because the decline of soil pH would reduce soil microbial activities but had neutral
effect on plant growth (Liu et al. 2014).
Arbuscular mycorrhizal fungi (AMF), belonging to the phylum Glomeromycota,
can form symbiotic associations with many terrestrial plant species and help their
host in nutrient uptake and improving tolerance to abiotic stress. Chen et al. (2017b)
examined the impacts of long-term N addition on the taxonomic and phylogenetic
diversity of AMF community in a temperate steppe. Nitrogen addition significantly
reduced AMF abundance, including root colonization and fungal biomass. Nitrogen
addition reduced both taxonomic and phylogenetic diversity of AMF. In another
study, however, they reported that 6-year N addition significantly altered AMF
communities in both soils and roots, but had no significant influence on the AMF
abundance (Chen et al. 2014). Nitrogen addition would also affect spore density of
AMF. In a Tibetan alpine meadow, Zheng et al. (2014) found that N addition rate,
irrespective of N forms, would reduce spore density of AMF by affecting soil
characteristics. Moreover, AM fungal extraradical hyphal density was reduced by
N addition in May but not in August. Together, those results suggest that the impacts
of N addition on the AMF abundance would be context dependent and always vary
across different years and seasons.
Nitrogen deposition could affect soil fauna diversity. Nematodes are one of the
most abundant and diverse groups of soil organisms, occupying key trophic positions in the soil food webs. In an old-field grassland ecosystem of norther China, N
addition reduced the generic richness of soil nematode and had no impact on their
abundance (Song et al. 2016). Nitrogen addition significantly altered the community
structure of soil nematodes by promoting the abundance of bacterivores and decreasing that of fungivores and omnivores-predators. Such a result is well consistent with
the finding that N addition would favour soil bacteria community instead of soil
fungal community. The accumulation of ammonium in soils following N addition
may account for the reduction of soil nematode richness, as indicated by the negative
relationship between herbivorous nematodes and soil ammonium concentrations
(Wei et al. 2012). The impacts of N addition on soil nematode communities might
be dependent on the addition rates. In an experiment with relatively lower addition
rates (20 and 50 kg N ha
À1 year
À1 ), Ruan et al. (2012) found that N addition had
moderate effects on the composition and structure of soil nematode community.
10.6 Impacts on Primary Productivity
Atmospheric N deposition has consequences on several fundamental ecological
processes, such as carbon cycling and climate regulation, and could lead to shifts
in fundamental Earth system processes including primary production. Using an
international multi-scale dataset, Stevens et al. (2015) found that atmospheric N
deposition is positively correlated with aboveground net primary productivity
10 Impacts of Nitrogen Deposition on China’s Grassland Ecosystems
237
effect on plant growth (Liu et al. 2014).
Arbuscular mycorrhizal fungi (AMF), belonging to the phylum Glomeromycota,
can form symbiotic associations with many terrestrial plant species and help their
host in nutrient uptake and improving tolerance to abiotic stress. Chen et al. (2017b)
examined the impacts of long-term N addition on the taxonomic and phylogenetic
diversity of AMF community in a temperate steppe. Nitrogen addition significantly
reduced AMF abundance, including root colonization and fungal biomass. Nitrogen
addition reduced both taxonomic and phylogenetic diversity of AMF. In another
study, however, they reported that 6-year N addition significantly altered AMF
communities in both soils and roots, but had no significant influence on the AMF
abundance (Chen et al. 2014). Nitrogen addition would also affect spore density of
AMF. In a Tibetan alpine meadow, Zheng et al. (2014) found that N addition rate,
irrespective of N forms, would reduce spore density of AMF by affecting soil
characteristics. Moreover, AM fungal extraradical hyphal density was reduced by
N addition in May but not in August. Together, those results suggest that the impacts
of N addition on the AMF abundance would be context dependent and always vary
across different years and seasons.
Nitrogen deposition could affect soil fauna diversity. Nematodes are one of the
most abundant and diverse groups of soil organisms, occupying key trophic positions in the soil food webs. In an old-field grassland ecosystem of norther China, N
addition reduced the generic richness of soil nematode and had no impact on their
abundance (Song et al. 2016). Nitrogen addition significantly altered the community
structure of soil nematodes by promoting the abundance of bacterivores and decreasing that of fungivores and omnivores-predators. Such a result is well consistent with
the finding that N addition would favour soil bacteria community instead of soil
fungal community. The accumulation of ammonium in soils following N addition
may account for the reduction of soil nematode richness, as indicated by the negative
relationship between herbivorous nematodes and soil ammonium concentrations
(Wei et al. 2012). The impacts of N addition on soil nematode communities might
be dependent on the addition rates. In an experiment with relatively lower addition
rates (20 and 50 kg N ha
À1 year
À1 ), Ruan et al. (2012) found that N addition had
moderate effects on the composition and structure of soil nematode community.
10.6 Impacts on Primary Productivity
Atmospheric N deposition has consequences on several fundamental ecological
processes, such as carbon cycling and climate regulation, and could lead to shifts
in fundamental Earth system processes including primary production. Using an
international multi-scale dataset, Stevens et al. (2015) found that atmospheric N
deposition is positively correlated with aboveground net primary productivity
10 Impacts of Nitrogen Deposition on China’s Grassland Ecosystems
237