The effects of N addition on diversity are also related to precipitation and exposed
years and rates of N application. Under low simulated precipitation, N enhanced the
richness and ecological dominance, while the decreased dominance was found under
higher precipitation (Guo 2016). Nitrogen addition can significantly decrease the
species richness in the same snow treatments on desert (Fan et al. 2013). The N
addition can reduce the richness from 17 to 11, with increases in Poaceae family and
decreases in Cruciferae family (Zhao et al. 2014a). The treatment effects on species
richness depended on the years of N exposed and N added rates (Zhou et al. 2018).
The longer periods the plants exposed to N addition, the more richness losses were
found.
11.6.2 Density and Coverage
Except for changes in richness and species dominance, N addition also affects the
plant density and coverage. Regardless of fertilizer types, population density was
reduced by the increasing levels of fertilization. Specifically, total density and
density of some species were significantly decreased by N treatments (Su et al.
2013b, 2014b). Nitrogen addition significantly reduced the density of ephemerals in
irrespective of snow depths in desert ecosystems (Fan et al. 2013). The vegetation
coverage and the plant density were significantly increased by NP and NPK additions, while no significant effects were found in other nutrient additions (Zhao et al.
2013). In desert regions, fertilization can affect productivity of herbaceous plants by
inhibiting population density. In moss biocrusts, although the moss shoot density
increased with increasing N, the population biomass was reduced by high N addition
(Zhang et al. 2016).
The water conditions also affect the N effects on density. In the Horqin Sandy
Land, N addition had no effects on root length density in dry season, while root
length density was affected by promoting root distribution into deeper soil layer in
wet season (Huang et al. 2009). Moreover, the effects of N addition on the density
also depended on the seasons and years to N exposed. Nitrogen addition showed
small effects on plant density in spring at the beginning year, with the years growth,
density was enhanced at lower rates of N addition and reduced by high N addition.
While, no obvious effects of N addition were found in summer (Zhou et al. 2018).
11.7 Conclusion and Outlook
Nitrogen deposition affected many ecosystem processes of deserts in China. From
microscale, the N deposition changed the soil availability and plant N uptake. High
N addition generally causes toxic effects on microbial activity. Soil enzyme activities, which related to the nutrient transformation, can be altered by N deposition,
while the responses depended on N addition rates and specific enzymes. Oxidative
256
X. Zhou et al.
years and rates of N application. Under low simulated precipitation, N enhanced the
richness and ecological dominance, while the decreased dominance was found under
higher precipitation (Guo 2016). Nitrogen addition can significantly decrease the
species richness in the same snow treatments on desert (Fan et al. 2013). The N
addition can reduce the richness from 17 to 11, with increases in Poaceae family and
decreases in Cruciferae family (Zhao et al. 2014a). The treatment effects on species
richness depended on the years of N exposed and N added rates (Zhou et al. 2018).
The longer periods the plants exposed to N addition, the more richness losses were
found.
11.6.2 Density and Coverage
Except for changes in richness and species dominance, N addition also affects the
plant density and coverage. Regardless of fertilizer types, population density was
reduced by the increasing levels of fertilization. Specifically, total density and
density of some species were significantly decreased by N treatments (Su et al.
2013b, 2014b). Nitrogen addition significantly reduced the density of ephemerals in
irrespective of snow depths in desert ecosystems (Fan et al. 2013). The vegetation
coverage and the plant density were significantly increased by NP and NPK additions, while no significant effects were found in other nutrient additions (Zhao et al.
2013). In desert regions, fertilization can affect productivity of herbaceous plants by
inhibiting population density. In moss biocrusts, although the moss shoot density
increased with increasing N, the population biomass was reduced by high N addition
(Zhang et al. 2016).
The water conditions also affect the N effects on density. In the Horqin Sandy
Land, N addition had no effects on root length density in dry season, while root
length density was affected by promoting root distribution into deeper soil layer in
wet season (Huang et al. 2009). Moreover, the effects of N addition on the density
also depended on the seasons and years to N exposed. Nitrogen addition showed
small effects on plant density in spring at the beginning year, with the years growth,
density was enhanced at lower rates of N addition and reduced by high N addition.
While, no obvious effects of N addition were found in summer (Zhou et al. 2018).
11.7 Conclusion and Outlook
Nitrogen deposition affected many ecosystem processes of deserts in China. From
microscale, the N deposition changed the soil availability and plant N uptake. High
N addition generally causes toxic effects on microbial activity. Soil enzyme activities, which related to the nutrient transformation, can be altered by N deposition,
while the responses depended on N addition rates and specific enzymes. Oxidative
256
X. Zhou et al.