Watershed nitrogen export model related to changing nitrogen balance and hydrology in the Changjiang River basin

Rivers play a key role in linking between terrestrial and aquatic ecosystems, and are becoming important sinks of nitrogen (N). However, quantifying N export from terrestrial ecosystems to rivers at large scales is still challenging, due to the heterogeneous characteristics of watersheds in hydrology, land use, geology, climatology, etc. N export from terrestrial ecosystems to rivers is basically controlled by N balances and driven by hydrological processes. Here a model of watershed export coefficient (E (ws) ) of dissolved inorganic nitrogen (DIN) is proposed to describe the dynamic processes of DIN export to the river from non-point sources (NPS), with emphasis on the changing N balance and hydrology in the Changjiang River basin during the period 1970-2003. We found a significant relationship between E (ws) and the relative N surplus degree, which can predict the variation of E (ws) in the basin. Our study shows that E (ws) increased from 0.11 to 0.61 across the whole basin during the period, indicating E (ws) was dynamic rather than static through time. The amounts of NPS-DIN export to the river (W (NPS) ) increased from 0.22 x 10(3) to 4.54 x 10(3) kg km(-2) year(-1) in response to the increasing watershed N surplus during 1970-2003. The quick increase of W (NPS) and E (ws) demonstrate the diminishing capacity of terrestrial ecosystems to retain N as N surplus increased as a direct result of human activities. Our research helps researchers and policy makers to understand the mechanism of river N level in response to watershed N balance and hydrology processes.