AGRICULTURE

THE STATE OF FOOD AND AGRICULTURE 2016
BOX 16
CARBON AND NITROGEN
IN THE AGRICULTURE SECTORS
The terms carbon cycle and nitrogen cycle
are used to describe the flow of those two
chemical elements, in various forms, through
the Earth’s atmosphere, oceans, terrestrial
biosphere and lithosphere.
It is estimated that up to 80 percent of the
total organic carbon in the terrestrial
biosphere, excluding fossil fuels, is stored in
soils, while about 20 percent is stored
in vegetation.
Plant growth produces an estimated 54 Gt of
carbon (or GtC), a year. The human
appropriation of this net primary production
– i.e. the quantity of carbon in biomass that
is harvested, grazed, burned or lost as a
result of human-induced land-use change –
has been estimated in the range of 15–20
GtC a year (Running, 2012; Krausmann et
al., 2013).
Oceans and coastal margins play a
significant role in the carbon cycle. It is
estimated that more than 90 percent of
global carbon is stored in aquatic systems.
Furthermore, around 25 percent of annual
GHG emissions are sequestered in aquatic
environments, primarily mangroves,
seagrasses, floodplain forests and coastal
sediments (Nellemann, Hain and Alder,
2008; Khatiwala et al., 2013). Aquatic
systems could, therefore, contribute
considerably to climate change mitigation.
Nitrogen is a major component of amino
acids, the building blocks of plant growth.
The use in agriculture of nitrogen, in plantuseable
forms, has increased rapidly with
the growing demand for food. In 2005,
farmers applied to crops an estimated 230
million tonnes of nitrogen in the form of
mineral fertilizer and manure. Global
leakages of nitrous oxide into the
environment may have already exceeded
biophysical thresholds, or planetary
boundaries (Rockström et al., 2009; Steffen
et al., 2015).
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