An experimental study of nitrogen flux in llamas: is 14N preferentially excreted?

Abstract

Nitrogen isotope analysis is now commonly used to investigate the diets, and to a lesser extent, the environments of ancient populations. These studies assume that mammals are predictably enriched in 15N over their food, and concomitantly, that 15N becomes increasingly concentrated as one moves up the food chain. The literature commonly states that this 15N-enrichment of mammalian tissues is due to preferential excretion of light nitrogen ( 14N), but there are few data to support this assertion. To address the gap, we conducted two nitrogen flux trials in which four llamas ( Lama glama) were fed high- and low-protein diets. The ratios of fecal nitrogen loss to urinary nitrogen loss were 0.30 and 0.88 on the high- and low-protein diets respectively. Feces were enriched in 15N by approximately +3‰ on both diets, whereas urinary nitrogen was depleted in 15N (−2.1‰) on the low-protein diet, but not significantly different from intake δ 15N on the high-protein diet. Most importantly, there was no statistically significant difference between dietary and total excreta δ 15N on either diet. Given these data and theoretical considerations, we argue that the nitrogen influx and efflux of adult mammals at steady state should be isotopically commensurate. However, during growth, diet change, thermal or nutritional stress, animals may not be at steady state and fractionation between intake and excreta δ 15N may occur.