Nitrogen fertilizer improves Salix matsudana growth and soil qualities

Abstract

Introduction: Soil contamination with heavy metals (e.g., Pb, Cd) poses severe environmental risks due to industrialization. Salix matsudana, a metal-tolerant woody plant, shows promise for phytoremediation, yet the synergistic role of nitrogen (N) fertilization in enhancing plant growth and soil remediation remains unclear. This study aims to elucidate how N fertilization optimizes S. matsudana's remediation efficiency.

Methods: We applied integrated physiological and multi-omics approaches to assess N fertilization effects on S. matsudana growth, Pb/Cd uptake, and rhizosphere properties. Physiological metrics (biomass, metal accumulation) were combined with microbial community analysis (16S rRNA sequencing) and metabolomic profiling (LC-MS/GC-MS) of rhizosphere soils under varying N concentrations.

Results: High N levels significantly increased plant biomass and Pb/Cd accumulation. Microbial diversity shifted, with enriched metal-mobilizing taxa. Metabolomics revealed elevated organic acids, correlating with improved metal bioavailability and soil health.

Discussion: N fertilization synergistically enhances phytoremediation by: (1) stimulating plant growth and metal uptake, (2) reshaping rhizosphere microbiomes for metal mobilization, and (3) promoting chelating metabolite secretion. These findings provide actionable insights for optimizing N-assisted phytoremediation strategies.

Keywords: Salix matsudana; metabolomic; nitrogen fertilization; phytoremediation; rhizosphere microorganism.

Figure 1

Effects of nitrogen fertilizer on Salix matsudana growth and uptake of heavy metals. (A) S. matsudana was transplanted into heavy metal-contaminated soil supplemented with four types of nitrogen fertilizers: ammonium nitrogen (AM), nitrate nitrogen (NI), a 1:1 mixture of ammonium and nitrate (AN), and urea nitrogen (CN). Each fertilizer was applied at four concentration levels (low, medium, high, and hyper-high). Scale bar: 10 cm. The control (CK) group consisted of S. matsudana grown in heavy metal-contaminated soil without nitrogen fertilization. (B) Plant biomass and Pb/Cd accumulation in S. matsudana under different nitrogen treatments. Data are presented as mean ± SD (*p < 0.05, Student’s t-test).

Figure 2

Nitrogen fertilizer enhances heavy metal accumulation in Salix matsudana roots. (A) Phenotypic observation of S. matsudana roots grown in heavy metal-contaminated soil with or without nitrogen fertilizer supplementation. Control (CK) represents roots grown in heavy metal-contaminated soil without nitrogen fertilizer application. Scale bar, 1 cm. (B) Biomass and heavy metal (Pb and Cd) accumulation in primary roots of S. matsudana grown in heavy metal-contaminated soil supplemented with four different nitrogen fertilizers. (C) Biomass and heavy metal (Pb and Cd) accumulation in lateral roots of S. matsudana under the same treatment conditions as discussed in subpart (B). Data are presented as mean ± SD; *p < 0.05 (Student’s t-test).

Figure 3

Effects of nitrogen fertilizer on the physicochemical properties of heavy metal-contaminated soil. (A) Soil pH after nitrogen fertilizer application. (B) Soil cation exchange capacity (CEC) following nitrogen fertilizer treatment. (C) Soil catalase activity in response to nitrogen fertilizer. (D) Soil organic matter content after nitrogen fertilizer amendment. (E) Urease activity in soil treated with nitrogen fertilizer. (F) Soil phosphorus content following nitrogen fertilizer application. (G) Soil potassium content after nitrogen fertilizer treatment. (H) Pb concentration in plant roots under nitrogen fertilization. (I) Cd concentration in plant roots under nitrogen fertilization. Data are presented as mean ± SD (*p < 0.05, Student’s t-test).

Figure 4

Effects of nitrogen fertilization on rhizospheric bacterial communities in Salix matsudana grown in heavy metal-contaminated soil. (A) Venn diagram illustrating the shared and unique bacterial taxa across (ASVs) four nitrogen fertilizer treatments. (B) Heatmap displaying the taxonomic composition of rhizosphere bacterial communities under different nitrogen fertilization regimes. (C) Relative abundance of dominant bacterial phyla in the rhizosphere of S. matsudana following nitrogen fertilizer application.

Figure 5

Influence of nitrogen fertilization on rhizosphere fungal communities in Salix matsudana grown in heavy metal-contaminated soil. (A) Venn diagram demonstrating the shared and unique fungal operational taxonomic units (ASVs) across four nitrogen fertilizer treatments. (B) Heatmap illustrating the composition of rhizosphere fungal communities under different nitrogen fertilization regimes. (C) Circos plot depicting the structure of fungal communities in nitrogen-amended soils. (D) Relative abundance of dominant fungal taxa in the rhizosphere of S. matsudana under nitrogen fertilization.

Figure 6

Metabolomic analysis of the effects of nitrogen fertilization on the rhizosphere soil of Salix matsudana grown in heavy metal-contaminated environments. (A) Venn diagram depicting the number of differentially accumulated metabolites (DAMs) in the rhizosphere soil of S. matsudana across four nitrogen fertilizer treatments. (B) Correlation analysis of differentially accumulated metabolites in the rhizosphere soil of S. matsudana. (C) Heatmap illustrating the relative abundance of differentially accumulated metabolites in the rhizosphere soil of S. matsudana under four nitrogen fertilizer regimes. (D) KEGG pathway enrichment analysis of differentially accumulated metabolites in the rhizosphere soil of S. matsudana under varying nitrogen fertilizer applications.

Figure 7

The proposed mechanism model of nitrogen fertilizer improves Salix matsudana growth and soil properties.