The Interactive Effects Of Watershed Land Use And Climate Change On Riverine Nitrogen Export In The Jiulong River Watershed Using In-situ Monitoring And INCA-N Model

Riverine dissolved inorganic nitrogen(N)is an important indicator for health of aquatic ecosystems.Most regions worldwide suffered from rapid increase in nitrogen fluxes,leading to the coastal eutrophication and degradation of freshwater ecosystem services.As one hotspot of riverine nitrogen export globally,the rivers in coastal China need thorough understanding of the response mechanism of hydrology and water quality to dual effects of climate and land use changes in order to formulate the sound strategies for watershed management.The Jiulong River Watershed,a medium-sized subtropical coastal watershed that has experienced climate change and intensive human disturbance,was selected as the research area.Associated with in-situ monitoring,statistical analyses,and modeling approaches,this study explored the interactive effects of climate change and land-use change on hydrology and riverine nitrogen export.The main findings are as follows:The relative impacts of climate variability and human activities on the change in mean annual runoff in the Jiulong River Watershed were analyzed using the Mann-Kendall test and hydrological sensitivity analysis during 1961-2013.Based on the results of Mann-Kendall test,the jumping point of runoff changes was in 1974.Results showed that the mean annual runoff during the altered period increased by 3.5%and 11.6%for the North and West Rivers,respectively,compared with that of baseline period.Climate change was identified as the dominant factor controlling the increased annual runoff,accounted for 58 mm(193%)and 90 mm(97%)of the changes in the North and West Rivers,respectively.Human activities played more important role in the reduction of annual runoff,particularly in North River.Coupled effects of climate variability and land use pattern on surface water quality were evaluated using an elasticity approach and watershed health indicators during 2010-2017.Results showed that temperature elasticity and precipitation elasticity of ammonium-N and nitrate-N varied substantially among land use patterns.The N concentrations were highly sensitive to extreme climate conditions,particularly at urban and agricultural sub-watersheds.The measure of risk indicators revealed that the watershed health index varying from good health to unhealthy status.Linear regression analysis was used to analyze the interactions among watershed characteristics,climate elasticity,and watershed health.Cropland and population had strong positive correlations with climate elasticity of NO3-N.Forest and geographic locations had strong negative correlations with climate elasticity of NO3-N.Watershed health significantly declined with increasing proportion of cropland and population density.These findings suggest that watersheds with intensive agricultural activities,densely populated area,and low elevation area would face a higher risk of climate instability and poor watershed health.The wet and dry atmospheric N depositions were evaluated from 2017 to 2018.The atmospheric nitrogen deposition exhibited a distinct spatio-temporal variability.Nitrogen deposition was higher in urban and agricultural watersheds,and lower in forest watersheds.The highest deposition was observed in spring and summer,resulting from higher precipitation intensity.The stable isotope of ?15N-NO3 range from-8.1 to 4.4‰ suggested that transport sector(vehicle exhausts)could be the potential sources of nitrate,meanwhile the higher molar ratio of NH4-N to NO3-N could be an indicator of NH3 emission from agricultural sources(fertilizer and livestock.INCA-N model was developed based on the database regarding hydrology,meteorology,land use/cover,water quality during 2014-2017,thereby identifying the source of N and the key processes of N cycling,and evaluating the relative importance of climate change and land use change with regards to nitrogen export.Results showed that soil nitrification,fertilizer application,and atmospheric N deposition were the main sources of nitrate-N and ammonium-N.Three main N removal processes was riverine N transport,plant uptake,and denitrification.Greater impacts were estimated by climate change compared to land-use change on nitrate-N exports.Land use change introduced greater impact on ammonium-N export.The coupled effects of climate change and land use change are projected to amplify watershed nitrogen export in the Jiulong River Watershed by 30,36,36%for nitrate-N and 32,48,and 71%for ammonium-N during the years of 2030s,2050s,and 2080s,respectively.The findings of this study can enhance our understanding of the eco-environmental effects of global changes in the subtropical coastal watershed and provide reference for watershed management and regional water security.