Soil salinization and nutrient deficiency have emerged as the major factors negatively impacting soil quality and primary productivity in the coastal saline-alkali soil of the Yellow River Delta. Biochar has been proposed as an efficient strategy for promoting plant growth and restoring degraded saline-alkali soil. However, knowledge is inadequate regarding the effects of adding Spartina alterniflora-derived biochar alone or in combination with effective microorganisms (EM) on the growth of Sesbania cannabina and soil quality in saline-alkali soil. To enhance this knowledge, a pot experiment with different EM treatments (without EM addition, EM-; with EM addition, EM+) and a gradient of biochar treatments (0%, B; 0.5%, B; 1.5%, B; and 3%, B; biochar weight/soil weight) was conducted. Our results showed that biochar addition alone and in combination with EM significantly increased seed germination, plant height, stem diameter, total biomass and plant nutrient uptake of S. cannabina. Biochar addition, EM addition and their interaction significantly decreased soil salt content efficiently and increased soil total carbon (TC), total nitrogen (TN), available phosphorus (AP) and available potassium (AK) but had little effect on soil pH. Biochar addition increased soil organic carbon, soil NH and NO-, microbial biomass carbon, and soil enzyme activities and these effects increased in strength when biochar and EM were present simultaneously. Of the treatments, the EM + B treatment had the largest effects in terms of inhibiting salinization, increasing soil fertility, elevating soil nutrients and enzyme activities, and improving plant growth. Moreover, the application of biochar and EM promoted the growth of S. cannabina by enhancing plant nutrient uptake, improving soil fertility (e.g., TN, AP, AK, NH and NO), and elevating soil enzyme activities (urease and alkaline phosphatase activity). Overall, the integrated use of an appropriate biochar rate (3%) and EM for coastal saline-alkali soil could be an effective strategy to ameliorate soil salinity, improve soil quality and promote plant productivity.
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