Temporal soil nitrogen dynamics with stand age of Betula platyphylla forests in the permafrost region of Northeast China
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Nitrogen (N) is a key nutrient regulating plant growth, forest productivity and ecosystem functioning in permafrost regions. Despite the critical role of succession in modifying soil N processes, how soil N dynamics respond to mid-successional Betula platyphylla forest development remains poorly understood. This study aimed to characterize soil N transformations across a 25- to 61-year stand-age gradient in permafrost forests of Northeast China. We determined soil inorganic nitrogen concentrations and net nitrogen mineralization (ammonization + nitrification) using in‑situ incubation experiments in 25-, 40- and 61-year-old mid-successional Betula platyphylla forests. Ammonium N increased significantly with stand age, whereas nitrate N concentration exhibited a decreasing trend. Generalized least‑squares (GLS‑AR1) modelling detected no significant main effect of stand age on soil inorganic N or net N mineralization rate (Rm). Mean Min-N remained stable across stands (8.39, 8.15 and 8.49 mg·kg⁻¹ for the 25-, 40-, and 61-year-old stands, respectively). Rm showed a numerical increase along the age gradient (0.11, 0.46, 0.67 mg·kg⁻¹·d⁻¹). Soil N dynamics were driven by significant stand‑age × soil‑depth and stand‑age × sample‑month interactions rather than stand age main effects. Inorganic‑N and mineralization rates were higher in topsoil (0–10 cm) than subsoil (10–20 cm). Ammonification dominated net N mineralization across all stands, contributing on average 3.88% (0–10 cm) and 4.18% (10–20 cm). Hierarchical partitioning analysis identified stand age as the primary driver of soil‑N variation, followed by sampling month, soil chemical properties and microbial biomass. Mid‑successional Betula platyphylla forest succession accelerates soil N turnover and modifies inorganic‑N composition without expanding total mineral‑N pools. These findings inform vegetation restoration and sustainable management of permafrost forest ecosystems.
