Biochar application improves karstic lime soil physicochemical properties and enzymes activity and enhances sweet tea seedlings physiological performance.

Karst lime soil, commonly found in rocky desert ecosystems of Southwest China, exhibits high pH, poor water retention, and intense erosion. To prevent further soil erosion and soil losses from these ecosystems, stabilization measures based on improved green infrastructure are needed. The present study aimed at elucidating the performance of sweet tea (Lithocarpus polystachyus) seedlings grown on this soil type upon biochar application. Biochar was classified into different particle sizes, viz. 0.25-0.5 mm (medium), 0.5-1 mm (coarse), 1-2 mm (gravel), and their mixture, and added at the concentrations of 1, 2, or 5% soil mass. The pH, moisture, and porosity of soil increased upon biochar application compared to control; however, soil bulk density significantly decreased. The activity of soil phosphatase was increased by biochar particle size. Biochar particle size and concentration significantly enhanced the soil organic carbon content, but they differently affected total and plant-available nutrients in the soil. Light-saturated photosynthesis was positively affected, while stomatal conductance, leaf transpiration, and the intercellular CO concentrations of sweet tea leaves were negatively affected by biochar particle size and/or concentration compared to control. Leaf chlorophyll and soluble protein contents were increased by biochar application. From these results, we conclude that biochar can improve soil properties and the performance of sweet tea seedlings grown on Karst lime soil. We suggest its application at a concentration of 2% soil mass for keeping a high physiological performance of sweet tea seedlings in this environment. The selection of the ideal particle size is context-specific and depends on the target outcome.