基于氮氧同位素的季节性冻融农业区冻土融化期硝酸盐来源解析与迁移机制OA

In the seasonal freeze-thaw agricultural regions of northern China,the spring freezing-thawing period is a critical stage for the output of nitrogen non-point source pollution.Freezing-thawing processes profoundly affect the migration paths and source-sink relationships of nitrate by altering soil hydrological processes and nitrogen transformation environments,while traditional concentration monitoring methods struggle to accurately analysis the multi-source mixed contributions under complex freeze-thaw conditions.This study focused on the Heidingzi River Basin,a typical seasonal freeze-thaw agricultural region,and conducted a systematic research during the frozen soil thawing period(March-April)in 2014.A total of 17 sampling sites were set up,including main channels,tributaries,rural drainage ditches,and confluence areas with different underlying surfaces.Water samples were collected 4 times,corresponding to the early snowmelt stage,late snowmelt stage,rainfall runoff stage,and late thaw stage,respectively.Combined with nitrogen and oxygen isotope(δ¹⁵N,δ¹⁸O)tracing technology,the sources and migration mechanisms of nitrate were analyzed.The results showed that the sources and migration paths of nitrate during the frozen soil thawing period exhibited significant stage-specific characteristics.During the early snowmelt stage(mid-March),rural domestic sewage and compost in areas near the river were the main contributors.The mean δ¹⁵N value in the main channel was 6.65‰,and the average nitrate concentration was 4.02 mg/L.In the late snowmelt stage(late March),the proportion of soil nitrogen input increased,coupled with denitrification in the river channel.The mean δ¹⁵N value in the main channel rose to 11.77‰,and the average nitrate concentration decreased to 3.81 mg/L.During the rainfall runoff stage(early April),nitrate from deep soil flowed into the river through interflow,leading to a significant increase in δ¹ ⁵ N and δ¹ ⁸ O values in the main channel,reaching 14.97‰ and 12.31‰respectively,with the nitrate concentration dropping to 2.68 mg/L.In the late thaw stage(late April),groundwater runoff became the dominant transport pathway.The mean δ¹⁵N value in the main channel reached 18.66‰,and the concentration decreased to 2.39 mg/L.This study reveals the nitrate migration paths during the freezing-thawing period in seasonal freezing-thawing agricultural regions based on nitrogen and oxygen isotope tracing,providing a key theoretical basis for the precise prevention and control of agricultural non-point source pollution in cold regions and offering a methodological reference for the application of isotope tracing in similar regions.