Use of 15N2 for direct measurement of free-living N2 fixation
Zhou, Qianchen
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Permalink
https://hdl.handle.net/2142/127367
Description
Title
Use of 15N2 for direct measurement of free-living N2 fixation
Author(s)
Zhou, Qianchen
Issue Date
2024-11-26
Director of Research (if dissertation) or Advisor (if thesis)
Mulvaney, Richard L
Department of Study
Natural Res & Env Sci
Discipline
Natural Res & Env Sciences
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
M.S.
Degree Level
Thesis
Keyword(s)
Biological N2 fixation
free-ling N2 fixation
15N2 incubation
Abstract
The use of 15N-labeled dinitrogen (15N2) affords the only direct means of measuring free-living nitrogen fixation (FLNF); however, progress in utilizing this approach has been impeded by methodological limitations and a lack of standardization regarding soil sampling, storage, and incubation. Such methodological constraints, including the presence of nitrogenous contaminants, a lack of atmospheric uniformity, and incomplete description of procedural details, are eliminated with a technique comprehensively described herein, which involves circulating 15N2 generated by hypobromite oxidation through a closed system that includes chemical (sulfuric acid-potassium permanganate) and cryogenic (isopentane-liquid N2) traps for atmospheric purification and an incubation chamber consisting of a desiccator equipped with a pressure gauge. Studies to evaluate the circulation system described showed that a uniform atmosphere was readily achieved with a 10 L desiccator by pumping for 30 min and that both chemical and cryogenic traps were necessary to ensure complete (98.8–99.6%) removal of gaseous contaminants subject to physicochemical retention by sterilized soil samples. The method proposed was successfully demonstrated in detecting the stimulatory effect of organic carbon (C) on FLNF in active soils and was subsequently utilized in a series of studies to systematically evaluate and standardize soil sampling, storage, and incubation parameters for optimizing isotopic measurement of this process. In these studies, incubations were conducted using four Illinois soils with respect to sampling depth, storage condition and period, surface exposure, moisture content, carbon source and pH, phosphorus (P) amendment, and incubation period. Among the major findings, diazotrophic activity was greatest with surface (0−7.5 cm) sampling, and storage effects were minimized when field-moist samples were kept for ≤ 1 d at room temperature (25°C) or in a refrigerator (5°C) with or without sieving (< 2 mm). In the presence of exogenous C (4 mg C g−1 dry soil), the rate of 15N2 fixation was maximized at ≥ 200% water-holding capacity and with a 3-d incubation period. A divergent preference regarding the form of organic C amendment was observed for soil samples collected before and after a 6-month interval, with simple sugars being preferred in the spring and organic acids in the fall. By standardizing several key parameters pertinent to the measurement of FLNF using the 15N2 method previously developed, the work reported can help clarify the ecological importance and agricultural potential of a process once believed to be a major source of soil N.
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