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

It remains difficult to assess the timing and amount of atmospherically deposited and microbial nitrate (NO 3 ‐N Atm and NO 3 ‐N Micro , respectively) reaching streams. To elucidate temporal variation of NO 3 ‐N Atm and NO 3 ‐N Micro , we measured nitrate concentrations, δ 17 O‐NO 3 − , and δ 18 O‐NO 3 − 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 17 O‐NO 3 − values, which were used to calibrate a statistical load model to estimate export of NO 3 ‐N Atm and NO 3 ‐N Micro . A hydrograph separation was used to estimate loss of NO 3 ‐N Atm and NO 3 ‐N Micro through base flow and direct runoff pathways. NO 3 ‐N Atm contributed 7–11% of the total nitrate yields and paralleled seasonal patterns of NO 3 ‐N Micro . Yields of NO 3 ‐N Atm and NO 3 ‐N Micro 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.