Comparison of high-M and high-G alginate macro-carriers: Effects on bioavailability and growth dynamics in agricultural soil treatments

Alginate-based carriers are widely used in agriculture for the controlled release of nutrients, pesticides, and bioactive compounds. This study provides a holistic assessment of alginate carrier performance in soil by integrating physicochemical, microbiological, and ecological assays in soil. Two formulations were tested high-G (high proportion of guluronic acid) and high-M (high proportion of mannuronic acid) alginate macro-carriers. The carriers were prepared as freeze-dried beads, representing a form suitable for long-term storage and practical field application. High-G alginate, characterized by stronger crosslinking, exhibited slower degradation and sustained nutrient release, whereas high-M alginate degraded more rapidly, allowing for faster bioavailability. Soil respiration analysis indicated that microbial activity was initially lower for high-G alginate but increased over time, ultimately surpassing high-M degradation rates. Physicochemical characterization revealed that high-G alginate beads retained more water and exhibited higher mechanical strength compared to high-M alginate beads. ICP analysis showed that high-M alginate retained more free calcium in solution and within formed beads, affecting mineral composition. Growth interference tests with lettuce indicated that high-M alginate exhibited slightly greater negative effects on root and shoot development at 30 days but lower negative effects on plant development at 70 days compared to high-G alginate. Additionally, VOC emissions analysis showed no substantial differences between the two alginate types, though key degradation markers were identified. These findings highlight the importance of considering alginate composition and degradation behavior to optimize its use in agricultural applications while minimizing unintended environmental effects.