9.6.2 Impacts of Nitrogen Deposition on Heterotrophic
Respiration
Heterotrophic respiration (Rh) is a sum of CO 2 efflux from microbial decomposition
of organic matter in all soil layers. Current understanding on the effects of N
deposition is mainly based on experiments with relatively high-level N additions
(generally >50 kg N ha
À1 year
À1 ) (see Sect. 9.2.2). Meta-analysis of experimental
results indicates that N deposition generally decreases decomposition of soil organic
matter and Rh in China’s forests and other regions in the world (Janssens et al. 2010;
Chen et al. 2015). However, experimental results indicate positive effects of N
deposition on soil respiration especially in N-limited temperate and boreal forests,
while receiving low levels of N deposition (Sun et al. 2014; Geng et al. 2017a; Yan
et al. 2018). In tropical and subtropical regions in southern China, the effect of N
additions on Rh is generally negative (Mo et al. 2007, 2008; Fan et al. 2014; Chen
et al. 2015), because of high-level background N availability from N mineralization
and N deposition (Zhang et al. 2011a; Du et al. 2016). However, this negative effect
of N deposition can be reversed when exposing to elevated CO 2 concentrations (Liu
et al. 2010).
The results of meta-analysis, only reporting an average effect size irrespective to
variations of N additions, might mislead our understanding of Rh response to N
deposition. A recent review suggests a non-linear effect of N deposition on Rh
(De Vries et al. 2014). In N-limited ecosystems, low-level N deposition tends to
increase Rh due to a stimulation on litter and fine root production and/or an increase
in substrate quality (e.g. lower C:N ratio) for microbial decomposers (Yan et al.
2018). When N deposition exceeds a certain threshold, the positive effect can be
deducted and even reversed due to (i) reduced C transfer to the rhizosphere, (ii) less
litter production due to nutrient imbalances and soil acidification and (iii) enhanced
formation of recalcitrant compounds following shifts in microbial community and/or
decomposing enzyme (Janssens et al. 2010).
The non-linear effect of N deposition on soil respiration, especially the positive
effect at low-level N deposition, has been increasingly evidenced by experimental
results (Allison et al. 2009; Geng et al. 2017a; Yan et al. 2018). For instance, an
incubation study of organic substrates from boreal ecosystems indicated a significant
increase of Rh in response to low-level N addition but no significant increase at the
highest level of N addition (Allison et al. 2009). Based on a N enrichment experiment with nine treatments (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N ha
À1 year
À1 ) in
a temperate forest in Changbai Mountain of Northeast China, soil respiration showed
a single-peak response to N additions, being significantly stimulated (+16.3%) by
low-level N addition (10 kg N ha
À1 year
À1 ) and inhibited (À27.7%) by high-level N
addition (140 kg N ha
À1 year
À1
) (Geng et al. 2017a). In a boreal coniferous forest in
the Greater Khingan Mountains of Northeast China, experimental results of 5-year N
addition (0, 25, 50 and 75 kg N ha
À1 year
À1 ) indicate a stimulation on soil
respiration (both autotrophic respiration and Rh) by low (25 kg N ha
À1 year
À1 ) to
medium N addition (50 kg N ha
À1 year
À1 ), while the positive effect diminishes at the
high-level N addition (75 kg N ha
À1 year
À1 ) (Yan et al. 2018). However, the
9 Impacts of Nitrogen Deposition on Forest Ecosystems in China
203
Respiration
Heterotrophic respiration (Rh) is a sum of CO 2 efflux from microbial decomposition
of organic matter in all soil layers. Current understanding on the effects of N
deposition is mainly based on experiments with relatively high-level N additions
(generally >50 kg N ha
À1 year
À1 ) (see Sect. 9.2.2). Meta-analysis of experimental
results indicates that N deposition generally decreases decomposition of soil organic
matter and Rh in China’s forests and other regions in the world (Janssens et al. 2010;
Chen et al. 2015). However, experimental results indicate positive effects of N
deposition on soil respiration especially in N-limited temperate and boreal forests,
while receiving low levels of N deposition (Sun et al. 2014; Geng et al. 2017a; Yan
et al. 2018). In tropical and subtropical regions in southern China, the effect of N
additions on Rh is generally negative (Mo et al. 2007, 2008; Fan et al. 2014; Chen
et al. 2015), because of high-level background N availability from N mineralization
and N deposition (Zhang et al. 2011a; Du et al. 2016). However, this negative effect
of N deposition can be reversed when exposing to elevated CO 2 concentrations (Liu
et al. 2010).
The results of meta-analysis, only reporting an average effect size irrespective to
variations of N additions, might mislead our understanding of Rh response to N
deposition. A recent review suggests a non-linear effect of N deposition on Rh
(De Vries et al. 2014). In N-limited ecosystems, low-level N deposition tends to
increase Rh due to a stimulation on litter and fine root production and/or an increase
in substrate quality (e.g. lower C:N ratio) for microbial decomposers (Yan et al.
2018). When N deposition exceeds a certain threshold, the positive effect can be
deducted and even reversed due to (i) reduced C transfer to the rhizosphere, (ii) less
litter production due to nutrient imbalances and soil acidification and (iii) enhanced
formation of recalcitrant compounds following shifts in microbial community and/or
decomposing enzyme (Janssens et al. 2010).
The non-linear effect of N deposition on soil respiration, especially the positive
effect at low-level N deposition, has been increasingly evidenced by experimental
results (Allison et al. 2009; Geng et al. 2017a; Yan et al. 2018). For instance, an
incubation study of organic substrates from boreal ecosystems indicated a significant
increase of Rh in response to low-level N addition but no significant increase at the
highest level of N addition (Allison et al. 2009). Based on a N enrichment experiment with nine treatments (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N ha
À1 year
À1 ) in
a temperate forest in Changbai Mountain of Northeast China, soil respiration showed
a single-peak response to N additions, being significantly stimulated (+16.3%) by
low-level N addition (10 kg N ha
À1 year
À1 ) and inhibited (À27.7%) by high-level N
addition (140 kg N ha
À1 year
À1
) (Geng et al. 2017a). In a boreal coniferous forest in
the Greater Khingan Mountains of Northeast China, experimental results of 5-year N
addition (0, 25, 50 and 75 kg N ha
À1 year
À1 ) indicate a stimulation on soil
respiration (both autotrophic respiration and Rh) by low (25 kg N ha
À1 year
À1 ) to
medium N addition (50 kg N ha
À1 year
À1 ), while the positive effect diminishes at the
high-level N addition (75 kg N ha
À1 year
À1 ) (Yan et al. 2018). However, the
9 Impacts of Nitrogen Deposition on Forest Ecosystems in China
203