The underlying mechanisms are not clear for the impact of N deposition on soil
enzymes. The microbial resource allocation hypothesis has been proposed as a
promising mechanism, being associated with microbial process of investigating
soil extracellular enzymes to obtain carbon and/or nutrients with increased N
availability (Allison and Vitousek 2005; Allison et al. 2010; Weintraub et al.
2013). Specifically, the hypothesis suggests that microbes would produce more
carbon- or phosphorus-degrading enzymes to maintain microbial metabolisms and
growth, while the production of N-degrading enzymes would be depressed by N
deposition. In China’s forests, N addition experiments show both consistent and
inconsistent results to the microbial resource allocation hypothesis. For instance, N
deposition decreases N-acetyl-β-glucosaminidase (associated with N cycling) activity while increases β-glucosidase (associated with C cycling) activity in two temperate forest soils (Zhou et al. 2017). In contrast to the prediction of resource
allocation hypothesis, Yang et al. (2015) found that the activity of acid phosphatase
in a Larix gmelinii plantation was depressed by N additions. In addition, Du et al.
(2014b) found that inorganic N addition not only decreased cellulase activity but
also decreased polyphenol oxidase activity in an adjacent temperature forest. Further
studies are needed to test the resource allocation hypothesis, the mechanisms of N
deposition in regulating soil C storage and the linkage between soil microbial
community structure and functioning.
9.5 Impacts of Nitrogen Deposition on Understory
Community
Elevated N deposition is now considered as one of the most detrimental drivers to
biodiversity loss (Sala et al. 2000). One main mechanism is that external N inputs
can cause asymmetric growth among different species and functional groups and
finally result in competition exclusion (Fig. 9.4, Bobbink et al. 2010). Specifically, N
deposition tends to increase dominance of nitrophile plants due to their advantages in
utilizing available N. Additionally, N deposition can result in negative effects, such
as soil acidification, nutrient imbalances and Al
3+ toxicity, and filter out the intolerant species (Fig. 9.4). Moreover, N deposition generally increases leaf N concentration and decreases resistant metabolites and makes plant more susceptible to
secondary stressors, such as pathogens, grazing and drought (Fig. 9.4).
Although the impacts of N deposition on biodiversity have aroused world-wide
concerns, there are only a few reports on the effect of N deposition on plant
biodiversity in China’s forests. In temperate forests, N deposition has been
evidenced to alter understory community composition, and the effect varied with
forest type and functional group. For instance, Du (2017) showed that 3-year N
additions exerted no significant effect on understory species richness in an
old-growth boreal forest, but resulted in an obvious increase in coverage of
graminoids and a decrease in coverage of dwarf shrub species. In an old-growth
9 Impacts of Nitrogen Deposition on Forest Ecosystems in China
199
enzymes. The microbial resource allocation hypothesis has been proposed as a
promising mechanism, being associated with microbial process of investigating
soil extracellular enzymes to obtain carbon and/or nutrients with increased N
availability (Allison and Vitousek 2005; Allison et al. 2010; Weintraub et al.
2013). Specifically, the hypothesis suggests that microbes would produce more
carbon- or phosphorus-degrading enzymes to maintain microbial metabolisms and
growth, while the production of N-degrading enzymes would be depressed by N
deposition. In China’s forests, N addition experiments show both consistent and
inconsistent results to the microbial resource allocation hypothesis. For instance, N
deposition decreases N-acetyl-β-glucosaminidase (associated with N cycling) activity while increases β-glucosidase (associated with C cycling) activity in two temperate forest soils (Zhou et al. 2017). In contrast to the prediction of resource
allocation hypothesis, Yang et al. (2015) found that the activity of acid phosphatase
in a Larix gmelinii plantation was depressed by N additions. In addition, Du et al.
(2014b) found that inorganic N addition not only decreased cellulase activity but
also decreased polyphenol oxidase activity in an adjacent temperature forest. Further
studies are needed to test the resource allocation hypothesis, the mechanisms of N
deposition in regulating soil C storage and the linkage between soil microbial
community structure and functioning.
9.5 Impacts of Nitrogen Deposition on Understory
Community
Elevated N deposition is now considered as one of the most detrimental drivers to
biodiversity loss (Sala et al. 2000). One main mechanism is that external N inputs
can cause asymmetric growth among different species and functional groups and
finally result in competition exclusion (Fig. 9.4, Bobbink et al. 2010). Specifically, N
deposition tends to increase dominance of nitrophile plants due to their advantages in
utilizing available N. Additionally, N deposition can result in negative effects, such
as soil acidification, nutrient imbalances and Al
3+ toxicity, and filter out the intolerant species (Fig. 9.4). Moreover, N deposition generally increases leaf N concentration and decreases resistant metabolites and makes plant more susceptible to
secondary stressors, such as pathogens, grazing and drought (Fig. 9.4).
Although the impacts of N deposition on biodiversity have aroused world-wide
concerns, there are only a few reports on the effect of N deposition on plant
biodiversity in China’s forests. In temperate forests, N deposition has been
evidenced to alter understory community composition, and the effect varied with
forest type and functional group. For instance, Du (2017) showed that 3-year N
additions exerted no significant effect on understory species richness in an
old-growth boreal forest, but resulted in an obvious increase in coverage of
graminoids and a decrease in coverage of dwarf shrub species. In an old-growth
9 Impacts of Nitrogen Deposition on Forest Ecosystems in China
199