Belowground
respiration
Soils are the largest carbon pool in terrestrial ecosystems,
containing more than two-thirds of total carbon in the terrestrial ecosystems.
Soil respiration (belowground respiration) is the major pathway of carbon transfer
from soil to atmosphere, and a tiny amount of change in soil respiration rate
may have profound impact on the atmospheric CO2 budget, thus understanding soil
respiration is crucial for the carbon balance of terrestrial ecosystems and for
the global carbon balance.
Soil respiration normally refers to the total soil CO2
efflux at the soil surface. It is the combination of biotic, chemical and
physical processes.
Biotic processes: rhizosphere respiration
(root and root exudates); microbial respiration; faunal respiration
Chemical process: chemical oxidation of
soil minerals, which is relatively small compared to other sources, but
pronounced at high temperature.
Physical process: Soil CO2 degassing and
transport of CO2 through soil to the surface.
Various factors are identified to affect soil respiration
rate: soil temperature, soil moisture, root nitrogen concentrations, soil
texture, and substrate quantity and quality (Buchmann,
2000), among which soil temperature and moisture dominate.
Soil temperature is the most important factor in
regulating soil respiration and also the most intensively studied factor (Lin, et al., 1999, Winkler, el
al., 1996; Luo, et al., 2001; Carlyle and Than, 1988). Soil respiration increases
exponentially with increasing temperature, and this relationship is usually
described with exponential and Arrhenius equations (Lloyd
and Taylor, 1994).
Exponential equation: R = aebT= 
(Q10)T/10
exp
where R10 is the soil respiration at 10ºC, and T
is the absolute soil temperature (K)
Figure 1 describes
a typical response of soil respiration to soil temperature (adopted from Fang and Moncrieff, 2001). Q10, the times
of increase of soil respiration for every 10 ºC increase in temperature, which
describes the sensitivity of soil respiration to temperature, ranges from 1.3
to 5.6 (Raich and Schlesinger, 1992; Peterjohn et al., 1993; Simmons
et al., 1996). Usually a value of 2 is assumed for prediction of climate
change, which means that the soil respiration would double for every 10 ºC
increase in temperature. However, studies show that the value would not hold at
high temperatures. The sensitivity of soil respiration to temperature would
reduce at high temperature due to deactivation of enzymes. Even under not very
high temperature conditions, the acclimatization of soil respiration to
warming, i.e. temperature sensitivity of soil respiration decreases under
warming, has been recorded (Luo, et al., 2001).
It’s also found that Q10 varies with the depth of the active soil
layer and the depth at which temperature is measured (Swanson
& Flanagan, 2001; Kirschbaum, 1995).
Soil moisture is another important factor
influencing soil respiration. Soil CO2 efflux is usually low under dry
conditions due to low root and microbial activities, and is increasing with
soil moisture till some limit. In very high soil moisture condition, soil CO2
efflux is reduced due to limitation of diffusion of oxygen and suppression of
CO2 emissions. This relationship is sometimes described by a quadratic equation
(Bunnel, el at., 1977; Linn,
et al., 1984; Mielnik & Dugas, 2000). Figure 2 is an example of such relationship (from Mielnik & Dugas, 2000). Other equations are also
used to describe soil respiration-moisture relationship: linear, exponential
and hyperbolic equations (Norman, et al., 1992; Davidson, 1998; Liu, et al.,
2001; Schlentner & Van cleve, 1985; Carlyle & Than, 1988). Despite this, the
relationship between soil respiration and moisture is usually scattered, and
our understanding of this relationship the mechanisms underlying the
relationship is still limited, compared to that of respiration/temperature
relationship. This is partly due to the fact that most of the studies on this
relationship are field studies based on observations of seasonal and spatial
variations of soil respiration. Thus the effect of soil moisture is confounded
with other factors, such as temperature, root and microbial activities.
Soil temperature and moisture often interact to control the
rate of soil respiration, and it’s often hard to separate the effects of the
two. Studies show that soil respiration usually responds to the most limiting
factor, temperature or moisture. Soil respiration is not sensitive to
temperature under lower moisture (below 75%), but is more responsive at higher
moisture content (100-250%). Similarly, soil respiration is not sensitive to
moisture under lower temperatures (below 5 ºC) but more responsive at higher
temperatures (10-20 ºC) (Carlyle and Than, 1988).
The regulation of soil temperature and moisture to soil respiration is so
strong that the two effects can be coupled to make predictions of soil
respiration (see modeling section).