growth and consequent C sequestration. However, high-level N deposition has been
increasingly evidenced to cause N leaching, soil acidification, nutrient imbalance,
increased N 2 O emissions and decreased soil CH 4 uptake, which likely offset the
positive effect on ecosystem C storage over time. Meanwhile, N deposition has
changed both species composition and richness of plant and soil microbial communities, consequently altering C and nutrient cycling. Moreover, the effects of N
deposition are likely non-linear with N doses, and thus the assessment on the
threshold values for N deposition would have important implications for N emission
regulation and forest management options. We conclude that elevated N deposition
is altering ecosystem structure and function of China’s forests, especially in the
eastern and southern regions. In addition, we highlight research efforts to jointly
consider multiple abiotic factors (e.g. climate warming, CO 2 enrichment, drought
and increase in surface ozone) and biotic factors (e.g. insect outbreaks and invasive
species), when evaluating ecological impacts of N deposition in the future.
9.1 Introduction
Nitrogen (N) is an essential macronutrient that is externally introduced into natural
ecosystems via biological N fixation and atmospheric N deposition. Since the
agricultural and industrial revolutions, human activities have greatly altered global
N cycle, and anthropogenic reactive N creation has already exceeded the amounts of
biological N fixation in terrestrial ecosystems (Fowler et al. 2013). As driven by
growing fossil fuel consumption (e.g. power plants, transportation and industrial
activities), anthropogenic NO x emissions in China have increased from 1.3 Tg N
year
À1 in 1980 to more than 6.0 Tg N year
À1 in 2010 (Liu et al. 2013a). In the
meanwhile, NH 3 emissions have been more than doubled since the 1980s (from 5.6
to 14.5 Tg N year
À1 ), largely due to an absence of N management in agricultural
activities (Liu et al. 2011, 2013a). In line with increasing anthropogenic N emissions, national mean N deposition has increased by 60% from the 1980s to the 2000s
(Liu et al. 2013a). By synthesizing field measured data from 38 forest stands across
China, Du et al. (2014a) estimated that average bulk deposition and throughfall
deposition were 14.0 and 21.5 kg N ha
À1 year
À1
, respectively. Although elevated N
deposition in China may benefit forest growth and consequent carbon
(C) sequestration in many forest ecosystems, the adverse impacts from excess N
inputs have caused growing concerns, such as soil acidification, nutrient imbalances,
biodiversity loss and greenhouse gas emissions (Lu et al. 2010, 2011a, 2014; Du
et al. 2015, 2016; Chen et al. 2015; Tian et al. 2018).
As an external N input to forest ecosystems, N deposition can alter soil N
transformation processes and soil chemical properties (Gao et al. 2015). For
instance, N mineralization, nitrification and N 2 O emissions are generally increased
by N deposition (Lu et al. 2011b). However, other nutrients, especially phosphorus,
tend to be more limiting as a result of accumulation in biomass and organic matters
over time (Du and Fang 2014). Moreover, elevated N deposition, when exceeding
the demands of microbial organisms and plant growth, can contribute significantly to
186
E. Du et al.
increasingly evidenced to cause N leaching, soil acidification, nutrient imbalance,
increased N 2 O emissions and decreased soil CH 4 uptake, which likely offset the
positive effect on ecosystem C storage over time. Meanwhile, N deposition has
changed both species composition and richness of plant and soil microbial communities, consequently altering C and nutrient cycling. Moreover, the effects of N
deposition are likely non-linear with N doses, and thus the assessment on the
threshold values for N deposition would have important implications for N emission
regulation and forest management options. We conclude that elevated N deposition
is altering ecosystem structure and function of China’s forests, especially in the
eastern and southern regions. In addition, we highlight research efforts to jointly
consider multiple abiotic factors (e.g. climate warming, CO 2 enrichment, drought
and increase in surface ozone) and biotic factors (e.g. insect outbreaks and invasive
species), when evaluating ecological impacts of N deposition in the future.
9.1 Introduction
Nitrogen (N) is an essential macronutrient that is externally introduced into natural
ecosystems via biological N fixation and atmospheric N deposition. Since the
agricultural and industrial revolutions, human activities have greatly altered global
N cycle, and anthropogenic reactive N creation has already exceeded the amounts of
biological N fixation in terrestrial ecosystems (Fowler et al. 2013). As driven by
growing fossil fuel consumption (e.g. power plants, transportation and industrial
activities), anthropogenic NO x emissions in China have increased from 1.3 Tg N
year
À1 in 1980 to more than 6.0 Tg N year
À1 in 2010 (Liu et al. 2013a). In the
meanwhile, NH 3 emissions have been more than doubled since the 1980s (from 5.6
to 14.5 Tg N year
À1 ), largely due to an absence of N management in agricultural
activities (Liu et al. 2011, 2013a). In line with increasing anthropogenic N emissions, national mean N deposition has increased by 60% from the 1980s to the 2000s
(Liu et al. 2013a). By synthesizing field measured data from 38 forest stands across
China, Du et al. (2014a) estimated that average bulk deposition and throughfall
deposition were 14.0 and 21.5 kg N ha
À1 year
À1
, respectively. Although elevated N
deposition in China may benefit forest growth and consequent carbon
(C) sequestration in many forest ecosystems, the adverse impacts from excess N
inputs have caused growing concerns, such as soil acidification, nutrient imbalances,
biodiversity loss and greenhouse gas emissions (Lu et al. 2010, 2011a, 2014; Du
et al. 2015, 2016; Chen et al. 2015; Tian et al. 2018).
As an external N input to forest ecosystems, N deposition can alter soil N
transformation processes and soil chemical properties (Gao et al. 2015). For
instance, N mineralization, nitrification and N 2 O emissions are generally increased
by N deposition (Lu et al. 2011b). However, other nutrients, especially phosphorus,
tend to be more limiting as a result of accumulation in biomass and organic matters
over time (Du and Fang 2014). Moreover, elevated N deposition, when exceeding
the demands of microbial organisms and plant growth, can contribute significantly to
186
E. Du et al.