Effects of Hedysarum leguminous plants on soil bacterial communities in the Mu Us Desert, northwest China

Abstract This study assessed the influence of rhizocompartment types (i.e., root, rhizosphere soil, root‐zone soil, and intershrub bulk soil) on the diversity of soil microbial communities under desert leguminous plant shrubs. Moreover, the influence and variations of soil physicochemical factors in interactions among leguminous plants, soil, and microbes were investigated. Both 16S rRNA high‐throughput genome sequencing and conventional soil physicochemical index determination were used to characterize both the bacterial diversity and soil physicochemical properties in the rhizocompartments of two Hedysarum species (Hedysarum mongolicum and Hedysarum scoparium) in the Mu Us Desert of China. All nutrient indices (except total phosphorus and available phosphorus) in rhizosphere soil were uniformly higher than those in both root‐zone soil and intershrub bulk soil (p < .05). The bacterial community diversity in the root, undershrub soil (i.e., rhizosphere and root zone), and intershrub bulk soil also showed significant differences (p < .05). The bacterial community in the root is mainly composed of Proteobacteria, Actinobacteria, Bacteroidetes, Tenericutes, and Chloroflexi, among which bacteria of the Proteobacteria genus are dominant. Root endophyte and rhizosphere soil microbiomes were mainly influenced by soil nutrients, while bacterial communities in root‐zone soil and intershrub bulk soil were mainly influenced by soil pH and NH4 +‐N. The rhizocompartment types of desert leguminous plants impose a significant influence on the diversity of soil microbial communities. According to these findings, nitrogen‐fixing rhizobia can co‐exist with nonsymbiotic endophytes in the roots of desert leguminous plants. Moreover, plants have a hierarchical filtering and enriching effect on beneficial microbes in soil via rhizocompartments. Soil physicochemical factors have a significant influence on both the structure and composition of microbial communities in various rhizocompartments, which is derived from the interactions among leguminous plants, soil, and microbes.


| INTRODUC TI ON
In desert areas, the climate and environment are extremely harsh; the constant aridity, deficiency of soil nutrients and organic matter, and degradation of land all pose serious threats to human survival and biological diversity (Zhu, Li, Li, Liu, & Xue, 2010). In this context, the improvement of soil texture and fertility, and the restoration of ecological vegetation have become urgent issues (Wang et al., 2012). The genus Hedysarum Linn. comprises leguminous plants that are extensively distributed in deserts.
Their roots are characterized by a strong sprouting ability, a deep taproot, high stress resistance, and high nitrogen-fixing efficiency. The genus Hedysarum plays a vital role in improving soil fertility, fixing soil and sand, and restoring vegetation; these are pioneer plants, often used in the ecological restoration of deserts (Hou, He, Li, Wang, & Zhao, 2019;Xie, He, Wang, Hou, & Sun, 2017). Hedysarum mongolicum and Hedysarum scoparium are two species endemic to quicksand environments in Asian deserts with their main distribution in the desert areas of Northwest China Xie et al., 2017). Plants of this species have well-developed roots and can withstand both wind erosion and extreme aridity, thus making them effective pioneer species for vegetation restoration efforts in northwest China (Gao et al., 2014;Lai et al., 2016;Sun et al., 2017). In addition, H. scoparium can be used as forage grass to support animal husbandry, and as an oil extraction source of woody oil and fibrous plants. Yang et al. (2010) studied the effects of soil moisture on the growth of Hedysarum seedlings in desert areas and found that the rapid growth of roots during the seedling stage effectively adapts to extreme drought environments and controls the windward side of sand dunes, thereby slowing down the movement of sand dunes. Studies have also shown that Rhizobium sullae cannot survive independently under extreme drought or alkaline environments that Hedysarum plants can tolerate. When the bacteria establish a mutually beneficial symbiotic relationship with Hedysarum plants (Yates et al., 2015), they rely on the host plant to survive and in turn provide host plants with nitrogen for growth via nitrogen fixation (Sablok et al., 2017;Squartini et al., 2002).
The soil supports the growth of plants and supplies them with nutrients needed for growth. At the same time, with plants fixing external carbon sources, the physical, chemical, and biological characteristics of the soil are, in turn, improved (Pérez-Bejarano et al., 2010). Some previous studies pointed out that the distribution and growth of vegetation communities in desert ecosystems are restricted, with varying degrees, by the soil microorganisms (Mengual, Schoebitz, Azcón, & Roldán, 2014;Saul-Tcherkas & Steinberger, 2011). As a driving force in the transformation of soil organic matter and nutrients, soil microbes are pivotal in ensuring the stability and sustainability of ecosystems (Kennedy & Smith, 1995). Soil microbes are important components of belowground biota and are essential for a wide range of ecosystem processes such as decomposition, nutrient cycling, soil carbon storage, and maintenance of the soil structure (Kardol & Wardle, 2010;Zeller, Liu, Buchmann, & Richter, 2008;Zelles, 1999).
Rhizobia are Gram-negative bacteria that are extensively distributed in soil. They can infect and live on leguminous plants, form nodules in their roots, supply the nitrogen nutrients needed by leguminous plants during growth through nitrogen fixation from soil and air, and build a mutualistic symbiotic relationship with leguminous plants (Batut & Boistard, 1994;He & Zhu, 1998). This interaction is based on unique transcription and the signal transmission of symbiotic genes of the host plant and rhizobia, which in turn trigger the corresponding organogenesis and adaptive physiological response of the host plant (Sablok et al., 2017). Various studies have shown that R. sullae is an efficient nitrogen-fixing rhizobium that can enter the root system of the genus Hedysarum and establish a mutually beneficial symbiotic relationship with the host (Yates et al., 2015). Although R. sullae cannot survive in the extreme environment that the genus Hedysarum can tolerate, the bacteria can rely on the host plant to survive in an arid or alkaline environment (Sablok et al., 2017). And the interaction between such a wide range of plants and microorganisms mostly occurs in the rhizosphere soil, where plant root systems are in close contact with the soil.
In research on plant-associated microbes, the diversity of rhizospheric microbes has increasingly become a focus of attention (Bulgarelli et al., 2012;Edwards et al., 2015;Lundberg et al., 2012;Philippot, Raaijmakers, Lemanceau, & van der Putten, 2013). Since they inhabit narrow rhizocompartments that surround plant roots, microbial communities strongly affect the growth and nutrient absorption of plants, and even the structure and distribution of plant communities (Hoeksema et al., 2010;Ke & Miki, 2015;Maron, Marler, Klironomos, & Cleveland, 2011). In turn, plants regulate the soil pH and the structure of rhizocompartments via root exudates, thus substantially influencing microbial communities in the rhizosphere and root-zone soils (Gottel et al., 2011;Maron et al., 2011).
In addition, numerous rhizospheric microbes can enter root tissues and form an endophyte microbiome, which may form a substantially different community composition from that of the rhizospheric microbiome (Lundberg et al., 2012;Xie et al., 2017). Hiltner has defined this differential phenomenon as the "rhizosphere effect" (Hartmann, Rothballer, & Schmid, 2008). Rhizocompartments constitute a special micro-ecosystem. Differences in various soil factors among roots, rhizosphere soil, and nonrhizosphere soil can be used both as a reflection of the intensity of the rhizosphere effect and to characterize the degree of interaction between plants and soil. Assessment of these factors offers a feasible method for the quantitative study of bacterial diversity in the rhizocompartments of desert leguminous plants.
This study investigated the diversity of microbial communities of two Hedysarum Linn. desert leguminous plants and their undershrub rhizocompartments. The obtained data support desertification control, degraded land restoration and provide guidance for the development of future governance measures. The main hypotheses of this paper are as follows: (a) bacterial communities show hierarchical diversity and differential variations in undershrub and intershrub soils of desert leguminous plants; (b) the diversity of microbial communities in the rhizocompartments of desert leguminous plants is driven by interactions among soil factors, leguminous plants, and microbes in rhizocompartments.

| Research site
This study was conducted at the Yanchi Research Station (37°04′-38°10′N, 106°30′-107°47′E) in Ningxia Province, northwest China, in June 2018 ( Figure 1). The study site is located at the southern fringe of the Mu Us Desert, which is characterized by a typical semiarid continental monsoon climate with dry and warm climate throughout the year. The mean annual temperature is 8.1°C (range: −29.4°C to 37.5°C; relative humidity range: 49%-55%), and a mean annual rainfall of ~292 mm, 60%-70% of which falls between July and September (maximum in August). The local soil type is eolian sandy soil (Yu et al., 2015). The soil texture of the study area is sandy in the 0-1 m profile, with a mean bulk density of 1.5 g/cm 3 (Lai et al., 2016). Ecological degradation in this area is mainly caused by overgrazing, climate change, and wind erosion, which has resulted in the degradation of arid grasslands into sandy land. The existing vegetation was established via aerial seeding (mainly Artemisia ordosica, H. mongolicum, and H. scoparium), seedling planting (Caragana microphylla), and natural restoration, which began in 2001 (Lai et al., 2016). Currently, the major dominant xeric shrub species in this area include A. ordosica, H. mongolicum, Salix psammophila, H. scoparium, and C. microphylla.

| Sampling strategy and physicochemical analysis of soil
The common dominant xeric leguminous shrub species of this area were selected for analysis, namely, H. mongolicum and H. scoparium.
These two leguminous plants are both dominant xeric shrub species that naturally co-occur and are widely distributed in the desert areas of Northwest China (including the study area). The two shrub species selected for this study were collected from a sample plot, formed via aerial seeding and natural enclosure; the adaptability of plants of these species to a desert environment and their ecological restoration effects have received close attention (Gao et al., 2014;Lai et al., 2016;Sun et al., 2017). The sample plants were mainly distributed on the sunny slopes of immobilized dunes that had formed via natural enclosure. Three sample plots (100 × 100 m, 100 m apart) were established for each species.
For each shrub species, five shrubs were randomly selected from the immobilized sample plots and both plant root tissue and soil samples were collected. We had initially considered to include the legume nodule samples, but during the actual sampling, we found that the two Hedysarum spp. chosen, rarely have nodules in their root systems. Thus, we could not collect the nodules, as other studies have (Edwards et al., 2015;.
Occasionally, the nodule samples were available, but the 16S rRNA high-throughput gene sequencing analysis of the collected samples showed that the composition of the nodule endophytic community and the bacterial community in the root system involved were not significantly different. We believe that this might be to the sampling method described by .
After excavating to the roots, tweezers were used to pick up the soil particles adhering to the roots as rhizosphere soil to determine soil physicochemical properties. After that, roots were dug out and slightly shaken to remove large attached soil aggregates, and gloves were worn to collect large soil aggregates as root-zone soil, which were then put into aseptic bags. Root samples were obtained from the secondary or tertiary branches of plant roots.
Healthy and intact roots with an even thickness (5-8 cm) were removed and stored in sterile sample bags. Intershrub bulk soil was collected at the same sampling depth as root-zone soil, 10-40 cm below intershrub bare soil. All collected samples were stored in preservation boxes with dry ice and brought back to the laboratory for subsequent treatment.
Root samples were first oscillated for 1 min in 50 ml centrifuge tubes containing 25 ml PBS buffer (PBS-S; 130 mM NaCl, 7 mM Na 2 HPO 4 , 3 mM NaH 2 PO 4 , pH 7.0, 0.02% Silwet L-77); then, two replicates were rinsed with new cleaning fluid at an interval of 5 min. The turbid liquid was then filtered through a 100-µm nylon mesh cell strainer, and the remainder was centrifuged for 15 min at 3,200 × g to form a pellet, which contained fine sediment and microorganisms as the rhizosphere soil for molecular biological analysis. Sterile PBS buffer (25 ml) was added to a new sterile 50 ml tube, which contained the afore-mentioned root samples, the mixture was vortexed, and this step was repeated until the PBS buffer was clear. The washed roots were placed into new centrifuge tubes containing 25 ml PBS buffer for ultrasonic oscillation for 5 min; an interval of 30 s was used between the two replicates. After discarding the fluid, clean root samples were collected. The above-mentioned method for cleaning the root tissues has been verified by , who proved that this method could effectively avoid the contamination and damage of plant tissue samples by subsequent cleaning procedures and could guarantee the ability of the thus treated samples to represent the rhizocompartment properties of the shrub species under investigation. Next, in reference to the sample treatment method described by Sun et al. (2017) for bacterial analysis, each collected plant species was subjected to three replicates according to its three sample plots. Samples from the rhizocompartments of the five shrubs in the same sample plot were mixed, and these mixed samples were established according to the four rhizocompartment types of each plant species. Each mixed soil sample was homogenized and filtered through a 2-mm soil sieve to remove gravel and other impurities. Each mixed sample was divided into two subsamples: Part 1 contained 24 DNA samples (2 shrub species × 4 rhizocompartment types × 3 replicates), which were stored at −80°C for molecular biological analysis. Part 2 contained 18 soil samples (2 shrub species × 3 rhizocompartment types × 3 replicates), which were air-dried for physicochemical analysis.
To determine the physicochemical properties of the soil samples, a conventional approach was adopted to quantify total soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), available phosphorus (AP), ammonium nitrogen (NH 4 + -N), nitrate nitrogen (NO 3 − -N), and pH. SOC was quantified using the dichromate oxidation method (Walkley & Black, 1934). The TN concentration of soil samples was determined using a semi-micro Kjeldahl Apparatus Nitrogen Autoanalyzer (Du, Pan, Li, Hu, & Wang, 2011).
AP and TP were measured using an ultraviolet spectrophotometer (UV-2550; Shimadzu). NH 4 + -N and NO 3 − -N were measured using the indophenol blue method and the hydrazine sulfate method, respectively, and soil pH was recorded on a 1:1 (10 g:10 ml) soil/distilled water slurry. The soil water content (SWC) was determined gravimetrically after oven drying at 105°C for 24 hr. All these methods were analyzed following international standard methods

F I G U R E 2
Flowchart of the experimental design and sketch of rhizocompartment types (i.e., roots, rhizosphere soil, root-zone soil, and intershrub bulk soil) as adopted and published by the Institute of Soil Science, Chinese Academy of Sciences (1978).

| 16S rRNA gene amplification and sequencing
DNA was extracted from 0.5 g fresh samples using the EZNA Soil DNA Kit (Omega Bio-Tek) following the manufacturer's instructions and was stored at −80°C for later use. When extracting tissues in the root, we only collected the internal tissues of the root system (excluding any tissues from the root epidermis) for subsequent analysis. After thawing on ice, extracted DNA samples were centrifuged separately and fully mixed. Sample quality was determined using After sequencing using the Illumina MiSeq system (Illumina), the results were stored in the Fastq format. The quantitative insights into microbial ecology (QIIME) software (version 1.8; http://qiime. org/) were used to analyze original Fastq files and conduct quality control according to the following criteria (Fierer, Hamady, & Knight, 2008;Miao et al., 2016): (a) Base sequences with a quality score < 20 at the read tails were removed, and the window was set to 50 bp; if the mean quality score in the window was < 20, the posterior-end base sequences were discarded from the window; reads shorter than 50 bp were removed after quality control; (b) paired reads were assembled into one sequence according to the overlapping relationship between paired-end reads (minimum overlapping length: 10 bp); (c) the maximum allowable mismatch ratio of the overlapping areas of assembled sequences was set to 0.1, and sequences that failed to meet this criterion in pairs were removed; (d) samples were distinguished according to barcodes and primers at the head and tail ends of sequences, and sequence directions were adjusted based on the number of mismatches allowed by barcodes; (e) different reference databases were selected to remove chimeras according to the type of sequencing data (using Usearch; version

| Statistics and analysis
The

| Soil properties
In this experiment, soil samples were collected from undershrub (i.e., rhizosphere and root zone) and intershrub zones of H. mongolicum and H. scoparium. Variations were found in the soil physiochemical properties of rhizosphere, root-zone soil, and intershrub bulk soil in both species. In both cases, the pH of the rhizosphere soil was lower than that of the root-zone soil. The contents of all nutrients (except TP) in rhizospheres, root-zone soil, and intershrub bulk soil showed significant differences (p < .05) ( values in rhizosphere soil were uniformly higher than those in both root-zone soil and intershrub bulk soil (Table 1).
Overall, the nutrient contents (except TP) of the undershrub soil were higher than those of intershrub soil (and significant differences were observed; p < .05).

| Alpha and beta diversity of rhizocompartment bacterial communities
As shown in Table S1, all samples of the two Hedysarum Linn. leguminous plants achieved sound sequencing quality. The length distribution of high-quality sequences (i.e., clean reads) was mainly concentrated at 360-380 bp, accounting for 88.05% of the total number of sequences (i.e., reads). Good's coverage index had a high comparability for the sequencing depth of the four rhizocompartment types (root, rhizosphere soil, root-zone soil, and intershrub bulk soil), with a range of 95.57%-96.70%. This suggests that the sequencing depth was sufficient to reliably describe these plant rhizocompartment-related bacterial communities (Table S1). The Good's coverage indices of root samples were also significantly higher than those of undershrub and intershrub bulk soil samples (p < .05) (Table S1). According to related data, the Chao1 and Shannon indices and the observed OTUs of microbial communities in undershrub soil samples are both significantly higher than those in plant root tissues (p < .05) ( Table 2). By comparison, for the two desert leguminous plants, plant species and plant rhizocompartments jointly affect the alpha diversity of soil bacterial communities under and inter shrubs, but rhizocompartments play a leading role (Table 2). shrub species (R = .205, p = .009) ( Figure S1).

| Taxonomic composition of rhizocompartment bacterial communities and strategy for hierarchical filtration and enrichment of dominant microbial communities
In the four rhizocompartments of the two Hedysarum species, bacterial phyla mainly consisted of Proteobacteria, Actinobacteria, and Bacteroidetes (relative abundances > 1%), which accounted for > 90% of the total bacterial composition ( Figure 4 and Table S2).
The four investigated rhizocompartments had a significant influence on the relative abundances of major bacterial phyla. Proteobacteria showed a significant trend of hierarchical enrichment in the order of intershrub bulk soil < undershrub soil < root (p < .05), while Actinobacteria showed the opposite trend ( Figure 4).
For H. mongolicum, the major dominant bacterial orders of root endophyte microbes consisted of Rhizobiales, Burkholderiales, and Propionibacteriales. The relative abundances of Rhizobiales, Burkholderiales, Propionibacteriales, and Caulobacterales in microbes also showed a significant overall trend of hierarchical enrichment and filtration following the order of intershrub bulk soil < undershrub < root (p < .05) (Figure 4 and At the genus level, Rhizobiales genera had a relative abundance of 28% in roots, mainly consisting of Rhizobium and Bradyrhizobium (Table S2).
The bacterial communities in the rhizocompartments all originated from soil; however, there are varying degrees of significant differences between the bacterial communities in the four rhizocompartments. This identified OTUs with significant enrichment or reduction in intershrub bulk soil relative to the other three rhizocompartments. As shown in Figure   Note: Data are presented as mean ± standard deviation, n = 3. HM: Hedysarum mongolicum; HS: Hedysarum scoparium.
Different letters signify significant differences among four rhizocompartments (p < .05). ns: no significant difference.
The last three rows represent the F values of the interactions between plant species/rhizocompartment types (shrub species × rhizocompartments).

| Relationships between rhizocompartment bacterial communities and soil factors
In the investigated desert environment, the soil factors that affect the root and rhizosphere soil bacterial communities of the two  (Table 3).

| Composition of root endophytes
This study demonstrated that nitrogen-fixing bacteria occupied a dominant position in the roots of xeric leguminous plants, but also that many non-nitrogen-fixing microbes were present. These results The larger the circle of a species, the more the connections of the circle, and the more significant its correlation with other species, which represents the core species in the correlation. (a) co-occurrence network of bacterial communities in the root, (b) co-occurrence network of bacterial communities in the rhizosphere soil, (c) co-occurrence network of bacterial communities in the root zone, (d) co-occurrence network of bacterial communities in the intershrub bulk soil leguminous plant (Msaddak et al., 2018). It has also been reported that endophytic bacteria, which are known to exist in plant tissues, can facilitate plant growth (Rajendran, Sing, Desai, & Archana, 2008;Rivas et al., 2002) and that other root endophytes co-inoculated with rhizobia can accelerate the growth and nodulation of leguminous plants (Chen et al., 2015;Xu, Zhang, Wang, Chen, & Wei, 2014).
Together, these results demonstrate that nonsymbiotic endophytes in roots may coordinate with nitrogen-fixing rhizobia and exert various functions.

| Hierarchical filtration and enrichment strategy of rhizocompartments
The numbers of significantly enriched OTUs followed a decreasing hierarchical trend in the order of root < undershrub soil < intershrub bulk soil (p < .05). The majority of OTUs that were relatively decreased in roots also showed relative reductions in rhizosphere soil and root-zone soil ( Figure 5). This phenomenon can be considered as a hierarchical filtration strategy of desert leguminous plants on soil microbes . In the four rhizocompartments, bacteria were assessed at the phylum, order, and genus levels. The relative abundances of Proteobacteria, Rhizobiales, and other specific dominant bacterial taxa indicated an overall hierarchical enrichment trend in the order of intershrub bulk soil < undershrub soil < root (p < .05) (Figure 4). With respect to the mutualistic symbiotic relationship of rhizobia with leguminous plants, the enrichment trends discussed above suggest that the roots of desert leguminous plants exert a hierarchical enrichment and filtration effect for specific beneficial bacteria in the soil (Table 2, Figure 5). A similar hierarchical trend was also observed in the rhizosphere soil, root surface, and roots of rice (Edwards et al., 2015), as well as in the rhizosphere soil and internal root tissues of Arabidopsis thaliana (Lundberg et al., 2012). According to studies on the diversity of the rhizospheric microbiota of field-grown corn, significant differences exist between undershrub soil and intershrub soil in terms of both the enrichment and diversity of bacteria and the relative abundances of bacterial groups (Peiffer et al., 2013).  Table S3 TA B L E 3 Correlation between bacterial community and soil properties as shown by the Mantel test carbon-nitrogen ratio of soil and the variability of the ammonium nitrogen pool; moreover, the significance level is higher than that of any other environmental variable in soil (Nemergut, Cleveland, Wieder, Washenberger, & Townsend, 2010). Specifically, the undershrub enrichment trend of Proteobacteria toward roots is positively correlated with the available carbon pool of soil; furthermore, Proteobacteria are closely related to heterotrophic N-fixers, meaning that their presence can promote the utilization of ammonium nitrogen (NH 4 + pools) (Nemergut et al., 2010). In contrast, the relative abundance of Actinobacteria is positively correlated with the pH of soil (Lauber, Hamady, Knight, & Fierer, 2019;Nemergut et al., 2010).
Overall, the nutrient content of the rhizosphere soil was higher than that of intershrub soil (p < .05). Enrichment trend analysis of soil nutrients under the influence of shrub roots obtained similar results (Philippot et al., 2013). Both wild (Glycine soja) and cultivated (G. max) soybeans had higher alpha diversity of bacterial communities in the rhizosphere compared with intershrub bulk soil, suggesting that the nutrients enriched in the undershrub soil are needed by plants during growth (Chang et al., 2018). In the Mu Us Desert in Ningxia Province, China, the nutrients in the undershrub and intershrub soils of three different xeric shrub species (i.e., A. ordosica, S. psammophila, and C. microphylla) have also revealed a similar trend of enrichment from intershrub soil to undershrub soil .

| Interactions among desert leguminous plants, soil, and microbes
In this study, while soil pH mainly influenced the bacteria in the rootzone soil and intershrub bulk soil, its influence on the bacteria in root endophytes was relatively insignificant (Table 3 and Figure 7).
According to previous research, the influence of soil pH on the microbes of rhizosphere soil of soybean and alfalfa exceeded that of soil nutrients (Xiao, Fan, Wang, Chen, & Wei, 2017). guminous Hedysarum species is largely alkaline, the rhizosphere soil pH is lower than the root-zone soil pH (Table 1).
In contrast, only few soil microbes absorb and assimilate NO 3 − -N; in the co-existence of NO 3 − -N and NH 4 + -N, the latter inhibits the absorption of NO 3 − -N by microbes, since the assimilation of NO 3 − -N consumes energy (Wickramasinghe, Rodgers, & Jenkinson, 1985).
From the deepening of rhizocompartments from intershrub bulk soil to roots, the absorption and utilization of nutrients by roots, as well as the activity of soil microbes, are gradually strengthened. This results in a decrease of the NH 4 + -N content and has a significant influence of NO 3 − -N content in rhizosphere soil (Table 1). In addition, the relative increase of soil pH can increase the NH 4 + -N yield in soil, mainly because a higher pH improves the solubility of organic matter; this provides many matrices rich in carbon and nitrogen for the growth of microbes and thus promotes the mineralization of nitrogen (especially in the form of NH 4 + -N) (Narteh & Sahrawat, 1997).
According to previous studies, such changes in pH and nitrogen are related to the activities of soil microbes; during nitrogen assimilation, soil microbes usually show a preference for NH 4 + -N (Jenkinson & Parry, 1989) and can more efficiently absorb and utilize NH 4 + -N than NO 3 − -N (Recous, Mary, & Faurie, 1990). Because of this preferential adsorption by nitrogen-fixing microbes, NH 4 + -N is immobilized in large quantities, and the NH 4 + -N content of rhizosphere soil is significantly lower than that of root-zone soil and intershrub bulk soil.
Some studies have pointed out that organic carbon is the main driving factor for changing the soil microbial community structure (Shen, Zhang, Guo, Ray, & He, 2010), and this effect is mainly caused by labile organic carbon in the soil (Eilers, Lauber, Knight, & Fierer, 2010). The organic carbon in the soil of the desert area is mainly composed of labile organic carbon decomposed from plant leaves and root litters (Liu et al., 2015). Similarly, our data showed that the content of SOC in the rhizosphere soil of the two species of genus Hedysarum was significantly higher than that of the intershrub bulk soil (  (Kersters et al., 2006). Besides, studies have shown that soil microorganisms can transform soil-bound P in soil into water-soluble P and thus promote the absorption of P by plants (He & Zhu, 1998). In legume plants, P is also associated with complex signal transduction, energy conversion, respiration, and nitrogen fixation (Khan, Zaidi, Ahemad, Oves, & Wani, 2010). Studies have found that the relative abundance of phosphate-solubilizing bacteria in the rhizosphere is far greater than that in intershrub soil, which can be explained by the faster consumption of P in the rhizosphere (Laheurte & Berthelin, 1988;Pande, Pandey, Mehra, Singh, & Kaushik, 2017).
On the other hand, the effectiveness of soil moisture is a key factor that limits the diversity and productivity of plant communities in desert areas, because it can affect the quality and quantity of the plant litters and exudates entering the soil decomposed by soil microorganism. Also affected is the conversion efficiency of water-soluble soil nutrients, which in turn affects the overall changes in soil texture (Bai et al., 2008;Sun et al., 2017). Therefore, under H. mongolicum shrubs (i.e., rhizosphere and root zone), a high diversity of bacterial communities was observed (p < .05; Table 2), and the bacterial communities of the four rhizocompartments were significantly affected by each nutrient element (except for P) (p < .05; Table 3 and Figure 7).
The pH of soil has an important influence on its chemical, physical, biological, and other processes, especially on nitrogen cycling (Fan et al., 2018). Baggs, Smales, and Betaman (2010) have probed into the influence of long-term and short-term pH differences on N 2 O emission from soil, as well as on N 2 O emission from microbial sources (ammoxidation and denitrification).
According to their findings, the ammoxidation and nitrate nitrogen dissimilation (mainly denitrification) processes in soil generate N 2 O (Baggs, 2008), and variations of soil pH dominate N 2 O emission from microbial sources (ammoxidation and denitrification).
Furthermore, in a comparison of long-term soil pH differences, pH plays the most significant role, which can reflect the adaptability of microbial communities to their current habitat (Baggs et al., 2010).
According to Hartman, Richardson, Vilgalys, and Bruland (2008), pH is the optimal factor to predict variations of bacterial communities in soil, and the authors have also observed variations of the relative abundances of Acidobacteria and Actinobacteria across the pH scale at the phylum level (Hartman et al., 2008). Other studies have also pointed out that variations of the relative abundances of Acidobacteria, Actinobacteria, and Bacteroidetes in soil microbial communities are mainly driven by soil pH, which further affect the overall structural variability of bacterial communities (Lauber et al., 2019). According to the findings of a recent study that investigated the bacterial communities in poplar roots, pH has a significant influence on the genotypes of both bacterial and fungal communities, as well as their colonization and distribution in plant roots and stems . As a result, microbes in root-zone soil and intershrub bulk soil are mainly influenced by pH and NH 4 + -N.
In summary, these results demonstrate that interactions occur among plants, soil, and specific microbes based on the enrichment and absorption of nutrients by rhizocompartments. These interactions affect the diversity of soil microbial communities and change the physicochemical properties of undershrub soil and intershrub soil. However, soil physicochemical factors also have a significant influence on the structure and composition of microbial communities in various rhizocompartments.

| CON CLUS IONS
This study revealed that the soil physicochemical factors (especially the pH) have a significant influence on the structure and compo-

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study have been deposited in Dryad with https://doi.org/10.5061/dryad.m37pv mczk.