Box-modeling of 15N/14N in mammals

Abstract

The 15N/14N signature of animal proteins is now commonly used to understand their physiology and quantify the flows of nutrient in trophic webs. These studies assume that animals are predictably 15N-enriched relative to their food, but the isotopic mechanism which accounts for this enrichment remains unknown. We developed a box model of the nitrogen isotope cycle in mammals in order to predict the 15N/14N ratios of body reservoirs as a function of time, N intake and body mass. Results of modeling show that a combination of kinetic isotope fractionation during the N transfer between amines and equilibrium fractionation related to the reversible conversion of N-amine into ammonia is required to account for the well-established approximately 4 per thousand 15N-enrichment of body proteins relative to the diet. This isotopic enrichment observed in proteins is due to the partial recycling of 15N-enriched urea and the urinary excretion of a fraction of the strongly 15N-depleted ammonia reservoir. For a given body mass and diet delta15N, the isotopic compositions are mainly controlled by the N intake. Increase of the urea turnover combined with a decrease of the N intake lead to calculate a delta15N increase of the proteins, in agreement with the observed increase of collagen delta15N of herbivorous animals with aridity. We further show that the low delta15N collagen values of cave bears cannot be attributed to the dormancy periods as it is commonly thought, but inversely to the hyperphagia behavior. This model highlights the need for experimental investigations performed with large mammals in order to improve our understanding of natural variations of delta15N collagen.

Nutritional stress and body condition in the Great Gray Owl (Strix nebulosa) during winter irruptive migrations

“…This finding runs counter to the expectation of nitrogen enrichment in emaciated or starved individuals documented in several studies (Hobson et al 1993(Hobson et al , 1997Cherel et al 2005;Fuller et al 2005;Mekota et al 2006). To detect a shift in d 15 N, however, new muscle or liver proteins would need to be synthesized from a pool of amino acids with an enriched 15 N content derived from the catabolization of endogenous protein during phase III (Balter et al 2006).…”

The effect of dietary water soluble carbohydrate to nitrogen ratio on nitrogen partitioning and isotopic fractionation of lactating goats offered a high-nitrogen diet

“…However, there was little effect of dietary treatments on microbial protein synthesis, as indicated by both PD and MCP/FME calculations in this study. Third, feed N supplied in this study across treatments was similar, which would lead to limited changes in N isotopic fractionation (Balter et al, 2006). Period and infusion effect Lower MUN was observed in period 2 compared with period 1 (P <0.001), which is believed to be associated with a reduced urea intake in period 2 (Table 1).…”

Use of carbon and nitrogen stable isotopes to study the feeding ecology of small coastal cetacean populations in southern Brazil

“…The main reason for this trophic fractionation seems to be due to internal N metabolism, leading to 15 N enrichment in body proteins coupled with excretion of 15 N-depleted ammonia via urine (Balter et al 2006, Martínez del Rio et al 2009). There have been several recent revisions on differences between δ 15 N values of diet and consumers (Post 2002, McCutchan et al 2003, Vanderklift & Ponsard 2003, Vander Zanden & Fetzer 2007, Martínez del Rio et al 2009).…”