Microbial additive and biochar co-application enhances biotransformation of kelp waste into nitrogen-retentive, humified biofertilizer with high agronomic efficiency

Kelp processing generates substantial quantities of organic residues that require environmentally sustainable management. In this study, kelp waste was co-composted with corn starch residue for 42 days. Three treatments were established: a control (kelp + corn starch residue, C1), a microbial additive treatment (C2), and a combined biochar and microbial additive treatment (C3). The effects of microbial additive and biochar on nitrogen retention, humification, and agronomic performance were evaluated. Compared with the control, C3 reduced ammonia emissions by 65.26%, resulting in improved nitrogen conservation. It also promoted the transformation of humification precursors, including reducing sugars, amino acids, and polyphenols, thereby accelerating humification. At the end of composting, C3 exhibited the highest humic substances (184.07 g/kg), humic acid (HA, 144.90 g/kg), and HA/FA ratio (3.70), together with the lowest fulvic acid (FA, 39.17 g/kg). The microbial additive treatment (C2) also improved compost quality by reducing electrical conductivity and ammonia emissions, enhancing nitrogen conservation, increasing enzyme activities, and promoting humification and plant growth. The germination index reached 114.25%, 160.96%, and 161.87% in C1, C2, and C3, respectively, with both amended treatments showing markedly higher values than the control. Pot experiments further confirmed the superior agronomic performance of C3, with fresh weight, dry weight, plant height, root length, and SPAD value increasing by 59.46%, 64.54%, 26.20%, 2.70%, and 2.96%, respectively, compared with the control. Overall, the combined application of biochar and microbial additive effectively enhanced the biotransformation of kelp waste into a nitrogen-retentive, highly humified biofertilizer with improved agronomic performance.