Inorganic nitrogen immobilization in live and sterile soil of old-growth conifer and hardwood forests: Implications for ecosystem nitrogen retention

Rapid immobilization of inorganic nitrogen (N) in soil contributes to ecosystem N accumulation, even in old-growth and chronically-fertilized forests once thought to have poor N retention capacity. In old-growth conifer and hardwood stands in Pennsylvania, we tested the hypotheses that biotic and abiotic N immobilization are regulated by N form and forest type. We added ¹⁵NH₄⁺, ¹⁵NO₂^-, and ¹⁵NO₃^- to sterile (γ-irradiated) and live organic-horizon soil and define N immobilization as the mass of added ¹⁵N remaining in soil following extractions conducted 15 min, 24 h, and 21 days later. Immobilization of NO₂^- (19-25% of added N) occurred in sterile soils within 15 min and was little changed thereafter. Tracer NO₃^- immobilization was not observed, although soils had been pretreated (refrigerated) so as to quantify the lower limit of immobilization potential. Immobilization of NH₄⁺ (27%) occurred in live conifer soils by 21 days but not in other treatments. In 21-day incubations, tracer N immobilization was greater in NO₃^--poor and humic-rich soils. Immobilization was greater in sterile than in live soil, perhaps owing to artifacts of sterilization. Conifer stands exhibited more massive O-horizons, so NO₂^- immobilization per unit area was greater in conifer (1. 46 mg N m^-2) than hardwood (0. 43 mg N m^-2) stands, possibly accounting for lower N leaching from conifer forests. Areal immobilization rates appear to be fast enough to retain all N transformed to NO₂^-, so NO₂^- production may be a limiting step in soil N retention in old-growth ecosystems.