Nitrogen contents and natural 15N abundance of proteins in plants and soils: Analytical methods and preliminary insights

Proteins are pivotal organic nitrogen (N) species. Producing, accumulating and consuming mechanisms of proteins in plants and soils are critical information for understanding the nature and role of organic N pools in terrestrial N biogeochemistry. N contents and natural 15N abundance (δ15N) of total extractable proteins (EPN) can provide new insights into protein dynamics in plants and soils, whereas analytical methods of these parameters remain unestablished. Here we established a pretreating protocol for EPN in plants and soils and conducted a field investigation at the Mt. Dongling forest of western Beijing. We recommend the alkali dissolution (pH =14) and acid precipitation (pH = 2–5) for extracting and purifying proteins from both plant and soil samples. At the Mt. Dongling forest, leaf and soil EPN averaged 7.19 ± 0.20 and 0.15 ± 0.002 mg N g−1 dw, accounting for 56 ± 2% and 8 ± 0.2% in respective bulk N. The δ15N of EPN in leaves and soils averaged −3.0 ± 0.1‰ and 1.6 ± 0.2‰, differing from the respective bulk δ15N (−4.5 ± 0.1‰ and 1.9 ± 0.2‰). Preliminarily, leaf EPN is largely labile and its 15N enrichment relative to bulk N was attributed to the 15N discrimination during the transamination of amino acids. Differently, EPN is a small fraction in soil organic N pool, their similar δ15N values reflected a marginal N fractionation in organic N decomposition. Our analytical method and preliminary insights into EPN in plant-soil systems opened a new window to further explore protein synthesis and decomposition dynamics in the ‘gray box’ of organic N biogeochemistry under global changes.