Potential Application of Paenibacillus sp. C1 to the Amelioration of Soda Saline-Alkaline Soil

Abstract In this study, a saline-alkaline-tolerant bacterial strain named Paenibacillus sp. C1 was screened and isolated, and its basic properties were characterized. Its phosphorus dissolution, organic acid production, and application on ameliorating soda saline-alkaline soil were studied, and the changes of microbial communities after adding strain C1 with or without rice straws, (NH4)2SO4 and Ca3(PO4)2 were investigated. The results demonstrated that when the nitrogen, carbon, and phosphorus sources were (NH4)2SO4, glucose, and Ca3(PO4)2, respectively, strain C1 was in the best phosphorus-dissolving condition. The strain had good characteristics of acid production and phosphorus dissolution under the condition of high salinity and alkalinity. It could secrete cellulases, decompose rice and corn straws, and produce polysaccharides effectively. Strain C1 [especially with rice straws, (NH4)2SO4 and Ca3(PO4)2] could increase the content of available phosphorus (AP), organophosphorus (OP), total organic carbon (TOC), and polysaccharide (PS) and the activity of alkaline phosphatase (ALP) in saline-alkaline soil. Furthermore, adding strain C1 [especially with rice straws, (NH4)2SO4 and Ca3(PO4)2] could alter the bacterial flora of soil to make it adapt to high salinity and alkalinity better. Our study can be an important step toward the application of Paenibacillus sp. C1 to the amelioration of soda saline-alkaline soil and the investigation of its deep mechanisms.


Introduction
Soil salinization has become a major agricultural issue that restricts sustainable agricultural development and threatens food security in arid and semi-arid areas worldwide (Dong et al. 2021;Meena et al. 2019;Saifullah et al. 2018). It is reported that 1.13 billion hm 2 areas around the world are suffering from soil salinization (Wicke et al. 2011), and $77 million hm 2 areas have undergone secondary salinization to date (Shi et al. 2021). Notably, saline-alkaline soil can inhibit the growth of plants as a result of cellular damage through the degradation of nucleic acids and oxidation of lipids and proteins, ultimately causing cell death (Kohler et al. 2009;Xun et al. 2015). Saline-alkaline land, as an important potential land resource to compensate for decreasing cultivated land, plays a critical role in ensuring national food security and avoiding a global food crisis. Therefore, it is important to improve the physicochemical and biological characteristics of saline-alkaline soil to make the production system sustainable.
The pH value and salt content of soda saline-alkaline soil are high, while the organic substance content is relatively low. In soil with high salinity and alkalinity, the nutrient elements including nitrogen, phosphorus, and potassium needed by crops and plants are rarely utilized, resulting in a low level of soil fertility and a serious shortage of types and quantities of microorganisms capable of promoting the growth of crops and plants (Yang et al. 2010). Therefore, the lack of microorganisms makes it difficult to effectively improve soda saline-alkaline soil with the addition of fertilizers and organic materials in soil, inhibiting the nutrient absorption of crops and plants (Wu et al. 2001). In soil with severe salinization, the number of available microorganisms normally decreases with the salinization degree (Bai et al. 2019). Bacillus, Nesterenkonia, and Zhihengliuella are the main species in the soda saline-alkaline soil of Northeast China (Shi et al. 2012). The research of Zhu et al. (2019) showed that the activity of alkaline phosphatase (ALP) in saline-alkaline soil could be increased by 106.5% by adding the fermentation broth of Bacillus marmarensis to the saline-alkaline soil. After adding the fermentation broth, both the biomass of crops and the condition of salinealkaline soil were significantly improved. Liu et al. (2014) found that Klebsiella spp. could adapt to the environment of high salinity and alkalinity very quickly, improve the growth rate of plants in saline-alkaline soil, and remediate the saline-alkaline soil polluted by petroleum. The study by Bal et al. (2013) concluded that inoculation with the three 1-Aminocyclopropane-1-Carboxylate (ACC) deaminase containing plant growth-promoting rhizobacteria (PGPR) including Alcaligenes sp., Bacillus sp., and Ochrobactrum sp. could cause significant alleviation of stress-induced ethylene production and consequently improve the growth of rice under high salinity stress condition.
At present, the main strains used for ameliorating salinealkaline soil by microbial methods include photosynthetic bacteria (Liu et al. 2002), rhizosphere bacteria (Kearl et al. 2019), methanogens (Buan 2018), etc. There are two main problems in the existing microbial strains ameliorating saline-alkaline soil. On the one hand, the strain resources are scarce, and most of the microorganisms obtained from the researches have a single function, which cannot fully adapt to and improve soda saline-alkaline soil. On the other hand, the soil improvement effect is not ideal. Moreover, there are few studies on the biological amelioration of soda saline-alkaline soil in Northeast China, and most of the existing studies are on the influences of microorganisms on soil masses and aggregate composition. The abovementioned findings have almost exclusively focused on some cultured functional microorganisms in saline-alkaline soil or the single function of soil microbial communities, which made it difficult to meet the demands of the current agriculture development in saline-alkaline lands. Therefore, microorganisms with multiple functions should be developed in the biological improvement and utilization of soda saline-alkaline soil.
In this study, a saline-alkaline-tolerant bacterial strain named Paenibacillus sp. C1 was screened and isolated, and its basic properties were characterized. Simultaneously, we evaluated the effects of nitrogen, carbon, and phosphorus sources and researched the effects of salinity and alkalinity on the acid production and phosphorus dissolution of strain C1. Additionally, the cellulase activity of strain C1 and the content of extracellular polysaccharides while degrading straws were studied. Furthermore, we investigated the degradation process of straws by strain C1 in solution and soil, and analyzed the changes of main physicochemical parameters and microbial community in soil samples after adding this bacterial strain with (or without) (NH 4 ) 2 SO 4 and Ca 3 (PO 4 ) 2 . We hope that the results will provide a theoretical basis and technical support for improving soda salinealkaline soil by using strain C1, rice straws, (NH 4 ) 2 SO 4 , and Ca 3 (PO 4 ) 2 .

Soil samples and straws
Fresh soda saline-alkaline soil samples were collected in Songyuan, Jilin Province (44 52 0 49 00 N, 124 02 0 32 00 E) and kept at 4 C. Strain screening was completed within 48 h and bacterial DNA extraction was performed within 24 h. The main physicochemical parameters of soil samples were listed in Table S1. After natural air drying and removing impurities, soil samples were saved after sieving with a 2-mm sieve. Corn and rice straws were collected from the plant production base of Jilin Agricultural University. The straws were dried naturally, smashed, passed through a 2-mm sieve, and placed in a dry place.
Bacterial isolation, identification, and main culture media A bacterial strain named strain C1 was isolated from salinealkaline soil. The detailed procedures of bacterial isolation, culture, and identification are listed in Supporting Information (SI) 1.1 and 1.2. The detailed compositions of the main culture media and pH values are presented in SI 1.3.

Determination of phosphate concentration in fermentation broth
The solution and standard curve were prepared according to the method of Ramakrishnan et al. (2012), and the phosphorus-dissolving ability of strain C1 was determined by the molybdenum antimony colorimetric method. Firstly, the target strain was inoculated into the modified NBRIP culture medium, cultured at 30 C and 180 rpm in a constant temperature shaker, and made into a bacterial solution. Secondly, samples were taken at 6-h intervals and centrifuged at 4000 rpm for 10 min. Thirdly, 0.1 mL of supernatant was taken and added into a 50-mL colorimetric tube, then 2 mL of molybdenum antimony anti-chromogenic reagent was added. Finally, it was diluted with water to 50 mL, and the absorbance was measured after the chromogenic reaction for 20 min.
Analyses of acid production and phosphorus dissolution of strain C1 KNO 3 , NH 4 NO 3 , and NaNO 3 were respectively used instead of (NH 4 ) 2 SO 4 in the modified NBRIP medium to study the effects of nitrogen sources on the acid production and phosphorus dissolution of strain C1. After inoculation, the bacterial strains were cultured at 30 C and 180 rpm for 4 days, and each treatment was repeated three times. Samples were taken every 24 h to determine the pH value and phosphate content of the fermentation broth of strain C1. Likewise, maltose, lactose, and xylose were respectively used instead of glucose in the modified NBRIP medium to study the effects of carbon sources, while other procedures were the same. Similarly, FePO 4 and AlPO 4 were used instead of Ca 3 (PO 4 ) 2 in the modified NBRIP medium to study the effects of phosphorus sources on the phosphorus dissolution of strain C1, while other procedures were the same except that culture time was prolonged to 7 days.
In the study of the effects of salinity and alkalinity on acid production and phosphorus dissolution, the salinity of the modified NBRIP medium was adjusted to 0, 1, 3, 5, and 7% with NaCl, respectively, and the initial pH value of the modified NBRIP culture medium was adjusted to 8, 9, 10, and 11 with NaOH, respectively. After inoculation, the bacterial strains were cultured at 30 C and 180 rpm for 5 days, and each treatment was repeated three times. Samples were taken every 24 h to determine the pH value and phosphate content of the fermentation broth of strain C1.

Analyses of cellulase activity and exopolysaccharide production
To study the effects of salinity on the cellulase and exopolysaccharide production of strain C1, the straws were put into the modified straw degradation medium, and the salinity of the medium was adjusted to 0, 1, 3, 5, and 7%, respectively. Similarly, to study the effects of alkalinity, the pH value of the medium was adjusted to 8, 9, 10, and 11, respectively. After culture for 7 days, the cellulase activity of one part of the fermentation broth was determined by the DNS method, and the extracellular polysaccharide of the other part was collected by the alcohol precipitation method, and then its content was determined by phenol-sulfuric acid method (SI 1.4 and 1.5).

Soil experiment
To study the improvement effect of strain C1 on soda saline-alkaline soil with or without exogenous organic materials, four groups were set up (Table S2). In this experiment, 1 kg of soil was used in each group, and the soil culture container was a 1-L plastic basin. CK1 group contained 1 kg of saline-alkaline soil singly and C1 group was composed of soil, bacterial suspension, soil, and bacterial suspension sequentially from bottom to top, and the final concentration of bacterial cells in the soil was 1 Â 10 8 g/mL. In C1 þ RS group, 150 mL of bacterial suspension was sprayed on the surface of 30 g of rice straws. Then, they were added into the soil together with 1.2 g of (NH 4 ) 2 SO 4 and 1.3 g of Ca 3 (PO 4 ) 2 and mixed evenly. In CK2 group, the bacterial suspension was replaced with the same volume of sterile water, and other treatment processes were the same as those in C1 þ RS group. Each treatment was repeated six times and cultured at 28 C, and sterile water was added regularly to maintain the soil moisture content at about 25%.

Degradation experiment of rice and corn straws
The bacterial suspension of strain C1 was inoculated into the straw degradation liquid medium. After degradation for 10 days, the fermentation broth was removed. Then, the straws were taken out and washed with sterile water 2-3 times, dried in an oven at 50 C to constant weight, and observed by a scanning electron microscope (SEM, X-550, SHIMADZU, Japan). Additionally, the soil experiment was conducted using corn and rice straws with the same treatment in C1 þ RS group, and the straws were also washed, dried, and characterized by SEM analysis.

Determination of main physicochemical and biochemical parameters in soil
Main physicochemical and biochemical parameters in soil including the content of available phosphorus (AP), organic phosphorus (OP), total organic carbon (TOC), and polysaccharide (PS), and the activity of ALP were determined according to the procedures described in SI 1.6, 1.7, and 1.8.

DNA extraction and PCR amplification
Total genomic DNA was isolated from four groups (CK1, C1, CK2, and C1 þ RS) of fresh soil samples in the soil experiment utilizing the PowerSoil DNA Isolation Kit (Mobio Laboratories, Carlsbad, CA, USA). Subsequently, by using primers targeting the hypervariable V3 to V4 region of the bacterial 16S rRNA gene, the DNA extracts were amplified. The primers used in the amplification were 341 F 5 0 -ACTCCTACGGGAGGCAGCAG-3 0 and 805 R 5 0 -GGACTACNNGGGTATCTAAT-3 0 . A detailed description of PCR amplification is provided in SI 1.9.
Illumina MiSeq and sequence processing PCR amplicons were isolated from 1% agarose gels and purified using the AxyPrep DNA Gel Extraction Kit (Axygen Biosciences, USA), and quantified using QuantiFluor TM -ST (Promega, USA). After purification, amplicons were pooled at equimolar concentrations and sequenced on Illumina MiSeq platform (PE 300). A detailed description of sequence processing is presented in SI 1.10.

Basic properties of Paenibacillus sp. C1
Strain C1 was identified as Paenibacillus sp. based on the sequence similarity search in NCBI database and phylogenetic analysis with the type strains ( Figure 1). The stain showed above 97% sequence identity with more than one Paenibacillus sp. In the phylogenetic tree, the 16S rRNA gene sequence clustered with Paenibacillus amylolyticus strain S120 (AY509233.1). The growth curve showed that the logarithmic growth period of strain C1 was 9-18 h, and it grew slowly and entered a stable period after 21 h ( Figure  S1). At 24 h, the OD600 reached the maximum value of 0.892.
Strain C1 could grow vigorously on the modified NBRIP medium, and the formed phosphorus-dissolving circles and characteristics of strain C1 are shown in Figures S2(a,b), respectively. The phosphate concentration of fermentation broth increased with time and tended to be stable on the 4th day. The phosphate concentration in the fermentation broth was 141.70 mg/L, and the pH value dropped to 4.6.
Effects of nitrogen, carbon, and phosphorus sources on acid production and phosphorus dissolution The effects of different nitrogen sources on the acid production and phosphorus dissolution of strain C1 are shown in Figures 2(a,b), respectively. All the nitrogen sources could reduce the pH value in the fermentation broth, but it showed a slight effect of the types of nitrogen sources on the acid-producing ability of strain C1. However, there were large differences in the dissolved phosphate concentration of strain C1 with different nitrogen sources. When (NH 4 ) 2 SO 4 was used as the nitrogen source, the dissolved phosphate concentration was the highest (141.70 mg/L). By contrast, when NH 4 NO 3 was used as the nitrogen source, the dissolved phosphate concentration was the lowest (50.35 mg/L).
The effects of different carbon sources on the acid production and phosphorus dissolution of strain C1 are shown in Figures 2(c,d), respectively. Strain C1 could use glucose, maltose, lactose, and xylose to produce acid to dissolve Ca 3 (PO 4 ) 2 , and the pH values had almost the same decreasing trends. When glucose was used as the carbon source, the dissolved phosphate concentration was the highest, which was 141.70 mg/L, followed by xylose and lactose. However, the utilization effect of maltose was not high, and the dissolved phosphate concentration was only 52.89 mg/L. Strain C1 had a better utilization effect on monosaccharide than polysaccharide, and it also had good adaptability to different carbon sources.
The effects of different phosphorus sources on the phosphorus dissolution of strain C1 are shown in Figure 2(e). Compared with Ca 3 (PO 4 ) 2 , the phosphorus dissolution of strain C1 was greatly affected when the phosphorus source was AlPO 4 or FePO 4 . When AlPO 4 was used as the  phosphorus source, the dissolved phosphate concentration was 11.46 mg/L after culture for 7 days. However, when FePO 4 was used as the phosphorus source, the dissolved phosphate concentration of strain C1 was only 9.88 mg/L.

Effects of salinity and alkalinity on acid production and phosphorus dissolution
The effects of salinity and alkalinity on the acid production and phosphorus dissolution of strain C1 are shown in Figure 3. When NaCl concentrations increased from 1 to 7%, the pH value of the fermentation broth of strain C1 decreased gradually. With the increase of NaCl concentration, the pH value of the fermentation broth of strain C1 decreased slowly (Figure 3(a)). When NaCl concentration was 1%, the phosphorus-dissolving ability of strain C1 was higher than that in the control group (Figure 3(b)). When the NaCl concentration gradually increased to 7%, the phosphorus-dissolving ability gradually decreased. Strain C1 could grow normally when the pH value ranged from 8 to 11 and produce acid, which reduced the pH value of fermentation liquor to about 4.7 (Figure 3(c)). With the increase of initial pH value, the final dissolved phosphate content decreased (Figure 3(d)). When the initial pH values were 8 and 11, the maximum concentrations of dissolved phosphates were 141.70 and 88.81 mg/L, respectively.
Effects of salinity and alkalinity on cellulase activity and extracellular polysaccharide yields The cellulase activity of strain C1 and polysaccharide yields from rice and corn straws degraded by strain C1 are shown in Figure 4. Under the influences of different content of NaCl, the change trends of cellulase activity were similar, which increased with time (Figures 4(a,b)). When NaCl content was 1%, the cellulase activity of strain C1 degrading rice and corn straws peaked on the 4th and 5th day, and the maximum values were 1122.72 and 336.29 U/mL, respectively. When NaCl content was 7%, the cellulase activity of strain C1 degrading rice and corn straws reached 357.83 and 102.29 U/mL, respectively. Figure 4(c) shows the effects of salinity on the exopolysaccharide yields from rice and corn straws. Compared with the control, the exopolysaccharide content gradually decreased with the increase of NaCl content. When NaCl content was 1%, the exopolysaccharide yields from rice and corn straws degraded by strain C1 were 248.24 and 68.26 mg/L, respectively. When NaCl content was 7%, the exopolysaccharide yield from rice straws degraded by strain C1 was 76.23 mg/L, but it was hardly detectable from corn straws. Under the condition of different initial pH values, the cellulase activity of strain C1 had a similar change trend, which increased with time (Figures 4(d,e)). When the pH value was 11, the cellulase activity of strain C1 degrading rice and corn straws peaked on the 4th and 5th day, and the maximum values were 1481.49 and 498.51 U/mL, respectively. When the pH value was 8, the cellulase activity of strain C1 was the lowest. In the meantime, the enzyme activity of strain C1 gradually increased with the ascension of the initial pH value. Figure 4(f) shows the effects of alkalinity on the exopolysaccharide yields from rice and corn straws degraded by strain C1. The exopolysaccharide yield from rice straws degraded by strain C1 was higher than that from corn straws. When the initial pH value was 10, the exopolysaccharide yield from rice straws degraded by strain C1 was the highest (495.54 mg/L). By contrast, the extracellular polysaccharide from corn straws degraded by strain C1 reached the highest yield of 134.16 mg/L when the initial pH value was 11.

Degradation of straws in solution and soil
As shown in Figures 5(a,c), the waxy layers of the surfaces of the undegraded corn and rice straws were smooth and regular. Additionally, the straw structures were complete and closely connected, and the fiber bundles were evenly stretched. After the degradation by strain C1, the flat surface was disturbed, and the dense structure became loose ( Figures 5(b,d)). Simultaneously, the regular holes on the surface of corn straws were punched by strain C1, and the rice straws were decomposed into regular block structures. It can be seen from SEM ( Figures 5(e,g)) that corn and rice straws cultivated in soil have regular and complete surfaces under natural decomposition conditions, and some substances were attached to the surfaces. However, after degradation by strain C1, many clumps were attached to the surfaces of straws, and the regular structure of the straw disappeared ( Figures 5(f,h)).

Changes of main physicochemical and biochemical parameters in soil
The main physicochemical and biochemical parameters in different soil samples are listed in Table 1. It showed that adding C1 singly could significantly increase the AP content by 9.98% compared with the control. After adding strain C1 with rice straws, (NH 4 ) 2 SO 4 and Ca 3 (PO 4 ) 2 , the AP content increased by 40.48%. The OP content in C1 group had no significant change compared with that in CK1 group. However, the OP content increased by 54.73% in C1 þ RS group compared with that in CK2 group. There is no significant change in ALP activity when strain C1 was added singly. Nevertheless, the ALP activity increased significantly by 23.93% in C1 þ RS group compared with that in CK2 group. The TOC content in C1 group had no obvious change compared with that in CK1 group. By contrast, the TOC content increased by 31.26% in C1 þ RS group compared with that in CK2 group. Besides, adding C1 singly could increase the PS content significantly by 11.27% compared with CK1 group. Compared with that in CK2 group, the PS content of strain C1 increased significantly by 36.10% in C1 þ RS group.

Changes of the microbial community in salinealkaline soil
The diversity of the microbial community decreased after adding strain C1, rice straws, (NH 4 ) 2 SO 4 , and Ca 3 (PO 4 ) 2 in saline-alkaline soil, while the richness increased (Table S3). As shown in Figure 6(a), the dominant phyla of the bacterial communities differed across all groups. The bacterial sequences originated primarily from 12 phyla including Proteobacteria, Actinobacteria, Chloroflexi, Acidobacteria, Firmicutes, Bacteroidetes, Gemmatimonadetes, Cyanobacteria, Saccharibacteria, Planctomycetes, Nitrospirae, and  Verrucomicrobia, which accounted for more than 90% of the whole sequences. Among the dominant phyla, the relative abundances of Proteobacteria, Actinobacteria, and Firmicutes increased in C1 and C1 þ RS groups compared with those in CK1 and CK2 groups, respectively. The relative abundance of Bacteroidetes was higher in C1 group than that in CK1 group and maintained at a high level in CK2 and C1 þ RS groups. The relative abundance of Verrucomicrobia was at a close level in CK1, C1, and C1 þ RS groups, while it was lower in CK2 group.
The dominant genera of saline-alkaline soil bacteria were Sphingomonas, Skermanella, Pseudarthrobacter, Bacillus, RB41, Rubrobacter, Anaeroline, Microvirga, Gaiella, H16, Nocardioides, and Ensifer (Figure 6(b)). Among them, the relative abundances of Bacillus, Microvirga, and Ensifer were lower in C1 and C1 þ RS groups compared with those in CK1 and CK2 groups. By contrast, Skermanella and Sphingomonas had higher abundances in C1 group than those in CK1 group, while it was the opposite between CK2 and C1 þ RS groups. Pseudarthrobacter showed a higher abundance after the addition of stain C1 in soil, and the relative abundance of this genus was close to each other in CK2 and C1 þ RS groups. Besides, the relative abundance of Paenibacillus was higher in C1 group than that in CK1 group (Table S4).

Discussion
In the present study, a multifunctional saline-alkaline-tolerant bacterial strain named Paenibacillus sp. C1 was isolated and obtained by Gram staining from soda saline-alkaline soil. This strain could use glucose to produce acid to dissolve Ca 3 (PO 4 ) 2 , release PO 4 3À , produce cellulase, degrade straws, and release extracellular polysaccharides. According to the results of our study, when the nitrogen, carbon, and phosphorus sources were (NH 4 ) 2 SO 4 , glucose, and Ca 3 (PO 4 ) 2 , respectively, strain C1 was in the best phosphorus-dissolving condition.
Phosphorus-dissolving bacteria are widely distributed in soil, which can transform insoluble phosphates into soluble phosphates, playing an important role in the transformation of phosphorus-containing substances in soil (Sahay and Patra 2014). According to the existing studies, microorganisms can secrete a variety of organic acids during metabolism, and the organic acids chelate with metal cations, such as Ca 2þ , Fe 3þ , and Al 3þ to dissolve insoluble phosphates (Wang et al. 1994;Zhao et al. 2003). Besides, microorganisms can also release H þ to lower the pH value of their living environment through the assimilation of ammonium ions, thus dissolving insoluble phosphates (Ahuja et al. 2007;Lin et al. 2001). In this study, adding strain C1 reduced the pH value of the culture medium to dissolve insoluble phosphates, such as Ca 3 (PO 4 ) 2 , AlPO 4 , and FePO 4 . In general, due to the high pH value, large soil buffer capacity, high salinity, and poor nutrients in saline-alkaline land (Wang et al. 2011), few phosphorus-dissolving bacteria can adapt to the such environment or effectively increase the AP content in soil (Mahdi et al. 2021). Therefore, by adjusting the NaCl concentration and initial pH value in LB liquid medium, the salt-alkali tolerance test of strain C1 was carried out. The maximum level of tolerance to NaCl was 7%, and the  maximum level of tolerance to pH value was 11. The results suggested that the strain had a strong tolerance to salt and alkali stresses. The strain could survive under the condition of high salinity and alkalinity, so it was preliminarily concluded that the strain could be used in the amelioration of soda saline-alkaline soil. The influences of salinity and initial pH value on the cellulase activity of straw-degrading strain C1 and exopolysaccharide yields were studied. Strain C1 had good cellulase activity and there were high exopolysaccharide yields in the process of straw degradation when the salinity was 1% or 3%. Meanwhile, the cellulase activity gradually increased with the elevation of initial pH value, and exopolysaccharide content was maintained at a high level, suggesting that strain C1 had a good alkali tolerance in the process of straw degradation. To sum up, the salinity and initial pH value of the culture medium could affect not only the ability of strain C1 to degrade cellulose but also the exopolysaccharide yields, indicating strain C1 had a great potential to degrade straws and produce exopolysaccharides under high salinity and alkalinity.
Agricultural biomass is mainly made up of cellulose, hemicelluloses, and lignin, which are all tightly bound together to form a structural complex that makes the biomass resistant to degradation (Zahoor et al. 2021). The changes in the surfaces and structures of corn and rice straws indicated that strain C1 could secrete cellulases and degrade these two types of straws effectively. It should be noted that the clumps might be formed by the combination of degradation products of straws degraded by strain C1 with cellulases and exopolysaccharides produced by itself. The result further confirmed that strain C1 could act as a functional bacterial strain that decomposes organic substances including corn and rice straws in saline-alkaline soil. The distraction of structure and increase of pore spaces in the biomass substrate of straws were active and effective processes to develop degradation (Kim and Lee 2018;Rigual et al. 2019). Hence, the degradation was more efficient after the addition of strain C1 in this study.
It has been shown that phosphorus-dissolving bacteria with acid-producing and phosphorus-dissolving abilities can increase the AP content (Li et al. 2019). In addition, ALP can be produced to degrade OP under the alkaline condition in soda-saline soil, thus increasing the availability of phosphorus (Cui et al. 2021). In this study, as a phosphorus-dissolving bacterium, strain C1 was found to increase the AP content when it was added singly to soda saline-alkaline soil, but the OP content and ALP activity had no significant change. When strain C1 was added to soda saline-alkaline soil together with rice straw, (NH 4 ) 2 SO 4 and Ca 3 (PO 4 ) 2 , the AP and OP content and ALP activity all increased significantly. The reason might be that strain C1 secreted organic acid to dissolve inorganic phosphorus and release phosphate radicals. Through the degradation of rice straws, part of active phosphorus can be converted into OP that can be easily utilized by microorganisms, thus improving the ALP activity in soil (Xie et al. 2017), so it can further degrade OP and keep the AP content in soil at a high level. Meanwhile, it was found that adding strain C1 singly to soda saline-alkaline soil could not increase TOC content significantly. However, when strain C1 was added together with rice straws, (NH 4 ) 2 SO 4 and Ca 3 (PO 4 ) 2 , it secreted cellulase to degrade rice straws through the metabolism of strain C1, and the accumulation of organic carbon increased, which had a significant effect on the improvement of soil organic carbon content (Liu et al. 2020). The results showed that strain C1 could be colonized in soda saline-alkaline soil. However, when organic matters and other nutrients were deficient in soda saline-alkaline soil, strain C1 grew slowly and could not metabolize sufficiently to produce polysaccharides, which could not significantly change the composition and stability of aggregates. When organic matter (rice straws) and other nutrients [(NH 4 ) 2 SO 4 and Ca 3 (PO 4 ) 2 ] were added, the biological activity of strain C1 was  enhanced, and the decomposition products and polysaccharides produced by the degradation of rice straws cemented soil aggregates, thus promoting the formation of larger aggregates. The phylum Proteobacteria plays an important role in the cycling of nitrogen and energy through the soil ecosystem, so the abundance of this phylum in soil could be significantly increased by the application of nitrogen fertilizers (Fierer et al. 2012). Besides, it has been shown that the abundances of Proteobacteria and Firmicutes have positive correlations with hydrocarbon degradation (Li et al. 2016a), so their decreased abundances might result from the declined hydrocarbon content after the degradation by strain C1. Some studies have already shown that the bacteria of Bacteroidetes are widely present in different hypersaline environments and are resistant to salts (Keshri et al. 2013). Therefore, the increased abundance of this phylum suggested that strain C1 could improve the adaptability of bacteria to the stress induced by high salinity. Verrucomicrobia is categorized as an oligotroph with strong resilience to poor nutrient conditions (Pan et al. 2014). Its decreased abundance in CK2 group compared with that in CK1 group might be caused by the addition of nutrients including N and P. However, the abundances of Verrucomicrobia in C1 þ RS and CK1 groups were close, suggesting strain C1 utilized a certain quantity of nutrients, which would hardly affect the growth and reproduction of this phylum.
Many Bacillus species can degrade hydrocarbons (Borah and Yadav 2014) and utilize a broad range of hydrocarbons over a wide temperature and salinity range (Kumar et al. 2007). The decreased abundance of this genus indicated that there might be a competitive relationship between it and strain C1. The relative abundance of Skermanella in soil was associated with pH value and the content of phosphorus, carbon, and nitrogen (Van Wyk et al. 2017). The abundance of Skermanella was higher in CK2 group than that in CK1 and C1 groups, indicating that the addition of rice straw, (NH 4 ) 2 SO 4 and Ca 3 (PO 4 ) 2 increased the content of phosphorus, carbon, and nitrogenous substances (including nitrite and nitrate). Especially, its decreased abundance might result from strain C1-induced acidic environment. Chen et al. (2021) found that Pseudarthrobacter was correlated positively with methane and nitrogen metabolism. In this work, the relative abundance of this genus increased to a large extent, suggesting it participated in the metabolism of (NH 4 ) 2 SO 4 and nitrogenous substance decomposed from straws. Species of Microvirga are described as Gram-negative and strictly aerobic bacteria with the function of nitrogenfixing (Zilli et al. 2015). When (NH 4 ) 2 SO 4 was added to the soil, the nitrogen cycle in the system was promoted, thereby increasing the abundances of some nitrogen-fixing bacteria. However, when (NH 4 ) 2 SO 4 and strain C1 were added together, a certain amount of NH 4 þ was utilized by strain C1, which could reduce the dependence of soil on nitrogenfixing bacteria to some extent. The genus Sphingomonas showed a similar change trend to Microvirga, which might result from the fact that it also acts as a nitrogen-fixing bacterial genus (Videira et al. 2009). Ensifer strains are usually attached to legumes native to alkaline and semi-arid/saline soils (Sankhla et al. 2017). Additionally, Ensifer species are highly adapted to saline-alkaline soil (Li et al. 2016b). The higher abundances of this genus in CK2 and C1 þ RS groups might be caused by elevated salinity after adding (NH 4 ) 2 SO 4 and Ca 3 (PO 4 ) 2 . Meanwhile, its lower abundance in strain C1-treated soil samples compared with that in corresponding control groups suggested that strain C1 improved the alkaline environment. The varied abundances of bacteria indicated that adding strain C1 [especially with rice straws, (NH 4 ) 2 SO 4 and Ca 3 (PO 4 ) 2 ] could alter the bacterial flora of soil to make it adapt to a saline-alkaline condition better.

Conclusions
To conclude, the study demonstrated that when the nitrogen, carbon, and phosphorus sources were (NH 4 ) 2 SO 4 , glucose, and Ca 3 (PO 4 ) 2 , respectively, strain C1 was in the best phosphorus-dissolving condition. The strain had good characteristics of acid production and phosphorus dissolution under the condition of high salinity and alkalinity. Additionally, strain C1 could secrete cellulases, decompose rice and corn straws, and produce polysaccharides effectively. Strain C1 [especially with rice straws, (NH 4 ) 2 SO 4 and Ca 3 (PO 4 ) 2 ] could significantly increase the content of AP, OP, TOC, and PS and the activity of ALP in soil. Furthermore, phyla including Proteobacteria, Actinobacteria, Chloroflexi, etc., and genera including Sphingomonas, Skermanella, Pseudarthrobacter, etc. showed varied abundances under different treatments, suggesting that adding strain C1 [especially with rice straws, (NH 4 ) 2 SO 4 and Ca 3 (PO 4 ) 2 ] could alter the bacterial flora of soil to make it adapt to a saline-alkaline condition better. The results will contribute to the development of improved biological methods for the amelioration of soda saline-alkaline soil by using strain C1, rice straws, (NH 4 ) 2 SO 4 , and Ca 3 (PO 4 ) 2 .

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
This work was financially supported by the National Key Research and Development Project (2018YFD0300208).