Abstract
Although past increases in emissions and atmospheric deposition of reactive nitrogen (Nr) provided the impetus for extensive research investigating the effects of excess N in terrestrial and aquatic ecosystems, the Clean Air Act and associated rules have led to decreases in emissions and deposition of oxidized N, especially in the eastern U.S., but also in other regions of the world. Thus, research in the near future must address the mechanisms and processes of recovery for impacted forests as they experience chronically less N in atmospheric depositions. Recently, a hysteretic model was proposed to predict this recovery. By definition, hysteresis is any phenomenon in which the state of a property depends on its history and lags behind changes in the effect causing it. Long-term whole-watershed additions of N at the Fernow Experimental Forest allow for tests of the ascending limb of the hysteretic model and provide an opportunity to assess the projected changes following cessation of these additions. A review of 10 studies published in the peer-reviewed literature indicate there was a lag time of 3–6 years before responses to N treatments became apparent. Consistent with the model, I predict significant lag times for recovery of this temperate hardwood ecosystem following decreases in N deposition.
Highlights
Historic increases in atmospheric deposition of reactive nitrogen (Nr, primarily NH4 +and NO3 −, but including numerous other reactive species [1]), and modeled projections for future increases on a global scale, have led to a proliferation of studies on the effects of excess N on aquatic and terrestrial ecosystems over the past several decades
Galloway et al [2] estimated that the total global atmospheric deposition of NH4 + and NO3 - in terrestrial ecosystems increased from 17 Tg N yr−1 in 1860 to 64 Tg N yr−1 in the early 1990s
The (NH4 )2 SO4 additions were discontinued in 2019, creating the unique opportunity to examine experimentally the recovery of a temperate hardwood forest ecosystem following a significant decrease in N inputs [34]
Summary
NO3 − , but including numerous other reactive species [1]), and modeled projections for future increases on a global scale, have led to a proliferation of studies on the effects of excess N on aquatic and terrestrial ecosystems over the past several decades. Galloway et al [2] estimated that the total global atmospheric deposition of NH4 + and NO3 - in terrestrial ecosystems increased from 17 Tg N yr−1 in 1860 to 64 Tg N yr−1 in the early 1990s. They projected further increases to 125 Tg N yr−1 by 2050, a >7-fold increase during this time period. The (NH4 ) SO4 additions were discontinued in 2019, creating the unique opportunity to examine experimentally the recovery of a temperate hardwood forest ecosystem following a significant decrease in N inputs [34]
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