Abstract
Abstract. Reactive nitrogen (N) emissions have increased over the last 150 years as a result of greater fossil fuel combustion and food production. The resulting increase in N deposition can alter the function of ecosystems, but characterizing its ecological impacts remains challenging, in part because of uncertainties in model-based estimates of N dry deposition. Here, we use the Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric chemistry–climate model (AM3) coupled with the GFDL land model (LM3) to estimate dry deposition velocities. We leverage the tiled structure of LM3 to represent the impact of physical, hydrological, and ecological heterogeneities on the surface removal of chemical tracers. We show that this framework can be used to estimate N deposition at more ecologically relevant scales (e.g., natural vegetation, water bodies) than from the coarse-resolution global model AM3. Focusing on North America, we show that the faster removal of N over forested ecosystems relative to cropland and pasture implies that coarse-resolution estimates of N deposition from global models systematically underestimate N deposition to natural vegetation by 10 % to 30 % in the central and eastern US. Neglecting the sub-grid scale heterogeneity of dry deposition velocities also results in an underestimate (overestimate) of the amount of reduced (oxidized) nitrogen deposited to water bodies. Overall, changes in land cover associated with human activities are found to slow down the removal of N from the atmosphere, causing a reduction in the dry oxidized, dry reduced, and total (wet+dry) N deposition over the contiguous US of 8 %, 26 %, and 6 %, respectively. We also find that the reduction in the overall rate of removal of N associated with land-use change tends to increase N deposition on the remaining natural vegetation and facilitate N export to Canada. We show that sub-grid scale differences in the surface removal of oxidized and reduced nitrogen imply that projected near-term (2010–2050) changes in oxidized (−47 %) and reduced (+40 %) US N emissions will cause opposite changes in N deposition to water bodies (increase) and natural vegetation (decrease) in the eastern US, with potential implications for acidification and ecosystems.
Highlights
Fossil fuel combustion and food production release reactive nitrogen (N) to the atmosphere (Fowler et al, 2013)
While anthropogenic land use is estimated to reduce the overall N deposition in the contiguous US, we find that it tends to increase the surface concentration of reactive nitrogen species, which leads to greater N deposition on the remaining natural vegetation
Our study highlights the importance of accounting for surface heterogeneities and anthropogenic land use in modulating the magnitude and trend of N deposition
Summary
Fossil fuel combustion and food production release reactive nitrogen (N) to the atmosphere (Fowler et al, 2013). In the US, oxidized N deposition is projected to decrease as a result of effective controls on NO emissions, but deposition of reduced N (NHx ≡ NH3 + NH+4 ), primarily from agricultural emissions of NH3, is projected to remain elevated or even increase (Dentener et al, 2006; Ellis et al, 2013; Paulot et al, 2013; Lamarque et al, 2013; Li et al, 2016) This raises concerns of irreversible damage to sensitive biomes (Pardo et al, 2011; Meunier et al, 2016; Grizzetti, 2011; Dise, 2011), such as high-elevation lakes (Wolfe et al, 2003; Baron et al, 2012; Lepori and Keck, 2012) and organisms (e.g., lichen; Johansson et al, 2012)
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