Reduced nitrogen fertilization combined with microbial bio-inputs restores soil bacterial functional profiles and sustains wheat performance

Nitrogen (N) fertilization is a major driver of wheat productivity but also entails significant environmental costs, underscoring the need for strategies that reduce N inputs without compromising yield. Microbial bio-inputs are increasingly proposed as complementary tools to improve N use efficiency, yet their effects on soil microbial communities and functions under reduced N fertilization remain poorly documented in wheat agrosystems. In this study, we investigated the effects of two microbial bio-inputs, Sphingomonas sediminicola Dae20 and the cell-free supernatant of Bacillus velezensis GA1, applied alone or in combination under reduced N fertilization, on wheat performance, soil microbial functioning and bacterial community structure. A field experiment was conducted with five management treatments, including conventional or reduced N fertilization and reduced N combined with each bio-input or their combination. Grain yield and quality were measured, alongside soil microbial metabolic activity, functional richness, and bacterial community structure at tillering, stem elongation and ripening. Reducing N fertilization resulted in a yield decrease of approximately 20%, while grain quality remained unaffected. Both bio-inputs partially compensated for yield losses, and their combined application restored yields obtained under conventional fertilization, without increasing grain N and protein concentrations. Bio-input effects were mainly functional, as bacterial diversity remained stable across treatments. In contrast, microbial metabolic activity and functional richness declined sharply at the end of the crop cycle in untreated soils but were maintained in bio-input-treated soils. These late-season effects coincided with wheat senescence, suggesting that bio-inputs stabilize microbial functioning when rhizosphere resources become limiting. Together, our results indicate that microbial bio-inputs do not reshape soil bacterial diversity but act as functional buffers of the soil microbiome, contributing to improved crop performance under reduced N fertilization and highlighting the importance of integrating functional and temporal indicators when evaluating bio-input strategies for sustainable agroecosystems.