122
R. R. Shahin
was higher than −20 kPa (mostly during the wet season). During the drought season,
CO 2 emission rates were below 50 mg C m
−2 h
−1 [51].
The changes of soil moisture (SM) and temperature (T) in different seasons can
affect the trace gases carbon dioxide (CO 2 ), nitrous oxide (N 2 O), and methane (CH 4 )
that can be exchanged between the ecosystem and atmosphere [52]. This chapter
reports the impact of year-round SM status on GHG fluxes in three semiarid vegetation zones, having different SOM contents, in southeastern Arizona. Carbon dioxide
and N 2 O emissions were highly affected by available soil moisture and temperature.
During the rainy season (238 mm total rainfall), large differences in soil C content
did not correlate with variations in CO 2 emissions. The limited rains (95 mm total
rainfall) reduced CO 2 emissions by 19–40% as compared to the heavy rain season. Laboratory incubation experiments showed potentials for CH 4 oxidation from
0 to 45 cm soil layer. This suggested that when the soil surface becomes dry, CH 4
oxidation activity shifted depthwise in the sandy soils. In semiarid riparian soils of
southwestern Arizona, the predicted climate change shifts in annual precipitation
from arid to the wet season could reduce soil CO 2 and N 2 O emissions while enhancing CH 4 oxidation rates of, potentially acting as negative feedback for future global
warming.
As long as other factors such as soil organic carbon (SOC) availability and soil
moisture are not limiting, NO emission increases with soil temperature due to the positive effect of temperature on enzymatic activities and microbial turnover rates. Thus,
the temperature is not the main factor that controlling the amount of soil flux, but
rather a regulating factor of short-term changes [53]. Generally, it has been concluded
that NO emission inclines exponentially with soil temperature [25]. Based on a 5 year
dataset of continuous measurements of soil-atmosphere exchange of N 2 O, NO, and
CO 2 at the temperate, nitrogen-saturated Norway spruce forest site Höglwald, nitrous
gas NO emissions had significantly positive correlations with water-filled pores %
(WFPS) up to a soil temperature of 15 °C, but at soil temperatures above 15 °C,
highest NO fluxes were found at lowest WFPS values [54]. The correlation of GHG
gas fluxes with soil temperature was stronger than that with soil moisture. However,
soil moisture could become the crucial regulator of N 2 O emission. An increased
NO emission was noticed following soil re-wetting after long drought periods. The
NO:N 2 O ratio was controlled by WFPS rather than by soil temperature. A significant
positive correlation between soil temperature and NO:N 2 O ratio was observed only
when WFPS was below 45%. The highest NO:N 2 O ratio was found under conditions
favorable to nitrification (soil temperature around 15 °C; WFPS less than 40%). A
recent analysis of soil emission measurements over the period 1994–2010 at the
Höglwald site Samad et al. [33] confirmed these findings, specifying that NO fluxes
were highest when the temperature was high (more than 15 °C), but the soil moisture
was ranged between 24 and 30% (Fig. 5).
A study on the effect of soil temperature (at 5 and 10 cm depths) on soil
GHG fluxes, stated that soil temperature significantly influences CO 2 emissions
by inducing the accelerated decomposition of soil organic carbon, root respiration, and microbe respiration. Insignificant correlations were detected between soil
temperature and both NH 4 and NO emissions [55].
R. R. Shahin
was higher than −20 kPa (mostly during the wet season). During the drought season,
CO 2 emission rates were below 50 mg C m
−2 h
−1 [51].
The changes of soil moisture (SM) and temperature (T) in different seasons can
affect the trace gases carbon dioxide (CO 2 ), nitrous oxide (N 2 O), and methane (CH 4 )
that can be exchanged between the ecosystem and atmosphere [52]. This chapter
reports the impact of year-round SM status on GHG fluxes in three semiarid vegetation zones, having different SOM contents, in southeastern Arizona. Carbon dioxide
and N 2 O emissions were highly affected by available soil moisture and temperature.
During the rainy season (238 mm total rainfall), large differences in soil C content
did not correlate with variations in CO 2 emissions. The limited rains (95 mm total
rainfall) reduced CO 2 emissions by 19–40% as compared to the heavy rain season. Laboratory incubation experiments showed potentials for CH 4 oxidation from
0 to 45 cm soil layer. This suggested that when the soil surface becomes dry, CH 4
oxidation activity shifted depthwise in the sandy soils. In semiarid riparian soils of
southwestern Arizona, the predicted climate change shifts in annual precipitation
from arid to the wet season could reduce soil CO 2 and N 2 O emissions while enhancing CH 4 oxidation rates of, potentially acting as negative feedback for future global
warming.
As long as other factors such as soil organic carbon (SOC) availability and soil
moisture are not limiting, NO emission increases with soil temperature due to the positive effect of temperature on enzymatic activities and microbial turnover rates. Thus,
the temperature is not the main factor that controlling the amount of soil flux, but
rather a regulating factor of short-term changes [53]. Generally, it has been concluded
that NO emission inclines exponentially with soil temperature [25]. Based on a 5 year
dataset of continuous measurements of soil-atmosphere exchange of N 2 O, NO, and
CO 2 at the temperate, nitrogen-saturated Norway spruce forest site Höglwald, nitrous
gas NO emissions had significantly positive correlations with water-filled pores %
(WFPS) up to a soil temperature of 15 °C, but at soil temperatures above 15 °C,
highest NO fluxes were found at lowest WFPS values [54]. The correlation of GHG
gas fluxes with soil temperature was stronger than that with soil moisture. However,
soil moisture could become the crucial regulator of N 2 O emission. An increased
NO emission was noticed following soil re-wetting after long drought periods. The
NO:N 2 O ratio was controlled by WFPS rather than by soil temperature. A significant
positive correlation between soil temperature and NO:N 2 O ratio was observed only
when WFPS was below 45%. The highest NO:N 2 O ratio was found under conditions
favorable to nitrification (soil temperature around 15 °C; WFPS less than 40%). A
recent analysis of soil emission measurements over the period 1994–2010 at the
Höglwald site Samad et al. [33] confirmed these findings, specifying that NO fluxes
were highest when the temperature was high (more than 15 °C), but the soil moisture
was ranged between 24 and 30% (Fig. 5).
A study on the effect of soil temperature (at 5 and 10 cm depths) on soil
GHG fluxes, stated that soil temperature significantly influences CO 2 emissions
by inducing the accelerated decomposition of soil organic carbon, root respiration, and microbe respiration. Insignificant correlations were detected between soil
temperature and both NH 4 and NO emissions [55].