Biochar Increases Soil Moisture and Improves Tomato Resilience Under Field Conditions: Results from a Two-Year Field Study in Tuscany (Italy)

Biochar, a carbon-rich by-product of wood pyrolysis, improves soil structure, water retention, and plant growth. A two-year field experiment (2024–2025) was conducted in Poggibonsi (Tuscany, Italy) on tomato cv. “Canestrino” under contrasting climatic conditions. A single biochar application (15 t ha−1) was evaluated for its effects on soil properties, water dynamics, plant water status, and ecophysiological and tissue-level responses. From the results, it emerged that biochar improved soil quality by increasing organic matter (+7.7%) and the C/N ratio (+10.6%), while reducing bulk density (1.42 to 1.25 Mg m−3). Soil water content was higher in amended plots, particularly in 2024 (32.84% vs. 24.87%), with a smaller increase in 2025 (24.66% vs. 24.08%). Improved soil water availability enhanced plant water status, as shown by less negative leaf water potential under stress conditions. Microscopic analyses confirmed better xylem integrity in treated plants, with reduced formation of tyloses and improved hydraulic functionality during drought. Agronomic responses reflected climatic variability: yield increased in biochar in 2024, whereas in 2025 drought stress reduced productivity in both treatments, with no significant differences. Overall, biochar improved soil moisture retention, plant water status, and ecophysiological performance, with effects dependent on seasonal rainfall patterns and environmental stress intensity.