Isotopic evidence for seasonal and long-term nitrogen cycling in a subtropical basin of Southern China.

Knowledge about the origin and transformation of nutrients at different temporal scales in theriver ecosystem could provide a better understanding of nitrogen (N) cycles in the rivers andhave important implications for regional and global N cycling. In this study, a multi-isotopeapproach (δ13C, δ15N and 210Pb) was used to investigate the factors affecting seasonal andlong-term changes in N sources apportionment and N biogeochemical processes in theBeijiang River located in the Pearl River Basin, China. Sources apportionmentby a Bayesian model (Stable Isotopic Analysis in R, SIAR) showed significantseasonal variations. During the flood season, the dominated origins were non-pointsources such as soil N for dissolved nitrogen (DN, accounting for 38%), soil organicmatter for particulate organic matter (POM, 58%) and sedimentary organic matter(SOM, 31%) due to intense precipitation. During the non-flood season, fertilizerdominated in nitrate source (48%) and effluent detritus predominated in the POM (47%)and SOM (32%) pools. N transformation between the DN, POM and SOM wasinfluenced by seasonally variable hydrology. Our data suggested low discharge wasmore favorable for vertical mixing of the water column and sedimentation, whichwas evidenced by similar δ15N values in the DN, POM and SOM in the non-floodseason. A sedimentary history record of 65 years (1951-2015) showed the variation ofnitrogen in sediment was mainly affected by human activities. From 1999 to 2005, adecrease trend in δ15N value was observed due to promotion of aquatic plants growthafter impoundment of reservoir. From 2005 to 2011, the enrichment in δ15N wascaused by an increased in sewage and waster emission due to the development oftourism industry. From 2011 to 2015, the rising of pollution treatment fund bygovernment improved water environment, resulting in δ15N-depleted in sediment. At65-years temporal scale, increasing temperature and CO2 concentration had lessimpact on N cycles in river ecosystem, however, high discharge could increase Ncontents and flood events might increased δ15N values in sediment. The results alsoindicated that δ15N was more effective than δ13C on source distinguish at 65-yearstemporal scale. This study provided more detail information regarding the nutrientssources and improved our understanding of the N cycling processes at differenttemporal scales in the river ecosystem, providing a better understanding for N cyclesin the river ecosystem and a scientific basis for water environment management. [ABSTRACT FROM AUTHOR]