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

• Published in: Biogeochemistry
• Main Authors: Philben, Michael J., Billings, Sharon A., Edwards, Kate A., Podrebarac, Frances A., van Biesen, Geert, Ziegler, Susan E.
• Language: unknown
• Published: 2021
• Subjects: 59 BASIC BIOLOGICAL SCIENCES; Canada; Newfoundland
• Online Access: http://www.osti.gov/servlets/purl/1474866; https://www.osti.gov/biblio/1474866; https://doi.org/10.1007/s10533-018-0429-y

The interpretation of natural abundance δ 15 N 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 15 N 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 15 N. However, the “source” amino acids phenylalanine and hydroxyproline are degraded and recycled but not resynthesized. We therefore used their δ 15 N 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 15 N during decomposition. First, we measured amino acid δ 15 N 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 15 N of most amino acids increased with depth by 3–7‰, similar to the increase in bulk δ 15 N. However, the δ 15 N 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 δ 15 N following 400 days of laboratory incubation. This approach isolated the effect of decomposition on δ 15 N by eliminating plant N uptake and reducing leaching of N from the soil. Amino acid δ 15 N 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 15 N with soil depth likely results from the mycorrhizally-mediated transfer of 14 N from depth to the surface and accumulation of 15 N-enriched necromass of diverse soil ...