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.