Canadian Forest Service Publications
Amino acid δ 15 N indicates lack of N isotope fractionation during soil organic nitrogen decomposition. Philben, M., Billings, S. A., Edwards, K., Podrebarac, F. A., van Biesen, G., & Ziegler, S. E. (2018). Biogeochemistry, 138(1), 69-83.
Issued by: Atlantic Forestry Centre
Catalog ID: 40404
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"The interpretation of natural abundance d15N 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 d15N of hydrolysable amino acids to test the extent of fractionation associated with the depolymerisation 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 d15N. However, the ‘‘source’’ amino acids phenylalanine and hydroxyproline are degraded and recycled but not resynthesized. We therefore used their d15N to isolate the effects of N isotope fractionation during SON depolymerisation and ammonification. We used complementary field and laboratory approaches to evaluate the change in amino acid d15N during decomposition. First, we measured amino acid d15N changes with depth in the organic horizons of podzolic soils collected from the Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect (NL-BELT), Canada. The d15N of most amino acids increased with depth by 3–7%, similar to the increase in bulk d15N. However, the d15N 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 d15N following 400 days of laboratory incubation. This approach isolated the effect of decomposition on d15N by eliminating plant N uptake and reducing leaching of N from the soil. Amino acid d15N 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 d15N 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."
Plain Language Summary
The natural abundance of 15N (a kind of nitrogen) is used as a tool to understand a number of processes that occur in soils, however there are uncertainties about the way in which 15N is affected by various decomposition processes and this limits the utility of this tool for understanding how ecosystems work. In particular, δ15N is often observed to be greater in deeper soils, but it is unknown whether this because deeper soils have experienced more decomposition or for other reasons. We used complementary field and laboratory approaches to evaluate the change in amino acid δ15N during decomposition, selecting appropriate amino acids for understanding changes in δ15N abundance that occur during the breakdown of soil organic nitrogen (SON). We studied soils from the Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect (NL-BELT) including soils at different depths, and soils from a 400-day laboratory experiment. Our results indicate that δ15N does not change during decomposition, suggesting that variation in δ15N observed in field settings is the result of transportation between soils at different depths and not differences in decomposition. Understanding this will help scientists to use the natural abundance of δ15N more effectively in ecological research.