Geoscience Reference
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of organic, carbon-rich soils is known to stimulate rates of
decomposition and respiration, because it gives microor-
ganisms a greater access to both buried organic carbon and
oxygen (Smith, 2008b). Through such cultivation and distur-
bance, soils are estimated to have already lost 40-90 billion
tonnes of carbon since human intervention began (Lal, 1999).
Although these responses are mediated by microbial activity,
it is generally thought that changes in the structure and diver-
sity of terrestrial microbial communities will have little effect
on CO
2
production at the ecosystem level because, unlike CH
4
and N
2
O production, CO
2
production results from numerous
microbial processes Staddon et al. (2004). However, recent
findings have challenged this assumption by providing evi-
dence of a direct link between CO
2
fluxes and changes in the
structure and physiology of the microbial community (Carney
et al., 2007).
A prime cause of this uncertainty is the inherent complex-
ity and diversity of soil organic matter and the likelihood that
the temperature dependence of microbial decomposition of
soil carbon compounds of differing chemical composition
and substrate quality will vary (Rillig et al., 2002; Davidson
and Janssens, 2006). For example, there is evidence that the
temperature sensitivity of litter decomposition increases as
the quality of organic carbon consumed by microbes declines
(Fierer et al., 2005), which is consistent with kinetic theory
and indicates a greater temperature sensitivity for decompo-
sition of recalcitrant carbon pools (Knorr et al., 2005). There
is also a considerable potential for various environmental
constraints, such as physical and chemical protection of
organic matter, to decrease substrate availability for micro-
bial attack, thereby dampening microbial responses to warm-
ing (Davidson and Janssens, 2006).
Nitrous oxide
gas
Similar to CO
2
and CH
4
emissions, global N
2
O emissions have
a predominantly microbial basis. Natural and anthropogenic
sources are dominated by emissions from soils, primarily as
a result of microbial nitrification and denitrification (Reay
and Grace, 2007). For each tonne of reactive nitrogen (mainly
fertiliser) deposited on the Earth's surface, either naturally or
deliberately, 10-50 kg are emitted as N
2
O (Crutzen, 2007).
Several studies have been carried out to distinguish the relative
contributions of nitrification and denitrification to net N
2
O flux,
although little is known about the degree of microbial control of
these processes at the ecosystem level (Figure 18.1). Most N
2
O
produced by nitrification is a result of the activity of autotrophic
