Isotopic evidence for seasonal and long-term C and N cycling in a subtropical basin of southern China

Knowledge about the origin and transformation of nutrients at different temporal scales in river ecosystems can provide a better understanding of carbon (C) and nitrogen (N) cycles in rivers and bear important implications for regional and global C and N cycling. In this study, stable isotopes of C and N (delta C-13 and delta N-15) and a radioactive isotope of Pb-210 were used to investigate the factors affecting seasonal and long-term changes in N source apportionment and C and N biogeochemical processes in the Beijiang River located in the Pearl River basin, China. Source apportionment by a Bayesian model (stable isotopic analysis in R, SIAR) showed significant seasonal variations. During the flood season, the dominant origins were non-point sources, such as soil N for dissolved nitrogen (DN, accounting for 38%), soil organic matter for particulate organic matter (POM, 58%) and sedimentary organic matter (SOM, 31%), due to intense precipitation. During the non-flood season, fertilizer became the dominant nitrate source (48%), and effluent detritus prevailed in the POM (47%) and SOM (32%) pools. N transformation between the DN, POM and SOM was influenced by seasonally variable hydrology. Our data suggested that low discharge was more favorable for the vertical mixing of the water column and sedimentation, which was supported by similar delta N-15 values in the DN, POM and SOM in the non-flood season. A sedimentary history record of 65 years (1951-2015) showed that the variation of N in the sediment was mainly affected by human activities. From 1999 to 2005, a decreasing trend in the delta N-15 values was observed due to the promotion of aquatic plant cultivation after reservoir impoundment. From 2005 to 2011, the enrichment in the delta N-15 values was caused by increases in manure and sewage due to the development of the tourism industry. From 2011 to 2015, the increase in pollution treatment funds from the government improved the water environment, resulting in delta N-15 depletion in the sediment. At the 65-year temporal scale, increased temperature and CO2 concentration had less impact on the C and N cycles in river ecosystems, however, high discharge could increase the C and N contents, and flood events might increase the delta N-15 values in the sediment. This study provided more detailed information regarding the nutrient sources and improved our understanding of the C and N cycling processes at different temporal scales in river ecosystems.