Land use and climate variability amplifies watershed nitrogen exports in coastal China

Understanding how land use and climate variability interact could amplify riverine nitrogen exports is needed to develop effective watershed management strategies in coastal China, a region that is experiencing the most rapid urbanization globally. We investigated the interactive effects of land use and climate variability on riverine nitrogen exports. We analyzed long-term historical data (1961–2014) in two river reaches of the Jiulong River Watershed and spatial data (2010–2014) across a land use gradient spanning 17 monitored headwater streams. Results showed that annual riverine nitrate-N (NO3−-N) and ammonia-N (NH4+-N) exports have significantly increased from 1961 to 2014 coinciding with urban expansion. Nitrate-N exports were correlated with river flow during 1961–1980, while they were not correlated with river flow during 1981–2002 when fertilizer use rapidly increased in this region. Nitrate-N export was tightly correlated with river flow during 2010–2014 as fertilizer use decreased and there was conversion of agricultural lands to urban lands. Annual NO3−-N and NH4+-N exports for 17 headwater streams across land use showed significant inter-annual variability (2–10 times) during 2010–2014 across dry and wet years. There was greater inter-annual variability in N exports at the urban/agricultural sites than at the forest sites. Urban watersheds had the highest runoff and nitrate-N export during the flood season. Runoff was the “master variable” controlling nitrate export in urban watersheds across time and space. Given rapid urbanization, the interaction between climate variability and land use change can amplify river nitrogen loads and coastal water quality issues in the developing world unless more effective management of the watershed and associated riverine system is applied.