Processcs And Fluxes Of Nutrients And Gaseous Nitrogen Across The Interfaces Of The Jiulong River-Reservoir System

River is an important channel for nutrients export from watershed to the coast. The cascade dam developments for hydroelectricity generation have substantially changed the river hydrology and biogeochemistry of nutrient. This study targeted a typical subtropical river-the North Jiulong River. Xipi hydroelectric Reservoir on the middle reaches selected as the representative study site. A comprehensive observation and incubation were carried out to measure nutrients and gaseous nitrogen (both N2and N2O) across sediment-water-air interface. Furthermore, the study extended to the Mayang canal-pond-stream continuum, a source waterways to measured nutrient and gaseous N, allowing for a simple comparison with reservoir result. Based on the flied survey and incubation, the key biogeochemical processes and flux of nutrients and gaseous N in the river-reservoir system were examined and gaseous N removal by the river-reservoir was evaluated. The main findings and conclusions are summarized as following.First, nutrient concentrations and composition along the North Jiulong River were regulated by watershed nutrient loads, discharge and dam construction. The spatial pattern of nutrients in surface water was characterized as "higher in upstream and lower in downstream". DIN concentration ranged from133to1116μmol·L"(averaged331±242μmol·L-1), there was an obvious retention occurred in the reach where cascade hydropower stations installed. In general, NO3-N dominated DIN in the river (63±20%of DIN). NH4+-N became the main form of DIN (67%of DIN) during dry season in the upper reach where received a large magnitude of human and animal wastes around Longyan city. Under low flow condition (dry season), the water residence time was long and DIN concentration was enriched compared with high flow condition (DIN concentrations were ordered as February>March>June). Meanwhile, DIN retention was larger in dry season than in wet season. Moreover, a high sediment TOC/TN content and C:N ratio were found in the hydroelectric dam impacted reach, which facilitate sediment denitrification. TN and TP content in sediment were positively correlated with nutrients concentrations (i.e., DIN, DRP) of surface water, indicating nutrient pollution from watershed have impacted river.Second, N removal did occur along the river-reservoir system. The spatial and vertical distribution of gaseous N in water was controlled by the interactive effect of hydrology, geomorphology and biogeochemistry. Larger excess N2and N2O (AN2and△N2O) and emissions were found in the riverine zone where the cold effluents from upper dam were mixed with the shallow river. The lower△N2and△N2O observed in the lacustrine zone compared with the riverine zone was due to the gradually escape of excess N2and N2O to the atmosphere (gaseous N removal through emission) during fluvial transport downstream, and the dilution of tributary water with low gaseous N. A vertical variation of△N2in the water column was found in the lacustrine zone. Combined with lab and in-situ incubation results, we infered that excess N2mainly derived from denitrification that occurred in the sediment-water interface. In combaining with the messages form physicochemical characteristics and abundances of N cycling-associated bacteria in sediment samples, we concluded that an intensive denitrification typically occur in deep water (e.g., lower transitional zone and lower lacustrine zone). Different from N2,△N2O distributed evenly in the water profile, indicating N2O was mainly origin from nitrification in water column. Spatial variation of△N2was associated with temperature and DO level, while△N2O was mostly influenced by DIN concentrations. In general, NH4+-N and DRP in sediment of the lacustrine zone were released, while NO3-N and NO2-N were adsorbed to sediment. Nevertheless, nutrients fluxes across sediment-water interface varied with season. This suggested that the sediment play an important role in the N cycling in reservoir.Third, gaseous N removal only accounts for less than1%of N load, however, it contributs85%of N retention in the lacustrine zone of Xipi reservoir. Comparing with other systems in the world, DIN load in Jiulong River was fairy high, result in a low effectiveness of gaseous N removal. Despite N2O yield only accounts for1%of total (△N2+△N2O) and FN2O was55.1μmol·d)-1. Dissolved N2O was saturated as463±234%, suggesting that the reservoir was an N2O source to atmosphere.