Geoscience Reference
In-Depth Information
pathogens and by modifying nutrient availability in the soil
(van der Heijden et al., 2008).
Approximately 120 billion tonnes of carbon are taken
up each year by primary production on land (Hymus and
Valentini, 2007) and ~119 billion tonnes of carbon are emit-
ted, half by autotrophic (mainly plant) respiration and half
by heterotrophic soil microorganisms (Reay and Grace,
2007). Together, the land and oceans constitute a net sink
of ~3 billion tonnes of carbon each year, effectively absorb-
ing about 40% of the current CO
2
emissions from fossil fuel
use (Figure 18.1). In addition, 1-2 billion tonnes of carbon
are added to the atmosphere each year (Drigo et al., 2007)
through changes in land use (predominantly tropical defores-
tation). Furthermore, because soils store ~2000 billion tonnes
of organic carbon, their disturbance by agriculture and other
land uses can greatly stimulate the rates of organic matter
decomposition and net emissions of CO
2
to the atmosphere
(Smith, 2008a,b). For example, deep ploughing or drainage
(CH
2
O)
n
Organic matter
Aerobic
photosynthesis
Aerobes
Chemolithotrophs
Methanotrophs
Oxygenic
Low oxygen
CH
4
CO
2
Methanogenesis
Acetogens
Anaerobes
Methyl and
acetate
Anaerobic
photosynthesis
Organic matter
(CH
2
O)
n
FIGURe 18.1
Impact on global cycle (carbon and nitrogen
cycle).
