Simultaneous Application of Ammonium and Nitrate Nitrogen Enhances Phytoremediation Efficiency by Mediating Biomass and Bioavailability of Lead and Cadmium in Salix linearistipularis

This study aims to elucidate the effects and mechanisms of ammonium (NH4+-N) and nitrate (NO3−-N) nitrogen on the efficiency of Salix linearistipularis K. S. Hao in remediating heavy metal-contaminated soils. Thus, the effects of 15 fertilization treatments (comprising three nitrogen levels and five nitrogen form ratios) on Pb and Cd accumulation, soil properties, microbial structure, and metabolic characteristics were investigated using a pot experiment. The results indicated that Pb and Cd accumulation were the highest under the L12 treatment (60 kg N·hm−2·year−1, NH4+-N/NO3−-N = 1:2), whereas nitrate-only treatments, irrespective of concentration, resulted in a decrease in accumulation. In the L12 treatment, biomass increased by 87.0%, with Pb and Cd accumulation rising by 85.71% and 80.0%, respectively, suggesting that biomass may contribute predominantly to heavy metal accumulation. Additionally, NH4+-N/NO3−-N ratio had a greater effect on biomass than the nitrogen application amount. Microbial composition was altered, and the relative abundance of heavy metal-resistant microbes increased. However, the amount of nitrogen fertilizer had a stronger impact on microbial variation. Under different nitrogen application rates and NH4+-N/NO3−-N ratios, the formation or disappearance of unique metabolic pathways related to amino acids and carbohydrates was observed. Furthermore, both microbial metabolism and the bioavailability of Pb and Cd were positively correlated with nitrogen levels and NH4+-N/NO3−-N ratios. These findings indicate a potential association between shifts in microbial metabolism and the bioavailability of heavy metals. Therefore, the simultaneous application of ammonium and nitrate nitrogen in appropriate ratios can enhance the remediation efficiency of S. linearistipularis by boosting biomass and heavy metal bioavailability via microbial metabolism. The findings of this study not only provide novel insights into improving the phytoremediation efficiency of woody plants through fertilization strategies but also lay a theoretical foundation for the effects of nitrogen fertilization on nutrient cycling in metal-contaminated soils.