7.014 lecture 20: biogeochemical cyclesweb.mit.edu/7.01x/7.014/documents/lec_20.pdf · 1 7.014...
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7.0147.014
Lecture 20: Biogeochemical CyclesLecture 20: Biogeochemical Cycles
April 1, 2007April 1, 2007
Global Nutrient Cycling - Biogeochemical Cycles
Useful Conversion Factors
1012 g = 1 teragram = 1 Tg
109 g = 1 gigaton = 1 gt
106 g = 1 metric ton = 1 tonne
Mean Residence Time (MRT)
= pool size / flux in or out (or their average) of pool
Fractional Turnover = 1 / MRT
= fraction that is removed and replaced per unit time
OCEAN
Dead OrganicMatter
MarineBiomass
Sinking
Dead OrganicMatter
Bioelementsin Solution
Uptake
Weathering
Volatile ElementsVolatile ElementsPrecipitation
Decomposition
Runoff Losses
TerrestrialBiomass
Death
Evaporation
H2O andvolatile
elementsonly
Pools (or Pools (or ““reservoirsreservoirs”” or or ““compartmentscompartments””)) FluxesFluxes
Solar Energy BudgetSolar Energy BudgetTotal From Sun 100%
(1.3 x 1021 kcal per year)
Reflected 30%
Absorbed (heat) 47%
Used in Evaporation 23%
Used in Winds & Currents 0.2%
Used in Photosynthesis 0.002%
Used toDrive theCycles
SolarRadiation
UV radiation
Absorbedby ozone
Absorbedby earth
Visiblelight
Reflected byatmosphere
Radiated byatmosphere
as heat
Greenhouseeffect
Energy In = Energy Out
Source of energy to most ecosystems on Earth is Solar Radiation
Heat radiatedby earth
The Geologic Cycle (slow!)The Geologic Cycle (slow!)Powered by Solar & Geothermal EnergyPowered by Solar & Geothermal Energy
GEOTHERMAL
Magma MetamorphicRocks
SedimentaryRocks
Soils
SurfaceRocks
Igneous(Lava)
Igneous
Melting
WeatheringErosion
UpliftingUncovering
SOLAR
Critical forCritical forDriving theDriving the
CycleCycle
eg.Ca,K,Fe,P
2
““All the rivers run into the sea;All the rivers run into the sea;yet the sea is not full.yet the sea is not full.””
- Ecclesiastes 1:7- Ecclesiastes 1:7
The Global Water Cycle
Ice33,000,000
Soil Waters122,000
Groundwater15,300,000
Atmosphere13,000
Oceans1,350,000,000
111,000 71,000
River flow40,000
40,000
385,000 425,000
Net transport toland
Reference: Schlesinger, 1997
Pools (km3)
Fluxes (km3/yr)
=
The Global Phosphorus Cycle
Dust Transport
1.0
Soils200,000
Mineable rock10,000
P- Mines
Land plants3000
60Internalcycling
12
Sediments4x109
Riverflow21
1000Internalcycling
Oceans90,000
2
Fertilizers &Detergents
Pools (1012 g P)
Fluxes (1012 g P/yr)
Geologic TimeScales
(108 years)Reference: Schlesinger, 1997
crops
Negligibleatmosphericreservoir
Global Nitrogen- Transformations
ReducedReduced
OxidizedOxidized
AnoxicAnoxicOxicOxic
Organic - N NH4+
NO2-
NO3-
N2(gas)
N2O(gas)
NO(gas)
Ammonification Nitrogen Fixation
Denitrification
(anaerobic respiration)
Nitrification
(chemosynthesis)
Assim
ilatory
Nitrate R
eduction
ReleasesEnergy
Requires EnergyRequires Energy
3
The Global Nitrogen CycleN-fixation inlightening 10
Landplants3,500
1200
Soil organic – N12,000
IndustrialN-fixation
80
Internalcycling
Groundwater
Riverflow36
8000 Internalcycling Oceans
570,000
Permanent Burial10
Atmosphere4x109
BiologicalFixation 140
Denitrification<200
Denitrification110
Fluxes (1012 g N/yr)
Pools (1012 g N)
Reference: Schlesinger, 1997
Fertilizedcrops
20
Cultivatedlegumes
40
Natural sources Human sources
Am
ount
of n
itrog
en (g
igat
ons/
year
)
160
140
120
100
80
60
40
20
0
Nitrogen Fluxes
Lightning
Biologicalfixation
Fossil fuels
Nitrogenfertilizer
Nitrogen-fixing crops
140 Gt is equivalent to10 million trucks of dry
N fertilizer
Hubbard Brook Experimental Forest, New Hampshire
Choose two similar watersheds.Document nutrient levels in soil organic matter, plants, and streams.
Figure 51.10a upper Figure 51.10a lower
Clearcut Control
Devegetate one watershed and leave the other intact.Monitor the amount of dissolved substances in streams.
4
Devegetated
Net
dis
solv
ed s
ubst
ance
(kg/
ha)
1965–66 1966–67 1967–68 1968–69 1969–70
Control
1000
800
600
400
200
0
Year
Figure 51.10b
Nutrient runoff results
NO3- K+ , Ca+
Burning fossilfuel
The Global Carbon Cycle The Global Carbon Cycle –– Simplified! Simplified!(uncertainties abound)(uncertainties abound)
Burial0.1
Ocean40,000
92 90
Atmospheric Pool750
Measuredannual
increase+3.5
Soils1500
Net destructionof vegetation
1.5
10250
GPP
R50
Rivers1
Landplants560
Pools (1015 g C)
Fluxes (1015 g C/yr)
R
+2 in vegetation ? +2 in ocean ?
6
+3.5 increase in +3.5 increase in atmatm
+7.5 anthropogenic sources+7.5 anthropogenic sources
Year
CO
2 con
cent
ratio
n (p
pm)
360
350
340
330
320
310
1960 1970 1980 1990
Atmospheric COAtmospheric CO2 2 Concentrations are increasingConcentrations are increasing
Question:Question: Summer and winter areout of phase in the N. and S.Hemisphere. Why doesn’t thiscancel out the seasonal effect?
-450 -350-400 -300 -250 -100-150-200 0-50 50Thousands of years
400
150
200
350
300
250
Atm
osph
eric
CO
2 (pp
mV)
Ice Core Data Showing Changes inIce Core Data Showing Changes inAtmospheric COAtmospheric CO22 Concentrations Concentrations
5
CO2
nutrients: N, P, etc
Biogeochemical Cycles are Coupled to Each OtherBiogeochemical Cycles are Coupled to Each Other
Biosphere II ExperimentBiosphere II Experiment
Phot
osyn
thes
is O
Phot
osyn
thes
is O
22
Cement infrastructureCement infrastructureRich soilRich soil
Ca(OH)2 + CO CO2 2 →CaCO3 + H20
Why didn’t CO2increase?
Res
pira
tion
Res
pira
tion
CO
CO 22
Phot
osyn
thes
is C
O
Phot
osyn
thes
is C
O 22 Respiration
Respiration
O O22
gasp!
Δ O2
Δ CO2Understandable
Understandable
only inonly in
hindsighthindsight
6
““Biosphere IBiosphere I”” Experiment Experiment
Popu
latio
n si
ze (b
illion
s)Po
pula
tion
size
(billi
ons)
COCO22
NN22O (310 x COO (310 x CO22))
CHCH4 4 (21x CO(21x CO22))
Population sizePopulation size
380380
340340
300300
260260
18001800
14001400
10001000
60060000 10001000500500 15001500 20002000
CO
CO
22 an
d N
and
N22O
O
ppm
Vpp
mV
Time, Calendar years (A.D.)Time, Calendar years (A.D.)
CH
CH
44 pp
bVpp
bV
66
44
22
00
Falkowski and Tchernov 2004
Take Home MessagesTake Home Messages Cycles of essential elements for life on Earth (Cycles of essential elements for life on Earth (eg.C,N,Peg.C,N,P……))
sustain life, and are mediated by lifesustain life, and are mediated by life
The cycles are tightly coupled. You canThe cycles are tightly coupled. You can’’t change one withoutt change one withoutchanging the otherschanging the others
Human activities have accelerated the global fluxes of elementsHuman activities have accelerated the global fluxes of elementsto unprecedented rates in the history of the Earth, resulting into unprecedented rates in the history of the Earth, resulting inchanging concentrations in land, atmosphere, and water.changing concentrations in land, atmosphere, and water.
The long term effects of these changes on the Earth System areThe long term effects of these changes on the Earth System arenot easily understood or predicted.not easily understood or predicted.
Many environmental problems have their origin in acceleratedMany environmental problems have their origin in acceleratedbiogeochemical fluxes of life-essential elements, which, whenbiogeochemical fluxes of life-essential elements, which, whenout of balance, become out of balance, become ‘‘pollutantspollutants’’
““Some call it pollution, we call it lifeSome call it pollution, we call it life””
•• http://http://www.youtube.com/watch?vwww.youtube.com/watch?v=7sGK=7sGKvDNdJNAvDNdJNA