Effects of Biochar Applications Under Water Deficit Conditions on Soil Enzyme Activities and the Plant Antioxidant System in Spinach (Spinacia oleracea L.)

This study was conducted under greenhouse conditions to determine the effects of biochar applied at different rates on soil enzyme activities and the plant antioxidant system in spinach (Spinacia oleracea L.) under different levels of water deficit. The experiment was conducted under greenhouse conditions in a completely randomized design with four replications. Treatments included three irrigation levels [100% field capacity (FC), 75% FC, and 25% FC] and four biochar application rates (0, 1, 2, and 4% w/w). At the end of the experiment, soil urease, dehydrogenase, acid phosphatase, and alkaline phosphatase activities were determined. In plant samples, superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities, as well as malondialdehyde (MDA) content, were analyzed. Water deficit significantly reduced soil enzyme activities and increased oxidative stress indicators in spinach plants. Under severe water deficit conditions (25% irrigation), application of 4% biochar increased urease activity from 20.15 to 22.35 (10.9%) and dehydrogenase activity from 14.48 to 15.78 (9.0%) compared with the control treatment (p < 0.01). Acid phosphatase and alkaline phosphatase activities also increased by 3.2% and 1.3%, respectively. In contrast, MDA content decreased from 8.52 to 6.02 (29.3%), while SOD activity decreased from 7.67 to 6.88 (10.2%) under the same conditions (p < 0.01). The simultaneous reduction in MDA and antioxidant enzyme activities suggests alleviation of oxidative stress and a reduced requirement for antioxidant enzyme induction rather than suppression of the plant defense system. Correlation analysis revealed strong negative relationships between soil biochemical activity and plant stress indicators, with the strongest correlation observed between urease and CAT activity (r = −0.97, p < 0.001). The findings demonstrate that biochar application supports soil biological functions, enhances microbial activity, and alleviates oxidative stress in spinach under water-limited conditions. Therefore, biochar may serve as an effective soil management strategy for sustainable crop production under drought conditions.