causing large acid input into the soil (NH 4 NO 3 treated, H
+
N ¼ 7.4 keq ha
À1 year
À1 ;
NaNO 3 treated, H
+
N ¼ 4.4 keq ha
À1 year
À1
; reference, H
+
N ¼ 4.4 keq ha
À1 year
À1 ;
Huang et al. 2015). Despite the double N input in a long-term scale, the soil pH had
no significant decline (Huang et al. 2015).
The impact of N deposition on soil acidification is alleviated by N assimilation
(including plant N uptake and N immobilization) and denitrification. Consequently,
only part of the deposited N (1–25% of N deposition according to Zhu et al. 2016;
~22% of N input via throughfall, Fang et al. 2011) was regarded to be leached out,
depending on the N status (Fig. 8.9b). In the N-limited ecosystems, almost all the
deposited N was retained. In the N-saturated ecosystems, more deposited N could be
leached out. Threshold of N leaching in temperate region is considered of 8–10 kg N
ha
À1 year
À1 (Aber et al. 2003; Dise and Wright 1995). Subtropical forests in South
China, receiving elevated N deposition, have a higher threshold of N leaching
(26–36 kg N ha
À1 year
À1 , Fig. 8.9b; Yu et al. 2018). Several studies in the
N-saturated forest indicate that almost all deposited NH 4
+ nitrifies to NO 3
À in the
soil water (Larssen et al. 2011; Huang et al. 2015). The N transformation, especially
nitrification, still has vital impacts on the acidification of soil in the N-saturated
ecosystems. In addition, the plant uptake of NO 3
À is relatively low in the N-saturated
forests (Huang et al. 2014), leading to increasing NO 3
À leaching and serious
acidification of the soil in the rooting zone (Chen et al. 2004).
8.5 Acidification Buffering
The buffering mechanisms, including chemical weathering, BC deposition, and the
removal of S and N in ecosystems, vary according to climate, soil properties,
vegetation types, and historical deposition, and still remain unclear in China.
8.5.1 Neutralization by Chemical Weathering
Chemical weathering is one of the important mechanisms to buffer the acidification
through alkalinity export. There are different weathering mechanisms during the
acidifying period (Van Breemen et al. 1984). In the soil with pH >7, the weathering
mechanism is mainly as the dissolution of carbonate or bicarbonate minerals and
export Ca
2+ and HCO 3
À (Blake et al. 1999; Lu et al. 2014). When pH drops to 5.5–7,
the weathering processes function by the export of base cations (including Ca
2+ , Mg
2
+
, K
+
, and Na
+
) (Kirk et al. 2009; Lu et al. 2015). In the acidic soil with pH lower
than 5.5, the mobilization of Al
3+ dominate the neutralization mechanism (Gruba
et al. 2013). Thus, the impacts of acid deposition on soil and surface water acidification partly depend on the properties of soils, including BS and cation exchange
capacity (CEC) (Van Nevel et al. 2013; Tanikawa et al. 2014; Gruba and Mulder
2015; Van Breemen et al. 1984; Lu et al. 2015).
8 Contribution of Atmospheric Reactive Nitrogen to Acid Deposition in China
171
+
N ¼ 7.4 keq ha
À1 year
À1 ;
NaNO 3 treated, H
+
N ¼ 4.4 keq ha
À1 year
À1
; reference, H
+
N ¼ 4.4 keq ha
À1 year
À1 ;
Huang et al. 2015). Despite the double N input in a long-term scale, the soil pH had
no significant decline (Huang et al. 2015).
The impact of N deposition on soil acidification is alleviated by N assimilation
(including plant N uptake and N immobilization) and denitrification. Consequently,
only part of the deposited N (1–25% of N deposition according to Zhu et al. 2016;
~22% of N input via throughfall, Fang et al. 2011) was regarded to be leached out,
depending on the N status (Fig. 8.9b). In the N-limited ecosystems, almost all the
deposited N was retained. In the N-saturated ecosystems, more deposited N could be
leached out. Threshold of N leaching in temperate region is considered of 8–10 kg N
ha
À1 year
À1 (Aber et al. 2003; Dise and Wright 1995). Subtropical forests in South
China, receiving elevated N deposition, have a higher threshold of N leaching
(26–36 kg N ha
À1 year
À1 , Fig. 8.9b; Yu et al. 2018). Several studies in the
N-saturated forest indicate that almost all deposited NH 4
+ nitrifies to NO 3
À in the
soil water (Larssen et al. 2011; Huang et al. 2015). The N transformation, especially
nitrification, still has vital impacts on the acidification of soil in the N-saturated
ecosystems. In addition, the plant uptake of NO 3
À is relatively low in the N-saturated
forests (Huang et al. 2014), leading to increasing NO 3
À leaching and serious
acidification of the soil in the rooting zone (Chen et al. 2004).
8.5 Acidification Buffering
The buffering mechanisms, including chemical weathering, BC deposition, and the
removal of S and N in ecosystems, vary according to climate, soil properties,
vegetation types, and historical deposition, and still remain unclear in China.
8.5.1 Neutralization by Chemical Weathering
Chemical weathering is one of the important mechanisms to buffer the acidification
through alkalinity export. There are different weathering mechanisms during the
acidifying period (Van Breemen et al. 1984). In the soil with pH >7, the weathering
mechanism is mainly as the dissolution of carbonate or bicarbonate minerals and
export Ca
2+ and HCO 3
À (Blake et al. 1999; Lu et al. 2014). When pH drops to 5.5–7,
the weathering processes function by the export of base cations (including Ca
2+ , Mg
2
+
, K
+
, and Na
+
) (Kirk et al. 2009; Lu et al. 2015). In the acidic soil with pH lower
than 5.5, the mobilization of Al
3+ dominate the neutralization mechanism (Gruba
et al. 2013). Thus, the impacts of acid deposition on soil and surface water acidification partly depend on the properties of soils, including BS and cation exchange
capacity (CEC) (Van Nevel et al. 2013; Tanikawa et al. 2014; Gruba and Mulder
2015; Van Breemen et al. 1984; Lu et al. 2015).
8 Contribution of Atmospheric Reactive Nitrogen to Acid Deposition in China
171