Biochar Improves Soil Fertility in Sandy Nutrient-Poor Soil, While Wood Distillate Modulates Nutrient Dynamics and Plant Physiological Responses in Basil (Ocimum basilicum L.) Plants

The progressive decline in fertility in nutrient-poor sandy soils has increased interest in soil conditioners that improve nutrient availability. Rising costs and the need to restore biological fertility have shifted attention towards fertilizers that not only enhance productivity but also improve soil biological activity. This study aims to evaluate the effects of biochar (3% w/w; BC) and wood distillate (one irrigation intervention per week at 2% v/v; WD), applied individually or in combination (BC + WD), on a nutrient-poor soil, evaluating soil fertility and basil plant physiology and growth but also antioxidant responses in a pot experiment. Soil NPK content and enzymatic activity were assessed, while plant growth, macronutrient uptake, gas exchange, and antioxidant system responses were monitored after 28 and 56 days of treatment. BC treatment, followed by BC + WD treatment, increased soil P availability by 36% and 37%, respectively, after 56 days compared to untreated soil (CNT). A similar pattern was evidenced for the exchangeable K and pH of the soil. Although BC led to a reduction in soil enzymatic activity, the BC + WD treatment enhanced urease and acid phosphatase activity after 56 days by 26% and 7%, respectively, compared to CNT. Similarly, P uptake by plants was improved by BC + WD after 56 days, while potassium, K, uptake increased in both the BC and BC + WD treatments by 38% and 75% at the final sampling. BC or BC + WD resulted in improved photosynthesis and gas exchange, while WD influenced responses related to redox balance and antioxidant activity over time. Moreover, BC + WD slightly stimulated an increase in dehydroascorbate reductase (+52%), ascorbate peroxidase (+78%), and glutathione reductase (+41%) activity compared to CNT, enforcing the plant antioxidant system. Therefore, the positive antioxidant responses were primarily attributed to the use of BC rather than WD. Both BC and BC + WD proved to be effective and sustainable soil conditioners with beneficial effects on soil P and K availability, as well as certain enzymatic activities. For plants, the effects were more pronounced with BC treatment, showing antioxidant responses within the first 56 days. In general, BC improved soil fertility, and WD acted as a modulator of nutrient dynamics and plant physiological responses, especially when combined with BC.