Introduction to
Biogeochemical Cycles
Chapter 4
All matter cycles...it is neither created
nor destroyed...
As the Earth is essentially a closed system with respect to matter, we can
say that all matter on Earth cycles .
Biogeochemical cycles: the movement (or cycling) of matter through a system
in general... we can subdivide the Earth system into:
atmosphere
hydrosphere
lithosphere
biosphere
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, practically speaking, a biogeochemical cycle
The Cycling Elements:
macronutrients : required in relatively large amounts
"big six": carbon
hydrogen
oxygen
nitrogen
phosphorous
sulfur
other macronutrients:
potassium
calcium
iron
magnesium
micronutrients : required in very small amounts, (but still necessary)
boron (green plants)
copper (some enzymes)
molybdenum (nitrogen-fixing bacteria)
Generalized Biogeochemical Cycle:
Biogeochemical cycles are part of the
larger cycles that describe the functioning of the whole Earth (not just
the surface parts)
Geological cycle consists of:
tectonic cycle
rock cycle
hydrologic cycle
biogeochemical cycles
We will focus on the hydrologic cycle
and the biogeochemical cycles. These are the cycles in which humans interact
the most.
Hydrologic cycle: introduction
(more later with Chapters 19 and 20)
Box model:
Reservoirs, fluxes and residence
times
Reservoirs: km3 %
Atmosphere 12,700 .001
Ocean 1,230,000,000 97.2
Land surface
lakes 123,000 .009
rivers
and streams 1,200 .0001
Land subsurface
(ground water) 4,000,000 .31
Ice (glaciers) 28,600,000 2.15
Fluxes: km 3 /yr
P: precipitation total 496,000
land 111,000
ocean 385,000
E: evaporation total 496,000
land 71,000
ocean 425,000
T: transpiration included in evap
(plant evaporation)
R: surface runoff 26,000
SR: sub surface runoff
liquid 12,000
ice 2,000
I: infiltration 14,000
S: springs 2,000
Compare with
total human use 3,000
Notes:
-- More precipitation falls on the land than evaporates or transpires (40,000
km 3 /yr).
-- Excess precipitation leaves as runoff and subsurface runoff
-- Less precipitation falls on the ocean than evaporates or transpires (40,000
km 3 /yr).
-- Oceans export water to the land by the atmosphere
-- humans use 12% of surface runoff
Residence times:
atmosphere RT = 12,700 km 3
(relative to sum of in fluxes) 496,000
km 3 /yr
= 0.03 yr or 9 days
-- note that since in fluxes equal
out fluxes, the RT is the same relative to the sum of the out fluxes
-- this is an important RT, as anything that is removed from the atmosphere
by rain or snow will also have an RT in the atmosphere nearly equal to this
more residence times:
Ocean:
RT = 1,230,000,000 km 3 (relative to evap)
425,000 km 3 /yr
= 2,900 years
this applies to the whole ocean (which can be separated into the surface
and deep water) and does not incorporate circulation
Streams and rivers:
RT = 1,200 km 3 with respect to outflow
26,000 km 3 /yr
= .05 yr or 17 days
this is the average, but it does give a good idea of the time that water
spends in rivers and streams before it flows into the ocean
Ground water:
RT = 4,000,000 km 3 with respect to outflow
12,000 km 3 /yr
= 330 years
once again, this is the average... oldest ground waters can be 10,000 to
40,000 years old... but this average does tell us that:
-- ground waters are generally old compared with human lifetimes (we tend
to view them as "eternal")
-- ground waters have large sizes and long residence times... hard to pollute,
but once polluted, hard to clean up
more on water later in Chapters 19 and 20...
Introduction to the carbon
cycle
The carbon cycle is one of the most
important to humans because it is important to our existence:
-- one of the primary elements forming
human tissues
-- necessary to plants, the basis of human food
and because it is important to the
climate system which sets the background for our environment:
-- carbon dioxide (CO 2 ) and methane (CH 4 ) are greenhouse gases which
help set global temperatures
Basic Carbon cycle:
Box Model:
Reservoirs, Fluxes and Residence Times
Fluxes: (in billions of metric
tons/year )
Land Plants
P: photosynthesis 120
PR: plant respiration 60
SR: soil respiration 60
SF: plants to soils 60
FFF: fossil fuel formation 0.0001
FFB: fossil fuel burning 6
DEF: deforestation 2
Ocean
D: dissolving 107
E: exsolving 103
CP: carbonate formation 4
W: weathering 0.6
Volcanoes
V: 0.1
Notes on fluxes:
-- CO 2 increase in the atmosphere:
Flux to the atmosphere:
Plant respiration + soil respiration + fossil fuel burning + deforestation
+ ocean exsolving + weathering...
60+60+6+2+103+0.6 = 231.6 bmt/yr
Flux from the atmosphere:
Plant photosynthesis + ocean dissolving...
120 + 107 = 227 bmt/yr
...difference is buildup of carbon
dioxide in the atmosphere of about 4 bmt/yr (book says 3...)
More on fluxes...
-- human caused fluxes are small, but persistent
-- largest fluxes are between land plants and atmosphere, and the ocean
and the atmosphere
-- flux of carbon out of fossil fuels
(FFB) is 60,000 times faster than flux into fossil fuels (FFF)
-- flux to atmosphere from FFB and
DEF
(6 + 2 bmt/yr) is greater than accumulation
of carbon in the atmosphere (about 4 bmt/yr)... this is because the ocean exchange works by diffusion ...
Flux by diffusion = k (C air -C ocean )
(C is concentration or amount,
k is a constant)
if (C
air -C
ocean ) goes up, flux goes up
if (C air -C ocean ) goes down, flux goes down
if (C air -C ocean ) reverses, flux reverses
even more on fluxes...
-- photosynthesis is the basis
of life on Earth...
carbon dioxide + water + sunlight _
organic material (sugar) + oxygen
-- respiration is the reverse of photosynthesis...
organic material + oxygen =
carbon dioxide + water + energy
animals and plants respire, releasing
energy for other activities... decay is also a form of respiration
Reservoirs: billions of metric tons
Atmosphere: 720
Ocean: 39,000
Carbonates: 100,000,000
Fossil fuels: 4,000
Land plants: 560
Soils: 1500
Notes on reservoirs:
-- most carbon is in rocks (carbonates and other sediments)
-- most carbon not in rocks is in the ocean
-- about 3 times more carbon in soils than in land plants
Residence times: (years)
(all relative to sum of out fluxes)
Land plants ~ 5
atmosphere ~ 3
soils ~ 25
Fossil fuels ~ 650
oceans ~ 350
carbonates ~ 150 million
Notes on residence times:
-- some in fluxes are not balanced by out fluxes
...the atmosphere and fossil fuels,
for example... so RT's are slightly different (and reservoirs are growing...
or shrinking)
-- the RT of carbon in the air (mostly carbon
dioxide , but some methane ) is long enough that
the air is well mixed (atmosphere mixes in about 1 year)
-- the RT of soils is the average RT... some parts cycle very slowly (1,000's
of years), some parts very rapidly (a few weeks to months... leaves, for
example)
-- the RT of fossil fuels reflects all FF's suspected to exist... this is
a combination of:
... recoverable
... unrecoverable
(both physically and economically)
RT's of recoverable FF's:
coal: ~ 350 years
oil: ~ 40 years
natural gas: ~ 60 years
More notes on residence times:
-- ocean RT also reflects the average, which combines the surface water (short RT, few
months to years) and deep water (long RT, 200 to 400 years)... average is weighted
towards deep water, as this is most of the water
-- ocean RT reflects the circulation of the ocean ( deep water formation )
Still more on fluxes/residence times:
-- Anthropogenic flux (FFB and DEF) to atmosphere ~ 8 bmt/yr , but atmospheric
increase is only ~ 4 bmt/yr
Question: Where does the missing 4 bmt/yr go?
Two possibilities: Photosynthesis vs. Ocean
uptake
- -Important to know this
because the residence times are so different
Carbon => plants recycles quickly
( <70 yr ) to atmosphere
Carbon => ocean recycles slowly ( >300 yr ) to atmosphere
Carbonate - Silicate Cycle
Long term cycle of the carbon cycle,
tied with the rock (silicate) cycle
Time scale for this cycle is millions to hundreds of millions of years,
so not a major concern of humans...
On this time scale, carbon cycling by plants, oceans and the atmosphere
is thought to be in balance (s teady
state or equilibrium )... so carbon dioxide levels in the atmosphere are
thought to be controlled by weathering rates and rates of volcanic eruptions
Weathering rates are thought to be
controlled by rate of tectonic uplift...
--more uplift, more weathering, less
atmospheric carbon dioxide
May explain the slow decline in atmospheric
carbon dioxide from levels of several thousand parts per million (ppm) about
100 million years ago, to 280 ppm in the pre-industrial time.
During this time, the Tibetan Plateau and
Rocky Mountain Plateau were raised by tectonic activity...
Also may provide long term negative feedback to keep carbon dioxide levels
from getting too high...
warming _ more evaporation _ rain _ weathering _ carbonate _ removes carbon
dioxide from atmosphere _ cooling
Introduction to the Nitrogen
Cycle
Important cycle because:
-- nitrogen is a necessary nutrient
-- nitrogen is part of acid rain
The Cycle:
Some terminology:
Limiting Nutrient - Amount
of an element necessary for plant life is in short supply
Nitrogen Fixation - Chemical
conversion of N 2 to more reactive forms, e.g.
NH 3 (ammonia)
or NO 3 - (nitrate)
Denitrification - Chemical
conversion from nitrate (NO 3 -) back to N 2
Box Model
Reservoirs: (in millions of metric tons
)
Atmosphere: 4,000,000,000
Land Plants: 3500
Soils: 9500
Oceans: 23,000,000
Sediments and Rocks: 200,000,000,000
Notes on Reservoirs:
- Buried sediments and rocks are the largest pool of nitrogen, but
this reservoir is a minor part of the cycle.
- Lots of nitrogen in the atmosphere (N 2
= 80%), but this form can't be used
by plants.
So nitrogen still a limiting nutrient
; need nitrogen fixation to make
it usable to plants.
Fluxes: (in millions of metric
tons/year )
Atmospheric
LF: Land Fixation 140
LD: Land Denitrification 130
OF: Oceanic Fixation 50
OD: Oceanic Denitrification 110
I: Industrial Fixation 100
FFB: Fossil Fuel Burning 20
BB: Biomass Burning 10
L: Lightning 20
Other
D: Decay 1200
G: Growth 1200
L-O: Land-to-Ocean 48
(Rivers 36)
(Dust 6)
(NOx 6)
O-L: Ocean-to-Land 15
(Sea Spray)
Burial: 10
Notes on Fluxes:
- Industrial fixation is used to make fertilizers to provide usable
nitrogen for crops. This flux is
comparable to natural fixation.
- Most flux is in land plants to/from
soils; plants recycle nitrogen since it's a limiting nutrient.
- Specialized bacteria and lightning are the only natural ways that nitrogen
is fixed.
Lightning may have been necessary for life to begin:
no life => no bacteria => no bacterial fixation => no usable nitrogen
=> no life...
More on fluxes:
How did agriculture survive before fertilizers?
- Early civilizations had to rely on natural regeneration of fixed nitrogen:
Annual floods bring fresh sediments (e.g., Nile Valley)
Slash/burn agriculture: once the soil nutrients are depleted, move on to a
new place
Crop rotation : certain crops (e.g. soybeans) are good at fixing nitrogen, others
(e.g. corn) use it up; plant on alternate years
Nitrogen chemical cycle
Terminology:
F = fixation , D = denitrification ,
O = oxidation
Residence Times
Major Reservoirs:
Atmosphere : 14 million yrs.
Land plants : ~ 3 yrs.
Oceans : ~ 20,000 yrs.
Soils: ~ 9 yrs.
Atmospheric pollutants:
NO x ~
4 days
N 2 O
120 yrs.
Notes on residence times:
-- Reservoirs where N 2 is the dominant form of nitrogen ( atmosphere, ocean ) have long residence times.
-- Reservoirs where fixed nitrogen is dominant ( soils, plants ) have short
residence times.
=> N 2 is very stable, but fixed
nitrogen compounds are very reactive
(that's why plants can utilize them)
e.g. a common fertilizer is ammonium
nitrate , which is also an explosive!
-- N 2
O ,
a strong greenhouse gas, doesn't go away quickly!
Sources of Nitrogen Pollution:
-- SMOG --
NO x
is a product of automobile exhaust
and other combustion sources
=> NO 2 is the chemical that gives
smog it's characteristic brown color
NO 2
also leads to ozone production in the troposphere ...
...ozone is needed in the stratosphere to protect
the surface of the earth from UV radiation, but in the troposphere it's a pollutant.
More on Nitrogen Pollution:
-- Acid Rain --
NO 2
in the atmosphere can react to give
nitric acid :
NO 2 + OH ---> HNO 3
SO 2
(sulfur dioxide) also reacts to produce
acids. SO 2 is often a product from
the burning of coal.
These acids are soluble in water:
=> acid rain
-- Acid rain is a problem downwind
of major industrial emissions
coal power-plants in midwest =>
acid rain in the eastern US
Still more on Nitrogen Pollution:
-- Eutrophication => increasing
the nutrients in a body of water
Most rivers and estuaries are nutrient
limited (either N or P ). Runoff carrying excess nitrate fertilizers enriches these bodies
of water.
However: Algae respond to this first!
Excess algae => deplete all O 2 in the water
=> other species die
So : fertilizer runoff damages
ecosystems. Untreated sewage also causes this problem.
The Phosphorus Cycle
Important because:
-- Phosphorus is a necessary, limiting nutrient
-- Phosphate runoff causes eutrophication
Box Model:
Reservoirs: (in millions of metric tons
)
Earth's Crust: 20,000,000,000
( recoverable : ~20,000)
Ocean: 100,000
Freshwater: ~100
Land Plants: ~3000
Soils: ~100,000
-- Note that most of the phosphorus
is in rocks that are unrecoverable.
Fluxes: (in millions of metric tons/yr
)
M: Mining 50 (humans)
F: Fertilization 50 (humans)
W: Weathering 10
R: Runoff 20
B: Burial 13
D: Decay 200
G: Growth 200
Other fluxes:
Ocean to land by sea spray 0.03
Ocean to land by guano 0.01
Industrial wastes 2
Notes on Fluxes:
-- Phosphorous has no stable gas phase, so addition of P to land is slow
(low rain P).
-- 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).
-- Natural transfer of P from ocean to land is very small... less than 0.03
mmt/yr for sea spray and 0.01 mmt/yr for guano.
-- 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.
Residence Times:
-- Ocean: 100,000 mmt / 20 mmt/yr = 5,000 years (with respect to
input).
Availability to marine organisms is limited by the fact that most P is in
the deep ocean. Main productivity areas
are near upwelling zones where deep water comes to the surface.
-- Land deposits:
For phosphate rocks in the U.S.:
2,200 mmt / 50 mmt/yr = 44 years
Longer if less concentrated deposits are mined (8,800 mmt / 50 mmt/yr =
175 years)... major issue is mining
techniques (strip mining used) with visual impacts and water pollution.
Review of Basic Concepts
in Nutrient Cycling
Notes:
-- Movement through the atmosphere
is generally rapid
-- Movement through the soils is generally slow
-- Movement from terrestrial biosphere to the ocean (via stream flow, usually)
must be replaced by movement either through the atmosphere (such as with
nitrogen and carbon) or by weathering (such as with phosphorous or calcium).
The atmospheric route is much faster!
Increased transport by stream flow severely disrupts the cycles of elements
without a gaseous phase.
Thought for the Day:
Humans clearly disrupt many, if not
all biogeochemical cycles...and in the process threaten many ecosystems.
In the absence of humans, are the biogeochemical
cycles stable?
Probably not...
Life has existed for about 3.5 billion years, and a complete breakdown has
not occurred since oxygen became available about 1.5 billion years ago.
Change is a part of natural biogeochemical
cycles resulting in periods of abundant biota and periods of scarce biota
(both ocean and land).