Linking Snow, Soil, and Stream During Snowmelt and Rain-On-Snow Events: Storage and Transport of NO₃^- Ions in an Acid-Sensitive Alpine Catchment (Tatra Mountains, Poland)

The purpose of the study was to examine the storage and transport of NO3- ions through snowpack, soils, and stream water in an acid-sensitive alpine catchment (Tatra Mountains, Poland) during snowmelt and rain-on-snow events. Samples of snowpack layers, near-surface soil horizons, and stream water were collected in the winter and snowmelt seasons of 2019. A laboratory experiment was conducted to determine the effect of temperature on the rate of soil nitrogen mineralization and nitrification. Our study has shown that snowpack is an important source of NO3- ions in the catchment. As the snow melts, the release of NO3- ions from snowpack occurs. A gradual and slow melting of snow starts even before the first snowmelt-induced increase in stream discharge. NO3- ions eluted from available snowpack are temporarily stored in soil, which is shown by a large increase in the concentration of water-soluble NO3- in the soil at thattime. NO3- ions are washed out of soils and supplied to streams during the first snowmelt event. This is demonstrated by a large increase in the streamwater NO3- concentration, termed an " NO3- pulse." The NO3- ion is a key acid anion responsible for the acidification of the studied stream during snowmelt season,as the NO3- pulse coincides with a decrease in bicarbonate alkalinity. Our field research and laboratory experiment have shown a minor role of mineralization and nitrification in NO3- production in soils in the winter and NO3- pulse formation in stream water during the early stages of the snowmelt season.