Chlorella-Bacillus biofertilizers interact with varying nitrate addition amounts to increase soil phosphorus bioavailability

Background and aims: Phosphate-solubilizing bacteria (PSB) possess significant potential for enhancing soil phosphorus bioavailability, but their efficacy may be constrained by carbon and nitrogen availability. Algae exhibit synergistic interactions with bacteria through producing active organic carbon. However, the influence of algae and PSB together on phosphorus bioavailability under varying nitrogen levels remains unclear. Methods: We conducted a microcosm experiment to explore the effects of applying Chlorella-Bacillus biofertilizer, along with various nitrate addition amounts, on both soil microbial community composition and phosphorus bioavailability. In addition, we employed a co-culture of Chlorella and Bacillus with different nitrogen concentrations to investigate the possible effects of nitrate addition on the production of organic anions and phosphatases of the Chlorella-Bacillus communities by using metabolomic analysis. Results: Chlorella-Bacillus biofertilizer significantly increased soil Olsen-P concentration via strong interactive effects correlating with the various nitrate addition amounts in the microcosm experiment. These effects were due mainly to reducing the algal diversity, soil pH, and changing dissolved organic matter (DOM) characteristics, especially increasing the humification index (HIX). The metabolomic analyses of co-culture confirm that pathways related to the biosynthesis of fatty acids and phosphatases production are enhanced when nitrogen levels are high. Conclusion: Chlorella-Bacillus biofertilizer has significantly interactive effects with nitrate addition on soil phosphorus bioavailability by influencing soil DOM, pH and production of organic anions and phosphatases. These insights are useful for optimizing bio-fertilizer-nitrate combinations for increasing phosphorus bioavailability thereby reducing chemical fertilizer requirements.