Biogeochemical cycles
• Our planet consists of many complex, large-scale,
interacting systems.
• System = a network of relationships among a group of
parts, elements, or components that interact with and
influence one another through the exchange of energy,
matter, and/or information
Feedback loops: Negative feedback
Feedback loop = a circular process whereby a system’s
output serves as input to that same system
In a negative feedback loop, output acts as input that
moves the system in the opposite direction.
This compensation stabilizes the system
Feedback loops: Positive feedback
• In a positive
feedback loop,
output acts as
input that moves
the system
further in the
same direction.
• This
magnification of
effects
destabilizes the
system.
Dynamic equilibrium, homeostasis
• Dynamic equilibrium or steady state = when processes
in a system move in opposite directions at equivalent
rates so their effects balance out… this can be large
forces pulling in opposite directions yielding small to no
change
• Homeostasis = tendency of a system to maintain constant
or stable internal conditions
• Earth’s climate and an animal’s body are examples of
homeostatic systems in dynamic equilibrium.
Dynamic equilibrium
• Many equilibrium states can exist… there is rarely
just one
• Linear versus non-linear changes = movement from
one steady state to another can be gradual, or abrupt
• Earth’s climate is an example of a system with many
steady states and abrupt changes (examples)
Closed and open systems
• Closed system = isolated and self-contained
• Open system = exchanges energy, matter, and
information with other systems
• The Earth is basically a closed system with respect to
mass, but open with respect to energy
• But any system is open if we examine it closely enough or
long enough.
System example: the Mississippi River Delta
“Dead Zone”
A systems approach allows us to expand or contract our
area of interest as problems require
Connected systems
• The farms of Minnesota and the Gulf of Mexico interact.
• Understanding the dead zone requires viewing farms in
the Mississippi River drainage and the Gulf of Mexico as
a single system.
• This holistic kind of view is necessary for comprehending
many environmental issues and processes… consider the
problems that compartmentalizing the government
agencies causes here…
Eutrophication
Key to the dead zone =
Eutrophication: excess nutrient enrichment in
water, which increases production of organic matter...
… which when decomposed by oxygen-using
microbes can deplete water of oxygen.
Increasing nitrogen inputs
Amount of nitrogen
fertilizer used rose
greatly, 1950–80
Nitrate concentrations
in Midwestern rivers in
1980–98 were much
more than in 1905–07.
Creation of the hypoxic dead zone
Nitrogen input boosts phytoplankton…
…which die and are decomposed by microbes that suck
oxygen from water, killing fish and shrimp.
Importance of systems
• The problem persisted because government agencies that
regulated pesticide use did not interact with those that
regulated fisheries… systems approach is not a hallmark
of government
• The problem continues to persist because of the linkages
of systems… if farming is regulated, excess nutrients still
in the soil and river sediments continue to leach out.
• Systems that are hard to pollute are often hard to clean
up… we will see this in many biogeochemical cycles
Earth’s structural spheres
• Lithosphere = rock, sediment, soil below Earth’s surface
• Atmosphere = air surrounding the planet
• Hydrosphere = all water—salt and fresh, liquid, ice, and
vapor
• Biosphere = all the planet’s living things, and the abiotic
parts of the environment with which they interact
Biogeochemical cycles
• Nutrients are elements and compounds that organisms
consume and require for survival.
• Nutrients stimulate production by plants, and the lack of
nutrients can limit production.
• Nutrients move through ecosystems in nutrient cycles or
biochemical cycles.
Nutrients
• Macronutrients are elements and compounds required in
relatively large amounts and include nitrogen, carbon,
and phosphorous.
• Micronutrients are nutrients needed in small amounts…
but just as critical for survival
Terminology and Concepts
• All matter cycles...it is neither created nor destroyed...
• Biogeochemical cycles: the movement (or cycling) of
matter and energy through a system
• by matter we mean: elements (carbon, nitrogen, oxygen)
or molecules (water)
• so the movement of matter (for example carbon) between
these parts of the system is a biogeochemical cycle
Terminology and Concepts
• Reservoir: a unique, definable collection of a material of
interest… also called a pool…
• Flux: the movement of a material of interest from one
reservoir to another
• Residence time: the average amount of time something
spends in a reservoir
• Residence Time = Reservoir size/flux
The importance of residence times
• Reservoirs with Long residence times are hard to pollute,
but once polluted, hard to clean up
• Reservoirs with Short residence times are easy to pollute,
but once polluted, easier to clean up… maybe…
• Key is that cycles exist, so long residence times reservoirs
can be connected to those with short residence times.
Example: CO2 and the ocean… “CO2 is forever”…
more on this later
Mississippi River “dead zone” exaample
The carbon cycle
The carbon cycle
How carbon (C) moves through our environment
• Producers pull carbon dioxide (CO2) from the air and use it
in photosynthesis.
• Consumers eat producers and return CO2 to the air by
respiration.
• Decomposition of dead organisms, plus pressure
underground, forms sedimentary rock and fossil fuels.
This buried carbon is returned to the air when rocks are
uplifted and eroded.
• Ocean water also absorbs carbon from multiple sources,
eventually storing it in sedimentary rock or providing it to
aquatic plants.
Key facts in the carbon cycle
• How much carbon is where?
• Most carbon is in rocks (carbonates and other sediments)
• Most carbon not in rocks is in the ocean
• There is more carbon in the atmosphere than in living
plants
• About 3 times more carbon in soils than in land plants
• There is 6 times more carbon in fossil fuels than in the
atmosphere.
• There is 8 times more carbon in fossil fuels than in living
plants
Human impacts on the carbon cycle
• We have increased CO2 in the atmosphere by burning
fossil fuels and deforesting forests.
• Atmospheric CO2 concentrations are the highest they
have been in at least 800,000 years… probably 30 to 40
million years
• This is driving global warming and climate change.
Human impacts on the carbon cycle
• We have increased CO2 in the atmosphere by burning
fossil fuels and deforesting forests.
• Atmospheric CO2 concentrations are the highest now in
over 800,000 years.
Key facts in the carbon cycle
• CO2 has a residence
time of years in the
atmosphere. It doesn’t
matter who produces
it, we all share it.
• Largest fluxes are
between land plants
and atmosphere, and
the ocean and the
atmosphere.These are
seasonal fluxes
(photosynthesis and
respiration).
Key facts in the carbon cycle
• Human caused
fluxes are
smaller
seasonally, but
larger annually.
We unbalance
the carbon cycle
by taking fossil
fuels, and
burning forests,
and converting
these to CO2 in
the atmosphere.
Key facts in the carbon cycle
• Flux of carbon out of fossil fuels (human
burning) is 60,000 times faster than flux into
fossil fuel (formation of coal, oil and natural
gas).
• Fossil fuels are clearly non-renewable.
• What took millions of years to form will take
only a couple of centuries to convert back to
CO2.
Key facts in the carbon cycle
•Flux to atmosphere from fossil fuel burning and deforestation is greater than
accumulation of carbon in the atmosphere. So only about half of our anthropogenic
CO2 stays in the atmosphere.
•Where does it go?
Some goes to plants and soils.
 Reforestation, enhanced plant growth from excess nutrients?
Some goes to the ocean, because the ocean exchange works by diffusion...
•
•
Flux by diffusion = k (Cair -Cocean)
(C is concentration or amount, k is a constant)
- if (Cair -Cocean) goes up, flux goes up
- if (Cair -Cocean) goes down, flux goes down
- if (Cair -Cocean) reverses, flux reverses
The phosphorous cycle
The phosphorous cycle
How phosphorus (P) flows through our environment
• P comes from rocks. Weathering releases phosphate
(PO43–) ions from rocks into water.
• Plants take up phosphates in water, pass it on to
consumers, who return it to the soil when they die.
• Phosphates dissolved in lakes and oceans precipitate,
settle, and can become sedimentary rock.
The phosphorous cycle
• Phosphorous has no stable gas phase, so addition of P to
land is slow (low rain P), and P is not well distributed.
• Most P in plants cycles between living and dead plants...
addition by weathering is small compared to cycling
within plants.
• Humans have greatly accelerated P transfer from rocks to
plants and soils (about 5x faster than weathering).
Human impacts on the phosphorus cycle
• We mine rocks containing phosphorus for inorganic
fertilizers for use on crops and lawns.
• Treated and untreated sewage discharge contains
phosphates, as do detergents.
• Those phosphates that run off into waterways can
boost algal growth and cause eutrophication, altering
the structure and function of aquatic ecosystems.
Key points in the phosphorous cycle
• Natural transfer of P from ocean to land is very small...
tiny amount from sea spray and bird guano… long term
flux is volcanic
• Sources for human mining are guano and very old (10 to
15 million years ago) rocks formed in shallow seas which
dried up (Florida's Bone Valley). Such rocks are not
forming today as rapidly...
• Phosphorous is a strongly limiting nutrient because it
cannot be transferred from the ocean to plants very
effectively.
The nitrogen cycle
The nitrogen cycle
How nitrogen (N) moves through our environment:
• Atmospheric N2 is fixed by lightning or specialized
bacteria, and becomes available to plants and animals in
the form of ammonium ions (NH4+).
• Nitrifying bacteria turn ammonium ions into nitrite (NO2–)
and nitrate (NO3–) ions. Nitrate can be taken up by plants.
• Animals eat plants, and when plants and animals die,
decomposers consume their tissues and return ammonium
ions to the soil.
• Denitrifying bacteria convert nitrates to gaseous nitrogen
that reenters the atmosphere.
Human impacts on the nitrogen cycle
• Haber and Bosch during WWI developed a way to fix
nitrogen artificially.
Synthetic nitrogen fertilizers have boosted agricultural
production since then.
• Today we are fixing as much nitrogen artificially as all
bacteria do naturally.
Human impacts on the nitrogen cycle
• NOx also from all combustion (cars, etc.)
Smaller flux, but more widespread
• We have doubled the amount of nitrogen nutrient in
the environment.
Most is nitrate or ammonia (water soluble…
residence time in the atmosphere?)
Eutrophication
Nitrous oxide (N2O) a key greenhouse gas
Human impacts on the nitrogen cycle
Nitrogen and the dead zone
Excess nitrogen flowing down the Mississippi River into the
Gulf causes hypoxia, worse in some regions than others.
The hydrologic cycle
The hydrologic cycle
How water flows through our environment:
• Water enters the atmosphere by evaporation from liquid
water and by transpiration from plant leaves.
• It condenses and falls from the sky as precipitation.
• It flows as runoff from the land surface into streams,
rivers, lakes, and eventually the ocean.
• Water infiltrates into aquifers, becoming groundwater,
the upper limit of which is the water table.
Key points in the hydrologic cycle
• Transpiration dominates on land, evaporation
dominates over open water (ocean)
• Water stays in the atmosphere for an average of about
a week (its residence time is 5 to 10 days)
• This means…
Figure 6.24
Key points in the hydrologic cycle
Anything in the atmosphere that is water soluble
(sulfate, nitrate…) will come out of the atmosphere
on the same time frame
The atmosphere dries rapidly. Without more
evaporation or transpiration, it cannot rain or snow.
Water cycles rapidly. The same water molecule that
falls as rain in California may be transpired a day
later and fall as snow in Colorado a few days after
that.
Key points in the hydrologic cycle
• Plants are critical to maintaining moist environments
Transpiration recycles moisture
Plants create their own favorable environment
When plants are removed, transpiration ceases, and
desertification can occur
• Physics…the amount of moisture the air can hold is
controlled by temperature… warmer air, more
moisture; cool the air, condensation occurs.
Figure 6.24
Key points in the hydrologic cycle
• The ocean exports water to the land via evaporation
and condensation
In areas where ocean winds blow over land, this
leads to more abundant rainfall.
In areas where ocean winds blow away from land,
this leads to very little rainfall (Northern Africa, the
Sahara)
• The land returns the water via rivers and
groundwater… ever wonder why rivers flow?
The rock cycle
Rocks change from one form to another over time.
• Igneous rock = of volcanic origin; cooled magma that
may flow across Earth’s surface as lava
• Sedimentary rock = mineralized sediments
(layers of mud, dust, or sand) formed by lithification
• Metamorphic rock = transformed by extreme heat or
pressure
The rock cycle
Plate tectonics
• The process by which plates of crust move across
Earth’s surface, atop its malleable mantle and
molten core
• Over millions of years, continents change
position.
• Movement = only 2–15 cm (1–6 in) per year
• Plate tectonics underlies earthquakes, volcanoes.
Global map of tectonic plates
Plate boundaries
Tectonic plates can meet in several ways.
Magma seeps
up beneath
ocean at
divergent
boundaries.
Earthquakes occur
at transform
boundaries where
plates slip alongside
each other.
At convergent boundaries
where one plate is subducted
beneath another, volcanoes
occur. Where two plates collide,
mountains are formed.