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Title: Amino acid δ 15N indicates lack of N isotope fractionation during soil organic nitrogen decomposition

Abstract

The interpretation of natural abundance δ 15N in soil profiles and across ecosystems is confounded by a lack of understanding of possible N isotope fractionation associated with soil organic nitrogen (SON) decomposition. We analyzed the δ 15N of hydrolysable amino acids to test the extent of fractionation associated with the depolymerization of peptides to amino acids and the mineralization of amino acids to NH 4 + (ammonification). Most amino acids are both synthesized and degraded by microbes, complicating interpretation of their δ 15N. However, the “source” amino acids phenylalanine and hydroxyproline are degraded and recycled but not resynthesized. We therefore used their δ 15N to isolate the effects of N isotope fractionation during SON depolymerization and ammonification. We used complementary field and laboratory approaches to evaluate the change in amino acid δ 15N during decomposition. First, we measured amino acid δ 15N changes with depth in the organic horizons of podzolic soils collected from the Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect (NL-BELT), Canada. The δ 15N of most amino acids increased with depth by 3–7‰, similar to the increase in bulk δ 15N. However, the δ 15N of the “source” amino acids did not change with depth, indicating lackmore » of N isotope fractionation during their depolymerization and ammonification. Second, we assessed the change in amino acid δ 15N following 400 days of laboratory incubation. This approach isolated the effect of decomposition on δ 15N by eliminating plant N uptake and reducing leaching of N from the soil. Amino acid δ 15N did not change during incubation despite extensive turnover of the amino acid pool, supporting our conclusion of a lack of N isotope fractionation during SON decomposition. Finally, our results indicate the often-observed trend of increasing δ 15N with soil depth likely results from the mycorrhizally-mediated transfer of 14N from depth to the surface and accumulation of 15N-enriched necromass of diverse soil microbes at depth, rather than as a direct result of SON decomposition.« less

Authors:
ORCiD logo [1];  [2];  [3];  [4];  [5];  [5]
  1. Memorial Univ., St. John's, NL (Canada); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  2. Univ. of Kansas, Lawrence, KS (United States)
  3. Canadian Forest Service, Corner Brook, NL (Canada)
  4. Memorial Univ., St. John's, NL (Canada); United States Dept. of Agriculture, Columbus, IN (United States)
  5. Memorial Univ., St. John's, NL (Canada)
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1474866
Grant/Contract Number:  
AC05-00OR22725
Resource Type:
Accepted Manuscript
Journal Name:
Biogeochemistry
Additional Journal Information:
Journal Volume: 138; Journal Issue: 1; Journal ID: ISSN 0168-2563
Publisher:
Springer
Country of Publication:
United States
Language:
English
Subject:
59 BASIC BIOLOGICAL SCIENCES; Soil organic nitrogen; Ammonification; δ15N; Amino acid stable isotopes; Nitrogen isotope fractionation

Citation Formats

Philben, Michael J., Billings, Sharon A., Edwards, Kate A., Podrebarac, Frances A., van Biesen, Geert, and Ziegler, Susan E. Amino acid δ15N indicates lack of N isotope fractionation during soil organic nitrogen decomposition. United States: N. p., 2018. Web. doi:10.1007/s10533-018-0429-y.
Philben, Michael J., Billings, Sharon A., Edwards, Kate A., Podrebarac, Frances A., van Biesen, Geert, & Ziegler, Susan E. Amino acid δ15N indicates lack of N isotope fractionation during soil organic nitrogen decomposition. United States. doi:10.1007/s10533-018-0429-y.
Philben, Michael J., Billings, Sharon A., Edwards, Kate A., Podrebarac, Frances A., van Biesen, Geert, and Ziegler, Susan E. Fri . "Amino acid δ15N indicates lack of N isotope fractionation during soil organic nitrogen decomposition". United States. doi:10.1007/s10533-018-0429-y. https://www.osti.gov/servlets/purl/1474866.
@article{osti_1474866,
title = {Amino acid δ15N indicates lack of N isotope fractionation during soil organic nitrogen decomposition},
author = {Philben, Michael J. and Billings, Sharon A. and Edwards, Kate A. and Podrebarac, Frances A. and van Biesen, Geert and Ziegler, Susan E.},
abstractNote = {The interpretation of natural abundance δ15N in soil profiles and across ecosystems is confounded by a lack of understanding of possible N isotope fractionation associated with soil organic nitrogen (SON) decomposition. We analyzed the δ15N of hydrolysable amino acids to test the extent of fractionation associated with the depolymerization of peptides to amino acids and the mineralization of amino acids to NH4+ (ammonification). Most amino acids are both synthesized and degraded by microbes, complicating interpretation of their δ15N. However, the “source” amino acids phenylalanine and hydroxyproline are degraded and recycled but not resynthesized. We therefore used their δ15N to isolate the effects of N isotope fractionation during SON depolymerization and ammonification. We used complementary field and laboratory approaches to evaluate the change in amino acid δ15N during decomposition. First, we measured amino acid δ15N changes with depth in the organic horizons of podzolic soils collected from the Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect (NL-BELT), Canada. The δ15N of most amino acids increased with depth by 3–7‰, similar to the increase in bulk δ15N. However, the δ15N of the “source” amino acids did not change with depth, indicating lack of N isotope fractionation during their depolymerization and ammonification. Second, we assessed the change in amino acid δ15N following 400 days of laboratory incubation. This approach isolated the effect of decomposition on δ15N by eliminating plant N uptake and reducing leaching of N from the soil. Amino acid δ15N did not change during incubation despite extensive turnover of the amino acid pool, supporting our conclusion of a lack of N isotope fractionation during SON decomposition. Finally, our results indicate the often-observed trend of increasing δ15N with soil depth likely results from the mycorrhizally-mediated transfer of 14N from depth to the surface and accumulation of 15N-enriched necromass of diverse soil microbes at depth, rather than as a direct result of SON decomposition.},
doi = {10.1007/s10533-018-0429-y},
journal = {Biogeochemistry},
number = 1,
volume = 138,
place = {United States},
year = {2018},
month = {2}
}

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