Linking snow, soil, and stream during snowmelt and rain-on-snow events : storage and transport of $NO_{3}^{-}$ ions in an acid-sensitive alpine catchment (Tatra Mountains, Poland)

The purpose of the study was to examine the storage and transport of $NO_{3}^{-}$ 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 $NO_{3}^{-}$ ions in the catchment. As the snow melts, the release of $NO_{3}^{-}$ ions from snowpack occurs. A gradual and slow melting of snow starts even before the first snowmelt-induced increase in stream discharge. $NO_{3}^{-}$ ions eluted from available snowpack are temporarily stored in soil, which is shown by a large increase in the concentration of water-soluble $NO_{3}^{-}$ in the soil at that time. $NO_{3}^{-}$ ions are washed out of soils and supplied to streams during the first snowmelt event. This is demonstrated by a large increase in the stream water $NO_{3}^{-}$ concentration, termed an "$NO_{3}^{-}$ pulse." The $NO_{3}^{-}$ ion is a key acid anion responsible for the acidification of the studied stream during snowmelt season, as the $NO_{3}^{-}$ pulse coincides with a decrease in bicarbonate alkalinity. Our field research and laboratory experiment have shown a minor role of mineralization and nitrification in $NO_{3}^{-}$ production in soils in the winter and $NO_{3}^{-}$ pulse formation in stream water during the early stages of the snowmelt season.