Sources and key processes controlling particulate organic nitrogen in impounded river–reservoir systems on the Maotiao River, southwest China

The δ15N of nitrate (NO3−) and particulate organic nitrogen (PON) was used to study the sources and fate of nitrogen in the impounded Maotiao River, southwest China. During months when the reservoirs thermally stratified, denitrification was the key process in the hypolimnion in Hongfeng and Baihua reservoirs. Based on the δ15N of PON and NO3−, PON in the epilimnion of reservoirs was dominated by NO3− assimilation during stratification. Most δ15N of PON was higher than that of NO3− in the reservoirs in October, indicating that PON in reservoirs was mainly derived from exogenous nitrogen input or denitrification rather than assimilation of NO3− in the epilimnion. Relationships between the molar carbon:nitrogen (C:N) ratio and δ15N-PON indicated the main sources of PON in reservoirs were from phytoplankton, produced in inflowing rivers and the reservoir under the joint influence of soil organic matter, denitrification, and exogenous nitrogen inputs. The δ15N in PON and NO3− increased gradually from upstream to downstream, by 8.21 ± 0.65‰ and 1.74 ± 3.66‰, respectively, suggesting an accumulative effect caused by the downstream waters. Overall, this study provides insight into the nitrogen biogeochemical cycling of river–reservoirs systems using stable nitrogen isotopes and C:N to specify the effects of river impoundment on nitrogen dynamics.