Inorganic nitrogen immobilization in live and sterile soil of old-growth conifer and hardwood forests: implications for ecosystem nitrogen retention
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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.
KeywordsEcosystem Nitrite Nitrogen immobilization Old-growth forest Retention Soil organic matter
We thank Amanda Conover, Sara Eckert, Kristine Jimenez, and Steven Kinneer for technical help in the laboratory and field, and thank Michael Castellano for technical help and insightful review of the manuscript. Comments from three anonymous reviewers improved the manuscript. Funding was provided by the Andrew W. Mellon Foundation.
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