddique et al., 2020). Nguyen et al. (2014) studied that microbial and enzymatic activities respond dynamically to changes in moisture, temperature, and salinity, altering nitrogen mineralization, phosphorus availability, and organic matter turnover throughout the growing season. These interactions create feedback loops in which biological processes both respond to and modify soil chemical and physical conditions (Li et al., 2022a). Understanding these temporal patterns is crucial for predicting soil behavior and designing adaptive management strategies in saline-sodic affected rice systems.

Temporal heterogeneity in sodic and saline-sodic soils generates shifting chemical, physical, and biological conditions that strongly influence rice establishment, nutrient uptake, and yield stability (Qadir et al., 2007; Ahmadi and Souri, 2020; Chhabra and Chhabra, 2021). Chemical properties, e.g., pH, EC, SAR, ESP, and nutrient availability, change over time, creating periods of intensified stress during sensitive rice growth stages (Qadir et al., 2006; Hafez et al., 2021). For example, EC often increased from the vegetative to ripening stage due to evapotranspiration, while pH fluctuates through water management and nutrient transformations, altering ion solubility and nutrient availability (Qadir et al., 2007; Barrera et al., 2020). Seasonal patterns have revealed rising EC in low-lying fields during January to September, while higher-elevation areas maintain relatively stable pH levels, highlighting stage-specific salt stress (Nguyen et al., 2014). Furthermore, temporal variation in micronutrient availability, such as Fe, Mn, and Zn across growth stages (pre-sowing, bunch formation, and flowering), further contributes to stage-specific nutrient limitation (Morales et al., 2011). Also, long-term temporal trend indicates gradual soil acidification under intensive rice systems, reflecting cumulative management effects (Bazihizina et al., 2012; Siddique et al., 2020; Huang et al., 2022).

Soil physical properties fluctuate in response to rainfall, irrigation frequency, and evapotranspiration, shaping root environment over time (Xu et al., 2020). Wet periods improve infiltration and reduce crusting, while dry periods increase compaction, lower porosity, and raise mechanical resistance (Grzesiak et al., 2002; Hafez et al., 2021). These shifts restrict root elongation, alter biomass allocation, and impair water and nutrient acquisition, contributing to yield losses (Grzesiak et al., 2002; Kumawat et al., 2022). Alternating waterlogging and drought can create paradoxical stress cycles, limiting effective root functioning and narrowing optimal management windows (Mukhopadhyay et al., 2023). This hydrological imbalance, combined with the sticky texture of wet sodic soils and extreme hardness when dry, limits suitable periods for tillage, planting, and irrigation (Roy et al., 2014; Hopmans et al., 2021). However, current studies have improved the measurement of individual soil physical changes and their impact on plant performance, yet significant gaps remain (Riaz et al., 2019). Many studies focus on single factors under short-term conditions without incorporating multi-scale spatial-temporal variability into predictive models (Guo et al., 2015).

Furthermore, microbial biomass, diversity, and enzymatic activity vary with rainfall, irrigation, and salt accumulation, influencing nutrient cycling and root interactions (Preece and Peñuelas, 2020). Qu et al. (2020) showed that dehydrogenase and β-glucosidase activities in saline sodic paddy soils peak after monsoon irrigation but drop by nearly 60% during dry-season salt buildup, influencing nitrogen and carbon availability across stages, while seasonal shifts in microbial community composition also alter root-microbe interactions. Similarly, microbial biomass carbon and enzyme activities such as dehydrogenase and phosphatase also fluctuate with crop growth stages, with peak activity during early vegetative growth when root exudation was high, and declines under late season saline-sodic stress (Deng et al., 2011; Singh et al., 2016; Otlewska et al., 2020). These seasonal shifts alter rhizosphere processes and amplify yield variability, particularly in rice systems reliant on shallow roots and microbial nutrient cycling (Kumawat et al., 2022). Besides, Kumar et al. (2022) highlighted that rice is especially sensitive to such biological heterogeneity due to their shallow root systems. Barrera et al. (2020) reported that zones of higher microbial activity coincided with improved aggregation and organic carbon retention, while adjacent barren patches remained dispersive and nutrient-poor. Similarly, Wijayanto et al. (2023) demonstrated that biologically active patches act as nutrient hotspots, whereas low activity zones restrict root growth, reinforcing feedback loops that intensify spatial and temporal yield variability (Wijayanto et al., 2023). Despite advances in measuring individual soil processes, significant gaps remain in integrating multi-scale spatial-temporal variability into predictive models and management frameworks (Chhabra and Chhabra, 2021). Addressing these gaps is essential for improving yield stability and sustainability in saline-sodic rice systems.

Organic amendments, derived from natural and waste resources such as farmyard manure, composts, crop residues, manures, and biochar, have been widely used to enhance the physical, chemical, and biological conditions of saline-sodic soils (Elmeknassi et al., 2024). In heterogenetic saline-sodic soils, variability in salinity, sodicity, and alkalinity generates fluctuating ionic stress and unevenly compacted soil layers that hinder root growth, water movement, and infiltration, leads to inconsistent root development and localized stress (Li et al., 2022b). By improving soil structure, nutrient availability, and microbial activity, organic amendments mitigate these complex challenges in rice production (Tejada and Gonzalez, 2006; Safdar et al., 2019). In these stratified soils, organic amendments not only enhance overall soil quality but also buffer variability across patches and depths, helping to stabilize the microenvironments that rice roots experience during sensitive reproductive stages. For instance, biochar has been shown to increase soil organic matter and stimulate microbial activity while simultaneously reducing bulk density and compaction, but its effects are not uniform (Phuong et al., 2020). Li et al. (2022b) reported that biochar enhanced rice root vigor and salt tolerance by lowering malondialdehyde (MDA) levels and improving the Na⁺/K⁺ ratio primarily in surface horizons, whereas deeper sodic layers remained less responsive. Similarly, Jin et al. (2018) found that biochar application improved biomass and yield, but the magnitude of the response varied with subsurface sodicity intensity. Furthermore, the combination of rice husk biochar and organic fertilizer significantly enhanced soil fertility, CEC, and reduced exchangeable Na⁺, although Wijitkosum (2020) reported that these improvements were more pronounced in zones with higher organic carbon accumulation. Besides, Sun et al. (2020) reported that biochar combined with fulvic acid enhanced porosity and water retention, but the magnitude of improvement varied depending on whether soil patches were previously compacted or moderately porous. Zhang et al. (2014) found that biochar with humic acid improved soil properties and crop yield, with greater benefits in surface saline crusts than in deeper sodic horizons. Dos Santos et al. (2021) further demonstrated that biochar derived from corn cobs and sugarcane bagasse improved pore connectivity and water movement, facilitating sodium leaching and reducing EC, SAR, and ESP. However, leaching effectiveness was inconsistent, where compact sodic subsoil layers restricted water flow. Moreover, the microbial dimension of heterogeneity adds further complexity. For example, Elmeknassi et al. (2024) reported that biochar improved microbial biomass and activity in the upper 15 cm of sodic soils but had little effect in subsoil layers, while Smaoui-Jardak et al. (2017) found that organic amendments restored enzymatic activity unevenly across field patches. Jin et al. (2020) found that compost stimulated beneficial microbes in some microsites but not in salt-accumulated patches. However, most studies remain short-term and surface-focused, with limited attention to subsoil processes, interannual variability, or the connections between multi-scale heterogeneity and rice yield.

Compost provides important insights into depth- and patch-specific amelioration of saline-sodic soils. For instance, Abdel-Fattah (2012) demonstrated that rice straw compost reduced EC, pH, SAR, and ESP, but improvements were greater in topsoil than subsoil layers. Similarly, Tejada and Gonzalez (2006) reported enhanced aggregation and structure stability following compost application although benefits were concentrated in patches with higher initial microbial activity, while compact sodic zones remained relatively resistant. Vermicompost, rich in Ca²⁺, further contribute to sodicity mitigation by promoting Na⁺ displacement through decomposition-induced acidification of CaCO₃, thereby reducing ESP (Rezapour et al., 2022). Juleel et al. (2023) reported improved rice yields through humus-soil-root interactions, but both studies highlight site specificity, with greater improvements in friable zones than in sodic hardpans. Luo et al. (2014) demonstrated that compost-biochar mixtures not only improved mean soil properties but also helped create more uniform root-zone conditions. Figure 4 has illustrated the benefits of compost application, especially saline-sodic soils, and its broader advantages straw incorporation and other organic residues also play key role in managing saline-sodic heterogeneity. Studies by Zhao et al. (2019) and Liao et al. (2020) showed that straw incorporation increased aggregate stability and disrupted capillary continuity, which prevents upward salt movement that typically re-salinizes surface horizons in stratified profiles. This disruption of vertical heterogeneity is critical in paddy systems, where alternating flooding and drying cycles create temporal fluctuations in salinity distribution (Wang et al., 2014). By enhancing porosity and stability, straw incorporation helps dampen these fluctuations and stabilize rice performance (Wijitkosum et al., 2020). Complementing soil-based organic amendments, foliar-applied humic biostimulants provide a plant-cantered approach to mitigate residual heterogeneity by stimulating root growth and nutrient uptake in zones where soil remediation is less effective, thereby strengthening rice performance under spatially variable saline-sodic conditions (Izquierdo et al., 2024). Other organic inputs including green manure, animal manures, whey, fly ash similarly exhibit site-specific benefits, with stronger effects in zones where salts remain mobile compared with structurally rigid sodic layers (Graber et al., 2006; Safdar et al., 2019; Yao et al., 2021).

Inorganic amendments such as gypsum, phosphogypsum (PG), calcium, chloride, calcite, elemental sulphur, and sulfuric acid are widely used to reclaim sodic and saline-sodic soils (Figure 5) (Tian et al., 2021). Inorganic amendments mitigate these constraints by modifying ion exchange processes, improving soil structure, and adjusting the chemical environment (Elmeknassi et al., 2024). Gypsum is the most commonly applied amendment because of its cost-effectiveness and high solubility. The Ca²⁺ ions it supplies replace exchangeable Na⁺ on soil colloids, lowering ESP and promoting the flocculation of dispersed clay particles (Abdel-Fattah, 2012). This restoration of soil aggregation reduces bulk density, enhances macroporosity, and improves water infiltration and salt leaching. In the heterogeneity fields, this mechanism is especially critical because sodic hotspots often form impermeable barriers while adjacent zones remain permeable (Rezapour et al., 2022). By stabilizing aggregates and reopening pore networks, gypsum restores hydraulic conductivity across the soil profile, promoting more uniform water movement and alleviating the patchy distribution of water and oxygen to rice roots (Lastiri-Hernandez et al., 2019). Moreover, increasing Ca²⁺ in the soil solution improves the Ca²⁺: Na⁺ ratios, which not only alleviates structural problems but also enhances plant ion uptake by limiting toxic Na⁺ accumulation in rice roots and shoots while promoting the absorption of essential nutrients such as K⁺ and Mg²⁺ (Kim et al., 2016). This is particularly relevant under heterogeneous conditions, where sodic microsites often exhibit nutrient depletion and ionic imbalance, whereas adjacent patches may not; gypsum helps reduce these disparities and ensures a more balanced nutrient supply across the field (Alcívar et al., 2018). The improved electrolyte balance in gypsum-treated soils also stimulates microbial activity, which is typically suppressed in alkaline sodic patches (Nan et al., 2016). This promotes more uniform processes such as carbon mineralization, enzyme activity, and nitrogen cycling, thereby reducing biological heterogeneity and supporting rice tillering and root development (Abdul Qadir et al., 2022).

Other amendments, such as sulfuric acid and elemental sulfur, act through complementary mechanisms that are particularly effective in sodic soils with low calcium carbonate solubility (Ilyas et al., 1993). Sulfuric acid reacts with soil CaCO₃, releasing Ca²⁺ ions that displace Na+ on exchange sites, a process that is especially valuable in alkaline microsites where pH is high and gypsum dissolution is limited (Abate et al., 2021). Elemental sulphur and pyrite undergo microbial oxidation under flooded conditions, generating in situ sulfuric acid, which mobilizes native Ca²⁺ and lowers pH (Abd El-Naby et al., 2019). These reactions are spatially variable but selectively target highly alkaline patches, thereby reducing heterogeneity in soil properties (Abd El-Naby et al., 2019). Similarly, ammonium sulfate contributes to reclamation not only as a nutrient source but also by releasing protons during NH₄⁺ uptake, which acidifies the rhizosphere and increases Ca²⁺ and Mg²⁺ availability, an advantage in sodic microsites where rice roots face nutrient lock-up (Abdel-Fattah, 2012). Together, these amendments lower ESP, also reducing carbonate and bicarbonate concentrations, thereby decreasing the residual sodium carbonate (RSC) index, a key indicator of sodicity stress in rice paddies.

Among these, PG is a by-product of sulfuric acid and phosphate rock reactions during phosphate production and is widely used as an inorganic amendment (Elmeknassi et al., 2024). It supplies both Ca²⁺ and SO₄^2- ions, which not only replace Na⁺ on exchange sites but also increase ionic strength in the soil solution, as a result suppressing clay dispersion (Tian et al., 2021). PG effectively ameliorates sodicity by supplying Ca²⁺ to replace exchangeable Na⁺, which improves aggregation, enhances salt leaching, modestly lowers soil pH, and ultimately promotes more uniform soil structure and reduces heterogeneity in saline-sodic paddies (Al-Enazy et al., 2018). A field study demonstrates that PG reduces ESP, improves the availability of P and K, and stimulates enzyme activities that regulate microbial communities (Alcívar et al., 2018). In rice paddies, where yield losses often result from uneven panicle development and poor grain filling in sodic-saline microsites, PG application enhances uniformity in yield components including panicle weight and seed-setting rate (Liu et al., 2024). This indicates that PG helps reduce the physiological heterogeneity of rice plants caused by spatial variability in soil constraints (Hasana et al., 2022). Although concerns have been raised about heavy metal accumulation, long-term trials confirmed that PG remains within safe limits when applied responsibly, though continued monitoring is recommended (Duan et al., 2023). The rationale for combining amendments lies in their complementary mechanisms, such as gypsum provides a sustained source of Ca²⁺, sulfuric acid accelerates the dissolution of native CaCO₃, and organic inputs enhance biological activity and carbon supply (Al-Enazy et al., 2018). For instance, gypsum combined with humic acid not only lowered ESP but also stabilized organic matter-mineral complexes that improved aggregation (Saifullah et al., 2018), while gypsum applied with sulfuric acid increased rice grain yields by 119% under severe sodic conditions (Huang et al., 2022). Such synergies are particularly valuable in heterogeneous fields, where no single amendment can simultaneously resolve structural, chemical, and biological disparities (Rezapour et al., 2022). However, reclaiming heterogenetic saline-sodic soils in rice paddies requires not only selecting appropriate amendments but also understanding their key role in reducing field variability, creating uniform root-zone conditions, and stabilizing yield formation under fluctuating stresses.

Organic and inorganic combined amendments are an effective strategy for ameliorating sodic and saline-sodic soils, where heterogeneity in salinity, sodicity, and alkalinity generates microsites of severe stress (Phuong et al., 2020). In contrast to single amendments, mixtures operate through complementary mechanisms e.g., organic inputs such as compost, farmyard manure, crop residues, or biochar enhance soil organic matter, microbial activity, and nutrient cycling, whereas inorganic amendments such as gypsum, PG, calcium chloride, elemental sulfur, or sulfuric acid supply soluble Ca²⁺ or protons that displace exchangeable Na⁺, decrease ESP, and facilitate the leaching of salts from the root zone (Figure 5) (Harris and Rengasamy, 2004). This dual action is particularly important in heterogeneous soils, where uneven Na⁺ accumulation causes localized clay dispersion and compaction; the incorporation of organic matter improves aggregation and pore connectivity, enabling Ca²⁺ from PG to move more uniformly through the soil profile and exert de-sodification effects across variable patches (Hasana et al., 2022). When organic matter decomposes, it produces low-molecular-weight organic acids and CO₂, which acidify the soil microenvironment, solubilize native CaCO₃, and increase the activity of Ca²⁺ in the soil solution, enhancing the effectiveness of inorganic amendments in displacing Na⁺ (Badia, 2000). In parallel, the organic component improves cation exchange capacity, buffers pH fluctuations, and creates conditions conducive to microbial growth, which accelerates the cycling of C, N, and S and fosters rhizosphere processes that support rice root tolerance under salinity-sodicity stress (Chi et al., 2012). For instance, Mahmoodabadi et al. (2013) demonstrated that applying PG at 30 t ha^-1 improved rice yield components by reducing ESP and enhancing soil physical properties. Details regarding different soil amendments, their application levels, and their specific effects on reclaiming salt-affected soils are provided in Table 2. Similarly, Kim et al. (2016) showed that combining PG with farmyard manure and wood peat further decreased soil pH and EC, increased nutrient availability, and stabilized yields through higher panicle weight and grain filling rates. These studies demonstrate that the interaction between organic and inorganic amendments is synergistic rather than merely additive, as organic matter improves soil structure and microbial activity, facilitating deeper and more uniform penetration of Ca²⁺ and SO₄^2- from PG, while the inorganic component rapidly decreases ESP, creating a more favorable ionic environment for rice roots. Importantly, this synergy helps overcome heterogeneity because in areas where high ESP causes clay dispersion and forms impermeable layers, gypsum or PG alone cannot percolate effectively, and when combined with organic amendments that improve aggregation and porosity, Na⁺ leaching becomes more uniform and spatial variability in sodicity stress is reduced (Elmeknassi et al., 2024). Abdel-Fattah (2012) reported that the application of ammonium sulfate in combination with organic amendments enhanced root-zone acidification, promoted the release of Ca²⁺ and Mg²⁺ from native minerals, and improved nutrient availability in sodic microsites. This process is particularly important in rice paddies, where standing water facilitates the vertical and horizontal redistribution of salts. Such mixtures also help reduce carbonate and bicarbonate concentrations, lower the RSC index, and establish a more favourable ionic balance across the field (Lastiri-Hernandez et al., 2019).

From a mechanistic perspective, the release of Ca²⁺ from gypsum or PG increases ionic strength in the soil solution, suppressing clay swelling and dispersion, while organic matter-derived polysaccharides act as binding agents, stabilizing aggregates and reducing variability in infiltration rates between different patches of the field (AbdEl-Mageed et al., 2020; Yazdanpanah et al., 2011). Over time, these structural improvements enhance water distribution and support more consistent root proliferation across microsites that would otherwise exhibit strong contrasts in porosity and aeration (Abate et al., 2021). The combined amendments also help in the mitigation of temporal heterogeneity. For example, long-term studies in Northeast China showed that the application of PG combined with organic residues not only reduced soil Na⁺, CO₃^2-, HCO^3-, ESP, but also maintained lower EC and improved microbial biomass. These findings suggest that mixture amendments contribute both to immediate chemical reclamation and to the long-term stabilization of soil function (Phuong et al., 2020). Furthermore, biochar addition in saline-sodic soils has been reported to increase microbial biomass and enzymatic activity in surface layers, but its effect in deeper layers remained limited unless combined with gypsum, which improved percolation and ion exchange throughout the soil profile (Alcívar et al., 2018). This demonstrates that the mixtures of amendments help to overcome the vertical stratification of reclamation effects, resulting in more uniform conditions for rice root systems and reducing the spatial patchiness of stress responses. Similarly, the chemical interactions between organic and inorganic components also play a critical role (Churka et al., 2013). For example, biochar in mixtures can adsorb phytotoxic compounds and salts, reducing their spatial concentration hotspots, while mineral additives like silicon-based amendments can enhance the stability of organic matter, showing its degradation and promoting sustained release of benefits (Nan et al., 2016). Studies in rice paddies have demonstrated that these mixtures not only improve soil health indicators but also mitigate yield variability across heterogeneous fields, with yield increases of 30-56% reported in treated areas compared to untreated controls (Graber et al., 2006). Importantly, such integrated strategies align with sustainability goals, as combining locally available organic residues with moderate rates of gypsum or PG reduces reliance on large single applications of chemical amendments, minimizes environmental risks associated with PG contaminants, and recycles on-farm organic wastes into soil improvement practices (Safdar et al., 2019).

Despite these benefits, relatively few studies have systematically examined the combined use of organic and inorganic amendments in heterogeneous saline-sodic soils. However, organic amendments alone can improve sodic soils; their effects are typically gradual, requiring longer timescales to achieve significant reclamation (Yao et al., 2021). In contrast, inorganic amendments such as gypsum act more rapidly but often lack long-term stability unless supported by organic inputs, as mentioned in Table 3. Although the combined use of gypsum, organic matter, and PGP microorganisms has been explored in some contexts, further research is required to clarify their synergistic role in reducing salinity and sodicity stresses under rice-based systems.