Identifying nitrogen sources in intensive livestock farming watershed with swine excreta treatment facility using dual ammonium (δN) and nitrate (δN) nitrogen isotope ratios axes.

We proposed a novel approach based on dual ammonium and nitrate nitrogen isotope ratios (δN and δN, respectively) axes to identify nitrogen sources in intensive livestock farming watersheds, especially those with swine excreta treatment facilities. The δN and δN values in water samples were measured monthly in 2016-2017. Soil and mineral fertilizers, sewage, sewage effluent, manure, and swine effluents were the five sources considered to identify nitrogen sources. The results showed that nitrogen pollution from agricultural activities was well reflected by the seasonal δN and δN patterns in the river, and microbial nitrification was suggested as the dominant nitrogen transformation process in the river. This study revealed that δN and δN axes provided better results than the traditionally used nitrate oxygen (δO) and δN axes for identifying nitrogen sources in agricultural watersheds with swine excreta treatment facilities. The mixing model results showed that stream water was severely contaminated with swine effluents (e.g., a mean minimum contribution of 31%), thus affecting the quality of the mainstream (p = 0.068 < 0.10). This study was the first successful application of dual δN and δN axes to better understand nitrogen sources in intensive livestock farming watersheds with swine excreta treatment facilities.