levels of N addition (60, 80, 100, 120 and 140 kg N ha
À1 year
À1 ) significantly
stimulated soil N 2 O emission, suggesting a non-linear effect on soil N 2 O emission
(Cheng et al. 2016). Moreover, the response of soil N 2 O may differ with N forms. In
a subtropical slash pine plantation in southern China, NaNO 3 and NH 4 Cl additions
(40, 120 kg N ha
À1 year
À1 ) both increased soil N 2 O emissions, while NH 4
+ additions
had a stronger effect than NO 3
_ (Wang et al. 2014a). Additionally, other factors such
as water availability may regulate the response of N 2 O emissions to N deposition. In
an old temperate forest at Changbai Mountain in Northeast China, experimental
rainfall reduction (À30%) significantly increased the soil N 2 O emissions (3.19% vs.
1.59%) in response to N addition (50 kg N ha
À1 year
À1 ) (Geng et al. 2017b).
Overall, N deposition can significantly change the rates of soil N transformations,
but its effects depend on N doses, N forms, treatment duration and forest types. To
date, however, our understanding of responses of soil N transformations to N
deposition is mainly based on net transformation rates. As net rate is the difference
between gross rate and the rate of N immobilization, conclusions from net rates
could not reveal more specific responses of soil N transformations to N deposition.
In addition, microbes are the main drivers of soil N transformations, but there are
very few studies to examine both soil N transformations and related microbes in
response to N deposition in China (Gao et al. 2016c; Tian et al. 2017a). Thus, linking
gross rates of soil N transformations with soil microbial community composition and
enzymatic activity will help to understand the mechanism of the effects of N
deposition on soil N transformations.
9.3.2 Impacts of Nitrogen Deposition on Soil Acidification
and Nutrient Imbalance
In China, the contribution of anthropogenic N to acid deposition has been given an
increasing concern because SO 2 emissions have declined since 2005 (Liu et al. 2011;
Fang et al. 2013). By synthesizing data on pH, sulphate, nitrate and ammonium in
bulk precipitation and throughfall in southern China’s forests, Du et al. (2015)
proposed the concept of ‘urban acid island’ and showed that acid deposition
exhibited a power-law increase with a closer distance to large cities (nonagricultural
population >0.5 million) (Fig. 9.2a). The rainwater pH, defined as the negative
logarithm of the hydrogen ion concentration, showed a logarithmic decline with a
closer distance to large cities (Fig. 9.2b). They further estimated a critical radius of
approximately 70 km for these urban acid islands, which covered an area of 0.70
million km
2 and accounted for nearly 30% of the land area in southern China.
Despite a neutralization capacity of NH 3 in the atmosphere, deposited ammonium
has a potential to generate substantial soil acidification. Overall, N deposition
accounted for 36% of deposition-induced soil acidification in forests in southern
China (Du et al. 2015).
External N inputs can cause soil acidification regardless of the forms (e.g. NH 4
+
or nitric acids), and the N-induced net change in soil acidity also depends on the
192
E. Du et al.
À1 year
À1 ) significantly
stimulated soil N 2 O emission, suggesting a non-linear effect on soil N 2 O emission
(Cheng et al. 2016). Moreover, the response of soil N 2 O may differ with N forms. In
a subtropical slash pine plantation in southern China, NaNO 3 and NH 4 Cl additions
(40, 120 kg N ha
À1 year
À1 ) both increased soil N 2 O emissions, while NH 4
+ additions
had a stronger effect than NO 3
_ (Wang et al. 2014a). Additionally, other factors such
as water availability may regulate the response of N 2 O emissions to N deposition. In
an old temperate forest at Changbai Mountain in Northeast China, experimental
rainfall reduction (À30%) significantly increased the soil N 2 O emissions (3.19% vs.
1.59%) in response to N addition (50 kg N ha
À1 year
À1 ) (Geng et al. 2017b).
Overall, N deposition can significantly change the rates of soil N transformations,
but its effects depend on N doses, N forms, treatment duration and forest types. To
date, however, our understanding of responses of soil N transformations to N
deposition is mainly based on net transformation rates. As net rate is the difference
between gross rate and the rate of N immobilization, conclusions from net rates
could not reveal more specific responses of soil N transformations to N deposition.
In addition, microbes are the main drivers of soil N transformations, but there are
very few studies to examine both soil N transformations and related microbes in
response to N deposition in China (Gao et al. 2016c; Tian et al. 2017a). Thus, linking
gross rates of soil N transformations with soil microbial community composition and
enzymatic activity will help to understand the mechanism of the effects of N
deposition on soil N transformations.
9.3.2 Impacts of Nitrogen Deposition on Soil Acidification
and Nutrient Imbalance
In China, the contribution of anthropogenic N to acid deposition has been given an
increasing concern because SO 2 emissions have declined since 2005 (Liu et al. 2011;
Fang et al. 2013). By synthesizing data on pH, sulphate, nitrate and ammonium in
bulk precipitation and throughfall in southern China’s forests, Du et al. (2015)
proposed the concept of ‘urban acid island’ and showed that acid deposition
exhibited a power-law increase with a closer distance to large cities (nonagricultural
population >0.5 million) (Fig. 9.2a). The rainwater pH, defined as the negative
logarithm of the hydrogen ion concentration, showed a logarithmic decline with a
closer distance to large cities (Fig. 9.2b). They further estimated a critical radius of
approximately 70 km for these urban acid islands, which covered an area of 0.70
million km
2 and accounted for nearly 30% of the land area in southern China.
Despite a neutralization capacity of NH 3 in the atmosphere, deposited ammonium
has a potential to generate substantial soil acidification. Overall, N deposition
accounted for 36% of deposition-induced soil acidification in forests in southern
China (Du et al. 2015).
External N inputs can cause soil acidification regardless of the forms (e.g. NH 4
+
or nitric acids), and the N-induced net change in soil acidity also depends on the
192
E. Du et al.