ammonium concentrations can inhibit CH 4 oxidation by competing for methane
monooxygenase (Bodelier and Laanbroek 2004). In a subtropical slash pine plantation in southern China, NaNO 3 and NH 4 Cl additions (40, 120 kg N ha
À1 year
À1 )
both decreased soil CH 4 uptake, while the latter had a stronger inhibition effect
(Wang et al. 2014b). In a temperate deciduous forest in northern China, a 5-year
experiment (0, 50 and 150 kg N ha
À1 year
À1 ) indicates that N additions (NaNO 3 ,
(NH 4 ) 2 SO 4 and NH 4 NO 3 ) decreased annual CH 4 uptake and the inhibiting effect
went down in the order (NH 4 ) 2 SO 4 > NH 4 NO 3 > NaNO 3 (Yang et al. 2017).
Moreover, the effect of N deposition on soil CH 4 uptake is likely non-linear across
N doses. In a boreal forest in Northeast China, experimental results indicate an
increase of soil CH 4 uptake by low-level N addition (10 kg N ha
À1 year
À1 ), while
higher N additions (20 and 40 kg N ha
À1 year
À1 ) exerted no significant effect
(Xu et al. 2013). In a temperate forest in Changbai Mountain of Northeast China,
a multiple-level N enrichment experiment (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N
ha
À1 year
À1 ) also indicates that low-level N addition (10 kg N ha
À1 year
À1 )
significantly stimulated soil CH 4 uptake, while high-level N addition (140 kg N
ha
À1 year
À1 ) significantly inhibited it (Geng et al. 2017a). At a large scale, the
overall effect depends on the sum of increase in soil CH 4 uptake by low-level N
deposition and the reduction of soil CH 4 uptake by high-level N deposition. This
calls for global-scale efforts to assess the effect of N deposition on climate feedbacks
by simultaneously considering the responses of C sequestration and soil CH 4 uptake.
9.7 Conclusions and Outlook
By synthesizing existing experimental, observational and modelling results, we
conclude that continuously increasing N deposition has significantly altered the
structure and function of China’s forest ecosystems, especially in the eastern and
southern regions. Experimental results and modelling estimates generally indicate a
fertilization effect of N deposition on forest growth and consequent C sequestration.
Meanwhile, elevated N deposition has contributed significantly to soil acidification
and nutrient imbalances, which will likely diminish the positive effect on forest
productivity over time. Furthermore, N deposition can change both species composition of plant and microbial communities, which in turn alters C and nutrient
cycling. The impacts of N deposition, however, vary remarkably with background
N availability, ecosystem type and forest age (Du and Fang 2014; De Vries et al.
2014; Lu et al. 2014; Chen et al. 2015; Tian et al. 2016). Uncertainties remain in
long-term effects of N deposition in China, because most of current N addition
experiments have been initiated around the 2010s and rarely exceed 10 years. The
effects of N deposition are likely non-linear with N doses and experimental duration
(e.g. De Vries et al. 2014), and thus further research is needed to improve our
understanding of N thresholds. As a result of emission abatement policy, N deposition in China will likely decrease following the trends in Europe and the USA
(Waldner et al. 2014; Du et al. 2014c; Du 2016). There is an increasing need to
206
E. Du et al.
monooxygenase (Bodelier and Laanbroek 2004). In a subtropical slash pine plantation in southern China, NaNO 3 and NH 4 Cl additions (40, 120 kg N ha
À1 year
À1 )
both decreased soil CH 4 uptake, while the latter had a stronger inhibition effect
(Wang et al. 2014b). In a temperate deciduous forest in northern China, a 5-year
experiment (0, 50 and 150 kg N ha
À1 year
À1 ) indicates that N additions (NaNO 3 ,
(NH 4 ) 2 SO 4 and NH 4 NO 3 ) decreased annual CH 4 uptake and the inhibiting effect
went down in the order (NH 4 ) 2 SO 4 > NH 4 NO 3 > NaNO 3 (Yang et al. 2017).
Moreover, the effect of N deposition on soil CH 4 uptake is likely non-linear across
N doses. In a boreal forest in Northeast China, experimental results indicate an
increase of soil CH 4 uptake by low-level N addition (10 kg N ha
À1 year
À1 ), while
higher N additions (20 and 40 kg N ha
À1 year
À1 ) exerted no significant effect
(Xu et al. 2013). In a temperate forest in Changbai Mountain of Northeast China,
a multiple-level N enrichment experiment (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N
ha
À1 year
À1 ) also indicates that low-level N addition (10 kg N ha
À1 year
À1 )
significantly stimulated soil CH 4 uptake, while high-level N addition (140 kg N
ha
À1 year
À1 ) significantly inhibited it (Geng et al. 2017a). At a large scale, the
overall effect depends on the sum of increase in soil CH 4 uptake by low-level N
deposition and the reduction of soil CH 4 uptake by high-level N deposition. This
calls for global-scale efforts to assess the effect of N deposition on climate feedbacks
by simultaneously considering the responses of C sequestration and soil CH 4 uptake.
9.7 Conclusions and Outlook
By synthesizing existing experimental, observational and modelling results, we
conclude that continuously increasing N deposition has significantly altered the
structure and function of China’s forest ecosystems, especially in the eastern and
southern regions. Experimental results and modelling estimates generally indicate a
fertilization effect of N deposition on forest growth and consequent C sequestration.
Meanwhile, elevated N deposition has contributed significantly to soil acidification
and nutrient imbalances, which will likely diminish the positive effect on forest
productivity over time. Furthermore, N deposition can change both species composition of plant and microbial communities, which in turn alters C and nutrient
cycling. The impacts of N deposition, however, vary remarkably with background
N availability, ecosystem type and forest age (Du and Fang 2014; De Vries et al.
2014; Lu et al. 2014; Chen et al. 2015; Tian et al. 2016). Uncertainties remain in
long-term effects of N deposition in China, because most of current N addition
experiments have been initiated around the 2010s and rarely exceed 10 years. The
effects of N deposition are likely non-linear with N doses and experimental duration
(e.g. De Vries et al. 2014), and thus further research is needed to improve our
understanding of N thresholds. As a result of emission abatement policy, N deposition in China will likely decrease following the trends in Europe and the USA
(Waldner et al. 2014; Du et al. 2014c; Du 2016). There is an increasing need to
206
E. Du et al.