Identifying seasonal sources and processes controlling nitrate in a typical reservoir-type headwater watershed of Eastern China using stable isotopes

Abstract

Reservoirs serves as a crucial sources of drinking water for approximately 3 billion people globally. Despite their significance, the water quality in these reservoirs faces considerable threats due to intensive anthropogenic activities within the watersheds, particularly in agricultural reservoir-type headwater watersheds. This study investigated nitrate stable isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-), hydrochemistry, and stable water isotopes (δ²H-H₂O and δ¹⁸O-H₂O) in river water to identify nitrate sources and processes in the Tianmu Lake Basin. Results indicated a notable increase in NO₃^-N concentration during low-flow period of March, which subsequently declined during high-flow period of July. Additionally, NO₃^-N concentrations exhibited a decreasing trend from upstream to downstream. Isotopic evidence and MixSIAR model highlight the significant influence of agricultural activities on nitrate sources. The contribution of soil nitrogen (SN) remains a consistent nitrate source (29–33 %) across seasons and watersheds due to nitrogen accumulation. During low-flow period of March, intensive spring cropping operations led to an increased contribution from chemical fertilizer sources (CF; 32 %), elevating NO₃^--N concentrations. Conversely, manure and sewage (SW) emerged as the primary nitrate sources (34 %) due to reduced cropping operations and the flushing of poultry manure during high-flow period of July. Spatial variations showed that tea garden in the upstream regions contributed to a higher CF-derived NO₃^--N presence (33 %), while downstream regions, characterized by aquaculture and poultry farming, showed a significant SW contribution (38 %). Nitrogen cycling processes, influenced by high temperatures and dissolved oxygen (DO), facilitated extensive nitrification as the characteristics of increasing nitrate concentrations in the river. The correlation between NO₃^--N and δ¹⁵N-NO₃^- showed the presence of denitrification with a slower rate, resulting in a low nitrate removal efficiency. This study underscores the necessity to limit nitrogen inputs at headwaters to enhance nitrate removal efficiency throughout the watershed management cycle.