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Nitrogen deposition, along with habitat losses and climate change, has been identified as a primary threat to biodiversity worldwide (Butchart et al., 2010; MEA, 2005; Sala et al., 2000). The source of this stressor to natural systems is generally twofold: burning of fossil fuels and the use of fertilizers in modern intensive agriculture. Each of these human enterprises leads to the release of large amounts of biologically reactive nitrogen (henceforth contracted to "nitrogen") to the atmosphere, which is later deposited to ecosystems. Because nitrogen is a critical element to all living things (as a primary building block of proteins among other biological molecules), nitrogen availability often limits primary production and is tightly recycled in many natural ecosystems. This is especially true in temperate ecosystems, though it may also be true for some areas in the tropics that are not phosphorus-limited (Adams et al., 2004; Matson et al., 1999). Thus, the large increase in availability of this critical nutrient as a result of human activity has profound impacts on ecosystems and on biodiversity. Once nitrogen is deposited on terrestrial ecosystems, a cascade of effects can occur that often leads to overall declines in biodiversity (Bobbink et al., 2010; Galloway et al., 2003). For plants, nitrogen deposition can impact biodiversity generally through four processes: (1) stimulation of growth often of weedy species that outcompete local neighbors (termed "eutrophication"), (2) acidification of the soil and consequent imbalances in other key nutrients that favors acid tolerant species (termed "acidification"), (3) enhancement of secondary stressors such as from fire, drought, frost, or pests triggered by increased nitrogen availability (termed "secondary stressors"), and (4) direct damage to leaves (termed "direct toxicity") (Bobbink, 1998; Bobbink et al., 2010). For animals, much less is known, but reductions in plant biodiversity can lead to reductions in diversity of invertebrate and other animal species, loss of habitat heterogeneity and specialist habitats, increased pest populations and activity, and changes in soil microbial communities (McKinney and Lockwood, 1999; Throop and Lerdau, 2004; Treseder, 2004). Citation Clark, Christopher M.; Bai, Yongfei; Bowman, William D.; Cowles, Jane M.; Fenn, Mark E.; Gilliam, Frank S.; Phoenix, Gareth K.; Siddique, Ilyas; Stevens, Carly J.; Sverdrup, Harald U.; Throop, Heather L. 2013. Nitrogen deposition and terrestrial biodiversity. In: Levin S.A. (ed.) Encyclopedia of Biodiversity, second edition, Volume 5, Waltham, MA: Academic Press. pp. 519-536.