Variation in nitrogen isotopic composition in the Selenga river watershed, Mongolia

The stable nitrogen (N) isotope ratio (δ ¹⁵N) has been used to examine the anthropogenic N input (i. e., septic water, wastewater, and manure) to aquatic ecosystems, because anthropogenic N generally has a δ ¹⁵N signature distinct from that found in nature. Aquatic organisms and the derived organic matter such as sediments are reported to become increasingly enriched in ¹⁵N as the human population density increases in watersheds. However, little is known about the relationship in steppe ecosystems, where the livestock population is greater than that of humans. Here, we conducted a preliminary study in the Selenga river mainstream watershed in Mongolia, which covers an area of approximately 300,000 km ². A multiple regression analysis revealed that the δ ¹⁵N of the riverine sediment was significantly affected by the human population density and more significantly by livestock population density. The population density, including both humans and livestock, significantly influenced δ ¹⁵N of the macrophytic Potamogeton spp. The results showed that δ ¹⁵N of riverine organic matter can be an indicator of the human and livestock population density, which is likely associated with the status of N cycles in livestock-dominated watersheds.