Isotopic assessment of NO₃⁻ and SO₄²⁻ mobility during winter in two adjacent watersheds in the Adirondack Mountains, New York

<jats:p>Biogeochemical cycling of N and S was examined at two watersheds in the Adirondack Mountains, New York, to better understand the retention and loss of these elements during winter and spring snowmelt. We analyzed stable isotope compositions of NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> (<jats:italic>δ</jats:italic><jats:sup>15</jats:sup>N‐NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>, <jats:italic>δ</jats:italic><jats:sup>18</jats:sup>O‐NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>) and SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> (<jats:italic>δ</jats:italic><jats:sup>34</jats:sup>S‐SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup>, <jats:italic>δ</jats:italic><jats:sup>18</jats:sup>O‐SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup>), along with concentrations and fluxes of NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> and SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup>, in precipitation, throughfall, snowpack, snowmelt, soil water, groundwater, and stream water. Isotopic results showed no evidence of NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> and SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> transformations in the forest canopy and snowpack; however, markedly decreased <jats:italic>δ</jats:italic><jats:sup>18</jats:sup>O values of NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> and SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> in forest floor water suggest that microbial processing occurred in organic soil horizons. Similarly low <jats:italic>δ</jats:italic><jats:sup>18</jats:sup>O values of NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> and SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> were observed in forest floor and mineral soil leachates, groundwater, and streams. Over the winter observation period, most of the NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> and SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> in stream water was from a watershed‐derived source, whereas atmospheric contributions were relatively minor. Despite differences in soil water NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> concentrations between watersheds, the isotopic composition of NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> (<jats:italic>δ</jats:italic><jats:sup>15</jats:sup>N‐NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>, <jats:italic>δ</jats:italic><jats:sup>18</jats:sup>O‐NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>) was similar, and indicated that in both watersheds most of the NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> was produced by nitrification in the forest soils. Although there was likely some contribution of SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> from microbial oxidation of carbon‐bonded sulfur, most of the stream water SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> appeared to be derived from weathering of S‐containing bedrock or parent material. The decreased <jats:italic>δ</jats:italic><jats:sup>18</jats:sup>O values of NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> and SO<jats:sub>4</jats:sub><jats:sup>2−</jats:sup> in upper soil horizons indicate that atmospheric deposition of N and S was not directly linked with stream water losses, even during winter and spring snowmelt.</jats:p>