Responses Of Riverine Nitrogen Exports And Eukaryotic Community Structure To Land Use Pattern And Hydrological Regime In The Jiulong River Watershed

Of particular concern is the increasing degradation and shortage of regional and global water resources caused by extensive land use and the combined impacts of climate change.Watershed land use pattern combined with hydrological regime could affect water quality,biodiversity,primary productivity,even key ecosystem service.These effects resulted from changes such as the nitrogen sources and transformation pathways.Therefore,a better understanding about how land use and hydrological regime affect the riverine exports,N transformation process and eukaryotic community structure is useful for developing sustainable watershed management strategies.In-situ monitoring,dual isotopic analysis of nitrate,18s rDNA high-sequencing,watershed model,geographic information technology,remote-sensing technology and statistical analysis are used in this study to explore the coupled effects of watershed land use pattern and hydrological regime on riverine nitrogen exports and eukaryotic community structure at different spatial and temporal scales in the Jiulong River Watershed,a typical medium-sized subtropical coastal watershed.The major findings are as follows:The responses of riverine nitrogen exports to watershed land use pattern and hydrological regime were clarified.Nitrate nitrogen was major form of dissolved inorganic nitrogen in the Jiulong River Watershed.Agricultural and urban watersheds had higher nitrogen exports and greater temporal variability than those in forest watershed.The seasonal fluctuations of most nitrogen concentrations and exports were more obvious in wet year.When compared to agricultural watershed,annual runoff had less effects on annual nitrogen exports in urban and forest watersheds.Land use had significant effects on nitrogen concentrations and exports,especially in wet year.When compared to land use types,runoff had limited effects on nitrogen concentrations.The results indicated that the spatiotemporal pattern of nitrogen exports were affected by land use pattern,as well as the characteristics of hydrological regime.Nitrate sources and N transformation processes in three types of watershed with different dominant land use types during four seasons were identified.The ?15N values of nitrate varied small but significantly between agricultural,urban and forest watersheds,indicating that difference in nitrogen sources was the primary determinant of the ?15N-NO3 variation.Significant seasonal variation of ?18O-NO3 reflected the seasonal differences in biogeochemical processes among three types of watershed.Nitrate was mostly derived from nitrification in spring and summer,whereas atmospheric deposition greatly influenced the isotopic composition in autumn and winter.Nitrogen transformation showed some differences among three types of watershed,especially in spring and summer.Fertilizer contributed greatly to the riverine nitrate,followed by soil N,atmospheric deposition and manure&sewage.The relative contributions of different source varied among different types of watershed and seasons.The responses of riverine eukaryotic community structure to watershed land use pattern and hydrological regime were investigated.Agricultural and urban watersheds had significantly higher diversity in dry and transition seasons,and higher richness in transition and wet seasons than those in forest watershed.The non-metric multidimensional scaling analysis further verified great spatiotemporal variations of eukaryotic community.Stramenopiles,Alveolata,Animalia,and Eukaryota,dominated the sequences reads for all sampling sites in three seasons.Agricultural watershed had highest relative abundant of Animalia,whereas Eukaryota was most abundant in urban watershed and forest watershed had highest relative abundance of Stramenopiles and Alveolata.The RDA and VPA analyses revealed that land use percentages had greater influences on eukaryotic community,especially alga,than those of hydrological regime.Compared to land use,nutrient(e.g.N,P)concentrations had higher proportional contributions.This study can advance the understanding on regional consequence of land use and climate changes in medium-sized subtropical coastal watersheds,which can be useful for developing strategies for sustainable watershed management.