Speakers - AFHWC2025

Abstract

Eucalypts, widely cultivated for their rapid growth, exhibit high nitrogen (N) consumption which can negatively impact environmental sustainability. Mixed forests are recognized as effective ecosystems for enhancing soil N cycling efficiency, yet the underlying mechanisms governing the synergistic regulation of soil physicochemical properties, microbial communities, and enzymatic processes in Eucalyptus-dominated mixed forests remain poorly understood. We established a pure Eucalyptus forest (PF) and three Eucalyptus-broadleaf mixed forests, Eucalyptus × Michelia macclurei (MF1), Eucalyptus × Alnus formosana (MF2), and Eucalyptus × Erythrophleum fordii (MF3), in suburban Nanning, Guangxi. After seven years of growth, comprehensive soil analyses (0-20 cm and 20-40 cm depths) revealed that mixed forests significantly elevated microbial biomass nitrogen (MBN) compared to the pure stand, indicating intensified microbial immobilization and turnover. Across all experimental treatments, a nitrification-dominated transformation pathway consistently emerged within a semi-closed N cycle. Notably, mixed stands containing N-fixing species (MF2 and MF3) exhibited enhanced total nitrogen (TN), ammonium-N (NH₄⁺-N), and nitrate-N (NO₃--N) concentrations, alongside a higher NO₃⁻-N/NH₄⁺-N ratio and reduced soluble organic nitrogen (SON), which collectively suggest accelerated organic N mineralization and microbial assimilation. Furthermore, MF2 and MF3 exhibited elevated activities of urease (UE), protease (PRO), and β-glucosidase (NAG), coupled with increased microbial diversity and shifts in key N-transforming taxa, such as increased Bradyrhizobium and Nitrospira and reduced Bacillus, Pseudomonas, and Ralstonia. Comprehensive analysis identified microbial diversity (Chao1, Bacillus), PRO, NAG, and available phosphorus (AP) as key predictors of NH₄⁺-N and NO₃--N availability. The PLS-PM analysis further revealed two primary regulatory pathways: NH₄⁺-N directly stimulated nitrification; while AP indirectly promoted NO₃--N accumulation via microbial diversity and enzyme activity,. These findings clarify the interactions driving N transformation in mixed Eucalyptus forests, providing theoretical insights for optimizing plantation management strategies to enhance soil N availability and ecosystem sustainability.