9.6 Impacts of Nitrogen Deposition on Forest C Balance
9.6.1 Impacts of Nitrogen Deposition on Net Primary
Productivity
Nitrogen is a macronutrient that widely limits plant growth (Elser et al. 2007), and
thus N deposition usually stimulates net primary productivity (NPP) in forest
ecosystems (Du and De Vries 2018). In Europe and North America, N deposition
on average increases NPP by 20–40 kg C per kg N in temperate and boreal forests
(Högberg 2007; De Vries et al. 2009, 2014). However, experimental results in China
indicate that the effect of N deposition varies largely across forest types. Nitrogen
deposition has been reported to accelerate growth of temperate and boreal forest in
China (Du and Fang 2014; Yan et al. 2018), but the experiment-based C:N response
ratios are generally lower than those in Europe and North America (Högberg 2007;
De Vries et al. 2009, 2014). In tropical and subtropical regions in southern China,
several studies showed that N deposition exertsed no significant effect on forest
growth at ecosystem scale, due to high background N availability, prevailing P
limitation and N-induced soil acidification (Fan et al. 2014; Tian et al. 2017c; Wu
et al. 2017; Jiang et al. 2018).
A few experimental studies reported tree radial growth in response to N deposition
in China, but there has been rare experimental quantification of the NPP response,
especially in natural and seminatural forests. Tree growth response to N deposition
varies significantly with individual size (e.g. diameter or height) (Liu et al. 2015b; Tian
et al. 2017a, b; Jiang et al. 2018). In a boreal forest with low-level background N
deposition (2–4 kg N ha
À1 year
À1
), 4-year N additions (20, 50, 100 kg N ha
À1 year
À1
)
did not affect radial growth of small larch trees (height <16.5 m), while larger
individuals (height >16.5 m) showed significantly growth acceleration (Liu et al.
2015b). In a subtropical forest in Anhui Province, Tian et al. (2017c) found that the
radial growth of small trees of Castanopsis eyrie and understory saplings decreased
significantly after 3-year N addition (50 and 100 kg N ha
À1 year
À1
), but growth rates
of large trees were not affected by N additions. In a long-term perspective, N
deposition thus likely shifts the tree size structure and species composition in these forests. In addition, N deposition can alter biomass C allocation, resulting in an increase
in aboveground biomass C pool and a decrease in belowground biomass C pool (Chen
et al. 2015).
The effect of N deposition on forest NPP also varies with stand age. Nitrogen
deposition is expected to dramatically increase NPP in young and middle-aged
forests, while the stimulating effect might decrease during the late developmental
stages (De Vries et al. 2014). In an old-growth boreal forest in Northeast China,
3-year N addition only led to a minor increase in NPP (3 kg C per kg N) due to
progressive P limitation (Fig. 9.5, Du and Fang 2014). Soil P availability declines
during forest development with increasing biomass P accumulation (Vitousek et al.
9 Impacts of Nitrogen Deposition on Forest Ecosystems in China
201
9.6.1 Impacts of Nitrogen Deposition on Net Primary
Productivity
Nitrogen is a macronutrient that widely limits plant growth (Elser et al. 2007), and
thus N deposition usually stimulates net primary productivity (NPP) in forest
ecosystems (Du and De Vries 2018). In Europe and North America, N deposition
on average increases NPP by 20–40 kg C per kg N in temperate and boreal forests
(Högberg 2007; De Vries et al. 2009, 2014). However, experimental results in China
indicate that the effect of N deposition varies largely across forest types. Nitrogen
deposition has been reported to accelerate growth of temperate and boreal forest in
China (Du and Fang 2014; Yan et al. 2018), but the experiment-based C:N response
ratios are generally lower than those in Europe and North America (Högberg 2007;
De Vries et al. 2009, 2014). In tropical and subtropical regions in southern China,
several studies showed that N deposition exertsed no significant effect on forest
growth at ecosystem scale, due to high background N availability, prevailing P
limitation and N-induced soil acidification (Fan et al. 2014; Tian et al. 2017c; Wu
et al. 2017; Jiang et al. 2018).
A few experimental studies reported tree radial growth in response to N deposition
in China, but there has been rare experimental quantification of the NPP response,
especially in natural and seminatural forests. Tree growth response to N deposition
varies significantly with individual size (e.g. diameter or height) (Liu et al. 2015b; Tian
et al. 2017a, b; Jiang et al. 2018). In a boreal forest with low-level background N
deposition (2–4 kg N ha
À1 year
À1
), 4-year N additions (20, 50, 100 kg N ha
À1 year
À1
)
did not affect radial growth of small larch trees (height <16.5 m), while larger
individuals (height >16.5 m) showed significantly growth acceleration (Liu et al.
2015b). In a subtropical forest in Anhui Province, Tian et al. (2017c) found that the
radial growth of small trees of Castanopsis eyrie and understory saplings decreased
significantly after 3-year N addition (50 and 100 kg N ha
À1 year
À1
), but growth rates
of large trees were not affected by N additions. In a long-term perspective, N
deposition thus likely shifts the tree size structure and species composition in these forests. In addition, N deposition can alter biomass C allocation, resulting in an increase
in aboveground biomass C pool and a decrease in belowground biomass C pool (Chen
et al. 2015).
The effect of N deposition on forest NPP also varies with stand age. Nitrogen
deposition is expected to dramatically increase NPP in young and middle-aged
forests, while the stimulating effect might decrease during the late developmental
stages (De Vries et al. 2014). In an old-growth boreal forest in Northeast China,
3-year N addition only led to a minor increase in NPP (3 kg C per kg N) due to
progressive P limitation (Fig. 9.5, Du and Fang 2014). Soil P availability declines
during forest development with increasing biomass P accumulation (Vitousek et al.
9 Impacts of Nitrogen Deposition on Forest Ecosystems in China
201