can expand the effects of N on herbaceous plants in this desert, which can be
evidenced by the pot experiments (Zhou et al. 2011a). In years with high precipitation, the increases in biomass production were more significant in perennial grasses
than annuals and perennial forbs under N enrichment, while the effects of N addition
on biomass production closely depended on inter- and intra-annual precipitation in
relatively dry years (Su et al. 2013a). Meanwhile, N addition induced different
effects on the growth of ephemeral plants under different snow depth cover in desert
ecosystems. For example, under 50% snow-removal treatment, N addition reduced
plant height and the biomass of the ephemeral plants, while under the snow treatment
of 100% and 150% increases, the growth of the dominant plants was stimulated by N
addition (Fan et al. 2013). Similar results were found at lower natural precipitation
(<180 mm) in a desert steppe; the above and below biomass were decreased with the
enhancements in levels of added N (Su et al. 2013b, 2014b), while at sandy
grassland ecosystems, where precipitation is relatively higher, N addition increased
the aboveground biomass significantly (Li et al. 2009; Wang 2012). The growthrelated indices, such as photosynthesis and photochemical efficiency, were also
affected by N. Nitrogen addition can increase photosynthetic rate and maximal
photochemical efficiency, especially under high-water supply (Li and Huang 2009;
Zhang et al. 2016; Zhou et al. 2010). The physiological responses to N addition also
depend on water condition. In nature precipitation, activities of oxidase-resistant
enzymes, contents of soluble protein, and soluble sugar in plants decreased with
increasing N, while in water addition condition, these indices increased and then
decreased (Zhang et al. 2018).
The growth of the nonvascular plants (biocrusts) with different types in desert
ecosystems has different sensitivities to N addition. In general, lower levels of N
addition can stimulate growth, while higher levels can exhibit negative effects. For
the three types of biocrusts in the desert, the sensitivity to N addition was
moss > cyanobacterial > lichen crust (Zhou et al. 2016a) (Fig. 11.3). In cyanobacterial
crusts, most biomass and growth indices were not affected by lower rates of N
(0–15 kg N ha
À1 year
À1
) but reduced at the relatively higher rates (30 kg N ha
À1
year
À1
). Growth and biomass of lichen crusts were not sensitive to N addition as
cyanobacterial crust, with only actual photochemical efficiency decreased at the high
rates of N addition (Zhou et al. 2016b). Low amounts of added N (0–5 kg N ha
À1
year
À1
) increased leaf size and shoot length, while high amounts (10–30 kg N ha
À1
year
À1
) suppressed almost all growth parameters (Fig. 11.4); low N increased moss
chlorophyll b, total chlorophyll content, and soluble protein concentrations, with
chlorophyll a and chlorophyll fluorescence not changed. High N stimulated moss
shoot density but decreased population biomass (Zhang et al. 2016). The sensitivity
of moss to N addition may also depend on the water content. Net photosynthetic rates
of moss increased when levels of added N was less than 2 kg N ha
À1 year
À1
, while the
rates are inhibited in low-level water content and enhanced in high water content
under higher N addition (>2 kg N ha
À1 year
À1
)(Yan et al. 2015).
11 Impacts of Nitrogen Deposition on China’s Desert Ecosystems
253
evidenced by the pot experiments (Zhou et al. 2011a). In years with high precipitation, the increases in biomass production were more significant in perennial grasses
than annuals and perennial forbs under N enrichment, while the effects of N addition
on biomass production closely depended on inter- and intra-annual precipitation in
relatively dry years (Su et al. 2013a). Meanwhile, N addition induced different
effects on the growth of ephemeral plants under different snow depth cover in desert
ecosystems. For example, under 50% snow-removal treatment, N addition reduced
plant height and the biomass of the ephemeral plants, while under the snow treatment
of 100% and 150% increases, the growth of the dominant plants was stimulated by N
addition (Fan et al. 2013). Similar results were found at lower natural precipitation
(<180 mm) in a desert steppe; the above and below biomass were decreased with the
enhancements in levels of added N (Su et al. 2013b, 2014b), while at sandy
grassland ecosystems, where precipitation is relatively higher, N addition increased
the aboveground biomass significantly (Li et al. 2009; Wang 2012). The growthrelated indices, such as photosynthesis and photochemical efficiency, were also
affected by N. Nitrogen addition can increase photosynthetic rate and maximal
photochemical efficiency, especially under high-water supply (Li and Huang 2009;
Zhang et al. 2016; Zhou et al. 2010). The physiological responses to N addition also
depend on water condition. In nature precipitation, activities of oxidase-resistant
enzymes, contents of soluble protein, and soluble sugar in plants decreased with
increasing N, while in water addition condition, these indices increased and then
decreased (Zhang et al. 2018).
The growth of the nonvascular plants (biocrusts) with different types in desert
ecosystems has different sensitivities to N addition. In general, lower levels of N
addition can stimulate growth, while higher levels can exhibit negative effects. For
the three types of biocrusts in the desert, the sensitivity to N addition was
moss > cyanobacterial > lichen crust (Zhou et al. 2016a) (Fig. 11.3). In cyanobacterial
crusts, most biomass and growth indices were not affected by lower rates of N
(0–15 kg N ha
À1 year
À1
) but reduced at the relatively higher rates (30 kg N ha
À1
year
À1
). Growth and biomass of lichen crusts were not sensitive to N addition as
cyanobacterial crust, with only actual photochemical efficiency decreased at the high
rates of N addition (Zhou et al. 2016b). Low amounts of added N (0–5 kg N ha
À1
year
À1
) increased leaf size and shoot length, while high amounts (10–30 kg N ha
À1
year
À1
) suppressed almost all growth parameters (Fig. 11.4); low N increased moss
chlorophyll b, total chlorophyll content, and soluble protein concentrations, with
chlorophyll a and chlorophyll fluorescence not changed. High N stimulated moss
shoot density but decreased population biomass (Zhang et al. 2016). The sensitivity
of moss to N addition may also depend on the water content. Net photosynthetic rates
of moss increased when levels of added N was less than 2 kg N ha
À1 year
À1
, while the
rates are inhibited in low-level water content and enhanced in high water content
under higher N addition (>2 kg N ha
À1 year
À1
)(Yan et al. 2015).
11 Impacts of Nitrogen Deposition on China’s Desert Ecosystems
253