Download (1) (15) Chemical cycling

(1) (15) Chemical cycling
(a)
(a)
"Chemical elements such as carbon and nitrogen are cycled
between abiotic and biotic components of the ecosystem. Photosynthetic
organisms acquire these elements in inorganic form from the air, soil, and
water and assimilate them into organic molecules, some of which are
consumed by animals. The elements are returned in inorganic form to the
air, soil, and water by the metabolism of plants and animals and by other
organisms, such as bacteria and fungi, that break down organic wastes and
dead organisms."
(b)
(b)
Chemical cycles may be divided into two broad categories
(i)
(i)
Those elements that have a gaseous form
(ii)
(ii)
Those elements that do not have a gaseous form
(c)
(c)
We will consider particularly
(i)
(i)
The carbon cycle
(ii)
(ii)
The nitrogen cycle
(iii)
(iii)
The phosphorus cycle
(2) (17) The Carbon cycle
(a)
(a)
See Figure 54.17, The carbon cycle
(b)
(b)
The carbon cycle is an example of biogeochemical cycle in
which the element (carbon) has a gaseous form, i.e., CO2, carbon dioxide
(c)
(c)
Carbon dioxide is converted to organic forms of carbon in the
Calvin cycle of primary producers
(d)
(d)
Organic carbon is converted back to carbon dioxide during
respiration
(e)
(e)
Not all fixed carbon is converted back to CO2 over medium-term
time scales since some ultimately is buried as oil, coal, or limestone (the
latter is calcium carbonate)
(f)
(f)
(The only error that I immediately notice in the image above is
that at least some of the CO2 released by volcanoes must, ultimately, have
a biotic source, and certainly all of the carbon released from volcanoes can
trace itself back to the rectangle in the lower right, i.e., the Earth)
(g)
(3) (18) The Nitrogen cycle [ammonification, nitrogen assimilation, denitrification,
nitrification, nitrogen fixing]
(a)
(a)
See Figure 54.18, The nitrogen cycle
(b)
(b)
The nitrogen cycle, like the carbon cycle, involves a gaseous
form, i.e., N2 or nitrogen gas
(c)
(c)
Nitrogen gas may be removed from the atmosphere, particularly
by bacteria, in a process called nitrogen fixing [which is relatively
expensive since nitrogen gas is quite stable]
(d)
(d)
Nitrogen gas may be returned to the atmosphere, again
particularly by bacteria, in a process called denitrification (a form of
anaerobic respiration);
(e)
(e)
More typically, bioavailable nitrogen is found as ammonium ion
+
(NH4 ), nitrate ion (NO32-), and various organic, nitrogen-containing
compounds (e.g., amino acids and nucleic acids)
(f)
(f)
Nitrate and ammonium ion are converted back and forth
between each other (and nitrite, NO22-), also by various bacteria via
processes termed nitrification and ammonification
(g)
(g)
The nitrogen cycle thus involves
(i)
(i)
Nitrogen fixing, the fixing of nitrogen from the
atmosphere [typically by free-living or plant-associated nitrogenfixing bacteria]
(ii)
(ii)
Assimilation, the uptake of ammonium ion and nitrate ion
from soil by plants and the uptake of organic nitrogen by animals
from plants (amino acids, nucleic acids)
(iii)
(iii)
Ammonification, the conversion of organic nitrogen back
to ammonium ion by decomposers (nitrogenous waste) (NH4+);
�The decomposition of organic nitrogen back to ammonium, a
process called ammonification, is carried out mainly by bacterial and
fungal decomposers.� (p. 1211, Campbell & Reece, 2002)
(iv)
(iv)
Nitrification, the various conversions of nitrogen within
the soil from ammonium ion (NH4+ NO22- NO32- ; note that this
represents an oxidation of nitrogen); �Although plants can use
ammonium directly, most of the ammonium in soil is used by certain
aerobic bacteria as an energy source; their activity oxidizes
ammonium to nitrite (NO22-) and then to nitrate (NO32-).� (p. 1211,
Campbell & Reece, 2002)
(v)
(v)
Denitrification, also by soil bacteria (N2; note that this
process involves the reduction of nitrogen); �Some bacteria can
obtain the oxygen they need for metabolism from nitrate (NO32-)
rather than from O2 under anaerobic conditions.� (p. 1211,
Campbell & Reece, 2002)
(h)
(h)
[A portion of the nitrogen
cycle as it occurs within fish tanks (here
nitrification is a good thing and
ammonification, the production and
subsequent build up of ammonium ion,
is a bad thing):�
(4) (19) The Phosphorus cycle
(a)
(a)
See Figure 54.19, The
phosphorous cycle
(b)
(b)
Unlike the nitrogen and
carbon cycles, the phosphorus cycle
does not involve a gaseous phase
(c)
(c)
As a consequence,
phosphorus tends to cycle more locally
rather than entering into the world-wide
cycling seen with nitrogen and carbon
(d)
(e)
(f)
(g)
(h)
(i)
(d)
The ultimate source of phosphorous is
phosphate minerals that make up rocks
(e)
The form in which phosphorus is
available is as the phosphate ion (PO43-)
(f)
Phosphorous is lost from ecosystems
by erosion; e.g., The Grand Canyon, an
impressive example of erosion in action 
(g)
Phosphorous can be gained by
ecosystems, sometimes significantly, particularly
via the movement of animals
(h)
"After producers incorporate phosphorus into biological
molecules, it is transferred back to the soil by the excretion of phosphate
by animals and by the action of decomposers on detritus."
(i)
Note that phosphorus is transferred to (and between) terrestrial
environments also by processes other than just bird pooping (i.e., guano)
including the migration of various others animals such as salmon, which
carry phosphorous from the sea back to their mother streams and on the
way are eaten by such things as bears which, yes, do go on to poop in the
woods: