0.1 Tg N year
À1 in 1980). About 8.7 Tg N year
À1 was estimated to denitrify to N 2
(and 0.2 Tg N year
À1 to N 2 O) within the hydrosphere in 2015.
5.3.4 NUEs of Two Most Important Food Subsystems
Here the NUE was defined as the Nr contained in useful outputs from a subsystem
divided by Nr inputs into that subsystem. NUEs of the two most important food
subsystems, cropland and livestock, were calculated during the past 35 years. The
NUE of industrial production can reach over 90% (Gu et al. 2013), because their
production processes take place under controlled conditions in factories. Other
subsystems with an NUE either have a very small N flux (such as aquaculture), or
NUE is undefined since they usually do not have any products (such as surface water
subsystem). Based on the calculation in this study, we found that the NUE of
cropland (NUE c ) in China decreased from ~42% in the 1980s to 37% in the early
2000s and then increased to ~42% in 2015, which was still much lower than that in
North America or Europe where it was typically above 50% (EPA 2011; De Vries
et al. 2011). China’s overall crop yield is about 10–20% lower, while the N input
intensity is at least double than in North America or Europe (De Vries et al. 2011;
Liu et al. 2010), suggesting that there is a large potential to produce more food with
lower N inputs by improved crop management in China (Chen et al. 2014).
The livestock NUE (NUE a ) showed a steady upward trend, increasing from 6% to
15% between 1980 and 2015; nevertheless, it is still lower than that in North
America or Europe where the NUE a is generally above 20% (Leach et al. 2012).
This increase of NUE a in China was mainly due to a decreased proportion of pig
production (from 63% to 38%) and an increased proportion (from 29% to 52%) of
poultry and dairy cattle relative to total animal production. Poultry and dairy cattle
have a greater feed conversion ratio (FCR, animal protein produced per feed protein
consumed) (Leach et al. 2012; Oenema 2006).
5.4 Atmospheric Reactive Nitrogen Budget in China
5.4.1 Emission of NH 3 , NO x , and N 2 O in China from 1980
to 2015
The four agricultural subsystems (cropland, livestock, grassland, and aquaculture)
are the main sources of NH 3 and N 2 O emission, representing ~90% and 50% of total
emission, respectively (Fig. 5.4c); besides agriculture, forest ranked second on N 2 O
emission, followed by industry and Nr-enriched surface water; fossil fuel combustion from industry was the largest source of NO x emissions, followed by human
subsystem.
5 Reactive Nitrogen Budgets in China
97
À1 in 1980). About 8.7 Tg N year
À1 was estimated to denitrify to N 2
(and 0.2 Tg N year
À1 to N 2 O) within the hydrosphere in 2015.
5.3.4 NUEs of Two Most Important Food Subsystems
Here the NUE was defined as the Nr contained in useful outputs from a subsystem
divided by Nr inputs into that subsystem. NUEs of the two most important food
subsystems, cropland and livestock, were calculated during the past 35 years. The
NUE of industrial production can reach over 90% (Gu et al. 2013), because their
production processes take place under controlled conditions in factories. Other
subsystems with an NUE either have a very small N flux (such as aquaculture), or
NUE is undefined since they usually do not have any products (such as surface water
subsystem). Based on the calculation in this study, we found that the NUE of
cropland (NUE c ) in China decreased from ~42% in the 1980s to 37% in the early
2000s and then increased to ~42% in 2015, which was still much lower than that in
North America or Europe where it was typically above 50% (EPA 2011; De Vries
et al. 2011). China’s overall crop yield is about 10–20% lower, while the N input
intensity is at least double than in North America or Europe (De Vries et al. 2011;
Liu et al. 2010), suggesting that there is a large potential to produce more food with
lower N inputs by improved crop management in China (Chen et al. 2014).
The livestock NUE (NUE a ) showed a steady upward trend, increasing from 6% to
15% between 1980 and 2015; nevertheless, it is still lower than that in North
America or Europe where the NUE a is generally above 20% (Leach et al. 2012).
This increase of NUE a in China was mainly due to a decreased proportion of pig
production (from 63% to 38%) and an increased proportion (from 29% to 52%) of
poultry and dairy cattle relative to total animal production. Poultry and dairy cattle
have a greater feed conversion ratio (FCR, animal protein produced per feed protein
consumed) (Leach et al. 2012; Oenema 2006).
5.4 Atmospheric Reactive Nitrogen Budget in China
5.4.1 Emission of NH 3 , NO x , and N 2 O in China from 1980
to 2015
The four agricultural subsystems (cropland, livestock, grassland, and aquaculture)
are the main sources of NH 3 and N 2 O emission, representing ~90% and 50% of total
emission, respectively (Fig. 5.4c); besides agriculture, forest ranked second on N 2 O
emission, followed by industry and Nr-enriched surface water; fossil fuel combustion from industry was the largest source of NO x emissions, followed by human
subsystem.
5 Reactive Nitrogen Budgets in China
97