In a grassland of northern China, a 4-year N addition significantly enhances the
rates of nitrification and N mineralization by 288% and 149% (Wang et al. 2015).
Furthermore, N addition enhances the concentration of microbial biomass N by 12%
but decreases that of microbial biomass C by 10%. Those results indicate that N
addition has positive effects on soil N transformation in this grassland ecosystem.
Similarly, Zhang et al. (2013a) and Ma et al. (2011) also reported that N addition
significantly stimulates the soil N mineralization processes in meadow steppe.
However, the impacts of N deposition on the soil N transformation would vary
among different plant communities and across different years. Wang et al. (2011)
examined how N additions affect microbial N transformation in grass-dominated,
herb-dominated, and grass-herb-mixed communities in old-field grassland of northern China. They found that soil net N mineralization rate was 177% and 69% higher
in mixed communities as compared to grass-dominated or herb-dominated communities, respectively. The concentration of soil microbial biomass N was also significantly different among the three communities, with the highest value being
observed in the grass-dominated community. The responses of microbial N transformations in soils under different communities to experimental N addition were
limited by soil water availability, highlighting the importance of interannual variation of precipitation in driving the impacts of N deposition on soil N transformation.
Actually, empirical evidence showed that rainfall is significantly correlated with soil
net nitrification in grasslands of China (Zhang et al. 2012).
The substantial changes of N transformation processes following N enrichment
resulted from the changes of soil microbial community composition and the composition of related functional genes. In a temperate grassland of northern China,
Ning et al. (2015) reported the high sensitivities of the abundance of N-related
functional genes to N deposition (Fig. 10.2). They found that the abundances of N
fixation gene nifH, nitrification gene archaeal amoA (AOA), and denitrification
genes nirS and nosZ all increased due to the enhancement of soil-available N at
treatments with low N addition rates but were suppressed by salt toxicity and
acidification at treatments with high N addition rates. Similarly, in an alpine grassland on the Tibetan Plateau, N addition significantly increased the abundance of
ammonia-oxidizing bacteria (AOB) and AOA, which is positively correlated with
potential soil nitrification rates (Tian et al. 2014).
The changes of soil physical and chemical properties following N enrichment
would have significant consequences on the growth and chemical traits of plants, an
important driver of nutrient cycling in ecosystems (Hobbie 2015). Results from a
meta-analysis with studies all over the world showed that biomass and N concentration were increased under N enrichment conditions across 456 terrestrial plant
species, with greater biomass increases in herbaceous than in woody species (Xia
and Wan 2008). Such a result indicated that grasslands are more sensitive to N
deposition than those ecosystems dominated by woody species. In a temperate
steppe, N addition significantly decreased C:N ratios but increased N:P ratios in
dominant species (Lü et al. 2012). However, such changes are highly dependent on
the water availability in semiarid grasslands. Nitrogen addition had no effect on
foliar C:N ratios in both green and senesced leaves and N:P ratios in senesced leaves
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X. Lü et al.
rates of nitrification and N mineralization by 288% and 149% (Wang et al. 2015).
Furthermore, N addition enhances the concentration of microbial biomass N by 12%
but decreases that of microbial biomass C by 10%. Those results indicate that N
addition has positive effects on soil N transformation in this grassland ecosystem.
Similarly, Zhang et al. (2013a) and Ma et al. (2011) also reported that N addition
significantly stimulates the soil N mineralization processes in meadow steppe.
However, the impacts of N deposition on the soil N transformation would vary
among different plant communities and across different years. Wang et al. (2011)
examined how N additions affect microbial N transformation in grass-dominated,
herb-dominated, and grass-herb-mixed communities in old-field grassland of northern China. They found that soil net N mineralization rate was 177% and 69% higher
in mixed communities as compared to grass-dominated or herb-dominated communities, respectively. The concentration of soil microbial biomass N was also significantly different among the three communities, with the highest value being
observed in the grass-dominated community. The responses of microbial N transformations in soils under different communities to experimental N addition were
limited by soil water availability, highlighting the importance of interannual variation of precipitation in driving the impacts of N deposition on soil N transformation.
Actually, empirical evidence showed that rainfall is significantly correlated with soil
net nitrification in grasslands of China (Zhang et al. 2012).
The substantial changes of N transformation processes following N enrichment
resulted from the changes of soil microbial community composition and the composition of related functional genes. In a temperate grassland of northern China,
Ning et al. (2015) reported the high sensitivities of the abundance of N-related
functional genes to N deposition (Fig. 10.2). They found that the abundances of N
fixation gene nifH, nitrification gene archaeal amoA (AOA), and denitrification
genes nirS and nosZ all increased due to the enhancement of soil-available N at
treatments with low N addition rates but were suppressed by salt toxicity and
acidification at treatments with high N addition rates. Similarly, in an alpine grassland on the Tibetan Plateau, N addition significantly increased the abundance of
ammonia-oxidizing bacteria (AOB) and AOA, which is positively correlated with
potential soil nitrification rates (Tian et al. 2014).
The changes of soil physical and chemical properties following N enrichment
would have significant consequences on the growth and chemical traits of plants, an
important driver of nutrient cycling in ecosystems (Hobbie 2015). Results from a
meta-analysis with studies all over the world showed that biomass and N concentration were increased under N enrichment conditions across 456 terrestrial plant
species, with greater biomass increases in herbaceous than in woody species (Xia
and Wan 2008). Such a result indicated that grasslands are more sensitive to N
deposition than those ecosystems dominated by woody species. In a temperate
steppe, N addition significantly decreased C:N ratios but increased N:P ratios in
dominant species (Lü et al. 2012). However, such changes are highly dependent on
the water availability in semiarid grasslands. Nitrogen addition had no effect on
foliar C:N ratios in both green and senesced leaves and N:P ratios in senesced leaves
222
X. Lü et al.