Engineered Biohydrogel Orchestrates Soil Hydro-Saline-Porous Microenvironments for Saline-Alkali Land Revitalization

Abstract

Soil salinization and compaction are tightly coupled degradation processes that severely limit agricultural productivity and ecological sustainability. However, simultaneously regulating soil water retention, salt removal, and structural reconstruction remains a major challenge for existing soil amendments. Herein, a hierarchically porous biomass-based double-network hydrogel (KGM–SA/B–Zn) is developed to synergistically regulate soil water–salt–structure interactions. The hydrogel integrates a dynamically borate-crosslinked konjac glucomannan (KGM) network with a Zn²⁺ crosslinked sodium alginate (SA) network, combining a highly hydrated porous framework with abundant ion-exchange sites. The KGM network provides efficient water storage and swelling-induced soil loosening, whereas the SA–Zn network enables selective sodium sequestration and reinforces structural stability. Together with the interconnected porous architecture, these complementary functions accelerate salt migration, enhance water retention, and reconstruct soil pore connectivity, thereby simultaneously mitigating salinity stress and soil compaction. Consequently, KGM–SA/B–Zn decreases exchangeable sodium content by 45%, significantly improves soil permeability and water-holding capacity, and promotes oat root development and biomass accumulation under saline conditions. Moreover, SA₂₀KGM₄₀ demonstrates exceptional environmental compatibility, achieving a biodegradation rate of 72.4% within 30 days. This work presents a multifunctional biomass-based hydrogel that integrates physical soil conditioning, chemical desalination, and structural reconstruction, providing a promising materials-design strategy for disrupting the salinization-compaction feedback loop and enabling the sustainable restoration of saline-alkali soils.

Graphical abstract

Keywords

Biomass-based hydrogel / Soil compaction / Water-salt regulation / Saline-alkali soil

Highlight

	● Biomass-based double-network hydrogel disrupts the salinization-compaction cycle.
	● Hierarchical pores enhance water retention and Na+ capture.
	● Double-network structure reconstructs soil pore architecture.
	● Exchangeable sodium decreases by 45% with improved oat growth.
	● Biodegradable hydrogel enables sustainable saline-soil remediation.

1 Introduction

Soil salinization represents one of the most pervasive forms of land degradation, threatening global food security and ecological sustainability by affecting over 20% of irrigated agricultural land worldwide [1-4]. Beyond the direct osmotic and ionic stresses imposed on plants, excessive accumulation of soluble salts, particularly exchangeable sodium ions, fundamentally disrupts soil architecture. Sodium-induced dispersion of clay particles triggers aggregate collapse, promotes compaction, and creates a cascade of structural failures including reduced porosity, impaired aeration, restricted water infiltration, and suppressed microbial activity [5-8]. Critically, salinization and compaction constitute a self-reinforcing cycle: structural degradation accelerates salt accumulation by impeding leaching, while escalating salinity further destabilizes soil aggregates [9]. Breaking this salinization-compaction feedback loop demands materials that can simultaneously sequester harmful ions and reconstruct soil pore networks, an objective that existing remediation technologies have yet to achieve.

Biomass-based hydrogels have emerged as promising candidates for soil amendment, offering renewability, environmental compatibility, and exceptional water-retention capacity through their highly hydrated polymer networks [10-12]. By absorbing and storing substantial amounts of water, these materials mitigate physiological drought and enhance crop resilience under saline conditions [13]. Furthermore, abundant hydroxyl and carboxyl functional groups provide active sites for ion adsorption and exchange, enabling localized reduction of salt concentrations in soil solutions [14,15]. However, current hydrogel-based strategies suffer from a fundamental disconnect: they optimize water conservation and ion sequestration as isolated functions while neglecting the structural dimension of soil rehabilitation. Conventional single-network hydrogels exhibit limited mechanical stability and typically collapse after repeated swelling-deswelling cycles, rendering them incapable of regulating soil pore architecture or resisting compaction forces [16,17]. Consequently, the coupled remediation of salinity and structural degradation remains beyond the reach of existing hydrogel systems.

Recent advances in hydrogel engineering have established that network architecture governs water transport, ion diffusion, and mechanical performance in an intimately coupled manner [18-20]. Double-network (DN) hydrogels, comprising two interpenetrating polymer networks with complementary mechanical and functional properties, have demonstrated remarkable potential for simultaneously enhancing swelling capacity and structural robustness [21-23]. Yet, the design principles underlying most reported DN systems remain misaligned with the specific physicochemical demands of saline-alkali soils: they prioritize either maximum water retention or mechanical reinforcement, but rarely address the dual requirements of ion capture and soil-structure regulation within a unified framework [24-26]. Increasing crosslinking density improves network stability but constricts water uptake and ion migration pathways, whereas highly swollen hydrogels sacrifice mechanical integrity and operational longevity in dynamic soil environments [27,28]. Achieving a harmonious balance among water retention, ion adsorption, mechanical durability, and soil-structure regulation therefore represents a critical materials-design challenge.

Herein, we report a hierarchically porous biomass-based double-network hydrogel (denoted KGM–SA/B–Zn) fabricated through selective crosslinking between a konjac glucomannan (KGM)-borate dynamic network and a Zn²⁺ crosslinked sodium alginate (SA) network, specifically designed to disrupt the soil salinization. The KGM framework provides a highly swollen porous skeleton that stores water and generates swelling pressure to loosen compacted soil matrices, while the SA–Zn network enhances mechanical stability and introduces abundant ion-exchange sites for selective sodium capture. Benefiting from the synergistic coupling between the double-network architecture and hierarchical pore structure, the KGM–SA/B–Zn hydrogel simultaneously promotes water retention, facilitates salt migration, and reconstructs soil pore networks. Systematic soil-column and plant-growth experiments demonstrate that the KGM–SA/B–Zn hydrogel reduces exchangeable sodium content by 45%, significantly improves soil permeability and water-holding capacity, enhances oat root development and biomass accumulation, and exhibits a biodegradation rate of 72.4% within 30 days. This study establishes a multifunctional remediation paradigm that integrates physical soil conditioning, chemical desalination, and biological promotion, offering a materials-driven strategy for breaking the salinization-compaction cycle and advancing the sustainable utilization of saline-alkali soils.

2 Experimental Section

2.1 Materials

Saline-alkali soil was collected from an agricultural field in Jiuquan, Gansu Province, China. After collection, the soil was air-dried at room temperature, and visible plant residues, stones, and other impurities were manually removed. The dried soil was then ground and passed through a 2 mm sieve prior to use. The initial electrical conductivity (EC) and pH of the soil were 2.4 dS·m⁻¹ and 7.463, respectively. The initial physicochemical properties of the untreated soil used as the control are presented in the corresponding control-group data in the Results section; Konjac glucomannan (KGM) was provided by BioBiome (RUIBIO); sodium alginate (viscosity: 200–500 MPa·s) was supplied by Damas-beta; sodium tetraborate decahydrate of analytical purity was obtained from Hunan Huihong Reagent Co., Ltd., and high-purity sodium tetraborate decahydrate (99.5%) was purchased from Shanghai Macklin Biochemical Co., Ltd.

2.2 Preparation of KGM–SA/B–Zn

Sodium tetraborate decahydrate (2 g) was dissolved in 100 mL of deionized water to obtain a borax solution with a concentration of 0.02 g/mL. Separately, konjac glucomannan (KGM) powder (1 g) and sodium alginate (SA) powder (1 g) were each dissolved in 100 mL of deionized water under magnetic stirring for 4 h, yielding 0.01 g/mL KGM and SA stock solutions, respectively. The KGM and SA solutions were then mixed at volume ratios of 2:1, 3:1, and 4:1, and the mixtures were stirred magnetically for 3 h to ensure complete homogenization. Subsequently, the borax solution was added to each KGM–SA blend at a volume ratio of 1:30 (borax:blend), and the resulting systems were stirred for an additional 2 h under magnetic agitation to achieve full crosslinking homogeneity. Each homogenized mixture was poured into a glass dish and dried in an oven at 50 °C until a film was formed. The obtained KGM–SA–borax (KGM–SA–B) films were then immersed in a 0.01 mol/L zinc acetate aqueous solution for 30 min. After removal, the surface-crosslinked hydrogel films were gently blotted dry with filter paper, yielding the final KGM–SA-based double-network hydrogels.

2.3 Characterization methods

2.3.1 Inductively coupled plasma spectroscopy

The soil leachates from soaked and unsaturated samples are each diluted 100-fold; typically, the samples must be filtered to remove suspended particles that could interfere with the analysis. The samples are then introduced into the ICP instrument via the sample introduction system. In the nebulizer, the sample is atomized into fine droplets, which are then introduced into the ICP plasma. In the high-temperature plasma, the sample droplets are vaporized and dissociated into atoms and ions. The ICP spectrometer analyzes the ions in the sample and measures their relative intensities. The concentrations of each element in the sample are calculated based on data from known standard solutions.

2.3.2 Simultaneous thermal analysis

Weigh 5–10 mg of the sample and distribute it evenly in an alumina crucible. Subsequently, under a nitrogen atmosphere, perform a programmed temperature ramp at a preset rate of 10 °C/min, starting at 30 °C and rising to 800 °C. During this process, the change in sample weight loss was recorded in detail to obtain its thermal weight loss profile.

2.3.3 Contact angle testing

The KGM–SA/B–Zn was cut into regular 5 mm × 5 mm slices and freeze-dried. Subsequently, in a constant temperature and humidity environment at 25 °C, a contact angle measuring instrument (JCY-2) was used to vertically dispense droplets of 1% sodium chloride, sodium bicarbonate, calcium chloride, magnesium chloride, and magnesium sulfate solutions onto the hydrogel surface using a micropipette. Upon contact with the sample, the solid–liquid interface morphology was automatically captured using the static drop method. Each sample was tested at three different locations, and the arithmetic mean was calculated. The static contact angle values for each electrolyte solution system were obtained through fitting calculations, thereby characterizing the differences in the effects of various ion types on the wetting properties of the hydrogel surface.

3 Results and Discussion

3.1 Design and fabrication of KGM–SA/B–Zn hydrogel

The KGM–SA/B–Zn hydrogel was fabricated through a sequential dual-crosslinking strategy involving borate-mediated dynamic crosslinking and Zn²⁺ coordination interactions. As illustrated in Fig. S1A, KGM and SA were first dissolved to form a homogeneous polysaccharide precursor solution. Borate ions were subsequently introduced to establish reversible B–O coordination bonds with the abundant cis-diol groups of KGM, generating the first dynamic network (KGM–SA–B). Finally, Zn²⁺ ions were incorporated to coordinate with the carboxyl groups of SA, resulting in the formation of a secondary ionic network and producing the final KGM–SA/B–Zn double-network hydrogel. The detailed synthesis parameters are summarized in Table S1. This molecular design is deliberately asymmetric: the KGM–borate network functions as a highly hydrophilic, dynamically adaptable scaffold that provides efficient water-storage channels and swelling-induced soil-loosening capacity, while the SA–Zn network serves as a rigid, ion-active framework that contributes mechanical reinforcement and abundant carboxyl-based ion-exchange sites. The integration of these complementary functions within a single hierarchically porous architecture constitutes the structural basis for the hydrogel's multifunctional remediation performance (Fig. 1A).

The microstructural evolution of the hydrogel was examined through SEM and TEM analyses. The pristine KGM–SA hydrogel displayed a loosely interconnected network with irregular pore morphology and broad pore-size distribution (Fig. 1B), attributable to insufficient intermolecular interactions between KGM and SA chains that result in localized defects and nonuniform network formation. Upon introduction of borate ions, the KGM–SA–B hydrogel displayed a markedly denser and more homogeneous porous structure (Fig. 1C). The formation of dynamic B–O bonds strengthened intermolecular crosslinking and suppressed uncontrolled pore growth, leading to reduced pore size and improved structural uniformity. Upon further incorporation of Zn²⁺ ions, the KGM–SA/B–Zn hydrogel exhibited a significantly refined porous architecture characterized by interconnected honeycomb-like microdomains (Fig. 1D) [29]. The strong coordination interactions between Zn²⁺ ions and alginate carboxyl groups further elevated the crosslinking density and facilitated ordered network assembly, driving a progressive structural transition from a loose, disordered framework to a compact, hierarchically organized architecture [30]. EDS elemental mapping confirmed that carbon (C), oxygen (O), boron (B), and zinc (Zn) elements were uniformly distributed throughout the entire hydrogel matrix (Fig. 1E), validating the successful integration of borate and Zn²⁺ into the polymer network [31]. The homogeneous distribution of B and Zn indicates that dynamic B–O bonds and Zn²⁺ coordination interactions are formed throughout the three-dimensional framework rather than being localized in specific regions [32]. This uniform dual-crosslinked structure ensures that the hydrogel provides abundant active sites for ion exchange and adsorption across the entire network volume.

TEM observations reveal a progressive microstructural evolution from a loosely organized network to a highly integrated hierarchical architecture upon sequential incorporation of boron and zinc ions. As shown in Fig. S2A, KGM–SA exhibits a relatively disordered porous framework composed of irregularly interconnected polymer chains. The presence of heterogeneous pore domains and rough internal surfaces suggests an insufficient crosslinking degree, resulting in localized structural defects and broad pore-size distributions. The introduction of boron markedly restructures the polymer network. In KGM–SA–B (Fig. S2B), the formation of borate-diol complexes promotes additional intermolecular crosslinking, driving the transition from a randomly distributed framework to a more compact and homogeneous layered architecture. Consequently, interlayer spacing is reduced, pore-size distribution becomes narrower, and structural heterogeneity is significantly suppressed. Such network densification indicates that boron-mediated dynamic crosslinking effectively constrains polymer-chain mobility and inhibits excessive pore growth during gel formation.

More importantly, the subsequent incorporation of Zn²⁺ further reinforces the network through strong coordination interactions with hydroxyl and carboxyl groups along the polymer backbone. As evidenced in Fig. S2C, KGM–SA/B–Zn develops a highly interconnected honeycomb-like porous structure characterized by refined pore dimensions and abundant interlayer junctions. This hierarchical architecture suggests the establishment of a dual-crosslinking network, in which dynamic borate bonds and Zn²⁺ coordination bonds cooperatively regulate network assembly [33]. The synergistic effect not only increases crosslinking density but also promotes a more uniform stress distribution throughout the matrix.

3.2 Bonding mechanism and structural construction of KGM–SA/B–Zn

To elucidate the formation mechanism of the KGM–SA/B–Zn, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses were employed to investigate the molecular interactions and hierarchical assembly process of the network. Initially, borax reacted with the cis-diol groups of KGM chains to generate a flexible primary network (KGM-B). As shown in Fig. 2A, The broadening of the hydroxyl stretching band at 3383 cm⁻¹ together with the emergence of the characteristic B–O–C vibration at 1020 cm⁻¹ confirms the formation of dynamic borate ester linkages [34]. Owing to the reversible nature of these covalent bonds, the KGM–B network provides abundant water-accessible domains while maintaining structural adaptability. Subsequently, Zn²⁺ ions coordinated with the carboxylate groups of SA to establish a rigid secondary network (SA–Zn). The red shift of the carboxylate stretching vibration at 1600–1650 cm⁻¹ indicates the formation of Zn-carboxylate coordination complexes, which serve as stable junction points within the polymer matrix (Fig. 2B) [35].

More importantly, FTIR spectra of the KGM–SA/B–Zn hydrogel (Fig. 2C) simultaneously exhibit the characteristic B–O–C band (1020 cm⁻¹) and Zn–O coordination signal without obvious peak displacement, suggesting that the two crosslinking motifs coexist without mutual interference [36]. This observation indicates excellent molecular-level compatibility between the dynamic KGM–B network and the rigid SA–Zn network, enabling the formation of an interpenetrating dual-network architecture [37]. Such a configuration integrates the high swelling capability of the KGM–B framework with the mechanical robustness imparted by the SA–Zn network, thereby establishing a synergistic balance between flexibility and structural stability.

The assembly mechanism was further verified by XPS analysis. As shown in Figs. 2D and 2E, the appearance of the characteristic B 1s peak at 191.5 eV confirms the successful incorporation of borate species into the hydrogel network [38]. Combined with the FTIR results, these findings demonstrate that dynamic borate ester bonds and Zn²⁺ coordination bonds constitute two distinct yet cooperative crosslinking pathways. Notably, the evolution of the O 1s spectra (Figs. 2F and 2G) provides direct evidence of interfacial reconstruction during network formation [39]. The proportion of C–OH species decreases significantly from 58.3% in pristine KGM–SA to 34.7% in KGM–SA–B–Zn, indicating the consumption of hydroxyl groups through borate esterification and metal coordination processes [40]. Meanwhile, the contribution of the C=O/O–Zn component increases from 28.5% to 49.2%, confirming the enhanced coordination interactions within the SA–Zn network. These changes suggest that the incorporation of borate and Zn²⁺ not only increases crosslinking density but also strengthens intermolecular interactions, leading to a more integrated and mechanically stable network structure.

The establishment of this dual-crosslinked architecture is expected to fundamentally influence the environmental durability of the hydrogel. Differential scanning calorimetry (DSC) and thermogravimetric analysis/derivative thermogravimetry (TG/DTG) analyses were therefore conducted to evaluate the effect of borate crosslinking on thermal stability. As shown in Figs. 2H and 2I, the KGM–SA–B–Zn hydrogel exhibits a pronounced endothermic transition within 150–200 °C, which shifts approximately 25 °C toward higher temperatures compared with KGM–SA–Zn. This behavior indicates that the introduction of borate ester bonds restricts polymer-chain mobility and increases the thermal energy required for network dissociation. Consistent with this observation, TG/DTG results reveal that the initial decomposition temperature increases from 218 °C to 245 °C, while the maximum decomposition temperature shifts from 305 °C to 328 °C after borax incorporation. Such improvements can be attributed to the cooperative stabilization effect of dynamic B–O–C covalent bonds and Zn²⁺ coordination interactions, which effectively suppress chain scission and thermal degradation. Moreover, cyclic compression tests (Fig. S3) demonstrate that the hydrogel maintains highly stable loading-unloading behavior over 30 consecutive compression cycles. The compression curves largely overlap with only negligible stress attenuation, while the hydrogel rapidly recovers its original shape after each unloading process, indicating excellent elastic recovery and fatigue resistance. This outstanding mechanical resilience originates from the reversible dissociation and reformation of dynamic borate ester bonds coupled with the Zn²⁺-coordinated alginate network, which efficiently dissipate mechanical energy while preserving the integrity of the three-dimensional framework. These results demonstrate that the synergistic integration of dynamic borate ester bonds and Zn²⁺ coordination bonds drives the hierarchical assembly of a structurally integrated dual-network hydrogel. [41]. The resulting architecture simultaneously provides high water-retention capability, rapid recovery after repeated deformation, mechanical robustness, and enhanced thermal stability, thereby establishing a durable functional platform for long-term saline-alkali soil remediation.

3.3 Swelling properties of KGM–SA/B–Zn

The swelling behavior of hydrogels plays a critical role in determining their water-retention capability, ion-transport efficiency, and soil-conditioning performance [42]. To elucidate the ion-responsive characteristics of the KGM–SA KGM–SA/B–Zn, the wettability and swelling behaviors of hydrogels with different KGM contents were systematically investigated in representative saline-alkali solutions. As shown in Figs. 3Ai, 3Aii and S4 all KGM–SA/B–Zns exhibited lower contact angles toward solutions containing divalent cations (Ca²⁺ and Mg²⁺) than toward monovalent salt solutions, indicating enhanced interfacial affinity. This behavior originates from the abundant hydroxyl and carboxyl groups distributed throughout the dual-network framework, which can interact strongly with multivalent ions through coordination and electrostatic interactions. Among the investigated samples, SA₂₀KGM₆₀ displayed the lowest contact angle in MgSO₄ solution, suggesting that an optimized crosslinking density facilitates efficient ion-polymer interactions while preserving sufficient network accessibility. In contrast, the slightly increased contact angle observed for SA₂₀KGM₈₀ indicates that excessive crosslinking restricts pore accessibility and reduces effective surface wettability. These results demonstrate that the KGM–SA/B–Zn possesses selective affinity toward divalent salt ions, a characteristic closely associated with its potential for capturing Ca²⁺ and Mg²⁺ species in saline-alkali soils.

The ion-selective wettability directly influences the swelling behavior of the hydrogel. As shown in Figs. 3Bi, 3Bii and S5i, the swelling capacity decreases progressively with increasing KGM content regardless of the salt environment. For example, the equilibrium swelling ratio in NaHCO₃ solution decreases from approximately 25 g·g⁻¹ for SA₂₀KGM₄₀ to 15 g·g⁻¹ for SA₂₀KGM₈₀, while corresponding values in CaCl₂ and MgCl₂ solutions decrease from 10 and 5 g·g⁻¹ to 6 and 3 g·g⁻¹, respectively. This trend reflects the increasing density of the borate-crosslinked KGM network, which restricts polymer-chain relaxation and limits water penetration. Consequently, the network expansion driven by osmotic pressure becomes increasingly constrained.

The type of dissolved ions further regulates hydrogel swelling through distinct physicochemical mechanisms (Figs. 3Ci, 3Cii and S5ii). Among all tested solutions, NaHCO₃ induces the highest swelling ratios, with SA₂₀KGM₄₀ reaching approximately 25 g·g⁻¹. The weak alkalinity of HCO₃⁻ promotes the deprotonation of carboxyl groups, increasing the concentration of negatively charged –COO– species and thereby enhancing electrostatic repulsion within the network. The resulting increase in Donnan osmotic pressure favors network expansion and water uptake. In contrast, divalent cations such as Ca²⁺ and Mg²⁺ act as secondary ionic crosslinkers by bridging neighboring carboxyl groups, effectively increasing network density and suppressing hydrogel expansion. As a result, substantially lower swelling ratios are observed in CaCl₂, MgCl₂, and MgSO₄ solutions.

The influence of ion concentration further highlights the adaptive nature of the dual-network architecture (Figs. 5Ai, 5Aii, 5Bi and 5Bii). The swelling ratio of SA₂₀KGM₄₀ reaches nearly 30 g·g⁻¹ in 3% NaHCO₃ solution, approximately 2.5-fold higher than that observed in NaCl at the same concentration. Conversely, strong ionic crosslinking induced by Ca²⁺ limits the swelling ratio to approximately 8 g·g⁻¹. Increasing the KGM content from 40% to 80% reduces the swelling ratio in NaHCO₃ from 30 to 15 g·g⁻¹, indicating that the synergistic reinforcement provided by borate ester bonds and Zn²⁺ coordination interactions effectively suppresses excessive network expansion under highly alkaline conditions. Notably, SA₂₀KGM₆₀ exhibits enhanced swelling performance in concentrated NaCl solutions, suggesting that an intermediate crosslinking density optimally balances network rigidity and flexibility, thereby mitigating charge-screening effects while maintaining sufficient water accessibility.

Swelling kinetics further support this mechanism. As shown in Figs. 3Ci and 3Cii, all hydrogels achieve 50%–70% of their equilibrium swelling within the first two hours, whereas the equilibrium time increases from approximately 6 h for SA₂₀KGM₄₀ to 12 h for SA₂₀KGM₈₀. The slower swelling kinetics observed at higher KGM contents indicate that the denser network architecture imposes greater diffusion resistance to water transport. Therefore, the swelling behavior of the KGM–SA/B–Zn is not solely controlled by osmotic pressure but is cooperatively regulated by ion-specific interactions and network-constrained diffusion.

To further verify the ion-capturing capability of the KGM–SA/B–Zn, elemental mapping analysis was conducted on SA₂₀KGM₆₀ after immersion in NaHCO₃ solution. As shown in Fig. 3D, carbon is uniformly distributed throughout the matrix, confirming the structural integrity of the interpenetrating network. More importantly, the distribution of Na and Zn reveals active ion-exchange and coordination processes within the hydrogel. The widespread presence of Na indicates its incorporation into the network through electrostatic interactions, while Zn remains concentrated around crosslinking domains, serving as both a structural stabilizer and a potential adsorption center. The coexistence of hydroxyl-rich KGM chains and carboxyl-rich SA chains generates a gradient distribution of adsorption sites, enabling ion capture through a combination of ion exchange, electrostatic attraction, and surface complexation. Such cooperative adsorption behavior endows the hydrogel with the ability to selectively immobilize salt-forming ions while maintaining its structural stability.

3.4 Soil improvement effects of KGM–SA/B–Zns

Owing to the synergistic integration of borate-crosslinked KGM networks and Zn²⁺ coordinated SA frameworks, the KGM–SA DN hydrogel functions as a multifunctional platform capable of regulating both the physicochemical and hydraulic properties of saline-alkali soils [43]. As shown in Fig. 4A, the electrical conductivity of soil leachates decreased markedly with increasing hydrogel dosage. Compared with the untreated soil, conductivity decreased by 18.7% and 42.5% at hydrogel application rates of 1% and 3%, respectively. Notably, SA₂₀KGM₈₀ reduced the conductivity to below 1.0 mS·cm⁻¹ at a 3% dosage, indicating efficient removal of soluble salt ions from the soil solution. This behavior originates from the abundance of hydroxyl and carboxyl groups distributed throughout the interpenetrating network, which provide ion-binding sites for electrostatic attraction, ion exchange, and surface complexation. Consequently, the ionic strength of the soil solution is substantially reduced, creating a more favorable environment for root development.

The decrease in soil pH further confirms the chemical regulation capability of the hydrogel (Fig. 4B). Following hydrogel incorporation, soil pH decreased from 7.46 to 6.7–7.2, approaching the range favorable for nutrient availability and plant growth. This buffering effect can be attributed to the gradual release of protons from residual carboxylic groups and the dynamic Na⁺/H⁺ exchange occurring within the hydrogel matrix. Among all formulations, SA₂₀KGM₈₀ exhibited the strongest buffering capacity, highlighting the contribution of increased functional-group density to alkali neutralization. The reduction in soil salinity is closely associated with selective ion sequestration. As shown in Fig. 4C, SA₂₀KGM₆₀ removed 16.52% of Na⁺, 42.71% of Ca²⁺, and 15.68% of Mg²⁺ from the soil solution. Such ion selectivity originates from the hierarchical dual-network architecture. The carboxyl-rich SA domains preferentially interact with multivalent cations through coordination and electrostatic interactions, whereas the porous KGM framework provides interconnected transport channels that facilitate ion diffusion and capture. The coexistence of hydroxyl and carboxyl groups further generates heterogeneous adsorption sites, enabling progressive ion exchange and stabilization of salt-forming ions within the hydrogel network. As a result, the hydrogel effectively interrupts the accumulation of soluble salts and mitigates secondary salinization.

Beyond chemical regulation, the hydrogel substantially alters the physical structure of saline-alkali soils [44]. As shown in Figs. 4D–4F, hydrogel-treated soils exhibited lower bulk-density increases, higher water-holding capacities, and enhanced volume stability compared with the untreated control. In particular, the bulk-density increase in the SA20KGM60 treatment was 24.3% lower than that of the control, while the maximum water content reached 50.57%. Meanwhile, the volume expansion of hydrogel-amended soils reached 68%, in sharp contrast to the 12.7% volume shrinkage observed in untreated soils. These results indicate that the hydrogel acts as a dynamic structural scaffold. Upon water absorption, the three-dimensional network expands and occupies pore spaces, preventing particle sedimentation and maintaining pore connectivity. Simultaneously, the hierarchical pore architecture promotes rapid water infiltration through macropores while retaining water within mesopores through capillary forces and hydrogen-bond interactions. The cooperative effect of these processes reconstructs soil porosity and improves aggregate stability. Consistent with these structural improvements, the physicochemical properties of the treated soil were also markedly enhanced (Table S2). Specifically, the organic carbon content increased from 0.70% to 0.99%, while the total salt content decreased from 12.0 to 8.2 g·kg⁻¹. Moreover, the soil basicity was reduced from 54.4% to 34.5%, indicating effective alleviation of soil salinization and alkalization. These results further demonstrate that the KGM–SA–B–Zn hydrogel simultaneously improves the physical structure and chemical quality of saline-alkali soil, thereby creating a more favorable environment for sustainable plant growth.

The reconstructed soil structure directly contributes to enhanced water retention. As shown in Fig. 4G, all hydrogel-treated soils retained moisture more effectively than the control, with SA₂₀KGM₆₀ maintaining approximately 68% of its initial water content after 72 h. This value is 18.5% and 12.3% higher than those of SA₂₀KGM₄₀ and SA₂₀KGM₈₀, respectively. The superior performance of SA₂₀KGM₆₀ suggests that an optimal balance between network flexibility and crosslinking density is critical for maximizing water-storage efficiency. Excessive swelling in low-crosslinked systems accelerates water release, whereas overly dense networks restrict water uptake. The intermediate crosslinking density therefore provides the most effective water-management capability. Long-term functionality is equally important for practical soil remediation. As shown in Fig. 4H, SA₂₀KGM₆₀ maintained a stable swelling ratio after six swelling-deswelling cycles in 0.9% NaCl solution, demonstrating excellent resistance to ionic shielding and network collapse. The dynamic borate bonds continuously reorganize under environmental stress, while Zn²⁺ coordination junctions preserve structural integrity. Such adaptive stabilization ensures sustained water-retention performance under saline conditions and minimizes pore blockage caused by salt crystallization.

The cumulative improvements in soil physicochemical properties ultimately translate into enhanced biological performance. As shown in Fig. 4I, oat seed germination increased dramatically following hydrogel treatment, reaching 80% in the SA₂₀KGM₄₀ group compared with only 20% in untreated soil. The enhanced germination performance can be attributed to the combined effects of salinity reduction, pH buffering, improved water availability, and increased soil porosity. By simultaneously regulating water and ion transport, the hydrogel creates a low-salinity, weakly alkaline microenvironment conducive to seed germination and early root development. Further evidence of soil reconstruction is provided by the changes in soil quality indicators. Hydrogel treatment significantly reduced soil alkalinity and exchangeable sodium content while increasing cation exchange capacity by approximately 30%.

The significant rise in soil organic carbon content (Figs. 5Ci–5Ciii), which is 25% higher than that in untreated soil, can be attributed to the gradual biodegradation of the polysaccharide-based hydrogel network and the enhancement of microbial metabolic activity. The accumulated organic matter not only promotes the formation and stabilization of soil aggregates but also provides abundant functional groups capable of complexing cations, thereby enhancing the immobilization of excessive salt ions. Concurrently, the substantial reduction in water-soluble salt content (Figs. 5Civ and 5Cv), with a maximum decrease of 45%, demonstrates the hydrogel’s ability to effectively regulate ionic dynamics within the soil matrix. Benefiting from the synergistic action of SA–Zn²⁺ coordination interactions and KGM-mediated hydrogen-bonding networks, the KGM–SA/B–Zn forms a dynamic adsorption framework that selectively captures mobile ions such as Na⁺ and Cl⁻, reducing their activity and mobility in the soil solution. Consequently, osmotic stress and ion toxicity imposed on plant roots are markedly alleviated. These improvements indicate that the hydrogel not only suppresses sodium accumulation on soil colloids but also introduces abundant active functional groups that enhance nutrient retention and exchange. Consequently, the soil exhibits improved structural stability, greater nutrient-buffering capacity, and increased resistance to salinization.

3.5 Microbial compatibility and environmental degradability of KGM–SA/B–Zns

Beyond physicochemical regulation, ecological compatibility is a critical prerequisite for the practical deployment of soil amendments in saline-alkali environments [45]. Therefore, evaluating the interactions between KGM–SA/B–Zns and representative microbial species is indispensable for assessing their environmental safety. The microbial toxicity assessment revealed negligible inhibitory effects of the KGM–SA/B–Zns on the growth of Escherichia coli and Staphylococcus aureus (Figs. 6A and 6B). Following exposure to hydrogel suspensions, bacterial colony numbers remained comparable to those of the control group, with the SA₂₀KGM₆₀ hydrogel retaining 98.2% and 96.5% of the viable colonies of E. coli and S. aureus, respectively. These results demonstrate the excellent microbial compatibility of the KGM–SA/B–Zn system. Such behavior can be attributed to the intrinsically benign nature of the hydrogel components, which are primarily composed of natural polysaccharides interconnected through reversible SA–Zn²⁺ coordination interactions and KGM-mediated hydrogen-bonding networks. Unlike conventional synthetic soil conditioners, the KGM–SA/B–Zn does not release cytotoxic intermediates or persistent contaminants during use, thereby minimizing interference with microbial metabolism and community activity.

The environmental fate of the hydrogel after fulfilling its soil-conditioning function was investigated through soil-burial degradation experiments. As shown in Fig. 6C, all hydrogel formulations underwent progressive decomposition under natural soil conditions, exhibiting degradation rates of 72.4%, 58.3%, and 65.1% for SA₂₀KGM₄₀, SA₂₀KGM₆₀, and SA₂₀KGM₈₀, respectively, after 30 days. The observed degradation behavior reflects the delicate balance between network stability and biodegradability established by the dual-network architecture. Specifically, increasing cross-linking density enhances structural integrity during the service period while simultaneously modulating enzymatic accessibility to the polymer chains. Consequently, soil microorganisms can gradually depolymerize the hydrogel matrix through the secretion of carbohydrate-degrading enzymes, such as cellulases and β-glucanases, converting the network into environmentally benign low-molecular-weight products. Notably, the degradation timescale closely matches the effective functional period of the hydrogel in soil (~15 d), suggesting a desirable “function-then-disappear” behavior. This temporally synchronized performance allows the hydrogel to provide sustained water retention and salt regulation during the critical stage of soil remediation, followed by gradual biodegradation without generating long-term residues. Such a dynamic balance between functionality, ecological safety, and environmental turnover represents a key advantage of the KGM–SA/B–Zn system.

3.6 Application of KGM–SA/B–Zns in saline-alkali soil remediation

The KGM–SA/B–Zn acted as a multifunctional soil-engineering platform capable of simultaneously regulating these interconnected processes through its dynamic dual-network architecture. SEM observations revealed substantial differences in soil morphology before and after hydrogel incorporation. The untreated saline-alkali soil exhibited a compact microstructure characterized by densely packed particles, fragmented pore channels, and poor pore interconnectivity, which are typical consequences of salt crystallization and particle cementation (Fig. 7A). In contrast, hydrogel-treated soil displayed a well-developed hierarchical porous architecture with significantly improved pore connectivity (Fig. 7B). Such structural evolution can be attributed to the swelling-induced expansion of the KGM–SA/B–Zn, which effectively alleviates interparticle compressive stress and creates interconnected transport pathways throughout the soil matrix. Moreover, EDS elemental mapping demonstrated the homogeneous distribution of characteristic hydrogel-derived elements, indicating that the dynamically cross-linked network formed through SA–Zn²⁺ coordination interactions and KGM-mediated hydrogen bonding enables uniform integration with the surrounding soil matrix (Fig. 7C). This homogeneous distribution prevents localized aggregation and pore blockage, thereby facilitating long-term structural stability. Collectively, these results suggest that the KGM–SA/B–Zn functions as a structural regulator capable of transforming saline-alkali soils from a compacted state into a highly interconnected porous medium.

The reconstructed pore architecture directly influenced water transport behavior within the soil. Compared with untreated soil, the hydrogel-amended soil exhibited a threefold reduction in water permeation time, decreasing from 30 to 10 min (Fig. 7D). This accelerated infiltration behavior indicates a substantial reduction in hydraulic resistance and confirms the formation of efficient water-conduction pathways. Unlike conventional soil conditioners that primarily improve water retention at the expense of permeability, the KGM–SA/B–Zn simultaneously enhances water storage and transport through its dynamically adaptive network. Such dual functionality is particularly advantageous for saline-alkali soils, where restricted infiltration and excessive evaporation frequently coexist. Therefore, the hydrogel establishes a balanced water-management system that promotes both water accessibility and moisture preservation within the rhizosphere.

Beyond physical restructuring, the hydrogel-induced regulation of water and ion transport generated a favorable root-zone microenvironment for plant growth [46]. The superior performance of the SA₂₀KGM₆₀ formulation demonstrates the importance of balancing network stability and dynamic responsiveness. Through synergistic water retention, salt immobilization, and pore stabilization, the hydrogel effectively reduced osmotic stress while maintaining sufficient oxygen diffusion and nutrient availability. Consequently, a low-salinity, well-aerated, and water-accessible rhizosphere environment was established, providing favorable conditions for sustained oat development.

The plant growth results further validate this microenvironmental regulation mechanism. (Fig. 8) Oats grown in hydrogel-treated soils exhibited significantly enhanced leaf length, leaf width, and root length throughout the cultivation period, with the SA₂₀KGM₆₀ group consistently showing the strongest growth promotion effects. The continuous increase in leaf length reflects the alleviation of osmotic constraints on cell expansion, resulting from improved soil water availability and reduced salt toxicity. Simultaneously, the enlarged leaf width indicates enhanced development of photosynthetically active tissues, which can be associated with improved oxygen diffusion and nutrient transport enabled by the interconnected pore network. Notably, the relatively lower performance of SA₂₀KGM₈₀ suggests that excessive cross-linking density may restrict network flexibility and water mobility, highlighting the critical role of optimizing network architecture for effective soil regulation. Root growth exhibited an even stronger response to hydrogel treatment. The substantial increase in root length can be directly linked to the reduction in soil compaction and the enhancement of pore connectivity. Upon swelling, the three-dimensional hydrogel framework redistributes mechanical stress within the soil matrix, lowering penetration resistance and facilitating root elongation. Meanwhile, the adsorption and immobilization of excessive Na⁺ ions mitigate ion toxicity at root tips, while the dynamic network suppresses secondary pore blockage caused by salt recrystallization. Together, these effects promote continuous root exploration and resource acquisition, ultimately supporting aboveground biomass accumulation.

4 Conclusions

In summary, we developed a hierarchically porous biomass-based double-network hydrogel (KGM–SA/B–Zn) that enables the synergistic regulation of soil water, salt, and structure for the integrated remediation of saline-compacted soils. The hydrogel combines a dynamically borate-crosslinked KGM network with a Zn²⁺ crosslinked SA network, thereby integrating high water-storage capacity, ion-exchange functionality, and mechanical robustness within a single material. Benefiting from the synergistic coupling between the double-network architecture and the interconnected porous structure, KGM–SA/B–Zn effectively enhanced soil water retention, accelerated salt migration and sodium sequestration, reconstructed soil pore connectivity, and alleviated soil compaction. Consequently, the hydrogel reduced exchangeable sodium content by 45%, significantly improved soil permeability and water-holding capacity, promoted oat root development and biomass accumulation under saline conditions, and exhibited favorable environmental compatibility with a biodegradation rate of 72.4% within 30 days. More importantly, this work demonstrates that rational hydrogel network engineering can simultaneously regulate multiple soil physicochemical processes, providing an effective strategy for breaking the salinization-compaction feedback loop. The proposed multifunctional biomass hydrogel offers a sustainable and environmentally friendly platform for saline-alkali soil restoration and provides new insights into the design of advanced soil-conditioning materials for climate-resilient agriculture.

References

[1] Cui Q , Xia J B , Yang H J . et al. Biochar and effective microorganisms promote Sesbania cannabina growth and soil quality in the coastal saline-alkali soil of the Yellow River Delta, China. Science of the Total Environment, 2021, 756: 143801

[2] Zhao Q Q , Bai J H , Gao Y C . et al. Shifts in the soil bacterial community along a salinity gradient in the Yellow River Delta. Land Degradation & Development, 2020, 31(16): 2255–2267

[3] Weng H Y , Wu M Y , Li X B . et al. High-throughput phenotyping salt tolerance in JUNCAOs by combining prompt chlorophyll a fluorescence with hyperspectral spectroscopy. Plant Science, 2023, 330: 111660

[4] Wang G Z , Ni G , Feng G . et al. Saline-alkali soil reclamation and utilization in China: progress and prospects. Engineering Agriculture, 2024, 11(2): 216–228

[5] Xu X , Guo L , Wang S B . et al. Effective strategies for reclamation of saline-alkali soil and response mechanisms of the soil-plant system. Science of the Total Environment, 2023, 905: 167179

[6] Huang G Z , Liu B S , Jiang X T . et al. The application of amendments improves properties of salt-affected soils across China. Soil and Tillage Research, 2025, 248: 106431

[7] Osman K T. Saline and sodic soils. In: Osman K T, ed. Management of Soil Problems. Cham: Springer, 2018, 255–298.

[8] Xing J , Li X Y , Li Z Q . et al. Remediation of soda-saline-alkali soil through soil amendments: microbially mediated carbon and nitrogen cycles and remediation mechanisms. Science of the Total Environment, 2024, 924: 171641

[9] Cruz N , Avellan A , Ruivo L . et al. Biomass ash-based soil improvers: impact of formulation and stabilization conditions on materials’ properties. Journal of Cleaner Production, 2023, 391: 136049

[10] Gunarathne V , Senadeera A , Gunarathne U . et al. Potential of biochar and organic amendments for reclamation of coastal acidic-salt affected soil. Biochar, 2020, 2(1): 107–120

[11] Yang Z X , An Y , He Y L . et al. A programmable actuator as synthetic earthworm. Advanced Materials, 2023, 35(36): 2303805

[12] Iftime M M , Ailiesei G L , Ungureanu E . et al. Designing chitosan based eco-friendly multifunctional soil conditioner systems with urea controlled release and water retention. Carbohydrate Polymers, 2019, 223: 115040

[13] Chen Q , Cao X F , Li Y Y . et al. Functional carbon nanodots improve soil quality and tomato tolerance in saline-alkali soils. Science of the Total Environment, 2022, 830: 154817

[14] Qi L , Xiao X , Liu T . et al. Functionally responsive hydrogels with salt-alkali sensitivity effectively target soil amelioration. Science of the Total Environment, 2024, 918: 170350

[15] Tefera B B, Bayabil H K, Tong Z H, et al. Using liquefied biomass hydrogel to mitigate salinity in salt-affected soils. Chemosphere, 2022, 309(Pt 1): 136480

[16] Gao H N , Mao J L , Cai Y D . et al. Euryhaline hydrogel with constant swelling and salinity-enhanced mechanical strength in a wide salinity range. Advanced Functional Materials, 2021, 31(4): 2007664

[17] Hu X X , Tang Y L , Tan L L . et al. Multi-scale microstructural construction in ultralight graphene aerogels enables super elasticity and unprecedented durability for impact protection materials. Journal of Colloid and Interface Science, 2024, 673: 333–345

[18] Li Y Q , Li X N , Chen C . et al. A rapid, non-invasive and non-destructive method for studying swelling behavior and microstructure variations of hydrogels. Carbohydrate Polymers, 2016, 151: 1251–1260

[19] Kuang J , Yuk K Y , Huh K M . Polysaccharide-based superporous hydrogels with fast swelling and superabsorbent properties. Carbohydrate Polymers, 2011, 83(1): 284–290

[20] Li M Y , Dong Y P , Wang M D . et al. Hydrogel/nanofibrous membrane composites with enhanced water retention, stretchability and self-healing capability for wound healing. Composites Part B: Engineering, 2023, 257: 110672

[21] Zhu T , Jiang C , Wang M L . et al. Skin-inspired double-hydrophobic-coating encapsulated hydrogels with enhanced water retention capacity. Advanced Functional Materials, 2021, 31(27): 2102433

[22] Song J S , Zhang H Y , Peixoto L . et al. Burying straw interlayers decreases CO₂ emissions in deep saline soil. Sustainable Production and Consumption, 2023, 43: 194–203

[23] Ma M L , Yang J M , Ye Z P . et al. A facile strategy for synergistic integration of dynamic covalent bonds and hydrogen bonds to surmount the tradeoff between mechanical property and self-healing capacity of hydrogels. Macromolecular Materials and Engineering, 2021, 306(2): 2000577

[24] Gao J , Tian H , Dong B . et al. Mechanism of stabilized sludge-driven remediation in saline-alkali soil: New insights from salt-discharge capacity and microbially mediated carbon/nitrogen cycles. Sci. Total Environ, 2024, 957: 177588

[25] Han R H , Zeng F , Xia Q Q . et al. Zwitterionic cellulose nanofibers-based hydrogels with high toughness, ionic conductivity, and healable capability in cryogenic environments. Carbohydrate Polymers, 2024, 340: 122271

[26] Yang H J , Tan L L , Yan J . et al. Solvent-induced dynamic bond regulation for constructing adaptive lubricating hydrogels with switchable tribological performance and environmental adaptability. Advanced Functional Materials, 2024, 34(52): 2411384

[27] Wang Z K , Yuan L , Tang C B . Sustainable elastomers from renewable biomass. Accounts of Chemical Research, 2017, 50(7): 1762–1773

[28] Baccile N , Messaoud G B , Zinn T . et al. Soft lamellar solid foams from ice-templating of self-assembled lipid hydrogels: organization drives the mechanical properties. Materials Horizons, 2019, 6(10): 2073–2086

[29] Li X Y , Jiang Y , Li Y . et al. Highly reversible zinc anode enabled by trehalose modified sodium alginate/carboxymethyl cellulose hydrogel electrolyte. Chemical Engineering Journal, 2025, 509: 161185

[30] Tao J X , Li G Q , Wu X Z . et al. Eugenol-mediated molecular regulation: Enabling strong fluorescence and antibacterial activity in bio-based fluorescent polymers. Chemical Engineering Journal, 2026, 538: 176865

[31] Wang M , Zhang X H , Liu B . et al. Immunotherapeutic hydrogel with photothermal induced immunogenic cell death and STING activation for post-surgical treatment. Advanced Functional Materials, 2023, 33(29): 2300199

[32] Li T , Wei H G , Zhang Y Y . et al. Sodium alginate reinforced polyacrylamide/xanthan gum double network ionic hydrogels for stress sensing and self-powered wearable device applications. Carbohydrate Polymers, 2023, 309: 120678

[33] Tan L L , Chen X , Han R H . et al. Multi-scale doping-Induced impedance matching of graphene aerogel enables ultra-broadband microwave absorption. Chemical Engineering Journal, 2025, 504: 158947

[34] Xu Q , Huang W J , Jiang L B . et al. KGM and PMAA based pH-sensitive interpenetrating polymer network hydrogel for controlled drug release. Carbohydrate Polymers, 2013, 97(2): 565–570

[35] Gao L T , Chen Y M , Aziz Y . et al. Tough, self-healing and injectable dynamic nanocomposite hydrogel based on gelatin and sodium alginate. Carbohydrate Polymers, 2024, 330: 121812

[36] Zhao Z H , Zhao P C , Zhao Y . et al. An underwater long-term strong adhesive based on boronic esters with enhanced hydrolytic stability. Advanced Functional Materials, 2022, 32(26): 2201959

[37] Zhang X , Li Y , Ma Z J . et al. Modulating degradation of sodium alginate/bioglass hydrogel for improving tissue infiltration and promoting wound healing. Bioactive Materials, 2021, 6(11): 3692–3704

[38] Zhang Y , Wang F F , Yu Y T . et al. Multi-bioinspired hierarchical integrated hydrogel for passive fog harvesting and solar-driven seawater desalination. Chemical Engineering Journal, 2023, 466: 143330

[39] Cheng H , Keerthika Devi R , Huang K Y . et al. Highly biocompatible antibacterial hydrogel for wearable sensing of macro and microscale human body motions. Small, 2024, 20(37): 2401201

[40] He J , Zhao J W , Jia W Z . et al. Constructing anisotropic and strong polysaccharide-based hydrogels with stretching-dehydration strategy: effect of sodium alginate, pectin, Gellan gum, and curdlan. Carbohydrate Polymers, 2025, 359: 123567

[41] Zhang L Y , Shen B H , Zheng C M . et al. Chitosan/oxidized sodium alginate/Ca²⁺ hydrogels: synthesis, characterization and adsorption properties. Food Hydrocolloids, 2024, 156: 110368

[42] Wang L Z , Yu G B , Li J C . et al. Stretchable hydrophobic modified alginate double-network nanocomposite hydrogels for sustained release of water-insoluble pesticides. Journal of Cleaner Production, 2019, 226: 122–132

[43] Albalasmeh A , Mohawesh O , Gharaibeh M . et al. Artificial neural network optimization to predict saturated hydraulic conductivity in arid and semi-arid regions. CATENA, 2022, 217: 106459

[44] Li J , Li L T , Zhang Y J . et al. Bio-based hydrogel particles with enhanced water retention properties for alleviating drought stress of saline soil. Journal of Cleaner Production, 2025, 518: 145960

[45] Liu Z Q , Zhang H C , Zhou R G . et al. Thermoplastic elastomer-reinforced hydrogels with excellent mechanical properties, swelling resistance, and biocompatibility. Advanced Science, 2025, 12(12): 2414339

[46] Chen X Y , Wang X , Pan T T . et al. Multifunctional hydrogels with intelligent regulation of water, light, and heat for soil improvement. Chemical Engineering Journal, 2026, 542: 177963

RIGHTS & PERMISSIONS

The Author(s) 2026. This article is published by Higher Education Press.