Nowadays, salinity is one of the most serious environmental problems affecting plant performance and metabolites. Biochar as a biological and sustainable amendment can be a valuable tool for improving soil health and plant growth traits under salinity stress and reducing its effects. The aim of this study was to investigate the effects of biochar (0, 1.5, and 3% by a mass percentage of the pot) on physicochemical properties and soil enzyme activity, as well as functional, physiological, and phytochemical traits of Thymus vulgaris L. under salinity stress (0, 2, 4, and 6 ds m−1 NaCl), were investigated. Biochar increases porosity, water-holding capacity, enzyme activity, phosphorus, and potassium nutrient content of the soil and reduces sodium uptake by the plant under salinity stress. Growth and performance traits were significantly increased under the influence of biochar and salinity conditions. The maximum fresh weight (104.87 g/plant) and dry weight (63.56 g/plant) of shoot were observed in the 3% biochar treatment in normal conditions. The highest content of essential oil (1.91%), thymol (63.51%), carvacrol (9.12%), and rosmarinic acid (15.05 mg/g DW) was observed at the highest levels of biochar and salinity. The activity of antioxidant enzymes and osmotic substances increased significantly (P < 0.01) in salinity conditions, which were reduced by adding biochar (P < 0.01). Generally, biochar as an organic and environmentally friendly material can be a suitable solution for increasing resistance to salinity in the garden thyme and enabling its cultivation and production in low-yielding and saline lands.
