Abstract ［Objectives］ This study was conducted to explore the interaction between nitrogen-fixing and phosphate-solubilizing strains and the optimal combination of different functional strains, in order to provide a theoretical basis for the development of PGPR compound fertilizers suitable for local environment.
　　［Methods］ In this study, 16S rDNA gene sequence analysis was used to identify fast-growing and competitive strains from pasture nodules and rhizosphere soils in Guizhou Province, and three representative Rhizobia and phosphorus-solubilizing bacteria were chosen for the test of bacterial combination when reducing 50% of nitrogen and 30% of phosphorus. The effects of different strain combinations on the plant height, root length, aboveground and underground biomass of Lotus corniculatus L. were investigated, and the total nitrogen and total phosphorus contents of the plants were determined.
　　［Results］ The mixed bacterial agents could promote the increase of root biomass, and the effects of A1, A3, B3 and C3 were the most obvious. The fresh weight and dry weight of the roots of L. corniculatus increased by 30.35%-168.45% and 26.43%-180.00%, respectively, and A3, B3, B2 and C3 had the best effects. The total phosphorus content of the plants  increased by 12.79%-55.25% compared with the CK2; and most of the bacterial agents with significant growth-promoting effects showed decreased total nitrogen contents, while those with non-significant growth-promoting effects showed significantly-increased total nitrogen contents, which were not as much as the CK1. Comprehensively, the most productive combination was C3, namely R27-2 Rhinohizobium fredii and P33-3 Stenotrophomonas rhizophila.
　　［Conclusions］ This study can provide a theoretical basis for the production and promotion of bacterial fertilizers.
　　Key words Growth-promoting bacteria; Lotus corniculatus L.; Biomass; Quality
　　The unreasonable application of chemical fertilizers to soil causes the loss of a large amount of nitrogen ions, fixation of phosphorus and a low utilization efficiency of nutrients, which further result in negative effects on environment, resources and biological chain. The research and development of environmentally friendly growth-promoting bacterial fertilizers is an environmentally friendly method to solve this problem and promote effective ways to form biodiversity［1］. Plant growth promoting rhizobacteria (PGPR) can promote plant growth through various metabolic pathways. For example, nitrogen-fixing bacteria provide nitrogen to crops through biological nitrogen fixation mechanism; and phosphate-solubilizing bacteria secrete their own metabolites to promote the decomposition of soil phosphorus pools to provide soluble phosphorus. In addition, phosphate-solubilizing bacteria can also interact with other beneficial bacteria to promote the production of iron bacteria［2］, and some strains also have disease resistance. Therefore, PGPR fertilizers will become a new force in "white agriculture", which is one of the future development directions of green agriculture［3］. In recent years, some scholars have proved that strain mixing can provide important nutrients and hormones for the growth of another strain［4］, or create favorable conditions for its continued growth, and some even depend on other microorganisms to survive［5］. Therefore, in the development and utilization of beneficial microbial resources and the research of bacterial fertilizers, to understand the interaction between growth-promoting strains and to screen the multi-functional combination of strains with wide adaptability, drought resistance and disease resistance are of great significance for the development and utilization of PGPR compound fertilizers. In this study, the strains selected from the rhizosphere of natural grasses were used to carry out mixed culture of rhizobia and phosphate-solubilizing bacteria to explore the interaction effects of strains in nitrogen fixation and phosphorus solubilization, so as to explore the best combination of different functional strains. This study will provide a theoretical basis for the development of PGPR compound fertilizers adaptive to local environment. 　　Lotus corniculatus L. is a perennial herb belonging to Lotus of Leguminosae, also known as Wuyecao and Wuzudou. It is native to the warm regions of Europe and Asia and is widely cultivated and used in various countries around the world. There are wild species distributed in central and northwestern Guizhou. It is one of the excellent leguminous pastures cultivated and used for grassy hill improvement in Guizhou Province. It can be harvested for 2 to 3 times a year, and has an average fresh grass yield of 22 500- 45 000 kg/hm2. The nutrient content of L. corniculatus ranks first in leguminous pastures. After ripening and harvesting, the protein content can reach 17.4%, and the feed value is extremely high. Furthermore, L. corniculatus has developed root system, and root nodules can grow on the lateral roots, which achieves the effects of increasing soil organic matter, increasing nitrogen absorption, improving soil and increasing soil fertility. L. corniculatus serving as a host plant is of great significance to the screening of combined bacterial fertilizers with the advantages of easy nodulation, more effective nodules, wide adaptability and strong resistance.
　　Materials and Methods
　　Test strains
　　The tested strains were bacteria (PGPR strains) isolated and screened from rhizosphere soils of wild grasses (with healthy roots) and fresh and mature nodules of natural leguminous plants collected from different areas of Guizhou Province (Table 1). The strains have the characteristics of fast growth, strong competitiveness, and certain growth-promoting effects after application to L. corniculatus. L. corniculatus seeds were purchased from Bente Grass Company.
　　Media
　　PKO (Pikovaskaia, s) inorganic phosphorus medium［6］: for separation and screening of phosphate-solubilizing bacteria and determination of phosphorus solubilization capacity; LB (lysozyme broth medium)［7］: for strain activation and strain preservation; improved YMA medium: for the isolation of rhizobia and the determination of nitrogenase activity, prepared from mannitol 10 g,  MgSO4 0.2 g, K2HPO4 0.25 g, KH2PO4 0.25 g, CaSO4 0.2 g, NaCl 0.1 g, yeast powder 1 g, Na2MoO4 (1%) 1.0 ml, MnSO4 (1%) 1.0 ml, ferric citrate (1%) 1.0 ml and distilled water to a volume of 1 000 ml, pH 7.0-7.2; solid medium, added with agar powder 16-18 g.
　　Determination of phosphorus solubilization and nitrogenase activity of strains
　　PKO (Pikovaskaia, s) inorganic phosphorus solid medium was used to isolate strains. The strains were cultured at 28 ℃ for 7 d, and the diameter of phosphate solubilization ring (D/d) of each strain was measured, and the strains with larger D/d values were selected. The strains were purified and stored (4 ℃) for later use［8-9］. 　　For each strain, 0.5 ml of the LB-activated bacterial suspension was inoculated into 50 ml of PKO inorganic phosphorus (calcium phosphate) liquid medium in three replicates, and the same amount of sterile water was used as the control. The culture was performed with shaking at 150 r/min under a temperature of 28 ℃ for 7 d. After culture, each culture liquid was centrifuged at  10 000 r/min under a temperature of 4 ℃ for 15 min, and 1 ml of the supernatant was used to determine the amount of solubilized phosphate by  Mo-Sb colorimetry［8-10］.
　　Each activated strain to be tested was inoculated on CCM  medium, and the nitrogenase activity was measured after the culture at 28 ℃ for 3 d. The determination was carried out using the NITS enzyme-linked immunosorbent assay kit of Qingdao Sci-tech Innovation Quality Testing Co., Ltd. With the OD value plotted on the abscissa and the concentration of the standard plotted on the ordinate, a standard curve was drawn. The absorbance  (OD  value) was measured at 450 nm using a microplate reader, and the concentration of nitrogenase (NITS) in each sample was calculated from the standard curve. The regression equation was  Y=95.842x-3.958 5, R2=0.997 2, which indicated that the measured value was close to the fitted formula, and the formula was more reliable.
　　Strain identification
　　Strains that grew fast on the culture dish and were moist and viscous were selected. Rhizobia were reactivated with YMA medium, and the phosphate-solubilizing bacteria were reactivated with LB medium. After growing to metaphase (4 d after inoculation to a culture dish), they were sent to Shanghai Personal Biotechnology Co., Ltd. for strain identification. DNA extraction: DNA extraction of each strain was carried out according to the instructions of the Personal Bio Bacteria Genomic DNA Kit. The 16S rDNA sequence was analyzed by PCR amplification analysis according to the method of Cheng et al.［12］.
　　Strain combination design
　　The test strains were inoculated on the solid LB slant. The activated Rhizobia were inoculated in YMA medium, and the phosphate-solubilizing bacteria were inoculated into LB medium. They inoculated strains were cultured at 28 ℃ and 150 r/min for 2-3 d. The concentration of the suspension of each strain was adjusted to 1×108 cfu/ml (λ=600 nm, OD value≥0.5) with sterile water. Each prepared bacterial suspension was used to form various treatments according to the method of Table 2. Various bacterial suspensions were mixed at a volume ratio of 1∶1, into a total  amount of 10 ml, and a total of 11 treatments were prepared. 　　Treatment of potted plants
　　L. corniculatus seeds free of pest damage were soaked with 75% ethanol for 2 min and washed with sterile water for 2-3 times. The treated seeds were then sterilized with 3% sodium hypochlorite for 10 min, followed by washing with sterile water for  5-6 times. The seeds were then sown in following sterilized medium: perlite+powdered rock phosphate (30 g). The medium was contained in pots, each of which had a total weight of 332-310 g (including the pot weight of 36.7 g) and was 9.5 cm in height and 14.5 cm in width. Each pot was then put in a black plastic bag to avoid mixing with other strains. The flower pots were placed in an artificial climate chamber at 23 ℃ during the day, 23 ℃ at night, with 16 h of illumination per day. The seedlings were sprayed with Hoagland complete nutrient solution (Shanghai Sinopharm). After 26 d of growth, 8 plants with the same growth were kept in each pot for treatment. For each treatment group, 10 ml of corresponding treatment liquid was centrifuged according to the design method of "Strain combination design", obtaining the supernatant, which was added with sterilized water to 30 ml, which was added into the flower pot. Each treatment was repeated 3 times, and a total of  90 ml of supernatant was added. The plants in various treatments were irrigated with Hoagland nutrient solution (50% N+70%P) every day to keep the substrate moist. Meanwhile, the control treatments, CK1 and CK2 were added with 30 ml of sterilized water per pot, respectively. The CK1 was irrigated with Hoagland complete nutrient solution every day to keep the substrate moist, and the CK2 was irrigated with Hoagland nutrient solution (50% N+70% P) to keep the substrate moist. The potted plants were grown for 94 d and the treatment time was 68 d.
　　Analysis of the growth-promoting effects of the growth-promoting bacteria groups on L. corniculatus
　　Effects of the growth-promoting bacteria groups on the aboveground and underground parts of L. corniculatus
　　Four plants with little difference were selected from each replicate, a total of 12 plants from each treatment. The absolute height of each plant from the ground to the highest point was measured with a ruler. All roots were taken out and rinsed to determine root length. After cutting (at the ground level), 4 plants with little difference were selected from each pot, and weighed as a whole for the fresh weights of the aboveground and underground parts. Each group was tested with 3 replicates. The plants were then subjected to deactivation of enzyme at 105 ℃, dried to constant weight at 65 ℃, and weighed for the dry weights of the aboveground and underground parts. 　　Determination of total phosphorus and total nitrogen contents in L. corniculatus of the growth-promoting bacteria groups
　　Total phosphorus was determined by H2SO4-H2O2 digestion-vanadium molybdate yellow colorimetric method. The total nitrogen content was determined by a fully automated Kjeldahl analyzer.
　　Data processing
　　Statistical analysis was performed using Excel 2003 and SPSS 18.0 software.
　　Results and Analysis
　　Identification, nitrogenase activity and phosphate solubilization capacities of growth-promoting strains
　　The better strains screened in the early stage were applied to the rhizosphere of L. corniculatus, to perform individual strain screening tests, by which several strains with good growth-promoting effects were screened. Six of them were analyzed for 16S rDNA sequence by PCR amplification. Specifically, rhizobia R70-1, R173-1 and R27-2 were Bradyrhizobium japonicum, Sinorhizobium meliloti and Sinorhizobium fredii, respectively. Their nitrogenase activity was in range of 139.64-240.02 IU/L and ranked as R173-1＞R70-1＞R27-2, and the differences were significant  (P＜0.01). Phosphate-solubilizing bacteria PD4-5, P35-2 and P33-3 were Pseudomonas kilonensis, Bacillus aryabhattai and Stenotrophomonas rhizophila, respectively. Their phosphate solubilization capacities ranged from 131.98 to 294.92 μg/ml and ranked as P33-3＞PD4-5＞P35-2, and the differences were significant (P＜0.01) (Table 3).
　　Effects of the mixed growth-promoting bacteria on the aboveground part of L. corniculatus
　　The effects of the mixed bacteria on the aboveground part of L. corniculatus were higher than that of the CK2. The fresh weight and dry weight of the plant increased by 30.35%-168.45% and 26.43%-180.00%, respectively, and the differences were significant (P<0.01) (Table 4). Compared with the CK1 irrigated with the complete nutrient solution, A3 and B3 showed the best growth-promoting effects (P<0.01) on the dry weight and fresh weight. Specifically, the dry weight was increased by 26.86% and 19.74%, respectively, the fresh weight was increased by 9.47% and 4.12%, respectively, and the differences were significant  (P<0.05). However, other treatments were not higher in the dry weight and fresh weight than the CK1 treatment. The growth-promoting effects on the plant height were different. Compared with the CK2, B1 increased the height by 29.05%, exhibiting a significant difference (P＜0.01), and its effect was the best; and A2 increased the height by 14.47%, showing a significant difference as well (P＜0.05). The growth-promoting effects of other treatments ranked as A3＞C3＞B2＞B3＞CK2, but the differences were not significant (P＞0.05). The plant heights of treatments A1 and C2 were lower than the CK2 by 21.54% and 13.50%, respectively, with significant differences (P＜0.05). The growth-promoting effects of all the combinations on the plant height of L. corniculatus were not as high as that of the complete nutrient treatment CK1. Combinations A3, B3, B2 and C3 were the best comprehensively from the terms of the dry and fresh weights and plant height. 　　Effects of the mixed growth-promoting bacteria on the root characters of L. corniculatus
　　Roots are an important tissue and organ for plants to absorb nutrients and water. Their growth and distribution affect crop growth and nutrient uptake. When plants receive less nitrogen and phosphorus at the same time, they will hinder the growth of plant roots. The different mixed bacterial agents had higher growth-promoting effects on roots than on plant height. Compared with the CK2, except A2 which was not different from the CK2 in root length, other treatments showed increasing trends of root fresh weight, dry weight and length, and the differences were significant (P＜0.01) (Table 5). After the inoculation of the mixed bacterial agent A1, the fresh weight and dry weight of roots increased by 33.67% and 34.94% compared with the CK1 irrigated with the complete nutrient solution, exhibiting significant differences  (P＜0.01); treatment A3 had no significant difference in the fresh weight of roots from the CK1 (P＞0.05), and its dry weight increased by 16.35% compared with the CK1, showing a significant difference (P＜0.01); B3 was the same with the CK1 on the root weight; and other treatments were all lower than the CK1. When plants suffer from stress, it will stimulate the roots to absorb nutrients from deeper soil. Combinations B1 and C1 increased the root length by 33.03% and 13.35% compared with the CK1, showing significant differences (P＜0.01); A3 increased the root length by 8.15% compared with the CK1, which was significant (P＜0.05); combinations C3, B3, C2, B2 and A1 had no significant differences from the CK1 (P＞0.05); and A2 was lower than the CK1, with a significant difference (P＜0.01). Comprehensively from the root length, fresh weight and dry weight, such four groups of mixed bacterial agents as A1, A3, B3 and C3 were best in the growth-promoting effect on the roots of L. corniculatus. It can be seen from the experiment that the interaction between the three phosphate-solubilizing bacteria and the three rhizobias had a strong growth-promoting effect on the roots of L. corniculatus.
　　Effects of the mixed growth-promoting bacteria on the total nitrogen and phosphorus in L. corniculatus plants
　　The total nitrogen and total phosphorus contents of plants are one of the important indicators to determine the quality of pastures. The different mixed bacterial agents provided different free-state nitrogen and phosphorus to plants. The application of the mixed bacterial agents combined with simultaneous reduction of 50% of nitrogen and 30% of phosphorus content were all lower then complete nutrients-treated CK1 in the total nitrogen content of L. corniculatus plants, and the differences were significant (P<0.05). Compared with the CK2, A2 and B2 increased the total nitrogen content by 13.00% and 6.67%, respectively, which were significant (P<0.05); C3, B1, C2 and C1 were not significantly different from the CK2 (P>0.05); and compared with the CK2, B3, A3 and A1 decreased the total nitrogen content by 8.33%, 10% and 35%, respectively, which were significant (P<0.05). For the total phosphorus content, except A1, all other combinations increased the total phosphorus content. Compared with the CK2, the total phosphorus content was increased by 12.79%-55.25%, exhibiting significant differences (P＜0.05). Compared with the complete nutrient solution CK1, C2, A3, C1 and A2 increased the total phosphorus content by 11.98%-23.04%, showing significant differences (P＜0.05); and C3, B1 and B2 also had an increasing effect on the total phosphorus content compared with the CK1, but the differences were not significant (P＞0.05). Comprehensively from the total nitrogen and total phosphorus contents in L. corniculatus plants, combinations A2, C3, C2 and B1 were the best (Fig. 1). 　　Discussion
　　The inoculation of plants with PGPR bacteria is a method to promote plant growth and increase crop yield and quality［12-13］. Some scholars have studied the growth-promoting effects of mixed inoculants on plants, and the results show that the growth-promoting effect on plants was significant. Esitken et al.［14］inoculated Pseudomonas BA-8 and Bacillus OSU-1 42 strains to European sweet cherry (Prunus avium), and found that the contents of N, P, K, Fe, Mn and Zn in the leaves of plants increased, and the cross-sectional area of trunks, the length of branches and the weight of the cherries increased. Meanwhile, good yield-increasing effects has achieved in economic crops and pastures including Saccharum, Hordeum vulgare, Malus domestica, Trifolium repens, Medicago sativa, Vicia sativa and Trifolium Pratense CV. Minshan［15-16］.
　　In the test of the growth-promoting effects of the mixed bacterial agents on L. corniculatus, the growth-promoting effects on the fresh weight, dry weight and root length of L. corniculatus were stronger than the CK2, with significant differences (P<0.01), except A2, which showed a root length the same as the CK2. Compared with the complete nutrient control CK1, A1 had a fresh weight significantly higher than the CK1 (P＜0.01), while A3, B3 and C3 were not significantly different from the CK1 (P＞ 0.05). The variation of the dry weight was the same as the fresh weight. Specifically, combinations B1, C1 and A3 were significantly higher than the CK1 (P＜0.05), while C3, B3, C2 and B2 were not significantly different from the CK1 (P＞0.05). Many studies have shown that in low-phosphorus stress, plants gain more phosphorus sources by promoting root development, major root length, lateral root length and density［17-19］. In this study, the nitrogen and phosphorus elements were reduced simultaneously, which hindered the growth of plant roots. The root length, fresh weight and dry weight of the CK2 in all combinations were the smallest, but after the application of the mixed bacterial agents, the root-promoting effects partially reached or exceeded that of the complete nutrient treatment. Root development directly affects crop yield, panicle number, weight of grains per panicle, and 1 000-grain weight［20］, which reflects that the application of the microbial  agents promotes the root system, and enhances plants stress resistance within a certain range.
　　The effects of the mixed bacterial agents on the aboveground biomass of L. corniculatus were significantly higher than the CK2 (P<0.05), and most of the plant heights were higher than the CK2 except those of A1, C1 and C2. Compared with the complete nutrient treatment CK1, A3 and B3 showed dry matter yields higher than the CK1 (P＜0.01), but lower plant heights, which might be caused by limited growth conditions. The amount of inorganic phosphorus that can be dissolved in the flower pots, the temperature and light intensity of the artificial climate chamber, the short time for growth in the flower pots and the large number of aphids in the later stage still need further investigation. 　　In this study, with the reduction of 30% of soluble phosphorus based on Hoagland nutrient solution, rhizosphere microorganisms could produce organic acid to dissolve phosphate rock in perlite into soluble phosphorus for plant growth. The bacterial combinations were significantly higher than the CK2 (P<0.05), except A1. And the phosphorus contents of the plants in some treatment were also better than the complete nutrient treatment. A2, C1, A3, C2 and C3 all showed an increase in the phosphorus content compared with the CK1 (18.52%-8.76%); and the phosphate solubilization capacities of the three phosphate-solubilizing strains ranked as 3> 1>2, and the differences were significant (P< 0.01). However, after mixed with rhizobia, the amounts of phosphorus absorbed by the plants were determined to have an order of A2>C1>A3>C2>C3, so the strains could not be judged only by the phosphate solubilization capacity. There are two different relationships between rhizobia and phosphate-solubilizing bacteria and between different strains of the same kind of bacteria, i.e., interaction and antagonism. There is also an interaction between bacteria and root exudates.
　　Soil nutrients can not only affect the growth and development of plants, but also can change microbial communities［21］. No application of nitrogen fertilizer inhibits the growth and development of sugarcane, including plant height, stem diameter and effective stem number, and no application of phosphate fertilizer significantly reduces the effective stem number of sugarcane and affects the nitrogen nutrition of sugarcane［22］. Low nitrogen can promote the occurrence of biological nitrogen fixation［23］. In this study, with the reduction of 50% of nitrogen element and 30% of soluble phosphorus, most of the mixed bacterial agents showed a nitrogen content in L. corniculatus higher than CK2; and except A1, A3 and B3, other treatments were all lower than in the nitrogen content than CK1. Among all the test strains, the nitrogenase activity of rhizobia was in order of B>A>C, and the phosphate solubilization capacities of the phosphate-solubilizing bacteria ranked as  3>1>2. The bacterial combinations with the highest nitrogen contents ranked as A2>B2>C3. It can be seen that the combination of rhizobium with the highest nitrogenase activity and the phosphate-solubilizing bacterium with the highest phosphate solubilization capacity was not the best for nitrogen uptake by plants. It is necessary to consider the interaction of mixed bacteria and the amount of soluble phosphate solubilized under the influence of host root exudates, and to comprehensively evaluate the effect of the mixed bacteria. Looking for a good growth-promoting agent with an effect of "1+ 1>2", conducting in-depth study on the action mechanism of PGPR and screening strain combinations with superimposed promoting effects can fully play the role of PGPR in promoting growth and improving quality［24］. 　　Conclusions
　　C3 had a yield equivalent to CK1 and good nutrition, and was thus the best bacterial combination comprehensively from plant height, aboveground and underground biomass, root length and nitrogen and phosphorus contents of plants. This bacterial agent used S. rhizophila, while some scholars have studied S. rhizophila DSM 14405T, which is a kind of microorganism suitable for controlling Cr (VI) pollution［25］. S. rhizophila can promote crop growth and improve plant stress resistance［26］, and has a good control effect on wheat take-all as a biocontrol agent［27］. S. fredii belongs to fast-growing rhizobia with high nodulation rate and broad-spectrum hosts［28］. Its original host plant is Pueraria lobata (willd.) Ohwi, which has a good yield-increasing effect on L. corniculatus. This study provides a basis for the production and promotion of bacterial fertilizers.
　　References
　　［1］LIU SZ, FENG X, YU J. Research progress of plant growth-promoting rhizobacteria and its environmental effects［J］. Hubei Agricultural Sciences, 2009, 48(11): 2882-2887.
　　［2］AHEMAD M, KHAN MS. Effect of fungicides on plant growth promoting activities of phosphate-solubilizing Pseudomonas putida isolated from mustard (Brassica campestris) rhizosPhere［J］. ChemosPhere, 2012, 86(9): 945-950.
　　［3］YAO T. Associative nitrogen-fixing bacteria in the rhizosphere of Avena sativa in an alpine region Ⅱ: Phosphate-solubilizing power and auxin production［J］. Acta Prataculturae Sinica, 2004, 13(3): 1-8.
　　［4］ZHANG Y, ZHU Y, YAO T, et al. Interactions of four PGPRs isolated from pasture rhizosphere［J］. Acta Prataculturae Sinica, 2013, 22(1):29-37.
　　［5］WANG FM, ZHANG Y, WU HQ. Preparation of phosphate-solubilizing and nitrogen-fixing bacteria and their yield-increasing effects on wheat［J］. Journal of Biotechnology, 1994, 4(4):15-18.
　　［6］TAN ZJ, XIAO L, XIE BY, et al. Microecological functions of pseudomonad［J］. Journal of Nuclear Agricultural Sciences, 2004, 18(1): 72-76.
　　［7］HAN WX, YAO T, XI LQ, et al. PGPR bio-fertilizers producing and its effect on Arena sativa growth and quality development［J］. Acta Prataculturae Sinica, 2008, 17(2): 75-84.
　　［8］SHU JH, WANG PC, LI XG, et al. Isolation and screening of inorganic phosphate-solubilizing bacteria and their effect on the growth of Bromus cartharticus［J］. Acta Prataculturae Sinica, 2017, 26(5): 75-84.
　　［9］LI FX, ZHANG DG, YAO T. Study on cultural conditions of plant growth promoting rhizobacteria in rhizosphere of oat in alpine region［J］. Journal of Gansu Agricultural University, 2004, 39(3): 316-320. 　　［10］LI WH, SHI JY. Isolation, purification, and phosphate-solubilizing capability of phosphorous bacteria in West Lake sediment［J］. Chinese Journal of Applied Ecology, 2006, 17(11): 2112-2116.
　　［11］CHENG C, YANG M, LI JX, et al. Biolog microbial identification system-study on the operating regulation of bacteria identification［J］. Food and Fermentation Industries, 2006, 32(5): 50-54.
　　［12］GLICK BR.The enhancement of plant growth by free-living bacteria［J］.Canadian Journal of Microbiology, 1995, 41: 109-117.
　　［13］BASHAN Y, ROJAS A, PUENTE ME.Improved establishment and development of three cacti species inoculated with Azospirillum brasilense transplanted into disturbed urban desert soil［J］. Canadian Journal of Microbiology, 1999, 45: 441-451.
　　［14］ESITKEN A, YILDIZ HE, ERCISLI S, et a1. Effects of plant growth promoting bacteria (PGPB) on yield, growth and nutrient contents of organically grown strawberryl［J］. Science Hortic Amsterdam, 2010, 124: 62-66.
　　［15］HU JC, XUE DL, MA CX, et al. Research advances in plant growth-promoting rhizobacteria and its applicatin prospects［J］. Chinese Journal of Applied Ecology, 2004, 15(10): 1963-1966.
　　［16］RONG LY, YAO T, MA WB, et al. The inoculant potential of plant growth promoting rhizobacteria strains to improve the yield and quality of Trifolium pratense cv. Minshan［J］. Acta Prataculturae Sinica, 2014, 23(5): 231-240.
　　［17］NARANG RA, ALTMANN T. Phosphate acquisition heterosis in Arabidopsis thaliana: A morphological and physiological analysis［J］. Plant & Soil, 2001, 234(1): 91-97.
　　［18］WILLIAMSON LC, RIBRIOUX SP, FITTER AH, et al. PhosPhate availability regulates root system architecture in ArabidoPsis［J］.Plant Physiology, 2001,126(2): 875-882.
　　［19］GUO ZH, HE LY, XU CG. Uptake and use of sparingly soluble phosphorus by rice genotypes with different P-efficiency［J］. Acta Agronomica Sinica, 2005, 31(10): 1322-1327.
　　［20］SHI QH, LI MY. Preliminary report on the relationship between root characteristics and shoots in rice［J］. Acta Agriculturae Universitatis Jiangxiensis, 1995, 17 (2):  110-115.
　　［21］PAUNGFOO-LONHIENNE C, YEOH YK, KASINADHUNI NR, et al. Nitrogen  fertilizer  dose alters fungal communities in sugarcane soil and rhizosphere［J］. Sci ReP, 2015, 5: 1-6.
　　［22］HUANG XC, HU HN, LI XX, et al. Effects of nitrogen, phosphorus and potassium fertilization application on growth and nitrogenase activity of sugarcane［J］. Sugarcane and Canesugar, 2019(3): 23-29. 　　［23］MUTHUKUMARASAMY R, GOVINDARAJAN M, VADIVELU M, et al. N-fertilizer saving by the inoculation of Gluconacetobacter diazotrophicus and Herbasirillum sp. in  micropropagated  sugarcane  plants ［J］. Microbiol Res, 2006, 161(3): 238-245.
　　［24］CAMACHO M, SANTAMARFA C, TEMPRANO F, et al. Co-inoculation with Bacillus sp. CECT 450 improves nodulation in Phaseolus vulgaris L［J］. Canadian Journal of Microbiology , 20 01,47 : 1058-1062.
　　［25］WU SM, GAO J, LIU Y, et al. Analysis of resistance to high concentration of Cr (VI) in Stenotrophomonas rhizophila DSM 14405T and its reducing properties［J］. Acta Scientiae Circumstantiae, 2019, 1-9.
　　［26］ALAVI P, STARCHER MR, ZACHOW C, et al. Root-microbe systems: The effect and mode of interaction of stress protecting agent (SPA) Stenotrophomonas rhizophila DSM14405T［J］. Frontiers in Plant Science, 2013, 4: 141.
　　［27］YANG CY, DENG YJ, LI W, et al. Screening and identification of biocontrol bacteria against wheat take-all［J］. Chinese Journal of Biological Control, 2018, 34(6): 873-881.
　　［28］SHI J, GAO Y, LIU Y, et al. A research review on Sinorhizobium fredii HH103［J］. Journal of Anhui Agricultural Sciences, 2016, 44(2): 7-8, 33.