BC deposition must be considered in evaluating the acidifying effects of atmospheric
deposition on soils (Larssen and Carmichael 2000). The neutralization by BC
deposition (37.1 keq ha
À1 ) effectively reduced the net accumulated H
+ input caused
by atmospheric deposition (39.3 keq ha
À1 ) between 1981–1985 and 2006–2010 at a
national scale. The neutralization by BC deposition had a high value of 51.7 keq
ha
À1 in North China (Zhu et al. 2016). As the main content of BC deposition, the Ca
2
+ deposition had high values in comparison with the S deposition of five forested
catchments in southern and southwestern China and to a large extent alleviated the
acidification of soil and surface water (Larssen et al. 2011). During the long-term
recovery processes of soil acidification, base cation deposition can supplement the
historical alkalinity loss in leaching (UBA 2004). The soil Ca
2+ sink mainly occurred
in the upper 30 cm of the soil and was attributed to the building up of CEC in the soil
(Yu et al. 2017c). In addition, the Ca
2+ concentrations in surface water were
extremely high, contributing to the high ANC of the forests stream in China.
8.5.3 Alkalinization by the Removal of S and N
The removal of S and N in catchment is another important mechanism for preventing
the acidification of soil and surface water. The vast net removal of SO 4
2À within the
catchment is possible result from the SO 4
2À adsorption in soil (Vogt et al. 2007;
Duan et al. 2013; Huang et al. 2015), which can co-adsorb H
+ and buffer the
acidification of soil. It coincides with relatively low SO 4
2À leaching in soil waters
in the catchment with Haplic Acrisol, even under high S deposition (Larssen et al.
2011; Yu et al. 2017c). The adsorption capacity is large in these soils with high
content of Al or Fe oxides and low pH (Vogt et al. 2007). SO 4
2À leaching decreased
by 22.6 g S m
À2 due to adsorption, buffering two-third of input acidity in the
NH 4 NO 3 addition plot at Tieshanping site (Huang et al. 2015). The soil acidity
was buffered by the huge SO 4
2À adsorption in soil, especially in the earlier years
(2001–2006) (Yu et al. 2017c). However, there are some reports showing much
higher SO 4
2À leaching in stream waters in southwestern China (Duan et al. 2016).
The outflow of SO 4
2À to the Upper Yangtze River from the Sichuan Basin in
southwestern China has exceeded the estimated total S depostion to the basin,
suggesting that the SO 4
2À adsorption capacity of the soil may have been approached
(Duan et al. 2016). Since the soil adsorption capacity for SO 4
2À may be relatively
small or the soil is saturated for SO 4
2À adsorption, the SO 4
2À
fluxes in soil water
were comparable to those in throughfall for some regions (Yu et al. 2017b). In
addition to adsorption, SO 4
2À reduction may be another potential contribution for
the large S sink, especially in the deep-soil or in the water-logged groundwater
discharge zones (Larssen et al. 2011). However, direct evidence for SO 4
2À reduction
needs more study in the future.
In China, there are general large N sinks in most forested catchments (Yu et al.
2016a, 2017b, c; Larssen et al. 2011; Zhu et al. 2013) and in the large region (Duan
et al. 2016), showing relatively lower N fluxes in stream water in comparison with
8 Contribution of Atmospheric Reactive Nitrogen to Acid Deposition in China
173
deposition on soils (Larssen and Carmichael 2000). The neutralization by BC
deposition (37.1 keq ha
À1 ) effectively reduced the net accumulated H
+ input caused
by atmospheric deposition (39.3 keq ha
À1 ) between 1981–1985 and 2006–2010 at a
national scale. The neutralization by BC deposition had a high value of 51.7 keq
ha
À1 in North China (Zhu et al. 2016). As the main content of BC deposition, the Ca
2
+ deposition had high values in comparison with the S deposition of five forested
catchments in southern and southwestern China and to a large extent alleviated the
acidification of soil and surface water (Larssen et al. 2011). During the long-term
recovery processes of soil acidification, base cation deposition can supplement the
historical alkalinity loss in leaching (UBA 2004). The soil Ca
2+ sink mainly occurred
in the upper 30 cm of the soil and was attributed to the building up of CEC in the soil
(Yu et al. 2017c). In addition, the Ca
2+ concentrations in surface water were
extremely high, contributing to the high ANC of the forests stream in China.
8.5.3 Alkalinization by the Removal of S and N
The removal of S and N in catchment is another important mechanism for preventing
the acidification of soil and surface water. The vast net removal of SO 4
2À within the
catchment is possible result from the SO 4
2À adsorption in soil (Vogt et al. 2007;
Duan et al. 2013; Huang et al. 2015), which can co-adsorb H
+ and buffer the
acidification of soil. It coincides with relatively low SO 4
2À leaching in soil waters
in the catchment with Haplic Acrisol, even under high S deposition (Larssen et al.
2011; Yu et al. 2017c). The adsorption capacity is large in these soils with high
content of Al or Fe oxides and low pH (Vogt et al. 2007). SO 4
2À leaching decreased
by 22.6 g S m
À2 due to adsorption, buffering two-third of input acidity in the
NH 4 NO 3 addition plot at Tieshanping site (Huang et al. 2015). The soil acidity
was buffered by the huge SO 4
2À adsorption in soil, especially in the earlier years
(2001–2006) (Yu et al. 2017c). However, there are some reports showing much
higher SO 4
2À leaching in stream waters in southwestern China (Duan et al. 2016).
The outflow of SO 4
2À to the Upper Yangtze River from the Sichuan Basin in
southwestern China has exceeded the estimated total S depostion to the basin,
suggesting that the SO 4
2À adsorption capacity of the soil may have been approached
(Duan et al. 2016). Since the soil adsorption capacity for SO 4
2À may be relatively
small or the soil is saturated for SO 4
2À adsorption, the SO 4
2À
fluxes in soil water
were comparable to those in throughfall for some regions (Yu et al. 2017b). In
addition to adsorption, SO 4
2À reduction may be another potential contribution for
the large S sink, especially in the deep-soil or in the water-logged groundwater
discharge zones (Larssen et al. 2011). However, direct evidence for SO 4
2À reduction
needs more study in the future.
In China, there are general large N sinks in most forested catchments (Yu et al.
2016a, 2017b, c; Larssen et al. 2011; Zhu et al. 2013) and in the large region (Duan
et al. 2016), showing relatively lower N fluxes in stream water in comparison with
8 Contribution of Atmospheric Reactive Nitrogen to Acid Deposition in China
173