Objective The objective of this paper is to utilize the amino groups in gelatin (GEL) molecules and the carboxyl groups in xanthan gum (XG) molecules to construct a composite wall material through electrostatic interaction, and then encapsulate the core material, fulvic acid (BFA), by using this wall material to prepare fulvic acid microcapsules (BFAM). The core goal is to address the inherent problems of BFA, such as its susceptibility to degradation and insufficient stability, and significantly enhance its stability. This will provide a material foundation for the subsequent expansion of BFA's effective application scenarios, especially in the agricultural field.

Methods BFAM was prepared by using the complex coacervation method, in which GEL and XG served as the composite wall materials and BFA was used as the core material. The key parameters affecting the preparation effect were initially screened through single-factor experiments. The process conditions were systematically optimized by orthogonal experiments, and the optimal preparation scheme was finally determined as follow: the reaction temperature was controlled at 60 ℃; the pH value of the system was adjusted to 3.5; the dosage of core material BFA was 200 mg, and the dosage of crosslinking agent glutaraldehyde was 1.5%.

Results Under the above-mentioned optimal conditions, the encapsulation efficiency of BFAM reached 94.73%, demonstrating excellent core material encapsulation efficiency. The results of Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy jointly confirmed that BFA was successfully encapsulated within the composite wall material formed by GEL and XG through electrostatic interaction. Moreover, the comprehensive stability of BFAM was significantly improved compared to the unencapsulated pure BFA. The thermogravimetric analysis results indicated that BFAM effectively reduced the hygroscopicity of BFA, and it enhanced its own thermal stability through the combined effects of physical barrier and chemical interactions (hydrogen bonding and covalent crosslinking). The environmental exposure test further demonstrated that BFAM retained its chemical properties more completely, fundamentally solving the problem of easy degradation and poor stability of traditional unmodified BFA. In the validation of agricultural application effects, the planting experiments with butterhead cabbage as the test crop showed that BFAM significantly promoted crop growth. Compared with the control group (water), the BFA group, and the blank capsule (M) group, BFAM group significantly promoted crop growth, with the maximum increase in plant height reaching 61.8% and the maximum increase in total chlorophyll content reaching 57.4%. This result is attributed to the unique "GEL+XG" dual nitrogen source synergy of BFAM. The natural amino acids contained in GEL can serve as a fast-acting nitrogen source for crop growth, while BFA, after being encapsulated and released slowly by the wall material, can continuously release nitrogen, humic acid, and other nutrients. The combination of the two achieves a "fast-acting+long-lasting" synergistic nutrient supply mode, which can more efficiently meet the nutrient requirements of crops throughout their growth cycle compared to the single application of pure BFA (which is prone to loss and does not provide sustained nutrition) or M (which only contains the wall material without core nutrients), thereby significantly improving crop growth indicators.

Conclusion The sustained-release effect of the composite wall material constructed by using GEL and XG, which BFAM relies on, enables it not only to prolong the action period of BFA and significantly reduce the loss of nutrients, but also maximumly exerts its fertilizer effect. This method fundamentally solves the problems of easy degradation, poor stability, and low utilization rate of traditional BFA in agricultural applications, significantly enhancing its stability and application effect. This study not only verified the feasibility and superiority of preparing BFAM using the complex coacervation method but also provided a practical technical solution for the efficient application of fulvic acid in agriculture. It laid a solid technical foundation for the research and innovation as well as the large-scale promotion of new type slow-release fertilizers based on fulvic acid. Meanwhile, the technical solution provided in this work can directly reduce the raw material waste of BFA in agricultural production, enhance its field utilization efficiency, and holds clear and reliable practical reference value for the technological transformation and actual production in related fields.