Effects Of Dropping Exogenous Substances On Cotton Physiological Growth And Soil Environment Under Salt Stress

Bacillus subtilis and rhamnolipids can improve the soil environment,promote nutrient absorption by plants,and reduce the harm of soil salinization to cotton growth.They have certain application prospects in agricultural production.In order to study the effects of Bacillus subtilis and rhamnolipid on the physiological growth of cotton and soil environment under drip irrigation without film under different salt treatments,a total of 8 treatments were set up:2 m S·cm^-1 salt+no exogenous control（S2CK）,2 m S·cm^-1 salt+15 kg·hm^-1Bacillus subtilis（S2B1）,2 m S·cm^-1 salt+30 kg·hm^-1 Bacillus subtilis（S2B2）,2 m S·cm^-1 salt+540 kg·hm^-1rhamnolipid（S2R3）5 m S·cm^-1 salt+no exogenous control（S5CK）,5 m S·cm^-1 salt+15 kg·hm^-1 Bacillus subtilis（S5B1）,5 m S·cm^-1 salt+30 kg·hm^-1 Bacillus subtilis（S5B2）,5 m S·cm^-1 salt+540 kg·hm^-1rhamnolipid（S5R3）.The effects of exogenous drip application on enzyme activity,microbial community structure,soil water and salt content during the whole growth period,and physiological growth of cotton were analyzed.The research conclusions are as follows:（1）Drip application of Bacillus subtilis and rhamnolipids has a significant promoting effect on increasing soil moisture and reducing soil EC value.High concentrations of Bacillus subtilis can enhance soil water retention,while rhamnolipids have a better effect on soil desalination.As the cotton growth period progresses,the temperature gradually increases,soil evaporation and cotton leaf transpiration intensify,and the water demand for cotton growth increases.In critical periods（such as the flowering and boll stage）,irrigation frequency needs to be increased,and the soil moisture content will also increase.After the salt is washed,the EC value of the cotton rhizosphere soil will decrease.By the cotton boll opening stage,the soil moisture content will decrease after irrigation,and due to the influence of evaporation,the soil will experience salt return.At the end of the growth period,compared with the S2CK treatment,the soil moisture content of S2B1,S2B2,and S2R3 treatments increased by 11.46%,17.50%,and 8.13%,respectively,and the soil EC value decreased by 3.00%,3.86%,and 12.02%,respectively.Compared with the S5CK treatment,the soil moisture content of S5B1,S5B2,and S5R3 treatments increased by 16.96%,19.74%,and 13.26%,respectively,and the soil EC value decreased by 6.77%,0.75%,and 6.20%,respectively.（2）Drip application of Bacillus subtilis and rhamnolipids significantly affects soil enzyme activity and microbial community structure.Compared with low salt stress,high salt stress significantly（P<0.05）reduces urease and sucrase activities,but significantly increases catalase activity;The effect of high salt stress on alkaline phosphatase activity is relatively small（P>0.05）.Under low salt stress,the promoting effect of rhamnolipids on soil urease,alkaline phosphatase,and catalase activities is more significant than that of Bacillus subtilis,but rhamnolipids reduce sucrase activity more significantly than Bacillus subtilis.Under high salt stress,Bacillus subtilis has a stronger promoting effect on urease activity than rhamnolipids.Only high concentrations of Bacillus subtilis significantly increase soil sucrase and alkaline phosphatase activities,while low concentrations of Bacillus subtilis and rhamnolipids both reduce soil catalase activity.After applying exogenous fertilizer,it was found that Proteobacteria（27.22%～38.91%）,Bacteroides（9.93%～20.47%）,Planctomycetes（8.39%～17.11%）and Actinomycetes（4.07%～10.85%）in cotton rhizosphere soil of each treatment were relatively abundant.Under high salt stress,Bacillus subtilis significantly reduced the relative abundance of Actinobacteria in the soil.The treatment with S5B1 and S5B2reduced the Actinobacteria by 27.27%and 23.38%,respectively,compared to the treatment with S5CK.rhamnolipids significantly increased the relative abundance of Proteobacteria in the soil,while the treatment with S5R3 increased the Actinobacteria by 29.73%compared to the treatment with S5CK.Dropping exogenous enriched Ascomycota（54.47%～85.06%）,Mucor（2.29%～33.16%）and Basidiomycota（3.96%～20.95%）.Under different salt stresses,the application of high concentrations of Bacillus subtilis significantly increased the relative abundance of Basidiomycota.The S2B2 and S5B2 treatments increased the relative abundance of Basidiomycota by 74.00%and 290.62%,respectively,compared to the S2CK and S5CK treatments.The rhamnolipids only significantly increased the relative abundance of Mucor under low salt stress,while the S2R3 treatment increased the relative abundance of Mucor by 249.05%compared to the S2CK treatment.（3）Drip application of Bacillus subtilis and rhamnolipids can both improve cotton plant height,stem diameter,leaf area,dry matter accumulation,photosynthetic characteristics,relative chlorophyll content（SPAD value）,and yield and quality.Under different salt stress,the promoting effects of different exogenous substances on various physiological growth indicators of cotton were as follows:30 kg·hm^-1 Bacillus subtilis>15 kg·hm^-1Bacillus subtilis>540 kg·hm^-1 rhamnolipids>no exogenous control.Under low salt stress,compared with S2CK treatment,S2B1,S2B2,and S2R3 treatments increased seed cotton yield by 17.00%,22.20%,and 11.99%,respectively,and increased quality break ratio intensity by 5.21%,6.92%,and 2.18%,respectively;Under high salt stress,compared with the S5CK treatment,the yield of seed cotton increased by 15.50%,17.82%,and 6.48%in S5B1,S5B2,and S5R3 treatments,and the strength of quality break ratio increased by 1.81%,3.36%,and 0.94%,respectively.