9.4 Impacts of Nitrogen Deposition on Soil Microorganisms
and Enzymes
9.4.1 Impacts of Nitrogen Deposition on Microbial Biomass
Soil microorganisms are an essential component and play a fundamental role in
forest ecosystems. Soil microorganisms tightly interact with plant roots and drive a
variety of ecosystem processes (e.g. C, N and P cycling) through their metabolic
machinery capability, nutrient uptake, mineralization or immobilization (Wardle
1992; Bohlen et al. 2001; Fang et al. 2015). Moreover, microbial diversity from
gene to ecosystem levels, encompassing genetic diversity (i.e. genetic materials and
information transmitted along phylogenetic generations), species diversity
(i.e. richness and evenness of specific taxon) and structural and functional diversity
of communities (Torsvik and Øvreås 2002), significantly shapes the aboveground
biodiversity of an ecosystem and regulates its structure, functioning and stability
(van der Heijden et al. 2008).
In China’s forests, a recent meta-analysis indicates an overall negative effect of N
deposition on microbial biomass C (MBC) and N (MBN) (Tian et al. 2018).
However, the effect of N deposition on soil microbial biomass is evidenced to
vary significantly with forest type and soil layer (Fig. 9.3). For instance, N additions
significantly decreased MBC in subtropical forest, while it showed no significant
response in tropical and temperate forests. MBN showed negative response to N
additions in subtropical and temperate forests, while a positive response is likely in
tropical forest. The negative effect of N deposition on microbial biomass is significant in surface soil layers (e.g., 0–10 and 10–20 cm), while it became insignificant
in deeper soil layer (e.g., >20 cm) (Fig. 9.3). Moreover, the responses of microbial
biomass vary with levels of N addition, showing a positive response to low-level N
additions (0–30 kg N ha
À1 year
À1 ), a neutral response to mediate-level N additions
(30–60 kg N ha
À1 year
À1 ) and a negative response to high-level N additions (>60 kg
N ha
À1 year
À1 ). In N-limited forest ecosystems, external N inputs could meet the
requirements of microbes and accelerate their growth, while excess N deposition can
exert an adverse effect on microbes due to the N-induced soil acidification and the
release of toxic ions (e.g. Al
3+ and Fe
3+ ) (Aber et al. 1998; Högberg et al. 2006; Lu
et al. 2014; Chen et al. 2016). Despite emerging experimental studies in China, the
critical dosages of N deposition which shifted from the positive to negative effects
on microbes remain unclear across forest types.
9.4.2 Impacts of Nitrogen Deposition on Microbial Diversity
Several approaches have been used to measure the composition of microbial community. The traditional culture and plate count method is limited to the culturable
soil bacteria, fungi and actinomycetes. As a robust fingerprinting of microbial
196
E. Du et al.
and Enzymes
9.4.1 Impacts of Nitrogen Deposition on Microbial Biomass
Soil microorganisms are an essential component and play a fundamental role in
forest ecosystems. Soil microorganisms tightly interact with plant roots and drive a
variety of ecosystem processes (e.g. C, N and P cycling) through their metabolic
machinery capability, nutrient uptake, mineralization or immobilization (Wardle
1992; Bohlen et al. 2001; Fang et al. 2015). Moreover, microbial diversity from
gene to ecosystem levels, encompassing genetic diversity (i.e. genetic materials and
information transmitted along phylogenetic generations), species diversity
(i.e. richness and evenness of specific taxon) and structural and functional diversity
of communities (Torsvik and Øvreås 2002), significantly shapes the aboveground
biodiversity of an ecosystem and regulates its structure, functioning and stability
(van der Heijden et al. 2008).
In China’s forests, a recent meta-analysis indicates an overall negative effect of N
deposition on microbial biomass C (MBC) and N (MBN) (Tian et al. 2018).
However, the effect of N deposition on soil microbial biomass is evidenced to
vary significantly with forest type and soil layer (Fig. 9.3). For instance, N additions
significantly decreased MBC in subtropical forest, while it showed no significant
response in tropical and temperate forests. MBN showed negative response to N
additions in subtropical and temperate forests, while a positive response is likely in
tropical forest. The negative effect of N deposition on microbial biomass is significant in surface soil layers (e.g., 0–10 and 10–20 cm), while it became insignificant
in deeper soil layer (e.g., >20 cm) (Fig. 9.3). Moreover, the responses of microbial
biomass vary with levels of N addition, showing a positive response to low-level N
additions (0–30 kg N ha
À1 year
À1 ), a neutral response to mediate-level N additions
(30–60 kg N ha
À1 year
À1 ) and a negative response to high-level N additions (>60 kg
N ha
À1 year
À1 ). In N-limited forest ecosystems, external N inputs could meet the
requirements of microbes and accelerate their growth, while excess N deposition can
exert an adverse effect on microbes due to the N-induced soil acidification and the
release of toxic ions (e.g. Al
3+ and Fe
3+ ) (Aber et al. 1998; Högberg et al. 2006; Lu
et al. 2014; Chen et al. 2016). Despite emerging experimental studies in China, the
critical dosages of N deposition which shifted from the positive to negative effects
on microbes remain unclear across forest types.
9.4.2 Impacts of Nitrogen Deposition on Microbial Diversity
Several approaches have been used to measure the composition of microbial community. The traditional culture and plate count method is limited to the culturable
soil bacteria, fungi and actinomycetes. As a robust fingerprinting of microbial
196
E. Du et al.