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.
All Science Journal Classification (ASJC) codes
- Environmental Chemistry
- Water Science and Technology
- Earth-Surface Processes