Improving the yield of greenhouse tomato by adjusting the coupling coordination degree of soil–water–root system using a biochar

The unreasonable management of water, fertilizer and pesticides has led to the deterioration of soil quality and a reduction in crop yields in facility agriculture. Due to its well-developed pores, large specific surface area, strong cation adsorption capacity and stable structure, biochar has the potential to improve soil quality and crop yield. Greenhouse experiments were conducted in 2021 and 2022 to study the effects of different application rates of biochar (0, 15, 30, 45 and 60 t·ha −1 ) on soil structure, water-holding capacity, tomato root growth (the soil–water–root system), and their coupling coordination degree ( D ). Compared to no addition of biochar, adding biochar increased the content of aggregates with a diameter greater than 0.25 mm by 1.4%-22.8% and the mean weight diameter by 1.5%–34.5%, and the soil available water content ( SAWC ) increased by 6.9%–35.7%. Biochar makes the root system of tomato thinner and longer, increasing the surface area density and volume density of the roots. Adding biochar to the soil increased the D value of the soil–water–root system by 25.7–86.0%. Tomato yield and water use efficiency increased by 28.8%–33.3% and 21.4%–29.6% with the addition of biochar, respectively. Based on the CRITIC–TOPSIS model, considering indicators such as soil–water–root system, tomato yield and water use efficiency, the optimal biochar application rate under the conditions of this experiment was 30 t·ha −1 . Furthermore, through the structural equation model, it was concluded that biochar primarily affected the D value by increasing SAWC , which then had an indirect effect on tomato yield. These research results are of great significance for the sustainable development of facility agriculture. • Biochar increased tomato yield 28.8%-33.3% by enhancing soil-water-root coordination. • Biochar enhanced soil-water-root system coordination to highly/extremely coordinated. • Comprehensive evaluation showed 30 t·ha⁻¹ was the optimal biochar application rate.