Nitrogen migration paths and source areas at different snowmelt periods in a seasonal freezing agricultural watershed
2022
Zhao, Qiang | Guo, Chenyao | Zeng, Qiang | Zhao, Hang | Liu, Yawen | Zhang, Jifeng | Huang, Jiesheng | Wu, Jingwei
Heidingzi, a typical seasonal freezing agricultural watershed in northeastern China. This study’s objectives were to: (1) confirm whether nitrogen source areas and migration paths have changed with the melting of snow and frozen soil; (2) find an effective way to recognize the change points and separate nitrogen export processes to different periods during snowmelt; and (3) identify the source areas, migration paths, and critical influencing factors of ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃⁻-N) in river water during these snowmelt periods. The nitrogen export process during snowmelt can be divided into two stages according to the relationship between δ¹⁸O and nitrogen concentration. Nitrogen concentration had a linear relationship with δ¹⁸O during early snowmelt periods, while they varied within a small range during the late stages. Paddy field area proportion (PFP), Rural area proportion (RAP), Average Slope (AS), Slope area proportion < 2° (SAP < 2°), and SAP (6–15°) greatly affected water discharge, nitrogen concentration, and yield. Lower slope areas along the river expended water and nitrogen via soil saturation and depression detention. The NH₄⁺-N and NO₃⁻-N in these areas were flushed into rivers by water from steeper areas (slope > 6°). Abundant nitrogen reserves near the river compensated for the negative impact of water shortage on nitrogen yield during early snowmelt periods. Water and nitrogen in rural areas came mainly from snow-scattered areas during early snowmelt periods, and from frozen domestic sewage or refrozen snowmelt in drainage ditches during late snowmelt periods.
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