Variations in microbial community structure and extracellular enzyme stoichiometry in heavy metal-contaminated soil under different fertilization regimes

Abstract

Nitrogen (N) deficiency significantly inhibits microorganism and plant growth in mining areas, posing challenges to bioremediation efforts. An analysis of the relationship between soil nutrients and enzyme activity offers insights into microbial metabolic limitations and community dynamics. In a comprehensive one-year experiment, two N fertilizers (ammonium chloride (AC) and urea (U)) along with nine distinct fertilization regimes were applied to heavy metal-contaminated soil. The results showed that N fertilization increased available nutrient contents (especially ammonium nitrogen (NH 4 + -N), nitrate nitrogen (NO 3 − -N) and available potassium (AK)) in the soil. Moreover, N fertilizers significantly enhanced the activities of six key enzymes (cellulase, invertase, amylase, urease, protease, and phosphatase), as well as the contents of three organic acids (oxalic, tartaric, and malic) in the soil. Interestingly, vector lengths decreased by 34.1 %, while vector angles increased by 50.6 % during the experimental period, indicating that N fertilization alleviated soil N and C limitations but exacerbated P limitation in soil microbial metabolic activities. Furthermore, N fertilization promoted the growth of copiotrophic bacteria (e.g., Proteobacteria and Bacteroidetes) while inhibiting the growth of oligotrophic bacteria (such as Acidobacteria), thereby improving microbial community resilience. Redundancy analysis, Pearson's correlation analysis and structural equation modeling revealed significant associations between the relative abundance of dominant soil microbial phyla (bacteria and fungi), extracellular enzyme activity and related stoichiometric ratios, highlighting the interconnectedness of microbial community composition and enzymatic processes. In summary, N addition induced notable alterations in soil properties, influencing enzyme secretion and microbial community structure. These findings suggest that optimized N fertilization regimes can alleviate nutrient limitations, enhance microbial activity, and improve microbial community stability, providing a practical strategy for accelerating bioremediation and ecosystem recovery in nutrient-deficient mining soils.