data on N fluxes in precipitation, throughfall and soil leaching from 69 forest
ecosystems at 50 sites throughout China, Fang et al. (2011b) estimated that on
average 22% of throughfall inorganic N inputs were leached out from forest
ecosystems.
In general, tropical and subtropical forests have a lower N retention capacity than
temperate and boreal forests. In four subtropical forested catchments of southern
China, Larssen et al. (2011) found that the average N fluxes were approximately
similar in soil solution as in throughfall deposition, suggesting minor N retention
rates in the soils. Generally, high-level N deposition often leads to large N leaching
losses, mainly as nitrate. Across an urban-rural transect (16.2–38.2 kg N ha
À1 year
À1
in bulk deposition) in southern China, Fang et al. (2011a) showed that throughfall N
deposition was correlated positively with NO 3
À leaching to stream and negatively
with pH values in soil and stream water. By synthesizing data from 255 forested
headwater streams in southern and northeastern China, Yu et al. (2017) concluded
that considerable NO 3
À leaching to stream waters did not occur unless N deposition
was higher than 25 kg N ha
À1 year
À1 . These findings are also supported by N
addition experiments (Fang et al. 2009; Lu et al. 2014; Huang et al. 2015).
Nitrogen deposition, as an important acid source, can decline soil buffering capacity, resulting in soil acidification and alteration of soil solution chemistry (Sun et al.
2006; Lu et al. 2014, 2015, 2018; Huang et al. 2015). In a subtropical mature forest, Lu
et al. (2014) found that long-term N addition significantly changed soil solution
chemistry and decreased soil pH in the surface layer. In a subtropical Masson pine
(Pinus massoniana) forest at Tieshanping, Southwest China, Huang et al. (2015)
found that long-term N addition (40 kg ha
À1 year
À1
) caused increasing molar ratio
of dissolved Al
3+ to base cations (Ca
2+
+Mg
2+
+K
+
), primarily due to the leaching of
base cations. They further found a significant decrease of the SO 4
2À concentration in
soil water due to the NH 4 NO 3 treatment, probably due to increased SO 4
2À adsorption.
Overall, high N inputs generally decrease soil solution pH, but increase leaching losses
of base cations such as Ca
2+ and Mg
2+
. In that case, however, cation-deficient plants
have been evidenced to maintain nutrient balance in part by increasing transpiration
and thereby altering hydrological cycling (Lu et al. 2018).
Dissolved organic matter has been recently recognized as an important component of terrestrial element cycling. Fang et al. (2009) reported dissolved organic N
(DON) and dissolved organic C (DOC) losses from soils in three typical forests in
subtropical China, which received very high long-term ambient atmospheric N
deposition with or without additional experimental N inputs. They found that
DON could be a significant pathway of N loss from N-saturated forests, while a
concurrent increase in DOC loss was observed only in the pine forest, even though
DOC:DON ratios declined in all three forests. After 7 years of N addition, however,
Lu et al. (2013) found that elevated N inputs significantly decreased DOC concentrations in soil solution and annual DOC effluxes from the primary rooting zone in
this old-growth forest and concluded that chemo-physical controls (solution acidity
change and soil sorption) rather than biological controls may mainly account for the
decreases.
9 Impacts of Nitrogen Deposition on Forest Ecosystems in China
195
ecosystems at 50 sites throughout China, Fang et al. (2011b) estimated that on
average 22% of throughfall inorganic N inputs were leached out from forest
ecosystems.
In general, tropical and subtropical forests have a lower N retention capacity than
temperate and boreal forests. In four subtropical forested catchments of southern
China, Larssen et al. (2011) found that the average N fluxes were approximately
similar in soil solution as in throughfall deposition, suggesting minor N retention
rates in the soils. Generally, high-level N deposition often leads to large N leaching
losses, mainly as nitrate. Across an urban-rural transect (16.2–38.2 kg N ha
À1 year
À1
in bulk deposition) in southern China, Fang et al. (2011a) showed that throughfall N
deposition was correlated positively with NO 3
À leaching to stream and negatively
with pH values in soil and stream water. By synthesizing data from 255 forested
headwater streams in southern and northeastern China, Yu et al. (2017) concluded
that considerable NO 3
À leaching to stream waters did not occur unless N deposition
was higher than 25 kg N ha
À1 year
À1 . These findings are also supported by N
addition experiments (Fang et al. 2009; Lu et al. 2014; Huang et al. 2015).
Nitrogen deposition, as an important acid source, can decline soil buffering capacity, resulting in soil acidification and alteration of soil solution chemistry (Sun et al.
2006; Lu et al. 2014, 2015, 2018; Huang et al. 2015). In a subtropical mature forest, Lu
et al. (2014) found that long-term N addition significantly changed soil solution
chemistry and decreased soil pH in the surface layer. In a subtropical Masson pine
(Pinus massoniana) forest at Tieshanping, Southwest China, Huang et al. (2015)
found that long-term N addition (40 kg ha
À1 year
À1
) caused increasing molar ratio
of dissolved Al
3+ to base cations (Ca
2+
+Mg
2+
+K
+
), primarily due to the leaching of
base cations. They further found a significant decrease of the SO 4
2À concentration in
soil water due to the NH 4 NO 3 treatment, probably due to increased SO 4
2À adsorption.
Overall, high N inputs generally decrease soil solution pH, but increase leaching losses
of base cations such as Ca
2+ and Mg
2+
. In that case, however, cation-deficient plants
have been evidenced to maintain nutrient balance in part by increasing transpiration
and thereby altering hydrological cycling (Lu et al. 2018).
Dissolved organic matter has been recently recognized as an important component of terrestrial element cycling. Fang et al. (2009) reported dissolved organic N
(DON) and dissolved organic C (DOC) losses from soils in three typical forests in
subtropical China, which received very high long-term ambient atmospheric N
deposition with or without additional experimental N inputs. They found that
DON could be a significant pathway of N loss from N-saturated forests, while a
concurrent increase in DOC loss was observed only in the pine forest, even though
DOC:DON ratios declined in all three forests. After 7 years of N addition, however,
Lu et al. (2013) found that elevated N inputs significantly decreased DOC concentrations in soil solution and annual DOC effluxes from the primary rooting zone in
this old-growth forest and concluded that chemo-physical controls (solution acidity
change and soil sorption) rather than biological controls may mainly account for the
decreases.
9 Impacts of Nitrogen Deposition on Forest Ecosystems in China
195