Combined Application of Biochar and Arbuscular Mycorrhizal Fungi Enhances Soil Fertility, Antioxidant Defense, and Growth Performance of Cocoa (Theobroma cacao L.)

Abstract

Cocoa (Theobroma cacao L.) productivity in tropical agroecosystems is constrained by declining soil fertility, nutrient limitations, and increasing environmental stress driven by climate variability. Sustainable soil management strategies that enhance soil quality and plant resilience are therefore essential for stable cocoa production. This study evaluated the effects of biochar (40 t ha-1) and arbuscular mycorrhizal fungi (AMF; 10 g plant-1), applied individually and in combination, on soil physicochemical properties, enzymatic activities, antioxidant responses, and field performance of young cocoa plants. The experimental soil was slightly acidic and nutrient-deficient with low organic matter content. Both sole and combined applications of biochar and AMF significantly improved soil chemical properties and plant growth compared with the control. Soil respiration was influenced by amendments, with basal respiration following the order Control > AMF > Biochar + AMF > Biochar, while substrate-induced respiration showed no significant treatment effects. Soil phosphatase activities (acid and alkaline phosphatase) were significantly enhanced by amendments, with the highest activities observed under the combined biochar + AMF treatment, indicating improved phosphorus cycling and nutrient availability. Biochar and AMF also altered plant biochemical and antioxidant responses. Biochar alone induced higher proline accumulation and antioxidant enzyme activities (SOD and GPx), suggesting a stress-priming effect. In contrast, the combined treatment enhanced photosynthetic pigments, soluble sugars, non-structural carbohydrates, and antioxidant enzymes (CAT, SOD, and GPx), indicating improved physiological performance and metabolic balance. Seasonal variation significantly influenced plant metabolism, with the dry season promoting accumulation of osmoprotectants and stronger antioxidant defense, while the rainy season increased chlorophyll, carotenoids, and phenolic content. Overall, the integrated application of biochar and AMF improved soil fertility, enhanced enzymatic and antioxidant functions, and promoted cocoa growth under field conditions. These findings demonstrate that biochar–AMF synergism is a promising climate-smart strategy for improving soil functionality and sustaining cocoa production in tropical agroecosystems.