Episodic, seasonal, and annual export of atmospheric and microbial nitrate from a temperate forest

<jats:title>Abstract</jats:title><jats:p>It remains difficult to assess the timing and amount of atmospherically deposited and microbial nitrate (NO<jats:sub>3</jats:sub>‐N<jats:sub>Atm</jats:sub> and NO<jats:sub>3</jats:sub>‐N<jats:sub>Micro</jats:sub>, respectively) reaching streams. To elucidate temporal variation of NO<jats:sub>3</jats:sub>‐N<jats:sub>Atm</jats:sub> and NO<jats:sub>3</jats:sub>‐N<jats:sub>Micro</jats:sub>, we measured nitrate concentrations, <jats:italic>δ</jats:italic><jats:sup>17</jats:sup>O‐NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup>, and <jats:italic>δ</jats:italic><jats:sup>18</jats:sup>O‐NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> in a stream draining a gaged, forested watershed in the central Appalachian Mountains throughout a range of hydrological conditions during two water years. From the isotopic data we calculated Δ<jats:sup>17</jats:sup>O‐NO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> values, which were used to calibrate a statistical load model to estimate export of NO<jats:sub>3</jats:sub>‐N<jats:sub>Atm</jats:sub> and NO<jats:sub>3</jats:sub>‐N<jats:sub>Micro</jats:sub>. A hydrograph separation was used to estimate loss of NO<jats:sub>3</jats:sub>‐N<jats:sub>Atm</jats:sub> and NO<jats:sub>3</jats:sub>‐N<jats:sub>Micro</jats:sub> through base flow and direct runoff pathways. NO<jats:sub>3</jats:sub>‐N<jats:sub>Atm</jats:sub> contributed 7–11% of the total nitrate yields and paralleled seasonal patterns of NO<jats:sub>3</jats:sub>‐N<jats:sub>Micro</jats:sub>. Yields of NO<jats:sub>3</jats:sub>‐N<jats:sub>Atm</jats:sub> and NO<jats:sub>3</jats:sub>‐N<jats:sub>Micro</jats:sub> in streamflow during the study period were dominated (61 and 85%, respectively) by base flow transport, suggesting that base flow is a major loss pathway for both nitrate species from this watershed.</jats:p>