Nitrogen effects on richness of fungi were mediated by water application in the
desert. Nitrogen addition plus water manipulation significantly shifted the relative
abundances of cultivable fungi (Jia et al. 2017). In the 0–2 cm soil depth, enhanced
fungal species richness was observed under N addition plus reduced water compared
to only N addition treatments. However, the N addition with water treatments did not
affect fungal diversity indices such as species richness, Shannon-Wiener index, or
evenness at the soil depth of 0–30 cm (Jia et al. 2017).
11.4 Impacts on Carbon Cycling and Greenhouse Gas
Emissions
11.4.1 Soil Respiration
On the desert steppe of Inner Mongolia, some studies found that N addition did not
obviously enhance the soil respiration rate (Shan et al. 2009). However, the N
addition (100 kg NH 4 NO 3 ha
À1 ) can increase the net ecosystem carbon exchange,
ecosystem respiration, and gross ecosystem productivity, and the effects on gross
ecosystem productivity were greater than ecosystem respiration in growing seasons
(Chen 2012). Soil microbes play a crucial role in carbon cycle and thus the
greenhouse gas emission. In the meadow steppe and the desert steppe, MBC was
the driving factor for the regional variation of ecosystem respiration (Liu et al. 2017).
The interaction of N and temperature can stimulate the soil respiration in this area
(Yang 2012). In another study in the desert steppe, the increase in soil temperature
and N did not change the mean rates of seasonal soil respiration (Zhang 2014).
Other environmental conditions can greatly affect the soil respiration in response
to N addition. For example, Wang (2012) found that the effects of N on soil
respiration in the desert steppe depended on the natural condition, especially the
precipitation (Wang 2012). In the Gurbantunggut Desert, although soil respiration
increased with rates of N addition in the first year, the weakened effects were
observed at high N treatments in the second year, and negative effects were found
in the third year (Zhou and Zhang 2014b). Similar trends in the total carbon
production were found among different N treatments in the first two growing
seasons. Thus, the effects of N addition on carbon sequestration depended on the
rates of N added and number of exposed years. Another study in the Gurbantunggut
Desert found that increasing N deposition and precipitation significantly increased
soil respiration, while warming reduced it due to variation in soil moisture (Yue et al.
2018). Further analysis shows that the higher soil respiration occurred at moderate
soil NH 4
+
-N contents (6.3–12.6 mg N kg
À1 ), while lower soil respiration occurred at
relatively lower (<6.3 mg N kg
À1
) or higher (>12.6 mg N kg
À1 ) contents of soil
NH 4
+
-N (Yue et al. 2018). Yue et al. (2018) also reported that soil respiration was
mediated by soil temperature, soil moisture, and content of soil NH 4
+
-N, with soil
temperature as the dominant factor.
250
X. Zhou et al.
desert. Nitrogen addition plus water manipulation significantly shifted the relative
abundances of cultivable fungi (Jia et al. 2017). In the 0–2 cm soil depth, enhanced
fungal species richness was observed under N addition plus reduced water compared
to only N addition treatments. However, the N addition with water treatments did not
affect fungal diversity indices such as species richness, Shannon-Wiener index, or
evenness at the soil depth of 0–30 cm (Jia et al. 2017).
11.4 Impacts on Carbon Cycling and Greenhouse Gas
Emissions
11.4.1 Soil Respiration
On the desert steppe of Inner Mongolia, some studies found that N addition did not
obviously enhance the soil respiration rate (Shan et al. 2009). However, the N
addition (100 kg NH 4 NO 3 ha
À1 ) can increase the net ecosystem carbon exchange,
ecosystem respiration, and gross ecosystem productivity, and the effects on gross
ecosystem productivity were greater than ecosystem respiration in growing seasons
(Chen 2012). Soil microbes play a crucial role in carbon cycle and thus the
greenhouse gas emission. In the meadow steppe and the desert steppe, MBC was
the driving factor for the regional variation of ecosystem respiration (Liu et al. 2017).
The interaction of N and temperature can stimulate the soil respiration in this area
(Yang 2012). In another study in the desert steppe, the increase in soil temperature
and N did not change the mean rates of seasonal soil respiration (Zhang 2014).
Other environmental conditions can greatly affect the soil respiration in response
to N addition. For example, Wang (2012) found that the effects of N on soil
respiration in the desert steppe depended on the natural condition, especially the
precipitation (Wang 2012). In the Gurbantunggut Desert, although soil respiration
increased with rates of N addition in the first year, the weakened effects were
observed at high N treatments in the second year, and negative effects were found
in the third year (Zhou and Zhang 2014b). Similar trends in the total carbon
production were found among different N treatments in the first two growing
seasons. Thus, the effects of N addition on carbon sequestration depended on the
rates of N added and number of exposed years. Another study in the Gurbantunggut
Desert found that increasing N deposition and precipitation significantly increased
soil respiration, while warming reduced it due to variation in soil moisture (Yue et al.
2018). Further analysis shows that the higher soil respiration occurred at moderate
soil NH 4
+
-N contents (6.3–12.6 mg N kg
À1 ), while lower soil respiration occurred at
relatively lower (<6.3 mg N kg
À1
) or higher (>12.6 mg N kg
À1 ) contents of soil
NH 4
+
-N (Yue et al. 2018). Yue et al. (2018) also reported that soil respiration was
mediated by soil temperature, soil moisture, and content of soil NH 4
+
-N, with soil
temperature as the dominant factor.
250
X. Zhou et al.