Solid inoculants: Bacterial immobilization routes on the degradation in the soil of alginate-starch-based...

Biofertilizers based on alginate and starch biopolymers are widely used for microbial immobilization due to their biodegradability and capacity to maintain cell viability. However, the effects of different immobilization routes on soil degradation and microbial release remain unclear. This study evaluated how the immobilization route of Bacillus safensis influences the performance and biodegradation of solid biofertilizers formulated with 2.5 % (w/w) medium-grade sodium alginate, 1.0 % (w/w) potato starch, and 0.2 g L^-1 of L-tryptophan. Three treatments were tested: RT1 (bacteria inclusion before dripping), RT2 (bacteria inclusion in the cross-linking solution), and RT3 (bacteria inclusion on pre-formed beads). RT1 exhibited the highest immobilization efficiency (83 % ± 3.5) and bacterial viability (4.7 × 10⁸ CFU g^-1 ± 3.1 × 10⁸), while RT3 resulted in enhanced microbial release, reduced matrix weight loss (38 % less than the control at day 30), and significantly higher soil microbial activity, as indicated by FDAse enzyme activity and soil respiration. Principal component analysis revealed that RT3 correlated strongly with total organic carbon (loading = 0.915) and microbial activity, suggesting greater carrier stability during biodegradation. These findings demonstrate that the immobilization route modulates both the degradation and bioactivity of alginate-starch matrices, with RT1 supporting prolonged microbial activity in soil ecosystems and RT3 providing faster activity in rhizosphere-targeted applications.