Microalgal fertilizers as sustainable alternatives to chemical fertilizers: Enhancing pepper yield and soil health through reduced synthetic input strategies.

Long-term excessive application of chemical fertilizers (CF) has led to a decline in soil fertility and the deterioration of soil health. Consequently, developing eco-friendly microbial fertilizers as alternatives to chemical inputs has become a global consensus for promoting sustainable agriculture. To explore the underexplored potential and soil-modulating mechanisms of microalgae-based fertilizers, this study investigates their feasibility of microalgae-based fertilizers as CF substitutes for enhancing the yield and quality of peppers (Capsicum annuum L.). We compared the application effects of three types of microalgae-based fertilizers, namely total microalgal biomass (TMB), microalgal extracts (ME), and microalgae-bacteria consortia (MBC, supplemented with Bacillus spp.) under different fertilization regimes (sole- and co-application with reduced compound fertilizer) on plant traits and soil microbial communities. The results indicated that all three microalgal fertilizers significantly promoted seed germination, plant growth, and fruit yield. Notably, sole application of TMB exhibited the most prominent growth-promoting effects with an increase of 13%-37% compared to the CF treatment. The T2 treatment (substituting 40% of CF) achieved the highest pepper yield, representing a 72% increase over the CF control. Regarding soil properties, microalgal fertilizers effectively adjusted soil pH, lowered electrical conductivity and exhibited desirable slow-release fertilizer characteristics. In particular, TMB significantly increased soil organic matter content and maintained a stable supply of available phosphorus during the fruit-harvesting stage. Analysis of microbial community structure and function revealed that the application of microalgal fertilizers enriched various plant growth-promoting microbes, such as Bacillus, Neurospora and Trichoderma, which possess functions in nutrient mineralization or biocontrol. Concurrently, these treatments significantly enhanced the activities of soil enzymes (urease, phosphatase, and sucrase) and functional pathways related to carbon and nitrogen metabolism. Our findings highlight that the microalgal fertilizers represent an optimal strategy for achieving chemical fertilizer reduction and efficiency enhancement while improving soil bio-fertility.