Long-term effects of nitrogen and phosphorus fertilizers on rhizosphere physicochemical characteristics and microbial composition in alfalfa

Abstract

Repeated fertilizer applications to different monoculture cropping systems can alter soil nutrients and microbial community structure. Here we investigate the impact of long-term (4 year) distinct nitrogen (N) and phosphorus (P) fertilizer treatments on rhizosphere physicochemical characteristic and soil microbial community composition in an alfalfa (Medicago sativa L.) cropping systems. N and P fertilizer significantly influenced the physicochemical properties and stoichiometry of alfalfa rhizosphere soil. Nevertheless, N and P fertilizers application on the rhizosphere bacterial and fungal community structures were inconsistent. Fertilizer application minimally metamorphose the rhizosphere bacteria and fungi richness (Sobs index) and diversity (Shannon index). Non-metric multidimensional scaling analysis (NMDS) revealed that fertilizer treatments have no significant influence the fungal community, however, they significantly altered the bacterial community. Bacterial dominant phyla, Actinobacteriota, Acidobacteriota, Chloroflexi, and Gemmatimonadota changed significantly, indicating that the composition of the bacterial community was more responsive to fertilizer application when compared to fungal community composition. Spearman correlation analysis demonstrated no significant correlation amidst soil factors and bacterial diversity, conversely, bacterial richness, fungal diversity and richness were significantly modified by soil factors (AP, AN, and C/N). Network analysis indicated that N application reduced the positive associations between bacteria and fungi, whereas P application enhanced the positive associations. In conclusion, fertilization changes soil fertility of alfalfa fields and the bacterial community composition. Additionally, tests on phosphate solubilizing bacteria (PSB) isolated from the rhizosphere soil of alfalfa demonstrated that these bacteria could significantly enhance the biomass of alfalfa.