Understanding gaseous nitrogen removal through direct measurement of dissolved N2 and N2O in a subtropical river-reservoir system

Dam construction within a river basin modifies hydrology and affects nitrogen (N) removal during transport to coast. In this study, direct measurements of dissolved N-2 and N2O were carried out in a subtropical river reservoir (Xipi) in southeast China to understand the effectiveness of reservoirs in removal of fixed N by N gas fluxes. Results showed that larger excess N-2 and N2O emissions were found in the riverine zone where effluents from the upper dam mix with the shallow river. Excess N-2, mainly derived from denitrification, occurred in the sediment below deep water, while N2O was largely produced from nitrification in the water column, particularly in dry season. Seasonal variation of excess N-2 was associated with temperature and DO level, while N2O production was controlled by DIN concentration. The gaseous N dynamics and distribution in the studied reservoir reflected an interactive effect of hydrology, geomorphology and biogeochemistry. In the reservoir lacustrine zone, gaseous N removal accounted for 85% of total retention and less than 1% of DIN loads. The negligible retention of N by the reservoir highlights the importance of appropriate watershed management practices to reduce N losses from terrestrial systems. (C) 2014 Elsevier B.V. All rights reserved.