Effects of straw biochar under different thermochemical treatments on tomato growth and fruit quality

Thermochemical technology can be expected to rapidly convert waste biomass into high-value products, such as biochar. However, the biochar varies in its properties and potential effectiveness as a soil amendment during plant cultivation. This study aims to explore the effects of straw biochar under different thermochemical treatments on tomato growth and fruit quality. Tomato straw from a greenhouse facility after harvest was selected as the feedstock for biochar production. Three thermochemical techniques were employed: medium-temperature slow pyrolysis (500 ℃, 2 h), superheated steam-assisted torrefaction (350 ℃, 10 min), and hydrothermal carbonization (190 ℃, 2 h), thus producing biochar labeled as pyrolyzed biochar (PYBC), torrefied biochar (TOBC), and hydrothermal carbonized biochar (HCBC), respectively. A systematic analysis was conducted to determine the physicochemical properties of the biochar. Their effects on the growth of tomato plants (Micro Tom) were further evaluated after incorporation into the growing substrate. The results indicated that both PYBC and TOBC were beneficial to plant growth. Notably, TOBC at a mass ratio of 10% shared the most outstanding effects, which significantly increased plant height, stem diameter, and leaf area (P < 0.05). Compared with the control without biochar (CK), the 10% TOBC treatment significanlty increased plant biomass, the number of fruits per plant and fruit yield. In addition, both PYBC and TOBC significantly improved fruit quality, lycopene by 20%-54% and vitamin C by 12%-24%, respectively. Compared with CK, TOBC further enhanced soluble sugar content (by 8%-17%) and sugar-acid ratio to 7.5~7.6, while reducing fruit hardness, indicating its potential for the nutritional value, flavor, and texture of tomatoes. A significant (P < 0.05) or highly significant (P < 0.01) positive correlation was observed between biochar electrical conductivity (EC) and plant height, stem diameter, leaf area, and yield. Among the three biochars, the TOBC with the highest EC value supplied sufficient mineral nutrients for plant growth. Significant positive correlations were also found among biochar EC, plant growth parameters (height, stem diameter, and leaf area), and key fruit quality components, such as soluble solids, soluble sugar, and vitamin C. The experimental data confirmed the hypothesis that TOBC enhanced plant growth by providing mineral nutrients, thereby facilitating the accumulation of photosynthetic assimilates and quality-related compounds in fruits. In contrast, HCBC negatively affected plant growth. Fruit vitamin C content was reduced for the high nitrate concentrations, indicating potential health risks associated with dietary intake. The NH₄⁺-N content in biochar was negatively correlated with stem diameter, leaf area, and vitamin C content, but positively associated with nitrate content. The high NH₄⁺N content in HCBC adversely affected tomato plants, leading to metabolic imbalances between growth and fruit quality. Principal component analysis identified the 10% TOBC treatment as the optimal for the high tomato yield and fruit quality. Therefore, superheated steam-assisted torrefaction can represent an ideal solution for the rapid disposal of tomato straw waste, while simultaneously generating biochar suitable as an effective and sustainable substrate additive. These findings can provide a theoretical foundation for the efficient utilization of tomato straw and its biochar, thus supporting the transition toward a circular bioeconomy in agriculture.