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

Abstract

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.