Evaluation of Biochar and Salicylic Acid Application on Soil Biochemical Properties and Some Growth Parameters of Borago officinalis L. Under Different Irrigation Regimes

Considering the negative consequences of water crisis in agriculture, strategies need to be developed to mitigate the adverse effects of drought stress. This study investigates the effect of applying biochar, as a stable carbon-rich soil conditioner, and salicylic acid, as an elicitor, on soil biochemical properties and some growth parameters of Borago officinalis L. under water deficit conditions. This experiment was conducted as a split plot factorial experiment based on a completely randomized block design (RCBD) with four replications. Factors were irrigation regimes as water deficit treatments, including 100%, 75% and 50% water requirement (WR) in the main plots and application of biochar (0, 5, and 10 ton ha−1) and foliar application of salicylic acid (0, 0.5 mM) in the sub-plots in two growing seasons (2016–2018). The results of this study indicated that application of 5 and 10 ton ha−1 biochar increased soil pH and soil organic carbon (SOC). Biochar was effective on soil nitrogen only in 75% WR, and increased the amount of ammonium in the soil by 5 ton ha−1 in all irrigation regimes but decreased the soil nitrate. Furthermore, decreasing irrigation regimes increased ammonium and decreased soil nitrate in both cropping years. Reducing the irrigation regimes negatively reduced the soil respiration rate, which was improved by the application of biochar at 5 ton ha−1 in both years. Application of salicylic acid alone and in combination with biochar mitigated the adverse effects of water deficit. Co-application of salicylic acid and biochar increased plant growth parameters, including (flowering branches dry matter, leaf dry matter, shoot dry matter, and the plant height), and SOC under water deficit conditions. It can be concluded that both alone and combined applications of treatments mitigate the adverse effect of water deficit by soil biochemical properties improvement and enhances growth parameters of borage.