5.3.2 N Output and Accumulation
N output increased from 18.2 to 52.9 Tg N year
À1 from 1980 to 2015. Most of the
output occurred as N 2 emission, which is environmentally benign. Meanwhile,
losses of Nr to air and water systems also increased from 6.1 to 24.3 Tg N year
À1
over the same period. Emission of N 2 O, a greenhouse gas with strong global
warming potential, increased from 0.6 to 1.5 Tg N year
À1 . Nr accumulation
increased from 10.3 to 28.8 Tg N year
À1 within China. Most of these accumulations
occurred in cropland with excessive use of N fertilizers, and both organic N and
nitrate have been observed in cropland soils (Qin et al. 2006; Ju et al. 2004). In
forests, N accumulation was found both in the forest biomass and soils given
massive input of N from N deposition and BNF (Schlesinger 2008). In groundwater,
nitrate also accumulated through leaching from soil N accumulation and landfills. In
human systems, a large amount of N-containing industrial products such as nylons
was found. Detailed N accumulation in different subsystems can be found in
following sections.
5.3.3 Nitrogen Budget of Subsystems
All the N inputs will eventually find their way out of China or accumulate within the
system, often after cascading through multiple subsystems. How the N fluxes
perform in each subsystem on a national scale remains unclear. Based on CHANS,
the simplified pathways by which Nr inputs cascaded among all the 14 subsystems in
1980 and 2015 are compiled in Fig. 5.2. In order to make this process much easier to
understand, we illustrate Nr dynamics in China through a simplified CHANS model
that includes six key subsystems, industry, cropland, livestock, human, atmosphere,
and hydrosphere (surface water and groundwater), and a summed “Others”
subsystem that includes grasslands, forests, urban lands, aquaculture, pets, etc.
This combined set of six subsystems is small enough to display (N fluxes are usually
lower than 0.1 Tg N) yet allows us to discuss the nitrogen budget relating to
industrial production, food security, and the environment.
Total Nr input to China boosted from 24.8 Tg N year
À1 in 1980 to 70.9 Tg N
year
À1 in 2015. Most new Nr entered nitrogen circulation within China through the
industrial subsystem, which was the largest N pool. Most of the absolute increase
was a result of expanded HBNF (from 12.6 to 48.5 Tg N year
À1 ), while the relative
importance of fossil fuel N increased over time since the annual growth rate of fossil
fuel-derived N was even greater. Small quantities (totalling 0.9 Tg N year
À1 in 2015)
of already-fixed Nr entered the industrial subsystem as raw material for the production of non-food goods (e.g. cotton and leather), which were not shown in figure due
to the undefined source. The largest output from the industrial subsystem was
fertilizer N (35.0 Tg N year
À1 or 72% of HBNF in 2015), 88% of which went to
cropping systems (Fig. 5.2) to improve the grain production. Manufactured goods
94
B. Gu and X. Zhang
N output increased from 18.2 to 52.9 Tg N year
À1 from 1980 to 2015. Most of the
output occurred as N 2 emission, which is environmentally benign. Meanwhile,
losses of Nr to air and water systems also increased from 6.1 to 24.3 Tg N year
À1
over the same period. Emission of N 2 O, a greenhouse gas with strong global
warming potential, increased from 0.6 to 1.5 Tg N year
À1 . Nr accumulation
increased from 10.3 to 28.8 Tg N year
À1 within China. Most of these accumulations
occurred in cropland with excessive use of N fertilizers, and both organic N and
nitrate have been observed in cropland soils (Qin et al. 2006; Ju et al. 2004). In
forests, N accumulation was found both in the forest biomass and soils given
massive input of N from N deposition and BNF (Schlesinger 2008). In groundwater,
nitrate also accumulated through leaching from soil N accumulation and landfills. In
human systems, a large amount of N-containing industrial products such as nylons
was found. Detailed N accumulation in different subsystems can be found in
following sections.
5.3.3 Nitrogen Budget of Subsystems
All the N inputs will eventually find their way out of China or accumulate within the
system, often after cascading through multiple subsystems. How the N fluxes
perform in each subsystem on a national scale remains unclear. Based on CHANS,
the simplified pathways by which Nr inputs cascaded among all the 14 subsystems in
1980 and 2015 are compiled in Fig. 5.2. In order to make this process much easier to
understand, we illustrate Nr dynamics in China through a simplified CHANS model
that includes six key subsystems, industry, cropland, livestock, human, atmosphere,
and hydrosphere (surface water and groundwater), and a summed “Others”
subsystem that includes grasslands, forests, urban lands, aquaculture, pets, etc.
This combined set of six subsystems is small enough to display (N fluxes are usually
lower than 0.1 Tg N) yet allows us to discuss the nitrogen budget relating to
industrial production, food security, and the environment.
Total Nr input to China boosted from 24.8 Tg N year
À1 in 1980 to 70.9 Tg N
year
À1 in 2015. Most new Nr entered nitrogen circulation within China through the
industrial subsystem, which was the largest N pool. Most of the absolute increase
was a result of expanded HBNF (from 12.6 to 48.5 Tg N year
À1 ), while the relative
importance of fossil fuel N increased over time since the annual growth rate of fossil
fuel-derived N was even greater. Small quantities (totalling 0.9 Tg N year
À1 in 2015)
of already-fixed Nr entered the industrial subsystem as raw material for the production of non-food goods (e.g. cotton and leather), which were not shown in figure due
to the undefined source. The largest output from the industrial subsystem was
fertilizer N (35.0 Tg N year
À1 or 72% of HBNF in 2015), 88% of which went to
cropping systems (Fig. 5.2) to improve the grain production. Manufactured goods
94
B. Gu and X. Zhang