sodium alginate and application in agriculture

Journal Pre-proof Hydrogel synthesis based on lignin/sodium alginate and application in agriculture

Bin Song, Hongxu Liang, Ruru Sun, Pai Peng, Yun Jiang, Diao She PII:

S0141-8130(19)38344-8

DOI:

https://doi.org/10.1016/j.ijbiomac.2019.12.082

Reference:

BIOMAC 14109

To appear in:

International Journal of Biological Macromolecules

Received date:

15 October 2019

Revised date:

5 December 2019

Accepted date:

10 December 2019

Please cite this article as: B. Song, H. Liang, R. Sun, et al., Hydrogel synthesis based on lignin/sodium alginate and application in agriculture, International Journal of Biological Macromolecules(2018), https://doi.org/10.1016/j.ijbiomac.2019.12.082

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© 2018 Published by Elsevier.

Journal Pre-proof

Hydrogel synthesis based on lignin/sodium alginate and application in agriculture Bin Songa, Hongxu Lianga, Ruru Suna, Pai Pengb, Yun Jiangc, Diao Shed,e,* College of Natural Resources and Environment, Northwest A&F University, Yangling 712100, China

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College of Forestry, Northwest A&F University, Yangling 712100, China

c

Department of Foreign Languages, Northwest A&F University, Yangling 712100, China

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State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A&F

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Institute of Soil and Water Conservation, CAS&MWR, Yangling 712100, China

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University, Yangling 712100, China

Journal Pre-proof Highlights 

A new green hydrogel is synthesized by crosslinking sodium lignosulfonate and sodium alginate.

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The physical structure of the hydrogel is characterized by SEM, BET and FTIR.

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The hydrogel degrades by 20% after being buried in soil for six months.

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The hydrogel reduces saturated soil hydraulic conductivity, increases soil available water content and reduces soil nutrient leaching loss.

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The hydrogel extends tobacco growth time for 9~14 days under extreme drought conditions.

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

Journal Pre-proof ABSTRACT：A new green hydrogel has been synthesized by crosslinking lignosulfonate (L), sodium alginate (SA) and konjaku flour (KJ). We have optimized the ratio of the three synthesized polymers using an orthogonal design of experiments and characterized this hydrogel using SEM, BET and FTIR spectra. We have added the hydrogel into soil and investigated its degradability in soil and its influence on chemical and physical properties of soil, such as maximum water holding capacity, saturated hydraulic conductivity, water retention curve and nutrient retention and evaluated its

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performance when applied in drought stress tests on tobacco plants. The results show that the

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maximum water absorption of optimized L/KJ/SA is 41.23 g/g. Adding the L/KJ/SA hydrogel to soil

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reduces saturated hydraulic conductivity, increases available water capacity of soil and reduces

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leaching of soil nutrient. The L/KJ/SA hydrogel can improve the photosynthetic capability of tobacco

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plants under drought stress and the levels of osmotic regulators such as proline and reducing sugar, and could prolong the growth time of tobacco plants up to 14 days, which significantly increases its

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buried in soil for 6 months.

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mass harvest. The L/KJ/SA hydrogel also has good degradability, which can degrade 20% when

KEYWORDS: sodium lignosulfonate; sodium alginate; hydrogel; soil Improvement; agricultural utilization

Journal Pre-proof 1. Introduction Shortage of water resources, desertification of soil and excessive use of fertilizers are the main factors leading to the degradation of cultivated land. In order to improve the efficiency of water use in agricultural soil, reduce water loss and restore soil quality, polymer hydrogels are widely used in soil improvement and regulation[1]. At present, most of the hydrogel products in the market are made from monomers or polymers of acrylic acid and polyacrylamide, which are derived from petroleum

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productions and difficult to degrade in soil or whose degradation products are potentially biologically

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toxic. For example, acrylic acid and acrylamide monomers may have toxic effects on human or

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animal respiratory and nervous systems[2]. Burkhard et al. find that the average degradation rate of

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polyacrylate hydrogels for 24 weeks in soil is only 0.45% [3]. Polyacrylate hydrogels may be degraded

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by some fungal species by mere 0.80~3.20% during the 11-14 weeks of fungal mineralization test[4].

effect on plant growth[5].

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Chemically synthesized hydrogels are less used in soil, because their excessive use can have harmful

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Therefore, in order to avoid potential toxicity and protect environment [6], polysaccharides and their derivatives, due to their unique properties, such as biodegradability, environmental and ecological friendliness, low cost and abundant sources, are potentially used as desirable alternatives to hydrogels made from petroleum products, as reported by Muniz and Beaucamp et al[7-8]. Current polymer hydrogels are mostly made from polysaccharides and petroleum derivatives and used in agriculture. For example, sodium alginate/polyacrylamide hydrogel developed by Elbarbary et al [9]. promotes both corn quality and quantity and sustains release of soil nutrients. Hasija et al[10]. synthesized a biodegradable hydrogel from agar/gum Arabic by microwave irradiation to significantly increase the sustainable water holding capacity of agricultural land. Fang et al [11]. synthesized a

Journal Pre-proof superabsorbent hydrogel with amino ethyl chitosan and acrylic acid by free radical polymerization, which absorbs and retains extremely large amounts of water under salt stress and is applicable in arid areas. However, pure hydrogels have poor mechanical properties and are easy to decompose, so they cannot exist in soil for a sufficient time[12]. Thakur et al. find that polysaccharide quickly degraded 43.3% after buried in soil for two weeks[13]. High degradability of the biogel is not desirable for their applications in soil. Therefore, more stable and still degradable biopolymer hydrogels, which supply

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water for one or more growth cycles for plant, are more meaningful products for agricultural

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purposes[10,14].

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Among many biopolymers, lignin is the second most abundant substance after cellulose and is a

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by-product of bio-refining industries such as papermaking, ethanol production, etc. [1]. It is reported that

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the paper industry alone produces up to 40-50 million tons of lignin waste annually[15], of which only 1-2% are used for high-end production, and the rest are burned as fuel or discharged directly into rivers . It contains many functional groups, such as phenolic hydroxyl, alcoholic hydroxyl, carbonyl,

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[1]

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carboxyl, etc.[16-17], which all have relatively great surface area[18]. Compared with other bio-renewable polymers, lignin has attributes such as antioxidation, antimicrobial, low degradability, high strength and high yield[19]. As a plant residue, lignin plays a key role in soil humification. It is generally considered as a major source of soil humus and is the most valuable material for soil remediation [18]. Previous studies have shown that lignin can increase the hardness of polymers [16,20]. These properties make lignin a good material for the synthesis of green hydrogels. Due to the extensive application of lignin in the fields of new materials, energy, biomedicine and pharmacy, adsorbent and environmental protection，the potential value of lignin in hydrogel preparation is beyond doubt [8,14-16,20-21]. The extensive utilization of lignin, in line with the utilization of residual value of lignin proposed by

Journal Pre-proof Roberto et al., have proposed that the utilization of lignin will improve the feasibility and sustainability of bio-based consumer goods, which can drive bio-based economy[17]. Since lignin itself is an insoluble polymer, in order to make it more suitable as a precursor of hydrogel, it is sulfonated to make its water-soluble. At the same time, studies have shown that alginate/konjac/xanthan gum has been well studied and applied as a kind of hydrogel [22]. In our study, we replaced xanthan gum with sodium lignosulfonate as the skeleton of hydrogel, and cross-linked it

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with sodium alginate to prepare a new hydrogel. The new hydrogels may be nontoxic and

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biodegradable, providing sufficient water for the growth cycle of plants [23]. We describe the

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crosslinkage methods and structural properties of hydrogels, and evaluate their degradation and their

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effects on soil and plants. Drought stress tests were carried out on tobacco plants and this hydrogel

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was used in the soil which provided feed to the plant. The plant development and physiological

tobacco plants.

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processes were carefully observed and measured to evaluate the impact of the hydrogel on the soil and

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2. Materials and Methods

2.1 Materials and Instrument

Sodium lignosulfonate was obtained from Macklin Biochemical Technology Co. Ltd(Shanghai, china); sodium alginate from Fuchen Chemical Reagent Co. Ltd(Tianjin, China); Konjac powder from Kangyuan Pharmaceutical Co. Ltd (Anhui, China)and Anhydrous calcium chloride from Guanghua Technology Co. Ltd (Guangdong, china). The hydrogel was scanned by scanning electron microscopy (Nova Nano SEM-450) at 15kv to observe its morphology. The hydrogel was mixed with potassium bromide and ground before being scanned for 16 times by TENSOR27 FTIR spectrometer (USA) in the range of 400 cm-1 and 4000 cm-1 at a resolution of 4 cm-1, and its FT-IR spectrum was measured.

Journal Pre-proof The porous structures and specific surface area of hydrogels were measured by a gas adsorption analyzer (TriStar 3000) made by Mike Company of USA.

2.2 Synthesis of the Hydrogel and Orthogonal experimental design This experiment uses a method of chemically crosslinking polymers in liquid phase. Sodium lignosulfonate was dissolved in deionized water, which was stirred at constant temperature (25℃, 500 r) for 10 minutes in a magnetic stirrer before sodium alginate and purified konjac powder was added.

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After being stirring for another 4 hours, anhydrous calcium chloride solution was added and the stirrer

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worked at low speed for 30 minutes. Then the mixture was frozen for 24 hours and dried under

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vacuum for 48 hours. The samples were collected and stored for use.

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In order to explore the optimal polymer ratio of sodium lignosulfonate/sodium alginate/konjak

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(L/KJ/SA) compound, which has a maximum water absorption capacity, we carried out an orthogonal optimization experiment in which the concentration of sodium lignosulfonate (L), sodium alginate

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(SA), konjak (KJ), and anhydrous calcium chloride were taken as investigation factors and the

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expansion rate of hydrogel in distilled water and 0.9% NaCl solution was used as an indicator (Table 1). An orthogonal analysis was designed to identify factors having a significant effect on the water and 0.9% NaCl of the L/KJ/SA hydrogels. In this study, a four-variable, four-level values matrix was constructed, with the target output parameter being the water and 0.9% NaCl. The orthogonal design, which led to an optimized combination (test 7) through 16 runs, is listed in Table1.

2.3 Determination of Water Absorption Capacity, Repeated Water Absorption Capacity and Water Diffusion Mechanism First, 1g dry sample (W0) was loaded into a nylon net bag and soaked in 100 mL distilled water for 24 h separately. Then it was removed out of the solution and hung vertically for 30 min to release

Journal Pre-proof extra water. Finally, the weight of hydrogel in the nylon net bag was determined (W 1). The water absorption ratio (q) of hydrogels is calculated by equation (1). In the determination of water absorption ratio, the samples were subjected to 10 rounds of adsorption and desorption tests. In each round, the measurement of adsorption and desorption is repeated 10 times; the effect of repetition on water absorption is studied. At the same time, the sample was placed in a 100 mL 0.9% NaCl solution. Through repeating the above experiment, the reusability of hydrogels in 0.9% NaCl solution was

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investigated.

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q = (𝑊1 − 𝑊0 )/𝑊0 × 100% (1)

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In the initial stage of hydrogel swelling (Wt/ We≤60%), the diffusion of water in the colloid can be

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represented by power function equation (2). In the equation, W t and We are the weight of water

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absorbed by the hydrogel at t time and at equilibrium, respectively; where k is a constant incorporating characteristics of the macromolecular network system and the penetrant, and n is the diffusion constant

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which is indicative of the transport mechanism. When n<0.5, water diffusion behavior conforms to

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Fickian model. In this case, water only diffuses through the porous structures in the colloidal networks. When 0.45≤n≤0.89, water diffusion behavior follows non-Fickian models, that is, water diffuses through porous structures and relaxed gel clusters. (𝑊𝑡 /𝑊𝑒 ) = 𝐾𝑡 𝑛 (2)

2.4 Effects of the L/KJ/SA hydrogel on Soil Properties and Crop Growth The effect of the hydrogel on soil properties, including maximum hydraulic holding capacity, saturated water conductivity, water characteristic curve and nutrient retention, was studied to evaluate the applicability of the hydrogel as the conditioner in soil. The loess soil used as the matrix (Ningxia, China) was air-dried and sifted through a sieve with 2-mm meshes. The properties are shown in Table

Journal Pre-proof 2.

2.4.1 Determination of Maximum Water Holding Capacity and Saturated Water Conductivity of Soil The air-dried loess soil was mixed with 0.375, 0.650 and 0.975 wt% hydrogel and the mixtures were evenly loaded into cutting sample rings, which was recorded as M1, M2 and M3. The soil without hydrogel (CK) was used as control. After the above mixtures were immersed in water for 48

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hours until saturation, the maximum water holding capacity was measured by weighing and the

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saturated hydraulic conductivity was determined by the constant-head method. Saturated hydraulic

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conductivity of soil can be calculated by Darcy's law (3) and Hazan's formula (4).

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𝐾𝑡 = 𝑄𝐿/𝐴𝑇𝐻 (3)

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𝐾𝑠 = 𝐾𝑡 /(0.7 + 0.03𝑡)(4)

In the formula, H is the height of the cutting sample ring (cm), L is the height of soil filling in the

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cutting sample ring (cm), Kt is the saturated hydraulic conductivity at t temperature (cm/min), Q is the

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water outflow (mL), A is the cross-sectional area of the cutting sample ring (cm2), T is the measurement duration (min). KS is the saturated hydraulic conductivity (cm/min) at standard temperature and t is the temperature at measurement time.

2.4.2 Determination of soil-Water Characteristic Curve The soil-water curve was plotted by soil-water characteristic curve system (Hitachi CR21R). The L/KJ/SA hydrogels with different compositions calculated according to soil bulk density and gel water absorptivity (0, 0.375, 0.650, 0.975wt%) were mixed evenly with commercial acrylamide hydrogel and loess soil (over 2 mm sieve). After saturation, the mixtures were separately added into the centrifuge to determine moisture content of the soil samples at different rotational speeds (310,

Journal Pre-proof 980, 1380, 1960, 2400, 2770, 3100, 4380, 6200, 7590 and 8770 r/min, which are set in accordance with soil matric potential). At the end of each centrifugation, the soil moisture content was calculated by mass method and fitted by Gardner curve[24]. θ = α𝑆 −𝑏 (5) In the formula, θ is volumetric water content (%), S is water suction (Bar), and α and b are fitting parameters.

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2.4.3 Determination of N, P and K Nutrient Preservation in Soil by Hydrogel

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In terms of the bulk density of 1.35 g/cm3, the air-dried soil sample was loaded at the bottom of

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the soil column for 10 cm. The loess and fertilizer (250-80-300 kg/ha of nitrogen, phosphorus and

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potassium) were then mixed with 0.375%, 0.650% and 0.975% L/KJ/SA hydrogels respectively. The

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above mixtures were filled in the cutting ring on the air-dried soil sample for 20 cm and recorded as M1, M2 and M3 respectively, the mixture with no added hydrogel is CK. Each treatment was

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repeated three times, and then the surface layer was covered by a small amount of fine sand (25 g) to

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prevent the surface soil layer from being disturbed by added water which may lead to the effect of pipe-wall. When leaching, water was added till the field capacity and the leaching solution was collected after 24 hours. Then the soil column was cultured indoors and leached once on the 1st, 3rd, 7th, 14th, 21st and 28th days respectively. The leaching solution was collected into a 500 mL volumetric flask. After being weighed, the contents of nitrate nitrogen, ammonium nitrogen, total phosphorus and available potassium were determined by 25 mL collected leaching solution in a 50 mL volumetric flask.

2.4.4 Effects of Hydrogels on Crop Growth under Extreme Drought Conditions Drought stress tests were conducted on potted plants in the Institute of Soil and Water

Journal Pre-proof Conservation, Northwest Agriculture and Forestry University from March to June 2019. The native tobacco seedlings were raised in early March and transplanted in pots when the third true leaf grew from the tobacco seedlings on May 10. Drought treatment was started when the tobacco plants grew at its best. The potting experiment used 7 treatments which included no L/KJ/SA hydrogel, acrylic hydrogel (bought from market) amendments(0.375%、0.650%、0.975%), and L/KJ/SA hydrogel amendments (0.375%、0.650%、0.975%), labeled as CK, S1, S2, S3, M1, M2, and M3, separately. The

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experiment started when no more water was added to the potted plants after sufficient water supply. It

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stopped when two of treated groups completely wilted. During the experiment, the photosynthetic rate

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of all tobacco plants was measured five times every three days. The plant physiological indicators

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were determined on the day of the end of the experiment.

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2.5 Biodegradation test

Degradation of synthetic hydrogels was studied with soil burial method. Acrylamide hydrogels

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bought on the market and L/KJ/SA hydrogels were put into 60-mesh nylon bags and buried in soil at

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depth of 6~8 cm. The soil moisture maintained 40% of its field capacity. In order to reduce errors caused by the operation, the test was repeated for 10 times and the burial time of hydrogel in soil was increased. The hydrogel was weighed once every 30 days. When taking out samples, the attached soil was moved gently by washing. Then, the samples were dried and weighted, and the soil degradation rate in a certain period of time was calculated. During sample filtration, a same amount of unburied hydrogels was used for lyophilization comparison to reduce errors.

3. Results and Discussion 3.1 Systhesis of L/KJ/SA hydrogels Sodium lignosulfonate functions as the three-dimensional scaffold through the interaction of

Journal Pre-proof hydrophilic and hydrophobic groups, and sodium alginate is fixed to the long chain scaffolds of lignin by crosslinking agent (Figure1). In the structure of the blended lignin sulfonate and alginate, there is a hydrogen bonding between the hydroxyl group of the alginate, the alcohol hydroxyl group and the ether oxygen atom of the lignin sulfonate, which to some extent determines the structure of the macromolecular chains of the blend. This was confirmed in a study by R. P. Dumitriu et al. [25]. In addition, we also predict that the interaction of the sulfonic acid group in the lignin sulfonate with the

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uronic acid sugar group of sodium alginate may increase the stability of the hydrogel skeleton. The

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non-covalent macromolecular interaction between konjac glucomannan and sodium alginate in konjac

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flour can promote colloidal structural stability[26]. At the same time, Ca2+ can crosslink with the G and

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M units of sodium alginate to form a gel[27]. Finally, we conclude that sodium alginate interacts with

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konjac glucomannan and Sodium lignosulfonate to form this L / KJ / SA hydrogel. Sodium alginate, as the source of colloid, directly affects the water absorption capacity of

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hydrogel. The swelling capacity of hydrogel increases with the addition of sodium alginate. Konjac

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flour, with the addition of crosslinking agent, will directly affect the strength of the reactions between lignin and sodium alginate. That is, a small amount of konjac flour will slow down the polymerization and sodium alginate cannot be completely grafted onto the sodium lignosulfonate scaffolds, whereas too much konjac flour will accelerate the reaction, so that sodium alginate can be grafted onto the sodium lignosulfonate scaffolds without swelling completely. At the same time, the concentration of Ca2+ affects the gelation effect of alginate. The high concentration of Ca 2+ causes the hydrogel surface to become gelatin like, tight structured and less porous, thus reducing the entry of water. However, low concentration will result in soft hydrogel with poor moisture retention capacity. These results are consistent with findings of Thombare et al.[28]. After the preliminary optimization screening, it was

Journal Pre-proof found that treatment 7 could cause the optimum swelling, that is, 1 g hydrogel could adsorb 41.13 g distilled water.

3.2 Repeated absorption test The water absorption of hydrogels increases with time and reached equilibrium within 90 minutes (Figure 2A). In the initial stage, water could quickly enter the L/KJ/SA hydrogels under osmotic pressure and the influence of hydrophilic groups. However, with the continuous entry of

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water, the three-dimensional network of the L/KJ/SA hydrogels continued to extend and the osmotic

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pressure tended to balance, which lead to the increase of water entry pressure and the slowdown of

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water absorption. When the swelling rate of hydrogel was less than 60% (n =0.40), Fickian diffusion

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was observed; when the swelling rate of hydrogel was between 60% and 100% (n=0.78), non-Fickian

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diffusion was observed (Figure 2B). As studied by Zonatto et al[29]., the initial n<0.5 indicates that the initial expansion rate is mainly controlled by the diffusion of water molecules, and the latter n>0.5

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indicates that the water molecules in this stage are mainly affected by the macromolecular chain

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relaxation in the hydrogel network and controlled by pore diffusion. In practical production and application, reusability of a gel is an important index. The total amount of water absorption of L/KJ/SA hydrogels in the tests maintained an increasing trend and the fifth increase was the highest 73% at 55.24 g/g (Figure 2C). In 0.9% NaCl solution, the change of water absorption of L/KJ/SA hydrogels was not obvious and the highest increase was only 32.92% (in the eighth test). In both cases, the water absorption of L/KJ/SA hydrogels increased in different extents, which indicates that after the adsorption-desorption process, the three-dimensional structures of the hydrogels was fully developed. The molecular chains’ extensibility was higher and the pores between layers became larger, which allowed more water’s entry. This means that L/KJ/SA hydrogel

Journal Pre-proof will be a reusable material.

3.3 Characterization of L/KJ/SA Hydrogels 3.3.1 Analysis of Scanning Electron Microscope In the scanning electron microscopic images, it can be seen that the L/KJ/SA hydrogels are formed by three-dimensional porous structures consisting of a large number of connected irregular sheets with some grain-shaped bulges on the surfaces, which indicates that lignin and sodium alginate

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have been linked together through the long chain structure of konjac. In findings of Xiao et al，the

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network structures are an important feature for the water absorption of superabsorbent resin [30]. The

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more predominant the scaffolds are, the more the chains extend, and the more water is absorbed.

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Therefore, large pore size and three-dimensional structures can ensure that L/KJ/SA hydrogels can

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absorb and store a large amount of water in a short time.

3.3.2 Infrared Spectroscopic Analysis

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By comparing FTIR spectra of sodium lignosulfonate and L/KJ/SA hydrogels, it can be

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confirmed that sodium lignosulfonate was successfully grafted onto sodium alginate by konjac. Fig. 4 shows the FTIR spectra of sodium lignosulfonate, L/KJ/SA and sodium alginate (SA). The spectra indicated that C-H stretching vibration is observed at 2977 cm-1 and 2908 cm-1, C-H bending vibration at 1398 cm-1, C=C skeleton vibration at 1600 cm-1, O-H free hydroxyl stretching vibration at 3671 cm-1, and C-O stretching vibration at 1300-1000 cm-1. Compared with the spectra of SA, L and L/KJ/SA, it is found that the carboxyl stretching vibration[21] of the SA spectrum appears obviously at L/KJ/SA at 1885 cm-1 and syringyl unit (the unique phenylpropane unit of lignin) vibration [31] appears at 1125 cm-1. These results indicate that lignin-based sodium alginate hydrogels have been successfully synthesized.

Journal Pre-proof 3.3.3 Specific Surface Area Analysis Table 3 shows that the Brunauer–Emmett–Teller (BET) surface area of L/KJ/SA hydrogel is 10.96 m2/g with an average pore size of 39.25 nm. The pore size differential distribution curves of L/KJ/SA hydrogels were obtained by Barrett-Joyner-Halenda (BJH) Analysis (Figure 5). It is obvious that mesopores and macropores are most dominating types in L/KJ/SA hydrogels which contribute to

3.4 Application of L/KJ/SA Hydrogel in Agriculture

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the rapid absorption and immobilization of water.

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3.4.1 Effects of L/KJ/SA on Saturated Hydraulic Conductivity and Maximum Water

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Holding Capacity

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Saturated hydraulic conductivity of soil (KS) is an important hydraulic indicator, which plays an

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important role in transporting soil moisture and solute [32]. The size of soil pore can affect water flow in soil. Previous studies have shown that the addition of exogenous substances in soil can block soil

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pores and thus reduce water infiltration[33]. With the increase of L/KJ/SA hydrogel content, the

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saturated hydraulic conductivity of soil (KS) decreases continuously (Figure 6). An L/KJ/SA hydrogel is a fluid colloid. In addition to its ability to absorb a large amount of water, its fluidity can also lead to blockage in soil pores, increase friction between soil particles, hydrogel and water, and decrease water infiltration, thereby reducing the loss of water during infiltration process. The addition of L/KJ/SA hydrogels can increase the maximum water holding capacity of soil (MWHC) in different degrees (Figure 6). The maximum water holding capacity of soil without hydrogel was 52.66%, that is to say, the maximum water holding capacity of 100 g dry loess was 52.66 g. The addition of 0.375% L/KJ/SA hydrogel could increase MWHC by 2.98% to 55.64%. The MWHC was further increased to 58.69% and 61.63%, by adding 0.650% and 0.975% L/KJ/SA

Journal Pre-proof hydrogels, respectively. With the increase of hydrogel content, the soil MWHC flattens out, which may be caused by the increased pressure between soil pores [33]. Since L/KJ/SA hydrogels belong to a pure biological macromolecule composite with lower compression strength than that of acrylic hydrogel, therefore, its increment of MWHC is lower than that of synthetic substances [34].

3.4.2 Effect of L/KJ/SA Hydrogels on Soil Water Characteristic Curve Table 4 lists fitting parameters of soil water characteristic curve under different treatments. It

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shows that the relationship between water suction and water content under different L/KJ/SA

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hydrogels amendments fits Gardner model[35], and the correlation coefficient R reaches a significant

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level. As Wang et al. have studied[36], the analysis of parameter α shows an obvious regularity, that is,

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α increases with the addition of hydrogel, which indicates that hydrogel can significantly improve the

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water holding capacity of soil. The more hydrogel is used, the greater improvement is obtained. Constant b is not analyzed here as it has a small range of variation.

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Soil water characteristic curve shows the relationship between the potential of soil water (or

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water suction) and soil water content. It depicts the relationship between soil water capacity and quantity. The soil water characteristic plot reflects the strength of soil water holding capacity, that is, the higher the curve is, the stronger the water holding capacity is. Conversely, the lower the curve is, the weaker water holding capacity is[37]. Usually, the water released under pressure of 0.10 MPa~1.50 MPa is called available water. Effective water content in hydrogel is an important index for evaluating its practical application. As shown in Fig. 7, the soil water curve moves upward from 0.375% to 0.975%. Especially when a 0.975% L/KJ/SA hydrogel is added, the available water under pressure between 0.1 MPa and 1.50 MPa is the largest, which indicates that the addition of L/KJ/SA hydrogel can increase the available water and better meet water demands for plants. With the increase of

Journal Pre-proof hydrogel content, soil retained water also increases. By comparison, it is found that the L/KJ/SA hydrogels help release more available water than the acrylamide hydrogel. Under the condition of 8 MPa water suction, the remaining water in soil samples treated with CK, M1, M2, M3, S1, S2 and S3 are 16.42%, 19.51%, 22.86%, 21.46%, 24.33%, 23.64% and 25.10%, respectively, which indicates acrylamide hydrogels retain most water in soil unavailable to plants, while the L/KJ/SA hydrogels behave better. Among all treatments, the 0.65% L/KJ/SA has the best effect, which can significantly

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improve the water supply to plants in soil.

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3.4.3 Effects of L/KJ/SA Hydrogel on Soil N, P and K Nutrients

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N, P and K are indispensable nutrients for plant growth, but their utilization rate in soil is quite

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low. Therefore, necessary exogenous substances can be added to improve soil nutrient conservation.

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The addition of L/KJ/SA hydrogel (0.325%, 0.650%, and 0.975%) can significantly reduce nitrate nitrogen leaching by 25.81%, 35.73%, and 38.83%, ammonium nitrogen leaching by 18.96%, 26.14%,

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and 32.35%, total phosphorus leaching by 14.60%, 25.62%, and 28.22% and available potassium

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leaching by 16.92%, 22.15% and 26.89%, compared with CK(Figure 7). DU et al.[38] find that polyacrylamide-acrylic acid water retaining agent could reduce loss of nitrate nitrogen by 39.62%, of ammonium nitrogen by 47.65%, of total phosphorus by 28.31% and of available potassium by 24.55%. And our study reveals that L/KJ/SA hydrogels have similar ability to retain water. Soil nutrients can enter the hydrogel molecular network along with water, thus reducing the leaching and infiltration process of fertilizer[29]. Mixture of hydrogels and fertilizers can reduce the leaching and infiltration of nutrients in soil, because nutrients can be encapsulated in polymer networks by hydrogels before being released. According to Thakur et al., lignin is an amphoteric absorbent with a high capacity to absorb anions and cations[1]. Its three-dimensional network has a certain capacity to

Journal Pre-proof absorb nutrients as hydrogels can change the texture of soil and effectively reduce the infiltration of water, thus decreasing the loss of nutrients[33]. Generally speaking, L/KJ/SA hydrogel can reduce the loss of nitrogen, phosphorus and potassium nutrients in soil to varying degrees. In addition, the capacity of nutrient retention also increases with increase of the L/KJ/SA hydrogel dose.

3.4.4 Effects of Hydrogels on Crop Growth under Extreme Drought Conditions Hydrogel can reduce the wilting time of tobacco and extend its growth time. As shown in Fig. 9,

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on the 11th day after the water supply is stopped, tobacco plants treated with CK and acrylamide

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hydrogels were less vigorous than plants treated with L/KJ/SA hydrogels. On the 15th day, tobacco

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plants treated with CK completely wilted. On the 19th day, the leaves of tobacco plants treated with

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acrylamide hydrogels completely withered and dried out. As shown in Fig. 10, under extreme drought

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conditions, commercial acrylamide hydrogels can extend the growth cycle of the tobacco plant for 2~4 days, and L/KJ/SA hydrogels for 10~14 days. Although the water absorption of acrylamide

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hydrogels is much greater than that of L/KJ/SA hydrogels, it is obvious that the water retained by

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acrylamide hydrogels cannot be used for plant growth. Yang and Banedjschafie et al. also found that adding hydrogels could prolong the growth of herbaceous and woody plants up to 9~10 days[39-40]. Hydrogels can form small reservoirs around the rhizosphere soil, which helps increase the growth time before it becomes wilted and prolong the growth time of plants under drought stress [41]. The net photosynthetic rate (Pn) decreases as the drought increases (Figure 11A). As shown in Fig. 11A, photosynthesis in the plants treated with M1, M2 and M3 is respectively 3.72, 4.20, and 4.41 μmol·m-2s-1 higher than in the control plants and it is respectively 34.94%, 87.95% and 98.04% higher than in plants treated with S1, S2 and S3, which means L/KJ/SA hydrogel can effectively replenish the plants’ lost water and slow down stomatal closure. The change in stomatal conductance

Journal Pre-proof (Gs) demonstrates that drought conditions suppressed the biological activities in the plants. L/KJ/SA hydrogels could counter the effect of drought conditions while high concentration of acrylamide hydrogel simple worsened the situation (Figure 11A). The intercellular CO2 concentration (Ci) is the highest in the plants treated with M3 (347.62 μmol·mol-1) and the lowest in the control plants (309.28 μmol· mol-1). The Ci is higher in plants treated with hydrogels than in plants without treatment. On the 10th day, the Ci in plants treated with M1, M2 and M3 is significantly (20.61%~33.46%) higher

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than in the control plants and much higher (22.26%~41.97%) than in the plants treated with S1, S2

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and S3, which means the hydrogel treatments can facilitate photosynthetic process. On the second day,

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the transpiration in the plants treated with S3 is the highest and the transpiration in the plants treated

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with M1 is the lowest among all the plants, which indicates that L/KJ/SA hydrogels can effectively

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decrease transpiration and therefore retain more water in soil for future growth. On the 10th day, transpiration in the plants treated with M1, M2 and M3 is greatly (36.62%~47.47%) higher than in the

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control plants and significantly (7.08%~39.93%) higher than in the plants treated with S1, S2 and S3,

activities.

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which suggests the L/KJ/SA hydrogels can sustain the plants’ strong growth and active biological

Water stress can affect photosynthetic rate of tobacco, leading to stomatal closure on leaf surface and reduction of intercellular CO2 concentration[42]. Yang et al. found that the degree of drought and the level of hydrogel application had significant effects on the photosynthetic characteristics of tobacco. Research of Gerardin et al. also showed that drought could affect the changes of stomatal conductance of tobacco plants[43]. Plant stomata are mainly affected by soil water status which may inhibit ABA[44] secretion by plant roots. This study also proved that L/KJ/SA hydrogel had a significant effect on improving photosynthetic characteristics of tobacco. With the increase of drought

Journal Pre-proof stress, photosynthetic characteristics decreased, while the addition of hydrogel can increase it. Proline and reducing sugar are important substances involved in osmotic adjustment in plants. They are a protective substance against tobacco stress, which can stabilize cell structure and counteract the effect of osmotic pressure[45]. The content of proline and reducing sugar can be used to identify the degree of drought in crops[46]. It can be seen from Fig. 12 that the proline content in the control plants is higher than that in plants treated with L/KJ/SA hydrogels while lower than in plants

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treated with acrylamide hydrogels. In an Abrantes’ study, as the degree of drought increased, the

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activity of phosphor dehydrogenase decreased in tobacco plants, and the decomposition of proline

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slowed down, which results in an increase in proline content[47]. Hamideh et al. found that proline

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could also reduce the effect of drought stress on sugarbeet and the concentration of proline could

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reflect the level of drought stress[48]. In this study, the content of proline in tobacco plants treated with L/KJ/SA hydrogel was lower than that in CK, S1, S2 and S3 plants, which indicated that L/KJ/SA

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hydrogel could alleviate the drought stress level of tobacco plants.

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The content of reducing sugar in tobacco plants varies with the degree of drought stress. When the degree of drought stress is high, plants will regulate osmotic pressure by increasing intracellular reducing sugar concentration to prevent cell dehydration and increase cell water absorption and retention capacity. It can be seen from Fig. 12 that reducing sugar content is highest in the control plants while lowest in the plants treated with M3. M1, M2 and M3 treatments can greatly decrease reducing sugar accumulation by 49.90%~62.01% than CK treatment does; they can significantly do by 18.83%~40.95% than S1, S2 and S3 treatments. The less reducing sugar accumulation in the cells reflects the decrease of drought level[49]. In general, L/KJ/SA hydrogels can effectively release adsorbed water for plant absorption and alleviate tobacco drought stress.

Journal Pre-proof 3.5 Soil degradation of L/KJ/SA hydrogels Chemical hydrogels, such as acrylic acid and polyacrylamide, are poorly degradable in soil. Most of the degraded products will cause soil pollution or harm to human or animal health through biological enrichment processes[2-4], so they cannot be widely used in soil. The L/KJ/SA hydrogel is a hydrogel formed by the polymerization of three natural biopolymers. Its biodegradability is better than other synthetic hydrogels. Fig. 13 shows the degradation process of L/KJ/SA hydrogels and

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acrylamide hydrogels in soils buried for 120 days. As can be seen from Fig. 11, the acrylamide

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hydrogels degrade only 2% in the 120 days, while the degradation of L/KJ/SA hydrogels is faster,

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which can be divided into two stages, each of which has 60 days. The L/KJ/SA hydrogels lose 6% of

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the weight in the first 60 days and 14% in the second 60 days. The slower degradation rate in the

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initial stage may be caused by the fast water absorption of hydrogels, which hinders the entry of oxygen and thus suppresses the growth of microorganisms in hydrogels. When L/KJ/SA hydrogels are

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buried in soil for a long time, its networks are destroyed by microorganisms or water erosion, which

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increases absorption and desorption of oxygen in soil. In addition, the organic components of crosslinking agents konjac powder and sodium alginate can be decomposed by microorganisms. Therefore, their interactions lead to the acceleration of later degradation. Saruchi

Sharma

and

others

synthesized

some

polysaccharide

hydrogels,

such

as

baicalin/polyacrylic acid/copolymerized methacrylic acid hydrogels, degraded 78.83% when buried in soil for 11 weeks. Compared with them, the slow decomposition of lignin could prolong the use time of hydrogels[50]. Burkhard et al. found that the average degradation rate of pure polyacrylate hydrogel was only 0.45%[3] after 24 weeks of soil burial. Polysaccharides on L/KJ/SA hydrogel could be decomposed, thus reducing the retention time of colloids in soil. Maize, wheat and other food crops as

Journal Pre-proof well as rape, tobacco and lettuce generally have a growth cycle of 5~8 months[37]. L/KJ/SA hydrogel degraded only 20% in 4 months, which can supply water continuously for 1~2 growth cycles of crops.

4. Conclusions The new bio-based hydrogel is composed of lignin and sodium alginate through crosslinkage, which has many attributes such as biodegradability, non-toxicity, water absorption and soil nutrient retention. The addition of L/KJ/SA hydrogel to soil can increase soil available water content (at 0.10

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MPa~1.50 MPa), increase soil MWHC (by 2.98%~8.96%) and reduce leaching amount of nitrate

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nitrogen, ammonium nitrogen, total phosphorus and available potassium (by 25.81%~38.83%,

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18.96%~32.35%, 14.6%~28.22% and 16.92%~26.89% respectively). In addition, the hydrogel only

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degraded 20% after six months of soil burial due to its good reusability and degradability. In the

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extreme drought test, L/KJ/SA hydrogel could prolong the growth time of tobacco plants for 9~14 days, improve the photosynthetic performance of tobacco under drought stress and affect the

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concentration of proline, reducing sugar and other intracellular regulators. Therefore, L/KJ/SA

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hydrogels can be potentially used as soil conditioners in agriculture.

Acknowledgements

This research was partially supported by the National Natural Science Foundation of China (31772390, 31700521), and “Weat Light Foundation of The Chinese Academy of Sciences (XAB2018A05)”. The authors

also acknowledge the anonymous reviewers for their invaluable insight and helpful suggestions.

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References [1] V. K. Thakur, M. K. Thakur, Recent advances in green hydrogels from lignin: a review. Int. J. Biol. Macromol. 72 (2015) 834-847. [2] H. Takigami, N. Taniguchi, Y. Shimizu, S. Matsui, Toxicity assays and their evaluation on organic polymer flocculants used for municipal sludge dewatering. Water Sci. Technol. 38 (7) (1998) 207-215. [3] B. Wilske, M. Bai, B. Lindenstruth, M. Bach, Z. Rezaie, H. G. Frede, L. Breuer, Biodegradability of a polyacrylate superabsorbent in agricultural soil. Environ. Sci. Pollut. Res. 21 (16) (2014) 9453-9460. [4] C. Mai, W. Schormann, A. Majcherczyk, A. Hüttermann, Degradation of acrylic copolymers by white-rot fungi. Appl. Microbiol. Biotechnol. 65 (4) (2004)479-487. [5] M. Sarvaš, P. Pavlenda, E. Takáčová, Effect of hydrogel application on survival and growth of pine seedlings in reclamations. Austrian J. For. Sci. 53 (5) (2007) 204-209.

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[15] M. N. Collins, M. Nechifor, F. Tanasă, M. Zănoagă, Valorization of lignin in polymer and composite systems for advanced engineering applications–a review. Int. J. Biol. Macromol. 131 (2019) 828-849 [16] M. Pishnamazi, J. Iqbal, S. Shirazian, G. M. Walker, M. N. Collins, Effect of lignin on the release rate of acetylsalicylic acid tablets. Int. J. Biol. Macromol. 124(2019) 354-359. [17] R. Rinaldi, R. Jastrzebski, M. T. Clough, J. Ralph, M. Kennema, P. C. Bruijnincx, B. M. Weckhuysen, Paving the way for lignin valorisation: recent advances in bioengineering, biorefining and catalysis. Angew. Chem. Int. Ed. 55 (29) (2016) 8164-8215. [18] H. El-Saied, A. I. Waley, A. H. Basta, O. El-Hadi, High water absorbents from lignocelluloses. II. Novel soil conditioners for sandy soil from lignocellulosic wastes. Polym. Plast. Technol. Eng. 43 (3) (2004) 779-795. [19] I. E. Raschip, G. E. Hitruc, C. Vasile, M. C. Popescu, Effect of the lignin type on the morphology and thermal properties of the xanthan/lignin hydrogels. Int. J. Biol. Macromol. 54 (2013) 230-237. [20] M. Pishnamazi, H. Hafizi, S. Shirazian, M. Culebras, M. N. Collins, Design of Controlled Release System for Paracetamol Based on Modified Lignin. Polymers.11(6) (2019) 1059.

Journal Pre-proof [21] M. Culebras, H. Geaney, A. Beaucamp, Bio-derived Carbon Nanofibres from Lignin as High-Performance Li-Ion Anode Materials. ChemSusChem. 12 (19) (2019) 4516-4521. [22] N. V. Nepovinnykh, O. N. Kliukina, N. M. Ptichkina, Hydrogel based dessert of low calorie content. Food hydrocolloids, 86 (2019) 184-192. [23] K. Heise, M. Kirsten, Y. Schneider, D. Jaros, H. Keller, H. Rohm, S. Fischer, From Agricultural Byproducts to Value-Added Materials: Wheat Straw-Based Hydrogels as Soil Conditioners. ACS Sustain. Chem. Eng. 7 (9) (2019) 8604-8612. [24] G. Ebrahim-Zadeh, H. Bayat, A. A. S. Sinegani, H. Z. Abyaneh, H. Vereecken, Investigating the correlation between soil tensile strength curve and soil water retention curve via modeling. Soil Tillage Res. 167(2017) 9-29. [25] R. P. Dumitriu, I. Stoica, D. S. Vasilescu, Alginate/lignosulfonate blends with photoprotective and antioxidant properties for active packaging applications. J. Polym. Environ. 26(3) (2018) 1100-1112. [26] S. E. Harding, I. H. Smith, C. J. Lawson, Studies on macromolecular interactions in ternary mixtures of konjac

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glucomannan, xanthan gum and sodium alginate. Carbohydr. Polym. 83(2) (2011) 329-338.

crosslinking and deacetylation. Food Hydrocolloids, 82 (2018) 363-369.

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[27] J. Li, J. Ma, S. Chen, Characterization of calcium alginate/deacetylated konjac glucomannan blend films prepared by Ca2+

[28] N. Thombare, S. Mishra, M. Z. Siddiqui, U. Jha, D. Singh, G. R. Mahajan, Design and development of guar gum based

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novel, superabsorbent and moisture retaining hydrogels for agricultural applications. Carbohydr. Polym. 185 (2018) 169-178. [29] F. Zonatto, E. C. Muniz, E. B. Tambourgi, A. T. Paulino, Adsorption and controlled release of potassium, phosphate and

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ammonia from modified Arabic gum-based hydrogel. Int. J. Biol. Macromol. 105 (2017) 363-369. [30] X. Wang, Y. Zhang, C. Hao, X. Dai, F. Zhu, C. Ge, Ultrasonic synthesis and properties of a sodium lignosulfonate–grafted poly (acrylic acid-co-acryl amide) composite super absorbent polymer. New J. Chem. 38 (12) (2014) 6057-6063.

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[31] K. R. Aadil, D. Prajapati, H. Jha, Improvement of physcio-chemical and functional properties of alginate film by Acacia lignin. Food Packaging Shelf Life. 10 (2016) 25-33.

[32] R. Liao, P. Yang, Z. Wang, W. Wu, S. Ren, Development of a Soil Water Movement Model for the Superabsorbent

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Polymer Application. Soil Sci. Soc. Am. J. 82 (2) (2018) 436-446. [33] S. Xu, X. Chen, J. Zhuang, Opposite influences of mineral-associated and dissolved organic matter on the transport of

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hydroxyapatite nanoparticles through soil and aggregates. Environ. Res. 171(2019) 153-160. [34] R. D. Lentz, Polyacrylamide and biopolymer effects on flocculation, aggregate stability, and water seepage in a silt loam. Geoderma. 241 (2015) 289-294.

[35] G. Ebrahim-Zadeh, H. Bayat, A. A. S. Sinegani, H. Z. Abyaneh, H. Vereecken, Investigating the correlation between soil tensile strength curve and soil water retention curve via modeling. Soil Tillage Res. 167(2017) 9-29. [36] Y. Wang, A. Zhang, W. Ren, L. Niu, Study on the Soil Water Characteristic Curve and Its Fitting Model of Ili loess with High Level of Soluble Salts. J. Hydrol. (2019) 124067. [37] S. S. Dorraji, A. Golchin, S. Ahmadi, The effects of hydrophilic polymer and soil salinity on corn growth in sandy and loamy soils. Clean-Soil Air Water. 38 (7) (2010) 584-591. [38] J. J. Du, C. L. Gou, Y. D. Cui, D. Qu, Effects of water retaining agent on ammonia volatilization and nutrient leaching loss from N, P and K fertilizers. J. Agro-Environ Sci. 26 (4) (2007)1296-1301. [39] L. Yang, Y. Yang, Z. Chen, C. Guo, S. Li, Influence of super absorbent polymer on soil water retention, seed germination and plant survivals for rocky slopes eco-engineering. Ecol. Eng. 62 (2014) 27-32. [40] S. Banedjschafie, W. Durner, Water retention properties of a sandy soil with superabsorbent polymers as affected by aging and water quality. J. Plant Nutr. Soil Sci.178 (5) (2015) 798-806. [41] D. Cheng, Y. Liu, G. Yang, A. Zhang, Water-and fertilizer-integrated hydrogel derived from the polymerization of acrylic acid and urea as a slow-release N fertilizer and water retention in agriculture. J. Agric. Food Chem. 66 (23) (2018) 5762-5769.

Journal Pre-proof [42] X. Yang, X. Shao, X. Mao, M. Li, T. Zhao, F. Wang, J. Guang, Influences of Drought and Microbial Water‐Retention Fertilizer on Leaf Area Index and Photosynthetic Characteristics of Flue‐Cured Tobacco. Irrig. Drain. (2019). [43] T. Gerardin, C. Douthe, J. Flexas, O. Brendel, Shade and drought growth conditions strongly impact dynamic responses of stomata to variations in irradiance in Nicotiana tabacum. Environ. Exp. Bot. 153 (2018) 188-197. [44] S. Fahad, A. A. Bajwa, U. Nazir, S. A. Anjum, A. Farooq, A. Zohaib, M. Z. Ihsan, Crop production under drought and heat stress: plant responses and management options. Front. Plant Sci. 8 (2017) 1147. [45] L. Chen, J. Meng, Y. Luan, miR1916 plays a role as a negative regulator in drought stress resistance in tomato and tobacco. Biochem. Biophys. Res. Commun. 508 (2) (2019) 597-602. [46] J. Dobra, V. Motyka, P. Dobrev, J. Malbeck, I. T. Prasil, D. Haisel, R. Vankova, Comparison of hormonal responses to heat, drought and combined stress in tobacco plants with elevated proline content. J. Plant Physiol. 2010, 167 (16) (2010) 1360-1370. [47] F. D. L. Abrantes, A. F. Ribas, L. G. E. Vieira, N. B. Machado-Neto, C. C. Custódio, Seed germination and seedling

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vigour of transgenic tobacco (Nicotiana tabacum L.) with increased proline accumulation under osmotic stress. J. Horticult. Sci. Biotechnol. 94 (2) (2019) 220-228.

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[48] H. Ghaffari, M. R. Tadayon, M. Nadeem, M. Cheema, J. Razmjoo, Proline-mediated changes in antioxidant enzymatic activities and the physiology of sugar beet under drought stress. Acta Physiol. Plant. 41 (2) (2019) 23.

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[49] M. N. Khan, J. Zhang, T. Luo, J. Liu, F. Ni, M. Rizwan, L. Hu, Morpho-physiological and biochemical responses of tolerant and sensitive rapeseed cultivars to drought stress during early seedling growth stage. Acta Physiol. Plant. 41(2) (2019)

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25.

[50] S. Sharma, V. Kumar, D. Pathak, H. Mittal, S. M. Alhassan, Experimental assessment of the utilization of a novel

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interpenetrating polymer network in different processes in the agricultural sector. J. Appl. Polym. Sci. 136 (28) (2019) 47739.

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Hydrogel synthesis based on lignin/sodium alginate and application in agriculture

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Figure captions:

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Figure 1. Synthesis of the hydrogel and surface of L/KJ/SA hydrogel in dry and wet forms.

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Figure 2. Study on water absorption of L/KJ/SA hydrogel. (A) Curve of expansion rate with time; (B) Log

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saline.

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(Mt/Me) - Log (t) plot of hydrogel; (C) Repeated absorption of hydrogel in distilled water /0.9% normal

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Figure 3. SEM images of the freeze-dried L/KJ/SA hydrogels.

Figure 4. Fourier-transform infrared spectroscopy (FTIR) spectra of sodium lignosulfonate (red), sodium alginate (blue) and L/KJ/SA hydrogel (black).

Journal Pre-proof Figure 5. Pore size distribution characteristics of L/KJ/SA hydrogels.

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Holding Capacity of Soil.

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Figure 6. Comparative effect of Treatments on Saturated Hydraulic Conductivity and Saturated Water

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Figure 7. Soil moisture characteristic curve.

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Figure 8. Reduction of nutrient leaching by different treatments: (A) nitrate nitrogen, (B) ammonium

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nitrogen, (C) total phosphorus, (D) available potassium.

Figure 9. Different treatments of tobacco growth after stopping water supply. M: L/KJ/SA hydrogel treatment; S: acrylic hydrogel treatment.

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Figure 10. Different treatment of tobacco plant wilting time.

Journal Pre-proof Figure 11. Effects of drought stress on photosynthesis of tobacco under different treatments.

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Figure 12. Effects of drought stress on physiological characteristics of tobacco under different treatments.

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Figure 13. Degradation of L/KJ/SA Hydrogel by Soil Burial Method.

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Table 1 Combination of Variables of the Orthogonal Experimental Design for L/KJ/SA hydrogel Preparation [16 Runs (4 Levels and 4 Factors)] Test number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

(A) Sodium lignosulfonate

(B) Konjac

(C) Sodium alginate

(D) Anhydrous calcium chloride

(g) 0.5 0.5 0.5 0.5 1 1 1 1 1.5 1.5 1.5 1.5 2 2 2 2

(g) 0.625 1.25 1.875 2.5 0.625 1.25 1.875 2.5 0.625 1.25 1.875 2.5 0.625 1.25 1.875 2.5

(g) 1.25 2.5 3.75 5 2.5 1.25 5 3.75 3.75 5 1.25 2.5 5 3.75 2.5 1.25

(g) 0.1 0.2 0.3 0.4 0.3 0.4 0.1 0.1 0.4 0.3 0.2 0.1 0.2 0.1 0.4 0.3

o J

n r u

l a

r P

e

o r p

f o

water

0.9% NaCl

(g) 17.9 24.73 29.34 36.46 24.05 14.68 41.23 23.78 28.55 30.07 14.14 23.09 20 15 21.72 21.39

(g) 7.46 9.33 13.56 19.8 8.79 7.64 22.59 13.16 14.89 19.88 7.98 6.46 22.01 18.39 13.56 7.69

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Table 2. General properties and soil substrates Soil Particle Composition Organic matter

TN

-1

g kg

pH

Loess soil

( g/kg)

(g/kg)

10.86

0.66

8.29

Gravel

Silt

Clay particle

81.61

17.77

3.62

l a

o J

n r u

Soil bulk density

(%)

(g/cm3)

26.10%

1.35

f o

ro

p e

r P

Field water holding capacity

Soil texture

Sandy loam soil

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Sample

L/KJ/SA

BET surface area

Table 3 Pore parameters of L/KJ/SA hydrogel Volume of Volume of Volume of micropore mesopore macropore ＜1.1 nm

2-50 nm

50-300 nm

(m2/g)

(cm3/g)

(cm3/g)

(cm3/g)

10.96

0.007

0.207

0.312

l a

o J

n r u

2-300 nm (cm3/g)

f o

o r p

e

r P

VBJH

0.107

Average pore size （4V/A) (nm) 39.245

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Table 4 Gardner model fitting function and different treatments Mathematical model

R

CK

θ=20.594S-0.147

0.978

M1

θ=23.792S-0.133

0.975

M2

θ=26.347S-0.105

0.966

M3

θ=31.050S-0.101

0.973

S1

θ=28.634S-0.100

0.982

S3

θ=29.190S-0.107

0.991

S3

θ=31.050S-0.101

0.973

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Treatments

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CRediT author statement Song Bin ：Conceptualization, Methodology, Validation, Writing - Original Draft, Writing - Review & Editing Liang Hongxu: Methodology, Software Sun Ruru: Visualization, Software

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Peng Pai : Conceptualization, Methodology, Supervision Jiang Yun : Writing - Original Draft, Articles embellish

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-p

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She Diao:Conceptualization,Funding acquisition, Project administration, Supervision

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13