species in desert ecosystems. The increased individual growth under N addition was
not always observed in productivity because of changes in the community structure.
With the increase in N added rates, abundance, richness, and density usually
decreased, and the effects were affected by the years exposed to N addition. More
water supply can expand the N effects on plant growth and diversity in desert
ecosystems. Lower levels of N addition also stimulated growth of nonvascular plants
(biocrusts), while higher levels exhibited negative effects.
11.1 Introduction
Desert ecosystems, which occupy almost one-quarter of the earth’s land surface, are
limited by nitrogen (N) and have a limited capacity to consume N (Clark and Tilman
2008; Vourlitis et al. 2007). Thus, the desert can highly respond to even small inputs
of N, and N deposition is generally more impactful in desert ecosystem compared to
other ecosystems (Adams 2003). As in other ecosystems, N deposition in desert
ecosystems can lead to changes in plant community, soil biogeochemistry, and
ecosystem productivity (Baez et al. 2007; Schaeffer and Evans 2005; Zhou et al.
2018).
Following water, N is the second driving factor in desert ecosystems, where dry
deposition is the dominant part of deposited N (Li et al. 2013). Thus, the high dry
deposition can lead to pulse effects on the desert ecosystems after rainfall (James and
Richards 2006). For example, the rainfall events elicited an intense carbon dioxide
(CO 2 ) release and magnify the influences of N addition on CO 2 (Zhou and Zhang
2014b). Water regulation on the effects of N deposition increases the complexity
when we evaluate changes in structure and function of the desert ecosystem under N
deposition.
In addition, biocrusts, which are a complex of bacteria (cyanobacteria), fungi,
algae, lichen, and mosses, often develop well on the desert soil surface. The coverage
of biocrusts can be up to 70% in some ecosystems (Evans and Johansen 1999), and
biocrusts are considered as an organizing principle in drylands (Belnap et al. 2016).
Biocrusts not only are the zone of high nutrient transformation but also can fix the
atmosphere N 2 (diazotrophy) to constitute a dominant source of N in the desert
(Evans and Ehleringer 1993). Nitrogen deposition can affect the biological activity
of biocrusts (Zhou et al. 2016b), which may further greatly influence the N supply of
the desert ecosystems, resulting in cascade effects of some ecosystem processes.
Therefore, the effects of increasing N deposition on biocrusts deserve to be evaluated
in desert ecosystems.
China has large area of desert, most of which distribute in the north of China.
North China is also under the threat of increasing N deposition (Li et al. 2013; Pan
et al. 2012). In recent years, varied N enhancement experiments were conducted in
these desert ecosystems (Guo 2016; Su et al. 2014a; Zhou et al. 2012). In this chapter,
based on the experimental results from different deserts of China, we firstly discuss
impacts of N addition on nutrient and carbon cycling. We then address the effects of
246
X. Zhou et al.
not always observed in productivity because of changes in the community structure.
With the increase in N added rates, abundance, richness, and density usually
decreased, and the effects were affected by the years exposed to N addition. More
water supply can expand the N effects on plant growth and diversity in desert
ecosystems. Lower levels of N addition also stimulated growth of nonvascular plants
(biocrusts), while higher levels exhibited negative effects.
11.1 Introduction
Desert ecosystems, which occupy almost one-quarter of the earth’s land surface, are
limited by nitrogen (N) and have a limited capacity to consume N (Clark and Tilman
2008; Vourlitis et al. 2007). Thus, the desert can highly respond to even small inputs
of N, and N deposition is generally more impactful in desert ecosystem compared to
other ecosystems (Adams 2003). As in other ecosystems, N deposition in desert
ecosystems can lead to changes in plant community, soil biogeochemistry, and
ecosystem productivity (Baez et al. 2007; Schaeffer and Evans 2005; Zhou et al.
2018).
Following water, N is the second driving factor in desert ecosystems, where dry
deposition is the dominant part of deposited N (Li et al. 2013). Thus, the high dry
deposition can lead to pulse effects on the desert ecosystems after rainfall (James and
Richards 2006). For example, the rainfall events elicited an intense carbon dioxide
(CO 2 ) release and magnify the influences of N addition on CO 2 (Zhou and Zhang
2014b). Water regulation on the effects of N deposition increases the complexity
when we evaluate changes in structure and function of the desert ecosystem under N
deposition.
In addition, biocrusts, which are a complex of bacteria (cyanobacteria), fungi,
algae, lichen, and mosses, often develop well on the desert soil surface. The coverage
of biocrusts can be up to 70% in some ecosystems (Evans and Johansen 1999), and
biocrusts are considered as an organizing principle in drylands (Belnap et al. 2016).
Biocrusts not only are the zone of high nutrient transformation but also can fix the
atmosphere N 2 (diazotrophy) to constitute a dominant source of N in the desert
(Evans and Ehleringer 1993). Nitrogen deposition can affect the biological activity
of biocrusts (Zhou et al. 2016b), which may further greatly influence the N supply of
the desert ecosystems, resulting in cascade effects of some ecosystem processes.
Therefore, the effects of increasing N deposition on biocrusts deserve to be evaluated
in desert ecosystems.
China has large area of desert, most of which distribute in the north of China.
North China is also under the threat of increasing N deposition (Li et al. 2013; Pan
et al. 2012). In recent years, varied N enhancement experiments were conducted in
these desert ecosystems (Guo 2016; Su et al. 2014a; Zhou et al. 2012). In this chapter,
based on the experimental results from different deserts of China, we firstly discuss
impacts of N addition on nutrient and carbon cycling. We then address the effects of
246
X. Zhou et al.