Data-Driven Decision Optimization of Humic and Fulvic Acid Treatments: Experimental Evidence on Enzyme Regulation and Nutrient Bioavailability in Calcareous Soils

[Purpose] This study explores the influence of humic and fulvic acids on soil enzymatic activities and nutrient bioavailability in calcareous soils, aiming to enhance understanding of their environmental roles in soil management and fertility. [Design/methodology/approach] Four humic substances—fulvic acid (FA), OMRI, SP85, and HS—were applied at varying concentrations (0–1000 mg·kg⠻¹) to calcareous soils. Enzymatic activities (urease, phosphodiesterase, acid and alkaline phosphatase) and the bioavailability of key nutrients (P, K, NH₄⠺, Mg, Ca, Cu, Zn) were assessed after 40 and 70 days. [Findings] Humic substances significantly affected soil enzymatic processes and nutrient availability. Urease activity was consistently inhibited, with the strongest effect observed under SP85. Phosphodiesterase and phosphatase activities were notably enhanced, especially under FA and HS treatments. Optimal phosphatase activity was observed at 800 mg·kg⠻¹, while excessive concentrations (e.g., 1000 mg·kg⠻¹ of SP85) led to decreased activity. HS treatment induced the greatest increase in phosphomonoesterase activity by day 70. Nutrient responses varied: OMRI improved P and Mg availability; HS increased K and Zn; FA promoted Cu. Calcium levels declined under all humic acid treatments. An application rate of 400 mg·kg⠻¹ was found to be optimal. [Practical implications] Appropriate application of humic substances can improve soil biochemical health, supporting efficient nutrient management in high-pH soils. [Originality/value] This study provides novel insights into the enzyme–nutrient interactions influenced by humic substances in calcareous soils. This study also contributes to Decision Sciences by providing a data-driven optimization framework for selecting the most effective type and concentration of humic substances to support evidence-based agricultural decision-making. This research is original in integrating soil biochemical experimentation with quantitative decision modeling, allowing readers to see how data-driven optimization links biological evidence with managerial decision-support. Academics may cite this paper for its methodological synthesis of experimental design and optimization analysis, while practitioners can adopt its findings to enhance real-world nutrient-management efficiency in calcareous soils.