Abstract
AbstractHuman activity has significantly increased the deposition of nitrogen (N) on terrestrial ecosystems over pre‐industrial levels leading to a multitude of effects including losses of biodiversity, changes in ecosystem functioning, and impacts on human well‐being. It is challenging to explicitly link the level of deposition on an ecosystem to the cascade of ecological effects triggered and ecosystem services affected, because of the multitude of possible pathways in the N cascade. To address this challenge, we report on the activities of an expert workshop to synthesize information on N‐induced terrestrial eutrophication from the published literature and to link critical load exceedances with human beneficiaries by using the STressor–Ecological Production function–final ecosystem Services Framework and the Final Ecosystem Goods and Services Classification System (FEGS‐CS). We found 21 N critical loads were triggered by N deposition (ranging from 2 to 39 kg N·ha−1·yr−1), which cascaded to distinct beneficiary types through 582 individual pathways in the five ecoregions examined (Eastern Temperate Forests, Marine West Coast Forests, Northwestern Forested Mountains, North American Deserts, Mediterranean California). These exceedances ultimately affected 66 FEGS across a range of final ecosystem service categories (21 categories, e.g., changes in timber production, fire regimes, and native plant and animal communities) and 198 regional human beneficiaries of different types. Several different biological indicators were triggered in different ecosystems, including grasses and/or forbs (33% of all pathways), mycorrhizal communities (22%), tree species (21%), and lichen biodiversity (11%). Ecoregions with higher deposition rates for longer periods tended to have more numerous and varied ecological impacts (e.g., Eastern Temperate Forests, eight biological indicators) as opposed to other ecoregions (e.g., North American Deserts and Marine West Coast Forests each with one biological indicator). Nonetheless, although ecoregions differed by ecological effects from terrestrial eutrophication, the number of FEGS and beneficiaries impacted was similar across ecoregions. We found that terrestrial eutrophication affected all ecosystems examined, demonstrating the widespread nature of terrestrial eutrophication nationally. These results highlight which people and ecosystems are most affected according to present knowledge, and identify key uncertainties and knowledge gaps to be filled by future research.
Highlights
Human activity has increased the deposition of nitrogen (N) by 10-fold or more over preindustrial levels for much of the developed world (Vitousek et al 1997, Galloway et al 2004, 2008)
Overview of all chains examined Nitrogen deposition affected 21 system-specific critical loads related to terrestrial eutrophication, which cascaded through 582 chains (Table 1)
This cascade was grouped into 76 Ecological Production Function (EPF) that affected 66 total Final Ecosystem Goods and Services (FEGS) (21 unique) and 198 regional beneficiaries (17 unique) across all ecoregions (Table 1), some of which were affected in many ecoregions and ecosystems
Summary
Human activity has increased the deposition of nitrogen (N) by 10-fold or more over preindustrial levels for much of the developed world (Vitousek et al 1997, Galloway et al 2004, 2008). Intentional inputs of N in the form of fertilizer application to crops have been a boon to mankind, partially responsible for supporting global population increases to over 7 billion. In many areas of the globe, increased fertilizer N is still needed to improve agricultural output (Vitousek et al 2009, Zhang et al 2015). Deposition is increasing or unchanged in the western United States, and there is a shift in the composition of deposition toward more reduced forms of N nationally (Li et al 2016). Even though deposition may be declining in some regions such as the east, these rates still far exceed pre-industrial rates and the estimated sensitivities of many ecological endpoints in the region (Baron et al 2011, Pardo et al 2011a)
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