Impact of Biochar Interlayer on Surface Soil Salt Content, Salt Migration, and Photosynthetic Activity and Yield of Sunflowers: Laboratory and Field Studies

Soil salinization presents a significant challenge, driven by factors such as inadequate drainage, shallow aquifers, and high evaporation rates, threatening global food security. The sunflower emerges as a key cash crop in such areas, providing the opportunity to convert its straw into biochar, which offers additional agronomic and environmental benefits. This study investigates the effectiveness of biochar interlayers in enhancing salt leaching and suppressing upward salt migration through integrated laboratory and field experiments. The effectiveness of varying biochar interlayer application rates was assessed in promoting salt leaching, decreasing soil electrical conductivity (EC), and enhancing crop performance in saline soils through a systematic approach that combines laboratory and field experiments. The biochar treatments included a control (CK) and different applications of 20 (BL20), 40 (BL40), 60 (BL60), and 80 (BL80) tons of biochar per hectare, all applied below a depth of 20 cm, with each treatment replicated three times. The laboratory and field experimental setups maintained consistency in terms of biochar treatments and interlayer placement methodology. During the laboratory column experiments, the soil columns were treated with deionized water, and their leachates were analyzed for EC and major ionic components. The results showed that columns with biochar interlayers exhibited significantly higher efflux rates compared to those of the control and notably accelerated the time required for the effluent EC to decrease to 2 dS m−1. The CK required 43 days for full discharge and 38 days for EC stabilization below 2 dS m−1. In contrast, biochar treatments notably reduced these times, with BL80 achieving discharge in just 7 days and EC stabilization in 10 days. Elution events occurred 20–36 days earlier in the biochar-treated columns, confirming biochar’s effectiveness in enhancing leaching efficiency in saline soils. The field experiment results supported the laboratory findings, indicating that increased biochar application rates significantly reduced soil EC and ion concentrations at depths of 0–20 cm and 20–40 cm, lowering the EC from 7.12 to 2.25 dS m−1 and from 6.30 to 2.41 dS m−1 in their respective layers. The application of biochar interlayers resulted in significant reductions in Na+, K+, Ca2+, Mg2+, Cl−, SO42−, and HCO3− concentrations across both soil layers. In the 0–20 cm layer, Na+ decreased from 3.44 to 2.75 mg·g−1, K+ from 0.24 to 0.11 mg·g−1, Ca2+ from 0.35 to 0.20 mg·g−1, Mg2+ from 0.31 to 0.24 mg·g−1, Cl− from 1.22 to 0.88 mg·g−1, SO42− from 1.91 to 1.30 mg·g−1 and HCO3− from 0.39 to 0.18 mg·g−1, respectively. Similarly, in the 20–40 cm layer, Na+ declined from 3.62 to 3.05 mg·g−1, K+ from 0.28 to 0.12 mg·g−1, Ca2+ from 0.39 to 0.26 mg·g−1, Mg2+ from 0.36 to 0.27 mg·g−1, Cl− from 1.18 to 0.80 mg·g−1, SO42− from 1.95 to 1.33 mg·g−1 and HCO3− from 0.42 to 0.21 mg·g−1 under increasing biochar rates. Moreover, the use of biochar interlayers significantly improved the physiological traits of sunflowers, including their photosynthesis rates, stomatal conductance, and transpiration efficiency, thereby boosting biomass and achene yield. These results highlight the potential of biochar interlayers as a sustainable strategy for soil desalination, water conservation, and enhanced crop productivity. This approach is especially promising for managing salt-affected soils in regions like California, where soil salinization represents a considerable threat to agricultural sustainability.