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).
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).