Hydrobiogeochemical control of temporal variation in atmospheric nitrate export from a temperate forest watershed
Atmospheric N deposition can have important influences on biogeochemical processes in terrestrial and aquatic ecosystems. However, the mechanisms governing the transport pathways and transformation processes of atmospheric N in forested watersheds remain uncertain, in part because of the lack of an unambiguous tracer of atmospheric N. This lack of understanding limits our ability to predict the response of ecosystems to atmospheric N deposition. The unique Δ17O values of atmospheric nitrate help to overcome this challenge. We measured nitrate concentrations and Δ17O and δ15N of nitrate-N in a stream (n=74 samples) draining a small gaged, forested watershed located in the central Appalachian Mountains over two consecutive water years. Samples were collected across a wide range of hydrologic conditions throughout each year. The relationships among nitrate concentrations, isotopic values and flow conditions were assessed using non-linear regression analysis. A statistical load estimator model was used to estimate daily yields of atmospheric and terrestrial nitrate (NO3-NAtm and NO3-NProc) from measured N concentrations and discharge values, and these daily yields were summed to calculate annual yields. Annual flow-weighted mean concentrations were computed from the annual yields and runoff values. We discuss the implications of these results for understanding of N cycling in temperate forests.
Nitrate-Δ17O values were >0 in 68 of 74 stream samples, confirming the presence of NO3-NAtm across a variety of flow regimes. The statistical load model predicted that the annual flow weighted mean concentration of NO3-NAtm ranged from 0.05-0.12 mg N L-1, and contributed 6-10% of the modeled annual total nitrate yields. NO3-NAtm concentrations generally peaked during rising limb periods (up to 0.33 mg N L-1) associated with summer rainstorm events and winter/spring snowmelt events. The results confirm the direct, rapid transport of atmospheric nitrate to the stream during quickflow periods. Further, we observed non-zero concentrations of NO3-NAtm (0.02-0.10 mg N L-1) consistently in baseflow. Thus, a second pathway allowing export of NO3-NAtm may involve its accumulation in the soil, subsequent transport through the soil matrix, and eventual release as groundwater discharge. Significant hyperbolic relationships between NO3-NProc and discharge, δ15N-NO3 and discharge, and NO3-NProc and δ15N-NO3 suggested that NO3-N pools become diminished and fractionated at greater depth in the soil. Overall, our results illustrate two pathways by which temperate forests may dynamically respond to declining atmospheric N deposition, which may help explain why many streams draining forested watersheds in eastern North America have exhibited declining nitrate-N yields in recent decades.