Objectives Sesbania cannabina, due to the high nitrogen fixing capacity and salt tolerance, is a pioneer plant in bioremediation of saline-alkali soil. We tried to increase the remediation efficiency of S. cannabina by application of montmorillonite and a prepared microbial inoculum.

Methods Pot experiment was used in the research. The growth substrate was prepared by mixing soil, vermiculite, and pearlite in mass ratio 3∶1∶1 and sterilized first, then NaCl was added to imitate 0, 100, and 200 mmol/L of salt stress levels, respectively. Under each salt stress level, four treatments were setup by adding montmorillonite (M), microbial inoculum (J), and montmorillonite and inoculum together (M+J), and blank control (CK). At 60 days of growth, Sesbania were harvested for measuring biomass, soil physicochemical properties.

Results Pseudomonadaceae sp. ST3 and Escherichia coli ND2B in the microbial inoculum were isolated stronger growth promoting capacity than the other endophytes. Under 0, 100, and 200 mmol/L stress levels, the plant height, aboveground dry biomass, photosynthetic efficiencies, salt-tolerance indices of S. cannabina were all in order of CK<M<J<M+J, all the index values in J+M treatment were significantly higher than in M and CK treatment, and some indices significantly higher than in J treatment. The indices in J and M treatments were not always significantly higher than in CK. Compared with CK under 100, and 200 mmol/L stress levels, the increment by M+J treatment were: 157.2%, 187.6% in plant height, 307.6%, 286.9% in above ground dry biomass, 46.9%, 40.7% in root length, 31.2%, 25.0% in root surface area, 60.7%, 72.8% in root dry weight, 100.28%, 138.00% in chlorophyll content, 43.72%, 40.81% in net photosynthetic rate of leaves, 118.26%, 192.11% in superoxide dismutase activity, 138.24%, 137.5% in peroxidase activity, 222.96%, 151.50% in catalase activity, MDA content in leaves was significantly reduced by 39.93% and 38.41%, the decrease rate of soil pH was by 3.82%, 3.82%. Compared with CK under 100, and 200 mmol/L stress levels, M+J treatment decreased soil salt content by 36.39%, and 39.04%, and significantly increased soil total N by 92.83%, and 87.56%, available P by 35.40%, and 37.28%, available K by 27.53%, and 30.59%, soil organic carbon by 17.95%, 21.85%, respectively.

Conclusions Separate application of montmorillonite and microbial agents shows a certain level of improvement effect on the growth and salt tolerance of S. cannabina, the decrease of salt concentrations, and the increase of organic carbon and nutrients. The combined use of montmorillonite and microbial inoculum exhibites the most significant and stable effect, so is the efficient measurement for boosting the bio-remediation of the S.cannabina on the saline-alkali soil.