Amino acid δ15N indicates lack of N isotope fractionation during soil organic nitrogen decomposition
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. 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.
KeywordsSoil organic nitrogen Ammonification δ15N Amino acid stable isotopes Nitrogen isotope fractionation
We thank Jerome Laganière, Thalia Soucy-Giguere, and Julia Ferguson for assistance with soil collection, preparation, and incubations. We also thank Jamie Warren, Catie Young, Andrea Skinner, Darrell Harris, Rachelle Dove, Alison Pye, and Amanda Baker for field and laboratory assistance. Helpful comments from two anonymous reviewers improved the manuscript. This research was funded by the National Science and Engineering Councils of Canada Discovery Grants program and Strategic Project Grants program (#397494-10), Centre for Forest Science and Innovation of the Newfoundland and Labrador Agrifoods Agency, Canada Research Chairs Programme, and the Canadian Forest Service of Natural Resources Canada.
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