Abstract
AbstractIncreasing rates of atmospheric deposition of nitrogen (N) present a novel threat to the biodiversity of terrestrial ecosystems. Many forests are particularly susceptible to excess N given their proximity to sources of anthropogenic N emissions. This study summarizes results of a 25‐yr treatment of an entire central Appalachian hardwood forest watershed via aerial applications of N with a focus on effects of added N on the cover, species richness, and composition of the herbaceous layer. Research was carried out on two watersheds of the Fernow Experimental Forest (FEF), West Virginia. The long‐term reference watershed at FEF (WS4) was used as a reference; WS3 was experimentally treated, receiving three aerial applications of N per year as (NH4)2SO4 totaling 35 kg N ha−1 yr−1, beginning in 1989. Cover of the herbaceous layer (vascular plants ≤1 m in height) was estimated visually in five circular 1‐m2 subplots within each of seven circular 400‐m2 sample plots spanning all aspects and elevations of each watershed. Sampling was carried out in early July of each of the following years: 1991, 1992, 1994, 2003, and 2009—2014, yielding 10 yr of data collected over a 23‐yr period. It was anticipated that the N treatment on WS3 would decrease species richness and alter herb layer composition by enhancing cover of a few nitrophilic species at the expense of numerous N‐efficient species. Following a period of minimal response from 1991 to 1994, cover of the herb layer increased substantially on N‐treated WS3, and remained high thereafter. There was also a coincidental decrease in herb layer diversity during this period, along with a sharp divergence in community composition between WS4 and WS3. Most changes appear to have arisen from unprecedented, N‐mediated increases of Rubus spp., which are normally associated with the high‐light environment of openings, rather than beneath intact forest canopies. These findings support the prediction that N‐mediated changes in the herbaceous layer of impacted forests are driven primarily by increases in nitrophilic species.
Highlights
Despite the success of the Clean Air Act of1970, and especially its amendments of 1977 and1990, in decreasing emissions of nitrogen (N)-bearing compounds into the atmosphere, atmospheric concentrations of reactive N (Nr) remain high, as does atmospheric deposition of Nr, in many regions of the world (Sutton et al 2014, Vet et al 2014)
The purpose of this study was to assess the effects of 25 yr of aerial applications of N on the herbaceous layer of a central Appalachian hardwood forest ecosystem
On the basis of N homogenization hypothesis, we predict the following: (1) added N will increase the spatial homogeneity of N availability, which will lead to increased sample homogeneity of the herb layer, resulting in N-mediated loss of species diversity and alterations in herb layer community composition, (2) the herb layer for the N-treated watershed will initially exhibit increases in total cover, (3) N-m ediated increases in cover will simultaneously involve increases in nitrophilous species and decreases in N-e fficient species, relative to the untreated reference watershed, and (4) both species richness and species evenness will decline, resulting in decreases in herb layer diversity
Summary
Despite the success of the Clean Air Act of1970, and especially its amendments of 1977 and1990, in decreasing emissions of nitrogen (N)-bearing compounds into the atmosphere, atmospheric concentrations of reactive N (Nr) remain high, as does atmospheric deposition of Nr, in many regions of the world (Sutton et al 2014, Vet et al 2014). Nr exist, including NH3, NH4+, NO, NO2, NO3−, 2N2O5, HNO3, and several forms of peroxyacetyl nitrates (Horii et al 2005), a fact that adds to both the complexity of the N cycle and the challenges of addressing problems associated with excess N in ecosystems. Many factors contribute to these problems, including N volatilization from animal feedlots and, more prominently, increases in anthropogenic N-fixation, primarily comprising fossil-fuel combustion and the Haber–Bosch process for production of reduced N in fertilizers. The latter is troubling because, despite recent decreases in emissions of oxidized. N, emissions of reduced N from use of fertilizer N are increasing in the United States (Pinder et al 2011). The sum of all forms of anthropogenic N fixation currently exceeds the amount of N fixed via natural (nonanthropogenic) processes (Vitousek et al 1997).
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