Decreased atmospheric NOx deposition improving ecological condition and water quality in many areas but may compromise other important ecosystem services
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Over the past three decades, declines in nitrogen oxide (NOx) emissions and downwind atmospheric deposition of these pollutants offer an ideal natural experiment to assess the trajectory of ecosystem response to decreased nitrogen inputs. Coinciding with these changes, emerging reports of decreased nitrogen loads to surface water in both forested, agricultural, and mixed land use catchments, improved ecological conditions in forests and grasslands, and the recovery of the acid-base status in many streams and lakes highlight that ecosystems may be readily responsive to improvements in air quality and decreased nitrogen/acidifying inputs. Despite these promising developments as well as important improvements in human health, many challenges remain. For example, stream N export in many forested watersheds have been seemingly been unresponsive to declines in deposition; with some paradoxically increasing nitrogen export. Both the temporal and spatial extent of this surprising response needs to be elucidated as many water quality restoration plans either assume water quality from headwater forested streams will either be improving or at least maintained. Likewise, forest growth will also likely decline since trees are no longer being “fertilized”. The associated forest carbon sink has indeed declined in the United States, and its possible that the elimination of the fertilizing effect of atmospheric N deposition may compromise domestic efforts in achieving net zero greenhouse gas emissions. Overall efforts to decrease NOx emissions and decrease atmospheric deposition have been successful, and this success has led to many documented improvements in ecological conditions in both terrestrial and aquatic environments. However, legacy effects of past eutrophication and acidification from nearly a century of high rates of atmospheric acidic deposition are beginning to emerge in certain areas. These variable observations of improvement, stasis, and degradation highlight the need to identify likely factors that predict ecosystem trajectories to decrease NOx emissions and atmospheric deposition.