Nitrogen addition experiments have evidenced that biologically available cations
(especially Ca
2+ and Mg
2+ ) in soils can be substantially decreased and might cause
consequent nutrient imbalances (Lu et al. 2018). In view of low soil P contents in
terrestrial ecosystems across most areas of China (Jiang et al. 1986; Han et al. 2005),
high-level N deposition versus P deposition likely shifts large areas of China’s
forests towards P limitation, especially in southern regions (Du et al. 2016). A recent
assessment indicates an imbalance of N and P supply by atmospheric deposition in
China’s forests (Du et al. 2016), with high N:P deposition ratios being twice to three
times of N:P ratio in tree leaves (Han et al. 2005) and three to four times of that in
current-year twigs (Yao et al. 2015). The N:P ratio in bulk deposition is also much
higher than critical N:P ratios related to relative P limitation in view of forest growth,
which are ~15 for coniferous forests and ~25 for deciduous forests (after Mellert and
Göttlein 2012). By compiling dataset on measurement of leaf N and P concentrations, a recent synthesis suggested that foliar N concentration of woody plants in
China’s nonagricultural ecosystems increased significantly from the 1980s to the
2000s, while leaf P concentration did not change significantly over the same period
(Liu et al. 2013a). Although enhanced N deposition often stimulates forest growth
and C sequestration (Högberg 2007; De Vries et al. 2009; Thomas et al. 2010), this
simulation may diminish when forest ecosystems become P-limited (Braun et al.
2010; Crowley et al. 2012; Li et al. 2015). Modelling results also indicated that
terrestrial C sequestration in China showed a lower response to per unit N deposition
in recent years (Tian et al. 2011).
9.3.3 Impacts of Nitrogen Deposition on Forest
Hydrochemistry
The hydrological cycle plays an important role in linking soils, plants and atmosphere and in transporting and reallocating nutrients in forest ecosystems. Elevated
N deposition increases N availability and thus disturbs the internal N cycle
(Gundersen et al. 1998; Aber et al. 1998; Fang et al. 2008; Lu et al. 2014).
Accordingly, forest hydrochemistry of throughfall, runoff and soil solution might
be changed, the degree of which depends on climate, species composition and initial
N availability of the forest ecosystem. Until now, water chemistry of inorganic N is
the focus of attention, followed by solution acidification and dissolved organic
matter in Chinese forest ecosystems, while most studies have been conducted in
southern China associated with rapid economic growth and high atmospheric N
deposition (Fang et al. 2008; Du et al. 2015, 2016).
In China’s forests, ammonium dominated inorganic N deposition in both bulk
deposition and throughfall, and mean bulk N deposition and throughfall N deposition were estimated as 14.0 and 21.5 kg N ha
À1 year
À1 , respectively, with a mean
NH 4
+
-N:NO 3
À -N ratio of ~2.5 (Du et al. 2014a). Canopy-captured dry deposition,
estimated as the difference between throughfall and bulk deposition, was thus
approximately half of the bulk deposition (Du et al. 2014a). By compiling published
194
E. Du et al.
(especially Ca
2+ and Mg
2+ ) in soils can be substantially decreased and might cause
consequent nutrient imbalances (Lu et al. 2018). In view of low soil P contents in
terrestrial ecosystems across most areas of China (Jiang et al. 1986; Han et al. 2005),
high-level N deposition versus P deposition likely shifts large areas of China’s
forests towards P limitation, especially in southern regions (Du et al. 2016). A recent
assessment indicates an imbalance of N and P supply by atmospheric deposition in
China’s forests (Du et al. 2016), with high N:P deposition ratios being twice to three
times of N:P ratio in tree leaves (Han et al. 2005) and three to four times of that in
current-year twigs (Yao et al. 2015). The N:P ratio in bulk deposition is also much
higher than critical N:P ratios related to relative P limitation in view of forest growth,
which are ~15 for coniferous forests and ~25 for deciduous forests (after Mellert and
Göttlein 2012). By compiling dataset on measurement of leaf N and P concentrations, a recent synthesis suggested that foliar N concentration of woody plants in
China’s nonagricultural ecosystems increased significantly from the 1980s to the
2000s, while leaf P concentration did not change significantly over the same period
(Liu et al. 2013a). Although enhanced N deposition often stimulates forest growth
and C sequestration (Högberg 2007; De Vries et al. 2009; Thomas et al. 2010), this
simulation may diminish when forest ecosystems become P-limited (Braun et al.
2010; Crowley et al. 2012; Li et al. 2015). Modelling results also indicated that
terrestrial C sequestration in China showed a lower response to per unit N deposition
in recent years (Tian et al. 2011).
9.3.3 Impacts of Nitrogen Deposition on Forest
Hydrochemistry
The hydrological cycle plays an important role in linking soils, plants and atmosphere and in transporting and reallocating nutrients in forest ecosystems. Elevated
N deposition increases N availability and thus disturbs the internal N cycle
(Gundersen et al. 1998; Aber et al. 1998; Fang et al. 2008; Lu et al. 2014).
Accordingly, forest hydrochemistry of throughfall, runoff and soil solution might
be changed, the degree of which depends on climate, species composition and initial
N availability of the forest ecosystem. Until now, water chemistry of inorganic N is
the focus of attention, followed by solution acidification and dissolved organic
matter in Chinese forest ecosystems, while most studies have been conducted in
southern China associated with rapid economic growth and high atmospheric N
deposition (Fang et al. 2008; Du et al. 2015, 2016).
In China’s forests, ammonium dominated inorganic N deposition in both bulk
deposition and throughfall, and mean bulk N deposition and throughfall N deposition were estimated as 14.0 and 21.5 kg N ha
À1 year
À1 , respectively, with a mean
NH 4
+
-N:NO 3
À -N ratio of ~2.5 (Du et al. 2014a). Canopy-captured dry deposition,
estimated as the difference between throughfall and bulk deposition, was thus
approximately half of the bulk deposition (Du et al. 2014a). By compiling published
194
E. Du et al.