Effects of Biochar and PGPR Application on the Physicochemical Properties and Humus Components of Soil Used for Planting Fruit Mulberry Seedlings Under Salt Stress

Biochar can act as a carrier and a soil carbon source for rapid colonization by plant growth-promoting rhizobacteria (PGPR). However, the effects of a combined application of biochar and PGPR on soil physicochemical properties, humus components, and their stability in the rhizosphere around fruit mulberry seedlings remain unclear. A pot experiment using 1-year-old fruit mulberry seedlings with five treatments (control (CK), salt stress (SS), salt stress + <i>Bacillus fexus</i> (SS+P), salt stress + biochar (SS+B), and salt stress + <i>B. fexus</i> + biochar (SS+P+B)) was conducted to analyze the variations in soil physicochemical properties and humic acid (HA), fulvic acid (FA), and humin (HM) contents in the soil when planting fruit mulberry seedlings. The results indicated that the SS treatment significantly reduced total soil porosity, non-capillary porosity, water stable macro-aggregates content, available potassium content, and pH value compared to CK, but increased the soil bulk density, capillary porosity, and available phosphorus content. The SS+P+B treatment significantly increased soil total porosity, non-capillary porosity, pH value, electrical conductivity, the water stable macro-aggregates, organic matter, HA and HM contents, the HA/FA and HA/HE (humus-extractable) ratios, and the activities of catalase and urease. It significantly increased the water stable macro-aggregates and the HA/HE ratio by 27.83% and 25.00%, respectively. However, it significantly decreased soil bulk density and capillary porosity by 9.93% and 20.64%, respectively, compared to the SS treatment. The results suggest that the simultaneous addition of biochar and <i>B. fexus</i> under salt-stress conditions improves the soil physicochemical properties and increases the humus components content and stability, which is of great significance for improving the soil quality of saline-alkali land and enhancing the productivity of fruit mulberry.