Temporal soil nitrogen dynamics with stand age of Betula platyphylla forests in the permafrost region, Northeast China

Purpose Nitrogen (N) is a primary limiting nutrient for plant growth, and its turnover plays a vital role in forest productivity and ecosystem functioning in permafrost regions. Methods We determined soil inorganic N (ammonium + nitrate) concentration and net N mineralization (ammonization + nitrification) in 25-, 40- and 61-year-old Betula platyphylla forests in the permafrost region of Northeast China. Field experiments were conducted from June to October 2018 using an in-situ incubation method. Results The ammonium nitrogen concentration increased significantly with stand age, whereas nitrate nitrogen concentration exhibited a decreasing trend. Soil inorganic N concentration showed no significant differences among the three stand ages, with mean values of 8.39, 8.15 and 8.49 mg·kg⁻¹ for the 25-, 40-, and 61-year-old stands, respectively. In comparison, net N mineralization rate increased significantly with stand age, with values of 0.11, 0.46, and 0.67 mg·kg⁻¹·d⁻¹ in the three age classes. Generally, soil inorganic N concentration and mineralization rate were significantly higher in the 0–10 cm soil layer than in the 10–20 cm soil layer, except for the mineralization rate in the 25-year-old stand. Ammonification dominated N mineralization across all stands, contributing 83.88% and 64.18% on average in the 0–10 cm and 10–20 cm layers, respectively. Soil temperature, total N and microbial biomass N were the key drivers governing the variations in soil N dynamic. Conclusion Our findings clarify the characteristics of soil N dynamics along stand development of boreal Betula platyphylla forests and provide practical implications for sustainable forest management in permafrost regions.