Azolla reshapes rhizosphere microbiomes and nutrient cycling in paddy fields

BACKGROUND: Soil quality is a critical determinant of agricultural productivity and sustainability. The symbiotic nitrogen fixation by Azolla plays a key role in enhancing soil quality. However, despite its potential as a green manure for enhancing soil quality, the role of Azolla in paddy systems remains inadequately characterized. This study aims to elucidate the effects of Azolla on soil quality by examining nutrient cycling dynamics and microbial community composition, along with their interactions. RESULTS: We integrated soil physicochemical analyses, enzyme activity assays, bacterial community profiling, co-occurrence network analysis, and correlation assessments to evaluate the effects of Azolla on soil microbial ecology. Rice monoculture (R) and rice-Azolla co-cultivation (RA) systems were established. RA significantly increased activities of carbon- and nitrogen-cycle-related enzymes by 3-44% (P < 0.05), while phosphorus-cycle-related enzyme activities decreased by 12-42%. Under high nitrogen fertilization, Azolla altered bacterial community structure and reduced alpha diversity. Notably, Azolla recruited specific functional taxa-including Haliangium, SC-I-84, Candidatus_Solibacter, Anaerolinea, and Sphingomonas-whose relative abundances were 1.03-1.33 times higher in RA than in R. CONCLUSIONS: This study elucidates the interactions between soil properties and microbial communities under Azolla application and uncovers the mechanisms by which Azolla enhances soil quality through nutrient cycling. Our findings demonstrate that Azolla, as a green manure, not only elevates soil nutrient content but also improves soil quality by driving microbe-mediated nutrient recycling. These results underscore the potential of Azolla as a sustainable alternative to conventional fertilization practices, offering novel insights into biofertilizer strategies for agricultural soil enhancement.