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

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 N over their food, and concomitantly, that N becomes increasingly concentrated as one moves up the food chain. The literature commonly states that this N-enrichment of mammalian tissues is due to preferential excretion of light nitrogen (N), 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 highand low-protein diets. The ratios of fecal nitrogen loss to urinary nitrogen loss were 0.30 and 0.88 on the highand low-protein diets respectively. Feces were enriched in N by approximately +3‰ on both diets, whereas urinary nitrogen was depleted in N ( 2.1‰) on the low-protein diet, but not significantly different from intake N on the high-protein diet. Most importantly, there was no statistically significant difference between dietary and total excreta N 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 N may occur. 2003 Elsevier Ltd. All rights reserved.