Evaluation of reclaimed treated wastewater on soil quality, microbial community and function in urban greening irrigation

Reclaimed treated wastewater (TWW) offers a sustainable irrigation alternative for urban greening amid freshwater scarcity. However, its impact on soil quality and microbial ecology in urban landscapes remains underexplored. This study assessed the effects of three TWW types and one TWW-purified water mixture on soil properties, heavy metal accumulation, enzyme activity and microbial community dynamics in Common Nandina (Nandina domestica Thunb.) systems over three- and five-month in Nanjing, China, using a metagenomic approach. The results showed that TWW irrigation significantly increased soil nitrogen, phosphorus content and pH (P < 0.05), with stronger effects observed at three-month (P < 0.05). Soil heavy metal content varied, with Cr and As exhibiting potential accumulation. Enzyme activity (N-acetyl-glucosaminidase and phosphatase) was positively influenced by TWW irrigation at three-month. Metagenomic analysis revealed significant shifts in microbial community composition, particularly fungi, with Mucoromycota increasing and Ascomycota decreasing under TWW. The Normalized Stochasticity Ratio indicated increased stochasticity in microbial assembly under TWW irrigation (P < 0.05). Functionally, TWW increased abundance of functional genes related to amino acid metabolism and peroxisomes (P < 0.05), but decreased degradation genes for aromatic compounds. TWW also increased the abundance of antibiotic resistance genes (ARGs), particularly those related to triclosan and glycopeptide, and plant pathogenic genera like Aspergillus and Fusarium. The findings suggest that while TWW irrigation improves certain soil properties and microbial functions in the short term, it may pose ecological risks from heavy metal and microbial contamination, offering key insights for sustainable urban irrigation strategies.