their results are in range of previous studies (De Vries et al. 2009, 2014), large
uncertainties remain in the estimates of ecosystem N retention fractions in biomass
compartments and soils.
Process-based modelling approaches have been conducted to assess the impacts
of N deposition on forest C sequestration at regional and/or national scales (Tian
et al. 2011; Lu et al. 2012). For instance, an estimate based on Dynamic Land
Ecosystem Model (DLEM) indicates that increased N deposition has resulted in a net
C sink of 6.51 Pg C (i.e. 0.062 Pg C year
À1 ) over the period 1901–2005 in China’s
terrestrial ecosystems, with 4.69 Pg C (i.e. 0.104 Pg C year
À1 ) occurring in the latter
45 years (Lu et al. 2012). Generally, forest accounted for 60% of N-induced C
storage, followed by grassland, shrubland and cropland, with contributions of 18%,
7% and 4%, respectively (Lu et al. 2012). Another process-based model (CEVSA2,
Carbon Exchange between Vegetation, Soil, and Atmosphere) estimates that elevated N deposition increased China’s forest C storage by 0.70 Pg C in China during
1981–2010 (i.e. 0.023 Pg C year
À1 ), which was much lower than the estimates
(0.062 Pg C year
À1 ) by Lu et al. (2012). Overall, future efforts are needed to improve
ecosystem models by incorporating new mechanisms of C–N interactions from
manipulation experiments and thus make better estimates on the effects of N
deposition on C cycles at regional and global scales.
9.6.4 Impacts of Nitrogen Deposition on Soil CH 4 Uptake
Methane (CH 4 ) is the second most important greenhouse gas after CO 2 . Soil CH 4 is
produced in the anaerobic layers by methanogens and can be oxidized into CO 2 by
methanotrophs in the aerobic conditions, and thus the net soil emission or uptake of
CH 4 depends on the sum of production and consumption processes (Le Mer and Roger
2001). Soil CH 4 uptake in global forests contributes to a significant CH 4 sink that
dominates CH 4 uptake by terrestrial ecosystems (Dutaur and Verchot 2007). Nitrogen
deposition increases soil N availability and consequently alters soil CH 4 uptake by
affecting microbial production-consumption processes. In China’s forests, N addition
experiments indicate varied results, including negative, neutral or positive effects on
soil CH 4 uptake (e.g. Zhang et al. 2008, 2011b; Wang et al. 2014b; Xu et al. 2013).
This inconsistency suggests that the effect of N deposition varies with forest type,
background N availability and other soil properties (e.g. P availability and soil
texture). For instance, NH 4 NO 3 additions (50, 100, 150 kg N ha
À1 year
À1
) decreased
soil CH 4 uptake by 6–32% in an old-growth tropical forest in southern China, while
the effect was insignificant in a rehabilitated plantation and a disturbed secondary
forest (Zhang et al. 2008). In this old-growth (sub)tropical forest, experimental results
further indicated that increased P availability partially mitigated the negative effect of
N deposition on soil CH 4 uptake (Zhang et al. 2011b).
The effect of N enrichment on soil CH 4 uptake also varies with N forms and N
doses. For instance, elevated soil nitrate concentrations can decline CH 4 production
by increasing redox potentials (Le Mer and Roger 2001), while increased
9 Impacts of Nitrogen Deposition on Forest Ecosystems in China
205
uncertainties remain in the estimates of ecosystem N retention fractions in biomass
compartments and soils.
Process-based modelling approaches have been conducted to assess the impacts
of N deposition on forest C sequestration at regional and/or national scales (Tian
et al. 2011; Lu et al. 2012). For instance, an estimate based on Dynamic Land
Ecosystem Model (DLEM) indicates that increased N deposition has resulted in a net
C sink of 6.51 Pg C (i.e. 0.062 Pg C year
À1 ) over the period 1901–2005 in China’s
terrestrial ecosystems, with 4.69 Pg C (i.e. 0.104 Pg C year
À1 ) occurring in the latter
45 years (Lu et al. 2012). Generally, forest accounted for 60% of N-induced C
storage, followed by grassland, shrubland and cropland, with contributions of 18%,
7% and 4%, respectively (Lu et al. 2012). Another process-based model (CEVSA2,
Carbon Exchange between Vegetation, Soil, and Atmosphere) estimates that elevated N deposition increased China’s forest C storage by 0.70 Pg C in China during
1981–2010 (i.e. 0.023 Pg C year
À1 ), which was much lower than the estimates
(0.062 Pg C year
À1 ) by Lu et al. (2012). Overall, future efforts are needed to improve
ecosystem models by incorporating new mechanisms of C–N interactions from
manipulation experiments and thus make better estimates on the effects of N
deposition on C cycles at regional and global scales.
9.6.4 Impacts of Nitrogen Deposition on Soil CH 4 Uptake
Methane (CH 4 ) is the second most important greenhouse gas after CO 2 . Soil CH 4 is
produced in the anaerobic layers by methanogens and can be oxidized into CO 2 by
methanotrophs in the aerobic conditions, and thus the net soil emission or uptake of
CH 4 depends on the sum of production and consumption processes (Le Mer and Roger
2001). Soil CH 4 uptake in global forests contributes to a significant CH 4 sink that
dominates CH 4 uptake by terrestrial ecosystems (Dutaur and Verchot 2007). Nitrogen
deposition increases soil N availability and consequently alters soil CH 4 uptake by
affecting microbial production-consumption processes. In China’s forests, N addition
experiments indicate varied results, including negative, neutral or positive effects on
soil CH 4 uptake (e.g. Zhang et al. 2008, 2011b; Wang et al. 2014b; Xu et al. 2013).
This inconsistency suggests that the effect of N deposition varies with forest type,
background N availability and other soil properties (e.g. P availability and soil
texture). For instance, NH 4 NO 3 additions (50, 100, 150 kg N ha
À1 year
À1
) decreased
soil CH 4 uptake by 6–32% in an old-growth tropical forest in southern China, while
the effect was insignificant in a rehabilitated plantation and a disturbed secondary
forest (Zhang et al. 2008). In this old-growth (sub)tropical forest, experimental results
further indicated that increased P availability partially mitigated the negative effect of
N deposition on soil CH 4 uptake (Zhang et al. 2011b).
The effect of N enrichment on soil CH 4 uptake also varies with N forms and N
doses. For instance, elevated soil nitrate concentrations can decline CH 4 production
by increasing redox potentials (Le Mer and Roger 2001), while increased
9 Impacts of Nitrogen Deposition on Forest Ecosystems in China
205