drivers of soil respiration of root and microbial ... - Unitus DSpace

1.3. Soil respiration sources
Soil respiration (Rs) is an important component of the ecosystem C budgets. After the
photosynthesis, CO2 efflux from soil remains the second largest C flux in most ecosystems and can
account for 60-90% of total ecosystem respiration (Goulden et al., 1996; Longdoz et al., 2000). On
the global basis, the estimate of the contribution of Rs to the atmospheric CO2 emission is about
75-80 GtC per year. Raich et al. (2002) estimated the mean global CO2 efflux from soil in the
period between 1980 and 1994 to be 80.4 GtC.
Due to the large quantity of C stored in the soil it has been hypothesized that relatively
small changes in Rs induced by climate change or land use change could rival the annual fossil
fuel loading of atmospheric CO2 (Jenkinson et al., 1991; Raich and Schlesinger 1992). The
realization that the soil could be a possible source of atmospheric CO2, together with the
continuous increase in atmospheric CO2 concentration has given a rise to numerous methods to
quantify this input.
Soil respiration is the result of the production of CO2 in soils from a combination of several
belowground processes (Ryan and Law 2005; Trumbore, 2006). The most important are the
biological activity of roots and associated microorganisms and the activity of heterotrophic bacteria
and fungi living on litter and in the root-free soil (Fig. 4). Non biological processes related to
chemical weathering in soils are estimated to be a net carbon sink of ca. 0.3 Gt yr-1 (Jacobson et al.
2000), thus being of less significance.
Kuzyakov (2006) suggested five main contributors to total CO2 efflux (Fig. 4) :
(1) microbial decomposition of SOM in the root free soil;
(2) microbial decomposition of SOM in root affected soil, associated with a priming effect;
(3) microbial decomposition of the dead plant residuals;
(4) microbial decomposition of the rhizodeposits in the rhizosphere;
(5) root respiration.
The dynamic of different components of soil respiration is controlled by different biotic and
abiotic factors, such as temperature, water availability, photosynthetic activity, or plant
phenological development. Heterotrophic processes control soil C storage and nutrient dynamics,
while autotrophic component reflect plant activity and supply of organic compounds to roots from
canopy (Hogberg et al., 2001; Singh et al., 2003; Binkley et al., 2006). In addition the response of
microbial and root components of soil respiration to changes in soil temperatures is different,
exhibiting various Q10 values (Zhou et al., 2007). Thus, the potential change in soil CO2 efflux
associated with global warming will largely depend on the relative contribution of root and
microbial-derived respiration tot total CO2 efflux. Therefore quantifying the components of soil
respiration is imperative for understanding the nature and extent of feedbacks between climate
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