powerpoint presentation 1final.pdf · -fixation deposition soil 15n n losses transformations...
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4
Origins of elements and isotopes
• Hydrogen burning FUSION: E = mc2
– 1H + 1H = 2H + + 0.422 MeV
– 2H + 1H = 3He + + 5.493 MeV
– 3He + 3He = 4He + 1H + 1H + 12.859 MeV
• Helium burning
– 4He + 4He + 4He = 12C
5
• CNO cycle – 12C + 1H = 13N +
– 13N = 13C + + +
– 13C + 1H = 14N
– 14N + 1H = 15O
– 15O = 15N + + +
– 15N + 1H = 12C + 4He
• Carbon burning – 12C + 4He = 16O
• Oxygen burning – 16O + 4He = 20Ne
• Neon burning – 20Ne + 4He = 24Mg
16
17
Isotope notation and fractionation
• δ notation (in “parts per thousand” or
“permil”)
δHX = [(Rsample/Rstandard – 1)] * 1000
Where H = heavy isotope mass, X = element,
R = ratio of heavy to light isotope of the element
Natural ranges in isotope ratios:
ca. 600‰ for δ2H
ca. 100‰ for δ13C, δ18O, δ34S
ca. 30‰ for δ15N
19
Further notes on δ notation
• Values can be positive or negative.
• Linearly related to percent (%) abundance of the heavier isotope.
• Convenient means of dealing with relatively small % changes (i.e. 1% change in abundance is 10‰).
• However, δ notation is not “exact” in all mathematical applications and isotopic ranges and Atom Percent (HAP), Fractional (F) and Ratio (R) nomenclature is typically used (see Fry chapter).
• However, as Biologists, δ-notation will cover all your needs unless you delve into “spiking” experiments!
20
Stable Isotope Standards
Standards by definition have a 0‰ value of the -scale
of interest. Internal lab standards must be corrected to
International reference standards. International
reference materials are distributed by the US National
Institute of Standards and Technology (NIST; formerly
the US National Bureau of Standards), and by the
International Atomic Energy Agency (IAEA).
NIST: <www.nist.gov>
IAEA: <www.iaea.or.at >
38
Some examples of isotopic fractionations in
the Biosphere
• Elements and isotopes circulate in the atmosphere and fractionation and mixing bring about characteristic isotope distributions.
• Large (well buffered) pools provide points of “stability”
– E.g. Ocean (H,O,S,C), atmosphere (N).
• Fractionation is the agent of change.
• Plants, microbes fix nutrients and change isotope distributions for C,N,S.
39
Carbon
40
Carbon cycle
• Active exchanges between atmosphere, terrestrial ecosystems, sea surface.
• Atmospheric CO2 (-7 to -8 o/oo)
• C3 Photosynthesis ~-20 o/oo fractionation (-28 o/oo plant tissue).
• C4, CAM Photosynthesis ~-5 o/oo(-13 o/oo).
• Ocean: dissolved CO2 ~ +8 o/oo, bicarbonate production ~ +1 o/oo (+1 o/oo).
• Planktonic photosynthesis ~-20 o/oo.
41
Plant C fixation
Enzymatic
fixation CO2
Diffusion
C3: Calvin cycle, RUBISCO
ribulose biphosphate carboxylase
C4: Hatch-Slack cycle, PEP
Phosphoenolpyruvate carboxylase
Diffusion Δδ = ~-4 o/oo
Rubisco Δδ = ~-29 o/oo
Diffusion Δδ = ~4 o/oo
Rubisco Δδ = ~6 o/oo
CAM: Crassulacean acid metabolism,
PEP into C4 acids at night, refixed by
Rubisco during day
42
C4
C3
Model estimates of plant organic 13C
Suits et al. (2005)
Worldwide distribution of C4 plants (C4 fraction)
Still et al. (2003)
What is the typical range of values for plant tissues?
47
But, water stress affects isotopic
discrimination …
Stewart et al. (1995)
13C of the atmosphere is changing
Francey et al. (1999)
49
Dave Keeling
A: photosynthetic assimilation
R: respiration
R
R
A
A
Bowling et al. (2005)
Isotopic patterns in a
subalpine forest: 3-
month averages
0
5
10
15
20
25
Ingeborg Levin, University of Heidelberg
http://www.iup.uni-heidelberg.de/institut/forschung/groups/kk/en/14CO2_html
59
Nitrogen
Utility of Nitrogen Isotopes
• isotope ratios are frequently used to identify sources of nitrate
• isotope ratios give information about geochemical processes and chemical reactions such as nitrification or denitrification
• for NO3- - isotopes of N and O can be
used - dual isotope tracer
Decadal changes…
Fertilizer Nitrate
-5 0 5 10 15 20 25 30 35
15
N - NO3(AIR)
-5
0
5
10
15
20
25
30
1
8O
- N
O3 (
VS
MO
W)
2004
1993
Denitrificatio
n -->
DO < 4 mg/L
Fertilizer Ammonium
Animal Waste
•Significant shift in 15N to lower values
(switch to inorganic
fertilizer use from
manure – BMP )
•Denitrification –
limited to suboxic
riparian zones and
deep (>30yr)
groundwater
62
Nitrogen Cycling
• Atmospheric reservoir of 0‰
• Because N is limiting, fractionation is generally low.
• Faster loss of 14N than 15N in particulate N
decomposition leads to increase in 15N with depth in
oceans and soil.
• So, plants that rely on soil N tend to be more enriched
than those depending on atmospheric N.
• Nitrification and denitrification are the key sources of
fractionation.
• Phytoplankton use N2 gas, ammonia and nitrate.
N2-Fixation Deposition
Soil 15N
N Losses
Transformations
Fertilizer
Transformations
Input
Models and Patterns of Soil 15N
Plant 15N
Patterns and Gradients of Plant 15N
Inorganic N
Mycorrhizae
Lecture – Part 1
What Controls Plant 15N?
Variation in Soil and Plants
Observations from Fry (1991)
1. Large variation
2. No correlation with precipitation
3. Soils more enriched than plants
4. N2-fixers near 0 ‰
General Trends in Soil 15N
Values are usually positive (but there are exceptions)
Amundson et al. (2003)
Soil
Depth
(m
)
1.2
1.0
0.8
0.6
0.4
0.2
0
0.5 0.4 0.3 0.2 0.1 0.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0
Soil 15 N (‰) Soil Nitrogen (mg N / g Soil)
Juniperus
Artemisia
Inter-Canopy
General Trends in Soil 15N
From: Evans and Ehleringer (1993)
Soil Nitrogen Transformations S
oil
Org
anic
Ma
tter Active Pool
PassivePool
Slow Pool NH4+ NO3
-
Mineralization Nitrification
Plants Microbes
Volatilization
NH3 N2O, NO NO, N2O, N2
Denitrification
AminoAcids
Högberg (1997)
Shearer and Kohl (1990)
Process Observed Discrimination (‰)
Mineralization 0
NH4+ : NH3 Equilibrium 20 to 27
Volatilization 29
Diffusion in Solution 0
Nitrification 0 to 35
Denitrification 0 to 33
Högberg (1997)
Shearer and Kohl (1990)
% Substrate Remaining
020406080100
15N
(‰
)
-20
0
20
40
60
Substrate
Product
Nitrogen Loss: Volatilization
Ammonia
Ammonium
Plant 15N Patterns and Gradients
Swap et al. (2004) Annual Precipitation (mm)
0 500 1000 1500
Le
af
15N
(‰
)
-2
0
2
4
6
8
10C3 (R
2=0.63)
C4 (R2=0.19)
Kalahari Transect
Nutrient availability varies
inversely with precipitation
N cycles in arid sites are
more open
72
Sulfur
73
Sulfur Cycle
• Sulfate in the ocean is the primary reservoir
that is 21o/oo heavier than primordial sulfur
(e.g. Canyon Diablo Troilite).
• Fixation by plants has a small isotope effect
but reduction in sediments and anaerobic
conditions has a large effect (30-70o/oo).
• Continental vegetation (+2 to +6o/oo) vs
marine plants (+17 to +21o/oo).
Utility of Sulfur Isotopes
• S isotope ratios are frequently used to identify sources of dissolved species
• isotope ratios give information about geochemical processes and chemical reactions
• for SO4= - isotopes of S and O can be
used
• Animal studies: marine vs. terrestrial, estuaries, marshes; S-amino acids …..
Common Dissolved Sulfur Forms
• sulfate - SO4=
• hydrogen sulfide (H2S), elemental S,
bi-sulfide
• other sulfur forms are generally
insignificant (sulfite, thiosulfite)
Sources of Sulfate
• dissolution of evaporites (gypsum,
anhydrite)
• oxidation of pyrite
• atmospheric precipitation (minor)
• volcanic emissions
• hydrogen sulfide from bogs, fossil fuel
combustion
78
Pichlmayer et al. (1998)
The hydrologic cycle
Evaporation
Condensation
Sublimation
Percolation
Infiltration
& Transpiration - of -
PRECIPITATION
Linking it to soils and plants
81
In preparation for the
Hydrospehere
93
The Global Meteoric Water Line, GMWL
After Clark and Fritz, 1997; GMWL defined by Craig, 1961
Condensation is an equilibrium process
so
Most precipitation values lie along a Global Meteoric
Water Line (GMWL) of slope ~8
(e2H / e18O = 8)
94
d-excess & the Global Meteoric Water Line
Isotopes Reflect Soil Water Use Patterns
Community (Interspecific) Water-use
Ehleringer et al., Oecologia, 1991
Central Pacific
High
Warmer Temperatures
Cold Subarctic Currents
Upwelling
Summer fog formation off the coast of west-central North America occurs when subsidence air
moved by the Central Pacific high pressure cell meets the warm air moving off of the continent
and cold water from subarctic Alaskan currents and deepwater upwelling
H L
Fog Drip in Redwood-Forest Communities
-80
-70
-60
-50
-40
-30
-20
-10
0
101992 1993 1994
Fog
Rainfall
Sequoia sempervirens
Oxalis oregana
Rhododendron macrophyllum
Polysticum munitum
Gaultheria shallon
J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D
Month of the Year
Sequoia sempervirens
Oxalis oregana
Rhododendron macrophyllum
Polysticum munitum
Gaultheria shallon
90
80
70
60
50
40
30
20
10
(a)
(b)
Complete dependence
On FOG! FOG
Dawson, 1998
Hydrogen exchange:
H C
H N
H O
Strong bonds
Weak bonds
Saskatoon Seasonality
5/7/90 1/31/93 10/28/95 7/24/98 4/19/01 1/14/04
-240
-200
-160
-120
-80
-40
D
(V
SM
OW
)
5/7/90 1/31/93 10/28/95 7/24/98 4/19/01 1/14/04
-35
-30
-25
-20
-15
-10
-5
(V
SM
OW
)
“Hydrogen Exchange” Problem
• 10-20% of keratin H will quickly (<24 hrs)
exchange hydrogen with ambient moisture
• D results vary among season and
between labs in different geographic
locations
• Dorg results not comparable among labs!
• Require new standardized procedures
Intercomparison Results
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Passerine Individual #
D
NHRC SINLAB
CPSIL
•18 passerine
feathers
•Cut along vein
and stem
•Keratin references
+/-2 at all labs
•Intra-sample
heterogenetity may
be an issue!
•Heterogeneity
may be larger than
the geographic
variance!
Other topics?
• Lipid Extraction
– C/N ratio (see Post et al.) vs removal.
• Your projects?