Combined Application of Waste-Derived Biochar and Plant Growth-Promoting Rhizobacteria (PGPR) for Mitigating Drought Stress: A Mini-Review

Drought stress is one of the most damaging abiotic stressors impacting global agricultural productivity and soil ecosystem integrity, especially in the semi-arid and arid areas. Drought results in a cascade of physiological and biochemical disturbances in plants, such as inhibition of photosynthesis, loss of turgor pressure, and impairment of osmoregulation, while also causing a degradation of soil health through losses in organic carbon, nutrient availability, and microbial community structure. This calls for integrated soil management approaches that can restore soil health and support plant resilience simultaneously under water-limited conditions. Biochar, generated through the thermal decomposition of agro-waste materials under oxygen-limited conditions, represents a promising circular bioeconomy strategy for valorizing agricultural residues into high-value soil amendments. Biochar is widely known to enhance the physicochemical properties of soils, including water retention, cation exchange capacity, and microbial habitat provision. Plant growth-promoting rhizobacteria (PGPR) have been established as an environmentally friendly means of enhancing plant growth through phytohormone production, ACC deaminase activity, and antioxidant enzyme activation under stressed conditions. The combined application of biochar and PGPR has garnered considerable scientific attention due to their complementary modes of action and, in some cases, beneficial combined effects. Several studies reported significant improvement in plant water status, photosynthetic efficiency, osmoregulation, soil enzyme activity, and nutrient retention. This article summarizes the present research on the soil and physiological effects of drought stress before exploring the basic mechanisms of biochar–PGPR interaction and their combined efficacy under drought conditions. Furthermore, the review calls attention to the future research priorities, such as field-scale validation, region-specific formulation development, and omics-based mechanistic investigations to maximize the full potential of PGPR and biochar combined as a sustainable approach for alleviating drought stress.