Nitrogen additions are expected to increase net nitrification, but experimental
results indicate that net nitrification shows varied (i.e. positive, neutral and negative)
responses to N additions in China’s forest ecosystems. These results suggest that the
responses of net nitrification to N additions are likely dose-dependent. For instance,
Hu (2009) found that low-level N additions (20–25 kg N ha
À1 year
À1 ) had no effect
on net nitrification in a broad-leaved Korean pine forest in Northeast China, while
high-level N additions (40–50 kg N ha
À1 year
À1
) significantly increased net nitrification. Moreover, responses of net nitrification to N additions also vary with forest
types and duration of N treatments. For instance, experimental results indicate that
short-term N additions had no effects on net nitrification in a mature and two
adjacent secondary subtropical forests, while long-term N additions significantly
decreased net nitrification in the mature forest but increased net nitrification in the
secondary forests (Chen et al. 2017). However, N additions likely exert little effects
on gross nitrification in both temperate and tropical/subtropical forests, when N
additions are not able to change nitrifiers, soil C:N ratio and soil pH (Gao et al.
2016a, b, c; Sun et al. 2016; Tian et al. 2017a). In detail, nitrification can be divided
into two components, i.e. heterotrophic nitrification and autotrophic nitrification.
These two processes could show different response to N additions. To improve
understanding of responses of nitrification to N additions, future studies should
separate contributions of heterotrophic nitrification and autotrophic nitrification
under elevated N deposition.
Nitrogen additions tend to exert neutral or negative effects on microbial N
immobilization. For instance, Gao et al. (2016a, b) found that NH 4
+ (40 kg N ha
À1
year
À1 ) additions decreased gross rates of NH 4
+ immobilization in a temperate
mixed forest and NO 3
À additions (20–40 kg N ha
À1 year
À1 ) decreased NO 3
À
immobilization in a subtropical pine forest. The reduction in microbial assimilation
of NH 4
+ may result from a decline in microbial requirements of NH 4
+ and an
increase in microbial utilization of organic N to meet C requirement (Gao et al.
2016c). Meanwhile, a decline in immobilization of NO 3
À may be associated with a
decrease in heterotrophic nitrification, as these two processes are functionally linked
in acidic soils of subtropical and tropical forests (Gao et al. 2016b; Zhang et al. 2013;
Zhu et al. 2013).
Soil N transformation processes, especially nitrification and denitrification, are
main sources of N 2 O production. Generally, N deposition is expected to accelerate
soil N 2 O emission via increasing soil N availability, especially when it exceeds N
demands of microbial immobilization and plant uptake. Manipulated N addition
experiments indicate that approximately 1–2% of external N inputs are emitted as
N 2 O from soils in temperate and subtropical forests in China (Zhou et al. 2016; Geng
et al. 2017b). However, effects of N deposition on soil N 2 O emissions can vary
significantly with background N availability and the level of N deposition. For
instance, experimental results indicate that high N addition (150 kg N ha
À1
year
À1 ) significantly increased N 2 O emission in an old-growth tropical forest,
while it exerted no effect on soil N 2 O emission in two adjacent younger forests
with low-N status (Zheng et al. 2016). In a temperate forest in Northeast China, a
multiple-level N addition experiment indicates that soil N 2 O emission showed no
response to low-level N additions (10, 20 and 40 kg N ha
À1 year
À1
), while higher
9 Impacts of Nitrogen Deposition on Forest Ecosystems in China
191
results indicate that net nitrification shows varied (i.e. positive, neutral and negative)
responses to N additions in China’s forest ecosystems. These results suggest that the
responses of net nitrification to N additions are likely dose-dependent. For instance,
Hu (2009) found that low-level N additions (20–25 kg N ha
À1 year
À1 ) had no effect
on net nitrification in a broad-leaved Korean pine forest in Northeast China, while
high-level N additions (40–50 kg N ha
À1 year
À1
) significantly increased net nitrification. Moreover, responses of net nitrification to N additions also vary with forest
types and duration of N treatments. For instance, experimental results indicate that
short-term N additions had no effects on net nitrification in a mature and two
adjacent secondary subtropical forests, while long-term N additions significantly
decreased net nitrification in the mature forest but increased net nitrification in the
secondary forests (Chen et al. 2017). However, N additions likely exert little effects
on gross nitrification in both temperate and tropical/subtropical forests, when N
additions are not able to change nitrifiers, soil C:N ratio and soil pH (Gao et al.
2016a, b, c; Sun et al. 2016; Tian et al. 2017a). In detail, nitrification can be divided
into two components, i.e. heterotrophic nitrification and autotrophic nitrification.
These two processes could show different response to N additions. To improve
understanding of responses of nitrification to N additions, future studies should
separate contributions of heterotrophic nitrification and autotrophic nitrification
under elevated N deposition.
Nitrogen additions tend to exert neutral or negative effects on microbial N
immobilization. For instance, Gao et al. (2016a, b) found that NH 4
+ (40 kg N ha
À1
year
À1 ) additions decreased gross rates of NH 4
+ immobilization in a temperate
mixed forest and NO 3
À additions (20–40 kg N ha
À1 year
À1 ) decreased NO 3
À
immobilization in a subtropical pine forest. The reduction in microbial assimilation
of NH 4
+ may result from a decline in microbial requirements of NH 4
+ and an
increase in microbial utilization of organic N to meet C requirement (Gao et al.
2016c). Meanwhile, a decline in immobilization of NO 3
À may be associated with a
decrease in heterotrophic nitrification, as these two processes are functionally linked
in acidic soils of subtropical and tropical forests (Gao et al. 2016b; Zhang et al. 2013;
Zhu et al. 2013).
Soil N transformation processes, especially nitrification and denitrification, are
main sources of N 2 O production. Generally, N deposition is expected to accelerate
soil N 2 O emission via increasing soil N availability, especially when it exceeds N
demands of microbial immobilization and plant uptake. Manipulated N addition
experiments indicate that approximately 1–2% of external N inputs are emitted as
N 2 O from soils in temperate and subtropical forests in China (Zhou et al. 2016; Geng
et al. 2017b). However, effects of N deposition on soil N 2 O emissions can vary
significantly with background N availability and the level of N deposition. For
instance, experimental results indicate that high N addition (150 kg N ha
À1
year
À1 ) significantly increased N 2 O emission in an old-growth tropical forest,
while it exerted no effect on soil N 2 O emission in two adjacent younger forests
with low-N status (Zheng et al. 2016). In a temperate forest in Northeast China, a
multiple-level N addition experiment indicates that soil N 2 O emission showed no
response to low-level N additions (10, 20 and 40 kg N ha
À1 year
À1
), while higher
9 Impacts of Nitrogen Deposition on Forest Ecosystems in China
191