The Agroecosystem Concept - Agroecology: The Ecology of Sustainable Food Systems - page 27

Atmospheric carbon (CO 2 )

Photosynthesis

Fire

Respiration

Combustion Respiration

Decay

Herbivores
and their
predators

Green plants

Fossil fuels

Death and waste

Death

Carbonification

Dead and nonliving
organic matter

Soil carbon
(organic matter)

FIGURE 2.3 The carbon cycle.

Atmospheric Nitrogen (N 2 )

Biological
fixation

Volcanic eruption

Lightning
fixation

Fire

Weathering

Denitrification

Herbivores
and their
predators

Green Plants

Death and waste

Death

Sediments

Uptake

Soil Nitrogen
NH 4 -- > NH 3 -- > NH 2 -- > NO 3 -- > NO 2

FIGURE 2.4 The nitrogen cycle.

are not absorbed or bound and (2) phosphates adhering to
soil particles may be removed by erosion. In both of these
cases, the phosphates leave the ecosystem and end up in
the oceans. Once phosphorus is deposited into the sea, the
time frame required for it to cycle back into terrestrial
systems enters the geological realm, hence the importance
of the localized cycles that keep phosphorus in the eco-
system (Figure 2.5).

In addition to the macronutrients, a number of other
chemical elements must be present and available in the
ecosystem for plants to grow. Even though they are needed
in very small quantities, they are still of great importance
for living organisms. They include iron (Fe), magnesium
(Mg), manganese (Mn), cobalt (Co), boron (B), zinc (Zn),
and molybdenum (Mo). Each of these elements is known
as a micronutrient .