NH4+-N and Low Ratios of NH4+-N/NO3−-N Promote the Remediation Efficiency of Salix linearistipularis in Cd- and Pb-Contaminated Soil

Abstract

(1) Background: the utilization of fast-growing trees for phytoremediation in heavy-metal-contaminated soil is increasingly recognized as an effective remediation method. Nitrogen (N) fertilizer enhances plant tolerance to heavy metals, yet the impact of various N levels and ammonium (NH4+-N)/nitrate (NO3−-N) ratios on the remediation of cadmium (Cd) and lead (Pb) by trees remains unclear. (2) Methods: the efficiency of Salix linearistipularis in remediating Cd- and Pb-contaminated soil was investigated using a pot experiment with three N levels (60, 120, 200 kg hm−1 year−1) and five NH4+-N/NO3−-N ratios (6/0, 4/2, 3/3, 2/4, 0/6) employed, resulting in 16 treatments including a control. (3) Results: the levels and ratios of NH4+-N/NO3−-N significantly affected the Cd and Pb uptake by S. linearistipularis. The highest increases in Cd and Pb in S. linearistipularis were observed for the N120-6/0 treatment, which increased by 104.36% and 95.23%, respectively. In addition, in the N120-6/0 treatment, the stem and leaf bioconcentration factors of Cd were significantly enhanced by 28.66% and 40.11%, respectively. Structural equation modeling revealed that the uptake of Cd and Pb was predominantly influenced by plant traits (biomass and root traits) rather than soil properties. (4) Conclusions: Our findings highlight the potential of the NH4+-N/NO3−-N ratio to regulate plant traits, thereby improving the phytoremediation efficiency of heavy-metal-contaminated soil.