compared to other studies, such as pasture, aquaculture, traffic sources, natural
source of BNF, urban vegetation, humans, and pets, even though the latter contributed a relatively small proportion of total emissions (~11%).
Our estimate of NO x emissions generally agrees well with previous estimates
during the past 35 years (MEPC 2016), only small variation with previous studies
(Kurokawa et al. 2013; IIASA 2012; FRCGC 2011; EDGAR 2010) in which only
fossil fuel combustion was considered (Fig. 5.6b). The small variations of estimates
may be explained by the slightly varied EFs for fossil fuel combustion and the
dominance of fossil fuel combustion as the major emission source in China (over
90%).
For N 2 O emission, our estimate corresponds to other studies (Tian et al. 2011;
EDGAR 2010) before the 1990s; however, the disparities of estimations among
different studies increase since the 1990s (Fig. 5.6c). The large variations may arise
from the inconsistencies of estimates on N inputs to the system (EDGAR 2010;
IIASA 2012), since the N 2 O emission is generally positively correlated to the Nr
loading (Lu et al. 2006; Chen et al. 2008). For example, our estimates of N 2 O
emissions are higher than those of Tian et al. (Tian et al. 2011). One possible
explanation can be that a higher Nr deposition rate (contributing about 30% of
total N 2 O emission in China) used by our study led to an increase in the estimate
of N 2 O emission from forests in China.
5.4.4 Comparison of the Main N Fluxes in China
with the World, the United States, and the EU27
From an international perspective, we compared our N budget of China with recent
studies for the world, the United States, and the EU27 in Table 5.1. National
(regional) budgets differ in many important respects, with higher rates of creation
of anthropogenic Nr in China (49.7 Tg N year
À1 in 2000) compared with the United
States (30.7 Tg N year
À1 ) and the EU27 (22.3 Tg N year
À1 ). The fact that China
produces more than twice Nr through HBNF and applies substantially more N
fertilizer than the United States and the EU27 (Paulot et al. 2014) is well recognized
(Table 5.1). As a result, higher Nr emissions to the atmosphere and fluxes to the
hydrosphere in China as well, especially the NH 3 emissions in China (calculated at
11.3 Tg N in 2000), are more than three times as those in the United States and
EU27, respectively. The great difference could also attribute to the poor farming
practices in China. The corresponding N leaching and runoff are no wonder the
highest in China.
Widespread and remarkably high levels of water pollution and air pollution in
many Chinese cities (Shao et al. 2006; Wu et al. 2016) are related to the intensive
agriculture and industry, which are closely intermingled with population centres in
China than in the United States or the EU27. Other differences are less well
constrained but are consistent both internally and with other sources of information.
5 Reactive Nitrogen Budgets in China
103
source of BNF, urban vegetation, humans, and pets, even though the latter contributed a relatively small proportion of total emissions (~11%).
Our estimate of NO x emissions generally agrees well with previous estimates
during the past 35 years (MEPC 2016), only small variation with previous studies
(Kurokawa et al. 2013; IIASA 2012; FRCGC 2011; EDGAR 2010) in which only
fossil fuel combustion was considered (Fig. 5.6b). The small variations of estimates
may be explained by the slightly varied EFs for fossil fuel combustion and the
dominance of fossil fuel combustion as the major emission source in China (over
90%).
For N 2 O emission, our estimate corresponds to other studies (Tian et al. 2011;
EDGAR 2010) before the 1990s; however, the disparities of estimations among
different studies increase since the 1990s (Fig. 5.6c). The large variations may arise
from the inconsistencies of estimates on N inputs to the system (EDGAR 2010;
IIASA 2012), since the N 2 O emission is generally positively correlated to the Nr
loading (Lu et al. 2006; Chen et al. 2008). For example, our estimates of N 2 O
emissions are higher than those of Tian et al. (Tian et al. 2011). One possible
explanation can be that a higher Nr deposition rate (contributing about 30% of
total N 2 O emission in China) used by our study led to an increase in the estimate
of N 2 O emission from forests in China.
5.4.4 Comparison of the Main N Fluxes in China
with the World, the United States, and the EU27
From an international perspective, we compared our N budget of China with recent
studies for the world, the United States, and the EU27 in Table 5.1. National
(regional) budgets differ in many important respects, with higher rates of creation
of anthropogenic Nr in China (49.7 Tg N year
À1 in 2000) compared with the United
States (30.7 Tg N year
À1 ) and the EU27 (22.3 Tg N year
À1 ). The fact that China
produces more than twice Nr through HBNF and applies substantially more N
fertilizer than the United States and the EU27 (Paulot et al. 2014) is well recognized
(Table 5.1). As a result, higher Nr emissions to the atmosphere and fluxes to the
hydrosphere in China as well, especially the NH 3 emissions in China (calculated at
11.3 Tg N in 2000), are more than three times as those in the United States and
EU27, respectively. The great difference could also attribute to the poor farming
practices in China. The corresponding N leaching and runoff are no wonder the
highest in China.
Widespread and remarkably high levels of water pollution and air pollution in
many Chinese cities (Shao et al. 2006; Wu et al. 2016) are related to the intensive
agriculture and industry, which are closely intermingled with population centres in
China than in the United States or the EU27. Other differences are less well
constrained but are consistent both internally and with other sources of information.
5 Reactive Nitrogen Budgets in China
103