Poly-γ-glutamic acid enhances the wheat yield, water use efficiency and soil physicochemical properties of the arid area in the Northwest China

ABSTRACT Poly-γ-glutamic acid (γ-PGA) is an environment-friendly super absorbent polymer that can be used as a soil conditioner, which is promising to improve crop productivity and cope with soil degradation. The objectives of this study are to study the effects of γ-PGA on soil physicochemical properties and winter wheat (Triticum aestivum L. cv. Xinong 975) production in arid region of Northwest China. According to the mass ratio of γ-PGA and soil, experiment treatments consisted of four different γ-PGA application rates, which were designated as 0 (CK), 0.05% (P0.05), 0.1% (P0.1), and 0.15% (P0.15), respectively. The mixed soil samples were uniformly filled into identical plastic pots. Each treatment had three replicates randomly distributed within 12 pots. The results indicated that γ-PGA significantly increased saturated water content by 6.3-11.5%, field capacity by 8.4-15.3%, and plant available water by 5.1-12.5% compared with the CK. γ-PGA increased soil NO3 −-N content and residue, and enhanced the proportions of soil macro-aggregates compared with the CK. γ-PGA increased winter wheat yield by 29.3-34.7%, and WUE by 21.2-33.3% compared with the CK. The main conclusion γ-PGA application amount of 0.05-0.1% can be used to improve soil physicochemical properties and winter wheat production in degraded soil.


Introduction
Healthy soil is the foundation of agricultural sustainable development. It can produce healthy crops that provided healthy food, and further improve human wellbeing (Wall et al. 2015). However, soil degradation is a worldwide problem often caused by soil mismanagement, frequent anthropogenic activities, land overuse and intensive agricultural activities (Wei and Yang 2010;Karlen and Rice 2015). Globally, more than one-third of cultivated land resources are suffering from soil degradation, which resulted in a 60% reduction in soil ecosystem services between 1950 and 2010 (Lal 2015). Generally, degraded land resources often exhibit lower crop productivity due to poor soil properties (Roohi et al. 2022). Recently, Gerland et al. (2014) has pointed out that the global population will exceed 9 billion in 2050, which will result in severe problems of food security. Under the dual of this study are therefore to (1) explore the effects of γ-PGA on FC, PWP, PAW, soil evaporation, the wilting time and storage time of the available water; (2) study influences of γ-PGA on available nitrogen content, soil aggregates and the stability of soil aggregates; and (3) analyze the effects of different γ-PGA application rates on crop growth, winter wheat yield and water use efficiency.

Poly-γ-glutamic acid and preparation of soil sample
Poly-γ-glutamic acid (γ-PGA) used in this study was a white powder with a particle size of 0.15 mm and a molecular weight of 700,000 g mol −1 and was purchased by Shandong Furuida Biotechnology Co., Ltd. γ-PGA was a biodegradable, nontoxic, super water-absorbing polymer, the maximum natural water absorption rate could reach 1108 times, and the absorption rate of soil water was 30-80 times (Inbaraj et al. 2009). Soil samples were collected from Lubotan, Fuping County, Shaanxi Province (latitude 34° 43' N, longitude 109°18′ E), which was considered as a representative degraded cultivated land in arid and semi-arid region of China (Jia et al. 2011). The experimental soils were taken at depths of 0-25 cm from a degraded cultivated land in the Lubotan. Soil samples were transported to the laboratory, air dried, crushed and passed through a 2-mm sieve (Liang et al. 2021). Soil particle size composition was determined using a laser particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd, UK). According to the international standard for classification of soil textures: clay, powder and sand with a volume fraction of 1.82%, 9.61% and 88.57%, respectively, were classified as sandy loam. Soil properties are shown in Table S1.

Experimental design
All pot experiments were performed in a rain-free shelter (3.5 m height, 5.0 m width and 7.1 m lengths) at Xi'an University of Technology from October 2018 to June 2019. Experiment treatments consisted of four different γ-PGA application rates, which were designated as 0, 0.05%, 0.1% and 0.15% according to the mass ratio of soil and γ-PGA, and labeled as CK (0), P0.05 (0.05%), P0.1 (0.1%), and P0.15 (0.15%), respectively. γ-PGA was evenly mixed into the prepared soil samples according to the four different γ-PGA application rates. The mixed soil samples were uniformly filled at a bulk density of 1.38 g cm −3 into identical plastic pots (67 cm length, 46 cm width and 45 cm depth) with a 40-cm height. All treatments were designed using a randomized with three replications, 12 pots in total.

Winter wheat cultivation and irrigation management
Winter wheat seeds (Triticum aestivum L. cv. Xinong 975) in this study were purchased by Northwest Agriculture & Forestry University, China. Winter wheat seeds were sowed at a row spacing of 11 cm in each pot on 1 November 2019. Planting density was approximately 500-600 plants m −2 , each plot was approximately 154-184 plants. Before sowing, pre-sowing irrigation (16.2 mm) was applied in all pots. This was to ensure that there was sufficient soil moisture in each pot to promote winter wheat germination after winter wheat sowing. Fertilizer applications and irrigation management were the identical for all pots. During the growing seasons from November 2018 to June 2019, irrigation amount of 162 mm, 260 kg ha −1 urea, 250 kg ha −1 diammonium phosphate and 150 kg ha −1 potassium sulphate were applied in each pot.

Soil water content and soil NO 3 --N concentration
Soil samples were collected from all pots with a manual auger at depths of 0, 10 20 and 30 cm at greening stage (6 March 2019), jointing stage (24 March 2019), heading stage (15 April 2019) and filling stage (3 May 2019), respectively. These soil samples were divided into two parts. One part was used to determine soil water content through an oven-drying method. Then, soil water storage (SWS) in 0-30 cm soil layers was calculated by the following equation (Liang et al. 2019): where SWS is soil water storage, mm; θ iÀ j is average soil water content in i-j soil layers (i = 0, 10 and 20 cm; j = 10, 20 and 30), cm 3 cm −3 ; θ m is soil water content in m cm soil depth (m = 0, 10 20 and 30 cm), cm 3 cm −3 .
Another part of soil sample was used to measure soil NO 3 --N. Soil NO 3 --N concentration were extracted from 5 g soil samples using 50 mL of 2 mol L −1 KCl solution and shaken for 1 h at a constant 25°C. After shaking stopped, the mixture was filtered with qualitative filter paper. After filtration, the extracts were tested by the Ultraviolet spectrophotometer (DR5000, HACH company, USA). Nitrate residue (NR) in every layer of soils was calculated by the following equation (Yang et al. 2017): where C N is the soil nitrate concentration (mg kg −1 ), ρ is the dry bulk density (g cm −3 ), h is the soil depth (cm) and 0.1 is the conversion coefficient.

Field capacity (FC), permanent wilting point (PWP) and plant-available water (PAW)
Field capacity (FC) was defined as soil water content when soil suction was −33 kPa (Ghorbani et al. 2017). PWP was defined as the soil water content under tension at −1500 kPa (Slatyer 1967). At harvest stage of winter wheat, soil samples were collected by inserting steel cylinders into the 0-10 cm soil layer in each pot, and then immersed in water for 48 hours until they were saturated to measure saturated soil water content. After saturation, a high-speed centrifugation (H-1400pf, Japan) was used to determine FC and PWP at matric potential of −33 kPa and −1500 kPa, respectively. Plantavailable water (PAW) was calculated as the difference between field capacity (FC) and permanent wilting point (PWP) (Ma et al. 2016).

Soil water evaporation
At harvest stage, soil samples were collected by inserting steel cylinders into the 0-10 cm soil layer in each pot plot, and then immersed in water for 48 h until they were saturated. These saturated samples were placed in the laboratory to ensure that soil moisture naturally evaporate at room temperature (24 ± 3°C). Then, a consecutive 28-day evaporation experiment was conducted to determine soil water evaporation. These saturated samples were weighed at 12-h intervals with an electronic balance (with a precision of 0.01 g), and further determining soil water content and cumulative evaporation volume during evaporation.

Water-stable aggregates and mean weight diameter (MWD)
Moreover, at harvest stage of winter wheat, soil samples were collected from all pots with an auger in 0-10 cm soil layer, air-dried and then passed through a 2-mm sieve without breaking the soil structure. Water-stable aggregates in all treatments were measured with wet-sieving method (Six et al. 1998). Water-stable soil aggregates were measured by a Wet Sieving Apparatus (08.13, Royal Eijkelkamp company, Netherlands). The soil aggregates were separated into three different size fractions, such as <0.5 mm (micro-aggregates), 0.5-1 mm (small macro-aggregates) and 1-2 mm (large macro-aggregates) (Liu et al. 2014). Mean weight diameter (MWD) of water-stable aggregates was calculated by the following equation (Van Bavel 1950;Dai et al. 2019): where MWD is mean weight diameter, mm; D i is the mean diameter of the ith fraction, mm; and W i is the percentage of sample dry weight of the i th fraction.

Plant height, leaf area index (LAI), aboveground dry matter and winter wheat yield
Three representative winter wheat plants were randomly selected at greening stage from each pot to monitor leaf area and plant height by steel tape at the jointing stage and harvesting stage. The leaf area index (LAI) was calculated by the method (Watson 1947): Additionally, three representative winter wheat plants were randomly cut from the soil surface in each pot to determine aboveground dry matter at the jointing and harvesting stages of winter wheat, respectively. The aboveground parts of fresh wheat plant samples were dried in ovens at 105°C for 2 h and afterwards at 75°C until the weights were constant. Winter wheat was manually harvested from each pot to determine grain yield of winter wheat.

Soil particle fractal dimension (SPFD)
Soil particle fractal dimension (SPFD) could reflect degree of dispersion of soil aggregates (Castrignanò and Stelluti 1999). Soil particle fractal dimension could be determined by the following equation (Tyler and Wheatcraft 1992;Bittelli et al. 1999): Eq. (5) can be rewritten as where x i is the mean particle diameter of the size class, mm; x max is the mean diameter of the largest particle, mm; M r < x i ð Þ is the mass of soil particles with a radius smaller than x i , g; M T is total mass of soil particles, g; SPFD is the soil particle fractal dimension. then and K = 3-SPFD; so, the Eq. (6) can be expressed as Value of K was calculated by the linear relationship between x and y, and then SPFD was obtained by the K value.

Water use efficiency
The water balance method was used to determine evapotranspiration (ET, mm): where P is the precipitation, mm; I is the amount of irrigation, mm; R is the runoff, mm; D is the deep leakage, mm; ΔSWS is the difference between SWS in 0-30 cm soil layer before sowing and SWS in 0-30 cm soil layer after harvesting, mm. The pot experiment was conducted in a rain-free shelter where prevented rain from entering all pots. In addition, planned irrigation occurred at the winter wheat growing season. Therefore, P, R and D are considered negligible (zero) in the study. Thus, Eq. (8) can be described as Water use efficiency (WUE) was determined by the equation: where Y is winter wheat yield, kg ha −1 ; WUE is water use efficiency, kg ha −1 mm −1 .

Exponential function
The two-parameter exponential function was used to fit the relationships between different γ-PGA application rates and soil water content during evaporation. The two-parameter exponential function could be expressed as where SWC represents soil water content during evaporation, cm 3 cm −3 ; T represents evaporation time, days; α and β represent the empirical coefficient, respectively. Eq. (11) can be rewritten as Therefore, the wilting time (T PWP , the time required to reduce saturated water content to PWP during evaporation) can be obtained when SWC is PWP. Moreover, T FC (the time required to reduce saturated water content to FC during evaporation) can be obtained when SWC is FC. T FC -T PWP (the difference between T FC and T PWP ) represents storage time of the plant-available water.

Statistical analysis
Analysis of variance (ANOVA) was performed via SPSS 22 on all data. The differences of mean values between γ-PGA treatments were compared by least significant differences (LSD) at P < 0.05. Excel 2016 was used to create figures and analyze data, respectively.

Saturated water content, FC, PWP and PAW
There were significant (P < 0.05) differences in saturated water content, FC, PWP and PAW among different γ-PGA application rates (Table S2). The saturated water content, FC, PWP and PAW were significantly (P < 0.05) higher in the γ-PGA application treatments (P0.05, P0.1 and P0.15) than that of the CK. Compared with the CK, saturated water content, FC, PWP and PAW in the γ-PGA application treatments significantly (P < 0.05) increased by 6.3-11.5%, 8.4-15.3%, 4.8-26.4% and 5.1-12.5%, respectively. This result indicated that γ-PGA application could improve soil hydraulic properties in the degraded soils. However, the PAW was significantly (P < 0.05) lower in the P0.15 treatment than that of the P0.1 treatment, which implied that γ-PGA application rate of 0.15% might be overused.

Cumulative soil evaporation and soil water content during evaporation
Different γ-PGA application rates had some influences on the dynamic changes of soil cumulative evaporation and soil water content during evaporation ( Figure S1). During the evaporation time from 0 to 1.5 days, no significant difference in cumulative soil evaporation was found among different γ-PGA application rates ( Figure S1a). After 1.5 days, cumulative soil evaporation was significantly (P < 0.05) lower in the γ-PGA application treatments than that of CK ( Figure S1a). There was significant (P < 0.05) difference in soil water content during evaporation among different γ-PGA application rates ( Figure S1b). The soil water content during evaporation was significantly (P < 0.05) higher in the γ-PGA application treatments than that of CK ( Figure S1b). The result indicated that γ-PGA application could decrease soil water loss caused by soil evaporation. The soil evaporation of corresponding treatments ended when evaporation times were 13.5, 15.5 , 20 and 27.5 days, for CK, P0.05, P0.1 and P0.15 ( Figure S1a), respectively. This was due to the fact that soil water content was constant when evaporation times were 13.5 , 15.5 . 20 and 27.5 days, for CK, P0.05, P0.1 and P0.15 ( Figure S1b), respectively. The result indicated that γ-PGA application decreased soil evaporation rate.

The storage time of PAW and wilting time
The relationships between different γ-PGA application rates and soil water content during evaporation were fitted by a two-parameter exponential equation (Figure 1). We found that the SWC showed a 'monotonic decreasing' curve change with increasing γ-PGA application rate. Moreover, the determinate coefficients (R 2 ) of the four fitting curves were more than 0.98, which meant that fitting accuracy of the two-parameter exponential equation was satisfactory (Figure 1).
To further obtain T PWP and T FC , the values of T PWP and T FC were analyzed based on the four fitting curves of the two-parameter exponential equations (Table S3). Therefore, T PWP and T FC were obtained by submitting PWP and FC into Eq. (12) (Figure 1 and Table S3). We found that T PWP , T FC and T FC -T PWP were significantly higher in the γ-PGA application treatments than that of the CK (Table S3). Compared with the CK, T PWP and T FC -T PWP in the γ-PGA application treatments significantly increased by 20.5-125.6% and 21.7-117.4% (Table S3), respectively. This result indicated that γ-PGA application could prolong the wilting time and storage time of the plant-available water.

Water-stable aggregates, MWD and SPFD
Significant (P < 0.05) differences in water-stable aggregates and MWD of soil aggregates were found among all treatments (Table 1). The proportion of micro-aggregates was significantly (P < 0.05) lower in γ-PGA application treatments than that of the CK. Compared with the CK, γ-PGA application treatments significantly (P < 0.05) enhanced the proportions of large macro-aggregates and small macro-aggregates. The MWD of soil aggregates was significantly (P < 0.05) higher in γ-PGA application treatments than that of the CK. These results indicated that γ-PGA application improved the formation of macro-aggregates and the stability of soil aggregates in degraded soil.
There were significant (P < 0.05) differences in the SPFD of water-stable aggregates among different γ-PGA application rates (Table S4). The SPFD of water-stable aggregates was significantly (P < 0.05) lower in the γ-PGA treatments than that of CK. The result indicated that γ-PGA application could improve the stability of water-stable aggregates.

Soil water distribution and soil water storage
Different γ-PGA application rates had some influences on the distribution of soil water content in 0-30 cm soil layer ( Figure S2a, 3c, 3e and 3 g). At the greening stage, soil water content under all the treatments changed between 0.2 and 0.3 cm 3 cm −3 and slightly increased in 0-30 cm soil layer ( Figure S2a). The CK treatment had low soil water content in 0-30 cm soil layer ( Figure S2a). At the jointing stage, soil water content in 0-20 cm soil layer were higher in the P0.05, P0.1 and P0.15 treatments than that of the CK, but the P0.15 treatment had the lowest soil water content at the 30 cm soil depth ( Figure S2c). At the heading stage, the γ-PGA application treatments (P0.05, P0.1 and P0.15) showed slightly higher soil water content in 0-30 cm soil layer compared with the CK (Figure S2e). At the filling stage, soil water contents under P0.05, P0.1 and P0.15 treatments were extremely high in 0-30 cm soil layer, and the highest soil water content was obtained with the P0.1 treatment ( Figure S2g).
There were significant (P < 0.05) differences in soil water storage among different γ-PGA application rates during different growing stages ( Figure S2b, 2d, 2 f and 2 h). Compared with the CK, γ-PGA application treatments (P0.05, P0.1 and P0.15) significantly (P < 0.05) increased soil water storage at greening ( Figure S2b), jointing ( Figure S2d), heading ( Figure S2f) stages and the highest soil water storage was obtained under the P0.15 treatment. At the filling stage, soil water storage was higher in γ-PGA application treatments than that of the CK, but the highest soil water storage was obtained under the P0.1 treatment ( Figure S2h).

Soil NO 3 --N distribution and residue
Different γ-PGA application rates had some influences on the distribution of soil NO 3 --N content in 0-30 cm soil layer at the different growing stages (Figure 2a, c, e and g). At greening, jointing, heading and filling stages, the soil NO 3 --N contents under all treatments obviously increased with depth in 0-30 cm soil layer. Soil NO 3 --N contents in 0-30 cm soil layer were greater in the γ-PGA application treatments (P0.05, P0.1 and P0.15) than that of the CK. Significant (P < 0.05) differences in the soil NO 3 --N residue were found among all treatments at the different growing stages (Figure 2b, d, f and h). Compared with the CK, γ-PGA application Source of variance γ-PGA *** *** *** *** treatments significantly (P < 0.05) increased soil NO 3 --N residue at the different growing stages, and the highest soil NO 3 --N residue in 0-30 cm soil was obtained under the P0.15 treatments.

Plant height, LAI, aboveground dry matter, winter wheat yield and water use efficiency
There were significant (P < 0.05) differences in the plant height, LAI, aboveground dry matter, winter wheat yield and water use efficiency among all treatment (Table S5). At the jointing and harvesting stages, γ-PGA application treatments (P0.05, P0.1 and P0.15) significantly (P < 0.05) increased plant height, LAI and aboveground dry matter compared with the CK. Additionally, compared with the CK, winter wheat yield and water use efficiency in γ-PGA application treatments significantly (P < 0.05) increased by 29.3-34.7% and 21.2-33.3%, respectively. However, the highest winter wheat yield and water use efficiency were obtained with the P0.1 and P0.05 treatments, respectively. The result indicated that excessive γ-PGA application rate (P0.15) did not increase crop yield and water use efficiency as expected.

Effects of γ-PGA on soil hydraulic properties
Ameliorating soil hydraulic properties (such as increasing FC, PAW, soil water storage and waterholding capacity of degraded soils) was promising agricultural practice that effectively improved resilience against drought (Yin et al. 2018;Liang et al. 2019;Obia et al. 2020). Some research have indicated that γ-PGA has a positive impact on soil hydraulic properties. For example, Guo et al. 2020) reported that γ-PGA could increase FC and decrease soil evaporation in sandy loam. Liang et al. (2019) indicated that γ-PGA could increase soil water storage in the 0-40 cm soil layer and prolong soil water retention time after each irrigation event due to the fact that γ-PGA could enhance waterholding capacity (FC and PAW) of sandy soil. Moreover, Yin et al. (2018) also reported that γ-PGA could increase moisture and nutrient levels of soil. In this present study, similar results were also found in the degraded silt loam. This might be attributed to the following three reasons: (1) γ-PGA was easily cross-linked with soil moisture due to the fact that it was an anionic polymer consisted of many peptide bonds and hydrophilic carboxyl groups (Tarui et al. 2005); (2) γ-PGA could be classified as super-absorbent polymer and hydrogel that could absorb and store soil water (Sung et al. 2005). Similarly, Abedi-Koupai et al. (2008) and Choudhary et al. (1995) also reported that the superabsorbent polymer and hydrogel could not only store irrigation water in soil but also reduce soil evaporation; (3) γ-PGA could enhance the stability of soil aggregates (Liang et al. 2021). This was due to the fact that the soils with well-stabilized soil aggregates structure had the excellent ability of storing nutrients and soil water (Elliott 1986Xu et al. 2015. These results implied that γ-PGA had the great potential to improve soil hydraulic properties of the degraded soil in agriculture production. However, in this present study, it was noteworthy that excessive γ-PGA application rate (P0.15) did not increase saturated water content and PAW in the silt loam as expected. Liang et al. (2019) also reported similar result in sandy soil. The result implied that excessive γ-PGA application rate might have a negative impact on the improvement of PAW in the degraded soil. Therefore, finding an appropriate γ-PGA application rate will be very important for ameliorating soil hydraulic properties of degraded silt loam.

Effects of γ-PGA on water-stable aggregates and stability
The well-stabilized soil aggregates structure was an important index that maintained soil quality and ecological health and promoting crop production (Jat et al. 2019;Wang et al. 2019;Yang et al. 2019). A laboratory reported by Guo et al. 2020) indicated that adding γ-PGA to sandy loam could increase the content of soil aggregates larger than 0.25 mm. Chen et al. (2018) also indicated that γ-PGA application could increase the proportions of water-stable soil aggregates (>0.25 mm) in silt loam. Liang et al. 2021) showed that γ-PGA application increased the proportions of water-stable macroaggregates (0.5-2 mm) and decreased the proportions of water-stable micro-aggregates (<0.5 mm) in sandy soil. In this present study, similar results were also found in silt loam. Tisdall and Oades (1982) reported that the formation mechanism of soil aggregates was mainly attributed to the binding agents resulted from the decomposition of soil organic matter and freshly added residue to the soil. Ashiuchi et al. (2003) and Chen et al. (2008) indicated that γ-PGA was a nitrogen-containing organic matter and easily decomposed by microorganisms in the soil. This implied that adding γ-PGA to soil might lead to the production of binding agents. Therefore, we speculated that the increase of macro-aggregates might be due to the fact that micro-aggregates were bonded together by the binding agents produced by γ-PGA. Moreover, γ-PGA often formed a hydrogel after absorbing water (Sung et al. 2005). We speculated that the hydrogel might also be used as a binding agent, thereby promoting the formation of macro-aggregates.
The stability of water-stable aggregates was determined by the MWD (Kemper and Chepil 1965) and SPFD of water-stable aggregates (Castrignanò and Stelluti 1999). Nimmo and Perkins (2002) and Liu et al. (2014) reported that the greater the MWD value, the stronger the stability of water-stable aggregates. Additionally, Wu and Hong (1999) also reported that the smaller the SPFD of waterstable aggregates, the more stable water-stable aggregates. In the present study, γ-PGA significantly increased the MWD of water-stable soil aggregates and decreased SPFD of water-stable soil aggregates. The result suggested that γ-PGA effectively improved the stability of water-stable aggregates in the degraded silt loam. The positive effect of γ-PGA on stability of water-stable aggregates may in this case be mainly determined by the strong binding capability of γ-PGA.
In the present study, it was noteworthy that γ-PGA significantly increased soil NO 3 --N contents and soil NO 3 --N residue in the degraded soil. This might be attributed to the following three reasons: (1) γ-PGA could enhance the stability of soil aggregates due to the fact that the soils with wellstabilized soil aggregates structure had the excellent ability of storing soil nutrients (Elliott 1986;Xu et al. 2015); (2) γ-PGA could improve soil water-holding capacity due to the fact that the soils with great water-holding capacity could enhance soil nutrient retention capacity; (3) γ-PGA could promote the transformation of urea in soil to available nitrogen through activating urease activity in soil (Zhang et al. 2017b). In addition, γ-PGA had the ability to absorb available nitrogen in the soil and release the absorbed available nitrogen to the soil again for crop growth (Bhattacharyya et al. 1998). This may imply that γ -PGA had a very positive influence on the conversion and utilization of soilavailable nitrogen. Therefore, γ-PGA application would be a very effective and promising solution to ameliorate the degraded soil.

Effects of γ-PGA on plant growth, winter wheat yield and water use efficiency
Some studies have indicated that γ-PGA has a positive influence on the improvement of plant growth, crop yield and water use efficiency (Xu et al. 2013;Zhang et al. 2017b;Chen et al. 2018;Liang et al. 2019Liang et al. , 2021. In the present study, γ-PGA significantly increased plant growth, winter wheat yield and water use efficiency in the degraded soil. The result may be likely attributed to the following four possible mechanisms: (1) γ-PGA improved PAW and soil water storage, decreased soil water loss caused by soil evaporation and prolonged the wilting time and storage time of plant available water in the degraded soil; (2) γ-PGA promoted the formation of soil aggregates and enhanced the stability of soil aggregates in the degraded soil; (3) γ-PGA increased available nitrogen in the degraded soil; (4) some research indicated that γ-PGA could improve crop yield through improving crop root activity (Zhang et al. 2017a) and enhancing plant nutrient uptake capacity (Zhang et al. 2017b).
However, in our study, crop yield and water use efficiency could not maintain an increasing trend with the increase of γ-PGA application rates. Other researchers also found similar results (Chen et al. 2018;Liang et al. 2019Liang et al. , 2021. This result implied that excessive γ-PGA application rate might have an adverse effect on the increase of crop yield and water use efficiency. This was likely due to the fact that the high γ-PGA application rate weakens the respiration of crop roots. In this study, it was noteworthy that the highest winter wheat yield and water use efficiency were obtained with the P0.1 and P0.05 treatments, respectively. This result implied that the range of 0.05-0.1% might be recommended as an appropriate γ-PGA application range to improve soil quality and winter wheat production in the degraded soil.

Conclusions
In this present study, we hypothesized that applying γ-PGA to the degraded soils had a positive influence on soil moisture properties, soil aggregate stability, available nitrogen, water use efficiency and wheat yield in degraded soil. This study demonstrated that γ-PGA application could increase FC, PAW and soil water storage; decrease soil evaporation; and prolong the storage time of PAW and the wilting time compared with the CK. γ-PGA application could promote the formation of water-stable soil macro-aggregates and enhance the stability of soil aggregates compared with the CK. γ-PGA application could increase soil NO 3 --N content and residue in the degraded soil compared with the CK. Moreover, γ-PGA application could improve plant height, LAI, aboveground dry matter, winter wheat yield and water use efficiency. However, the highest winter wheat yield and water use efficiency were obtained with the P0.1 and P0.05 treatments, respectively. Therefore, we recommended the range of 0.05-0.1% could be used as an appropriate γ-PGA application range to improve soil hydraulic properties and winter wheat production in the degraded soil.

Disclosure statement
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Funding
This study was supported by the National Natural Science Foundation of China (No. 42077011).