Study on the Enhancement of Nutrient Removal by Bacillus amyloliquefaciens and the Growth Physiology of Plants Adapting to Salt Stress in the Ecological Ditch System

Traditional ecological ditch technology is unable to enhance the water quality purification effect of highly salty water and improve the salt tolerance of plants. BA (Bacillus amyloliquefaciens) has a positive effect in alleviating salt stress and promoting plant growth. However, the mechanism by which BA enhances plant stress resistance and improves nutrient removal efficiency in the ecological ditch system under high salt stress conditions remains unclear. The experiment constructed two ecological ditches of [P(substrate + plants) and PM (substrate + plants + BA)], and four different salinity treatments (5 g/L, 10 g/L, 15 g/L and 20 g/L) were set up to investigate the removal efficiency of nitrogen and phosphorus in water, and plant growth and physiological indicators, antioxidant enzyme activities and inorganic salt ion concentrations under different salinity stresses by adding BA. It also clarified the relationship between plant growth physiology and the response of water nutrient salts. The results show that the PM treatments for purifying nutrients performed significantly better than the P treatments under different salinity stresses, the removal rates of TN, TP and NH4+-N increased by 13.61–22.50%, and the effect was the best at a salinity of 15 g/L. BA enhanced the salt tolerance by increasing the activities of SOD (superoxide dismutase), POD (peroxidase) and CAT (catalase) of plants, reducing the content of MDA (malondialdehyde), lowering the concentration of Na+ and increasing the concentration of K+, improving the growth and activity of plant roots. Meanwhile, through a comprehensive evaluation, it was determined that BA has the most significant promoting effect on the growth of reed (Phragmites australis), and stronger plant root system is the key to improving the overall purification efficiency of the system. This research provides a theoretical basis for enhancing the water purification effect of high salinity water through ecological ditch technology.