(Vitousek et al. 2013). Human population was ultimately constrained by food
production during that long period, and the anthropogenic alternation of global N
cycle was minor compared with natural N flows (Fowler et al. 2013). The case has
changed since the early 1900s due to the industrial conversion of N 2 to ammonia
(NH 3 ), known as the Haber-Bosch process, which has substantially increased global
food production and sustained the growth of population. In turn, the growing
population further drives an increase in consumption of energy and other natural
resources, resulting in various environmental issues across the globe. The burning of
fossil fuels for energy production has increased the emissions of nitrogen oxides
(NO x ), which is formed as a by-product during combustion. As a result, anthropogenic creation of reactive N has been dramatically accelerated (Gruber and Galloway
2008; Galloway et al. 2008). Specifically, global anthropogenic emissions of reactive N have risen from approximately 13 Tg N year
À1 in 1860 (Galloway et al. 2004)
to approximately 100–115 Tg N year
À1 in 2000 (Duce et al. 2008; De Vries et al.
2017), causing a fundamental change in the N cycle and a cascade of negative
impacts on earth systems (Galloway et al. 2003, 2008; Fowler et al. 2013).
As reactive N moves along its biogeochemical pathway, it causes a sequence of
effects, known as the N cascade (Galloway et al. 2003). The increase in reactive N
emissions to the atmosphere can cause air pollution via a combination of physical
and chemical processes. For instance, NH 3 and NO x are both involved in the
formation of haze, and nitrate and ammonium are major compounds of atmospheric
particulate matter (Zhang et al. 2012; Wu et al. 2016). Moreover, anthropogenic
emissions of NO x play an important role in the formation of tropospheric ozone (O 3 )
during photochemical pollution episodes (Crutzen 1988). In view of the fact that
SO 2 emissions have been successfully curbed in many countries, the contribution of
reactive N precursors (i.e., NH 3 and NO x ) to acid deposition has become increasingly important at a global scale (Galloway 2001; Dentener et al. 2006; Vet et al.
2014). These effects of reactive N on air pollution can expand to a larger extent via a
short- or long-distance transportation. Consequently, the occurrence of haze, O 3
pollution, and acid deposition can result in various negative effects, such as damage
to human health, an alteration to climate systems, and a reduction of ecosystem
function and services (Schulze 1989; Kampa and Castanas 2008; Ramanathan and
Feng 2009; Du et al. 2017).
When deposited to the biosphere, reactive N can exert both beneficial and
deleterious effects on land and aquatic ecosystems, depending on the level of N
deposition and background N availability. As net primary productivity is widely
limited by N availability in natural ecosystems (Vitousek and Howarth 1991; Elser
et al. 2007; Bai et al. 2010; Song et al. 2012), N deposition can thus stimulate plant
growth and increase carbon (C) sequestration in these N-limited ecosystems
(De Vries et al. 2009, 2014; Du and De Vries 2018). When exceeding a certain
critical load, N deposition can, however, exert negative impacts on ecosystem health
and function. For instance, N deposition can cause a loss of plant biodiversity and
ranks the third driver of biodiversity loss after land use change and climate change at
the global scale (Bobbink et al. 2010; Sala et al. 2000). Other negative effects of N
deposition, such as nutrient imbalances, soil acidification, and increasing availability
2
X. Liu and E. Du
production during that long period, and the anthropogenic alternation of global N
cycle was minor compared with natural N flows (Fowler et al. 2013). The case has
changed since the early 1900s due to the industrial conversion of N 2 to ammonia
(NH 3 ), known as the Haber-Bosch process, which has substantially increased global
food production and sustained the growth of population. In turn, the growing
population further drives an increase in consumption of energy and other natural
resources, resulting in various environmental issues across the globe. The burning of
fossil fuels for energy production has increased the emissions of nitrogen oxides
(NO x ), which is formed as a by-product during combustion. As a result, anthropogenic creation of reactive N has been dramatically accelerated (Gruber and Galloway
2008; Galloway et al. 2008). Specifically, global anthropogenic emissions of reactive N have risen from approximately 13 Tg N year
À1 in 1860 (Galloway et al. 2004)
to approximately 100–115 Tg N year
À1 in 2000 (Duce et al. 2008; De Vries et al.
2017), causing a fundamental change in the N cycle and a cascade of negative
impacts on earth systems (Galloway et al. 2003, 2008; Fowler et al. 2013).
As reactive N moves along its biogeochemical pathway, it causes a sequence of
effects, known as the N cascade (Galloway et al. 2003). The increase in reactive N
emissions to the atmosphere can cause air pollution via a combination of physical
and chemical processes. For instance, NH 3 and NO x are both involved in the
formation of haze, and nitrate and ammonium are major compounds of atmospheric
particulate matter (Zhang et al. 2012; Wu et al. 2016). Moreover, anthropogenic
emissions of NO x play an important role in the formation of tropospheric ozone (O 3 )
during photochemical pollution episodes (Crutzen 1988). In view of the fact that
SO 2 emissions have been successfully curbed in many countries, the contribution of
reactive N precursors (i.e., NH 3 and NO x ) to acid deposition has become increasingly important at a global scale (Galloway 2001; Dentener et al. 2006; Vet et al.
2014). These effects of reactive N on air pollution can expand to a larger extent via a
short- or long-distance transportation. Consequently, the occurrence of haze, O 3
pollution, and acid deposition can result in various negative effects, such as damage
to human health, an alteration to climate systems, and a reduction of ecosystem
function and services (Schulze 1989; Kampa and Castanas 2008; Ramanathan and
Feng 2009; Du et al. 2017).
When deposited to the biosphere, reactive N can exert both beneficial and
deleterious effects on land and aquatic ecosystems, depending on the level of N
deposition and background N availability. As net primary productivity is widely
limited by N availability in natural ecosystems (Vitousek and Howarth 1991; Elser
et al. 2007; Bai et al. 2010; Song et al. 2012), N deposition can thus stimulate plant
growth and increase carbon (C) sequestration in these N-limited ecosystems
(De Vries et al. 2009, 2014; Du and De Vries 2018). When exceeding a certain
critical load, N deposition can, however, exert negative impacts on ecosystem health
and function. For instance, N deposition can cause a loss of plant biodiversity and
ranks the third driver of biodiversity loss after land use change and climate change at
the global scale (Bobbink et al. 2010; Sala et al. 2000). Other negative effects of N
deposition, such as nutrient imbalances, soil acidification, and increasing availability
2
X. Liu and E. Du