(ANPP) across global herbaceous ecosystems. Furthermore, it is estimated that
ANPP would increase by 3% in response to an increase of 1 kg N ha
À1 year
À1
(Stevens et al. 2015).
The effects of N deposition on ANPP have been extensively investigated in
diverse grasslands in China (Bai et al. 2010; Li et al. 2015; Zhang et al. 2015) as
well as in grasslands all over the world. In contrast, few studies have evaluated
whether and how N deposition would affect belowground net primary productivity
(BNPP), which may contribute much more to the overall net primary productivity
than ANPP in grasslands. For instance, it has been estimated that BNPP accounted
for 80% of grassland net primary productivity (Peek 2007). Nitrogen addition
reduces BNPP in a semiarid grassland of China and increases the allocation of
primary productivity to upper soils (Xu et al. 2017), implying that N deposition
may accelerate carbon and nutrient cycling in this ecosystem and thus may increases
the risk of soil carbon and nitrogen losses. Using different methods including
rhizotron, ingrowth core, and soil monoliths, Bai et al. (2015) examined the
responses of BNPP to chronic low-level N addition (20 kg N ha
À1 year
À1 ) in a
temperate steppe of China at the levels of ecosystem, plant functional group, and
species. They showed that the 8-year N addition significantly decreased root productivity by ~30%. They further uncovered the mechanisms underlying the negative
impacts of N deposition on BNPP. This is because N deposition acidified soils and
increased the concentration of extractable soil Mn. While the forb species showed an
enhancement of foliar Mn concentrations, grasses showed neutral responses. The
accumulation of Mn in the leaves of forbs may account for the significant reductions
of photosynthesis rate because excessive accumulation of Mn can be toxic to plants
by targeting photosynthetic apparatus and processes. The study of Bai et al. (2015) is
among the first ones to elucidate the mechanisms by which chronic and low-level N
deposition would have negative impacts on root production and BNPP in grasslands.
10.7 Conclusions and Outlook
It is well established that increased N deposition is the most important drivers of
plant diversity loss, and in turn diversity loss will negatively affect ecosystem
functioning (Hautier et al. 2015; Humbert et al. 2016). For example, grass loss
will affect aboveground biomass production, while forb loss will affect plant species
richness (Li et al. 2018). Nitrogen deposition also can influence soil enzymatic
activities and microbial community properties and increases net N mineralization
rate and thus soil inorganic N availability, which are closely related with the
emissions of greenhouse gas (Jing et al. 2017; Calvo-Fernández et al. 2018).
However, the effects of N deposition on soil characteristics are varied across
ecosystems and these effects on soil C and N even controversial among different
studies (Calvo-Fernández et al. 2018; Zheng et al. 2018). In addition to the effects of
N deposition on plant community and soil, primary and secondary consumers will
also be negatively influenced (Stevens et al. 2018). But the researches in this area
238
X. Lü et al.
ANPP would increase by 3% in response to an increase of 1 kg N ha
À1 year
À1
(Stevens et al. 2015).
The effects of N deposition on ANPP have been extensively investigated in
diverse grasslands in China (Bai et al. 2010; Li et al. 2015; Zhang et al. 2015) as
well as in grasslands all over the world. In contrast, few studies have evaluated
whether and how N deposition would affect belowground net primary productivity
(BNPP), which may contribute much more to the overall net primary productivity
than ANPP in grasslands. For instance, it has been estimated that BNPP accounted
for 80% of grassland net primary productivity (Peek 2007). Nitrogen addition
reduces BNPP in a semiarid grassland of China and increases the allocation of
primary productivity to upper soils (Xu et al. 2017), implying that N deposition
may accelerate carbon and nutrient cycling in this ecosystem and thus may increases
the risk of soil carbon and nitrogen losses. Using different methods including
rhizotron, ingrowth core, and soil monoliths, Bai et al. (2015) examined the
responses of BNPP to chronic low-level N addition (20 kg N ha
À1 year
À1 ) in a
temperate steppe of China at the levels of ecosystem, plant functional group, and
species. They showed that the 8-year N addition significantly decreased root productivity by ~30%. They further uncovered the mechanisms underlying the negative
impacts of N deposition on BNPP. This is because N deposition acidified soils and
increased the concentration of extractable soil Mn. While the forb species showed an
enhancement of foliar Mn concentrations, grasses showed neutral responses. The
accumulation of Mn in the leaves of forbs may account for the significant reductions
of photosynthesis rate because excessive accumulation of Mn can be toxic to plants
by targeting photosynthetic apparatus and processes. The study of Bai et al. (2015) is
among the first ones to elucidate the mechanisms by which chronic and low-level N
deposition would have negative impacts on root production and BNPP in grasslands.
10.7 Conclusions and Outlook
It is well established that increased N deposition is the most important drivers of
plant diversity loss, and in turn diversity loss will negatively affect ecosystem
functioning (Hautier et al. 2015; Humbert et al. 2016). For example, grass loss
will affect aboveground biomass production, while forb loss will affect plant species
richness (Li et al. 2018). Nitrogen deposition also can influence soil enzymatic
activities and microbial community properties and increases net N mineralization
rate and thus soil inorganic N availability, which are closely related with the
emissions of greenhouse gas (Jing et al. 2017; Calvo-Fernández et al. 2018).
However, the effects of N deposition on soil characteristics are varied across
ecosystems and these effects on soil C and N even controversial among different
studies (Calvo-Fernández et al. 2018; Zheng et al. 2018). In addition to the effects of
N deposition on plant community and soil, primary and secondary consumers will
also be negatively influenced (Stevens et al. 2018). But the researches in this area
238
X. Lü et al.