the N deposition. In addition to N uptake by plant and N immobilization in soils
(both relatively small), the large N sink may be attributed to N denitrification
(Yu et al. 2016a; Zhu et al. 2013). Denitrification in soil converting NO 3
À to N 2
or N 2 O can neutralize the acidity and prevent the acidification of stream water and
soil. The occurrence of denitrification was proved by the enrichment of
15
N and
18
O
in the groundwater discharge zone (Yu et al. 2016a). Furthermore, the magnitude of
denitrification was quantified as large amount by natural
15 N isotopes (Fang et al.
2015) and of about 20% of the N deposition by nitrous oxide (N 2 O) emission
observation (Zhu et al. 2013). The loss of NO 3
À due to denitrification in the
groundwater discharge zone led to the higher pH in stream water than the soil
water on the well-drained hillslopes. Thus, the N deposition has stronger acidifying
effect on well-drained soils, where the denitrification may be relatively limited.
Although numerous studies have reported the plant growth would accelerate soil
acidification due to the uptake of base cation, including Ca
2+ , K
+ , and Mg
2+ , and
release of H
+ (Duan et al. 2004; Guo et al. 2010; Wang et al. 2012b), plant growth
might buffer acidification. A recent study showed the mature forest had relatively
larger acid buffering capacity than the pioneer or transitional forests (Jiang et al.
2016). In addition, some plants like algae showed important impacts on the water
chemistry, increasing the pH in the eutrophic Taihu Lake (Yu et al. 2016b).
8.6 Acid Rain Control and Future Prospects
Acid rain started since the 1980s and remains a serious environmental issue by so far
in China, in spite of emission abatements of SO 2 and NO x that has been
implemented. It results from air pollution and can lead to long-term acidification
of soil and surface water in a large scale. With large variation of space and time, the
acid deposition transforms the air pollution to the damage of terrestrial and aquatic
ecosystems, altering the biogeochemical cycling of many elements (e.g., S, N, H, Ca,
Mg, and Al).
Since 2006, the national emission of SO 2 has been reduced in China (also see
Fig. 2.1 in Chap. 2), resulting in a significant decrease of S deposition at a national
scale. Although acidification of surface water and soil was proved to a reversible
process in Europe and North America after the abatements of S emission (Strock
et al. 2014; Sutherland et al. 2015), there are limited reports of the recovery of
surface water and soil acidification, after SO 2 reducing in China. In general, the
recovery from acidification in surface waters showed a decreasing trend of SO 4
2À
concentrations and an increasing trend of pH in some rivers (Duan et al. 2011).
However, the long-term observation on throughfall, soil water, and stream water in
Tieshanping catchment (located near Chongqing city), a typical forested catchment
sensitive to acidification in southwestern China, demonstrated no significant recovery in the soil nor the stream under the decreasing S deposition in recent years
(Yu et al. 2017c). Moreover, modelling results by MAGIC model implied that there
is no significant increase of pH in soil water before 2050 with the current control of
SO 2 emission (Duan et al. 2013).
174
Q. Yu and L. Duan
(both relatively small), the large N sink may be attributed to N denitrification
(Yu et al. 2016a; Zhu et al. 2013). Denitrification in soil converting NO 3
À to N 2
or N 2 O can neutralize the acidity and prevent the acidification of stream water and
soil. The occurrence of denitrification was proved by the enrichment of
15
N and
18
O
in the groundwater discharge zone (Yu et al. 2016a). Furthermore, the magnitude of
denitrification was quantified as large amount by natural
15 N isotopes (Fang et al.
2015) and of about 20% of the N deposition by nitrous oxide (N 2 O) emission
observation (Zhu et al. 2013). The loss of NO 3
À due to denitrification in the
groundwater discharge zone led to the higher pH in stream water than the soil
water on the well-drained hillslopes. Thus, the N deposition has stronger acidifying
effect on well-drained soils, where the denitrification may be relatively limited.
Although numerous studies have reported the plant growth would accelerate soil
acidification due to the uptake of base cation, including Ca
2+ , K
+ , and Mg
2+ , and
release of H
+ (Duan et al. 2004; Guo et al. 2010; Wang et al. 2012b), plant growth
might buffer acidification. A recent study showed the mature forest had relatively
larger acid buffering capacity than the pioneer or transitional forests (Jiang et al.
2016). In addition, some plants like algae showed important impacts on the water
chemistry, increasing the pH in the eutrophic Taihu Lake (Yu et al. 2016b).
8.6 Acid Rain Control and Future Prospects
Acid rain started since the 1980s and remains a serious environmental issue by so far
in China, in spite of emission abatements of SO 2 and NO x that has been
implemented. It results from air pollution and can lead to long-term acidification
of soil and surface water in a large scale. With large variation of space and time, the
acid deposition transforms the air pollution to the damage of terrestrial and aquatic
ecosystems, altering the biogeochemical cycling of many elements (e.g., S, N, H, Ca,
Mg, and Al).
Since 2006, the national emission of SO 2 has been reduced in China (also see
Fig. 2.1 in Chap. 2), resulting in a significant decrease of S deposition at a national
scale. Although acidification of surface water and soil was proved to a reversible
process in Europe and North America after the abatements of S emission (Strock
et al. 2014; Sutherland et al. 2015), there are limited reports of the recovery of
surface water and soil acidification, after SO 2 reducing in China. In general, the
recovery from acidification in surface waters showed a decreasing trend of SO 4
2À
concentrations and an increasing trend of pH in some rivers (Duan et al. 2011).
However, the long-term observation on throughfall, soil water, and stream water in
Tieshanping catchment (located near Chongqing city), a typical forested catchment
sensitive to acidification in southwestern China, demonstrated no significant recovery in the soil nor the stream under the decreasing S deposition in recent years
(Yu et al. 2017c). Moreover, modelling results by MAGIC model implied that there
is no significant increase of pH in soil water before 2050 with the current control of
SO 2 emission (Duan et al. 2013).
174
Q. Yu and L. Duan