Microalgae inoculation should return to the root

Microalgae are emerging as sustainable biofertilizers that enhance soil fertility and plant productivity, yet their roles in medicinal plants remain largely unexplored. In this study, we evaluated the effects of three representative microalgae, Anabaena cylindrica (AC), Phormidium tenue (PT), and Chlorella vulgaris (CV), on the growth, quality, rhizosphere microbiome, and hormone regulation of Angelica sinensis. Field inoculation trials demonstrated that all three microalgae significantly promoted biomass accumulation and antioxidant capacity. AC and CV further enhanced the accumulation of ferulic acid and flavonoids, two key quality determinants. Microalgal inoculation significantly altered rhizosphere soil properties by increasing total organic carbon and alkali-hydrolyzable nitrogen, with AC uniquely elevating available phosphorus and Fe. Metagenomic analysis revealed that AC and PT stimulated nitrification while suppressing denitrification, thereby reducing nitrogen losses and stabilizing soil nitrogen pools. Distinct microbial taxa, including Rhodanobacter, Streptomyces, and Pseudomonas, were identified as major contributors to carbon and nitrogen cycling. Hormone metabolomics further showed that microalgal treatments reprogrammed phytohormone landscapes, with AC and CV increasing salicylic acid but reducing jasmonates, whereas PT exhibited opposite effects. Partial least squares path modeling indicated that salicylic acid acts as a central negative regulator of ferulic acid biosynthesis, while nutrient- and microbe-driven compensation modulated this inhibitory effect.