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).