Tracking riverine nitrate sources under changing land use pattern and hydrologic regime.

It remains challenging to identify nitrate sources in streams due to complications associated with anthropogenic inputs and in-stream biogeochemical processes. We used dual isotopic analysis of nitrate and a Bayesian isotope mixing model to explore the dynamics of nitrate sources and their associated transformations among three types of headwater watershed with different dominant land use types during four seasons in Jiulong River Watershed, a coastal China watershed. Nitrogen sources were the primary determinant of the δ15N-NO3 and seasonal differences in biogeochemical processes exhibited among watersheds. Nitrate was mostly derived from nitrification in spring and summer, whereas atmospheric deposition greatly influenced the isotopic composition in autumn and winter. Chemical fertilizer contributed the largest to the riverine nitrate, accounting for 36.9 ± 12.3%, followed by soil N (27.2 ± 4.4%), atmospheric deposition (23.9 ± 11.8%) and manure & sewage (12.0 ± 5.9%). This study reveals the seasonality of riverine nitrate sources under changing watershed land use patterns.