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Ecological applications of stable isotopes Howard Griffiths, University of Cambridge,

Ecological applications of stable isotopes Howard Griffiths, University of Cambridge, With acknowledgements to Jim Gillon, Kirsty Harwood, Andy Roberts, Kate Maxwell and Annie Borland, And particular thanks for the enthusiasm of the current team,

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Ecological applications of stable isotopes Howard Griffiths, University of Cambridge,

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  1. Ecological applications of stable isotopes Howard Griffiths, University of Cambridge, With acknowledgements to Jim Gillon, Kirsty Harwood, Andy Roberts, Kate Maxwell and Annie Borland, And particular thanks for the enthusiasm of the current team, Dr. Gary Lanigan (NETCARB), Nick Betson (NERC) and Joel Dunn, (Hortlink 212 with HRI East Malling) Theme for this morning’s talk: Ask not what isotopes can do for me….

  2. General applications • 13C: Organic material photosynthetic pathway • 13C: Real-time “instantaneous fractionation” • 13C: Mesophyll conductance • 13C/18O: Screening populations for drought tolerance • 13C/18O/D: Water sources for plants, evaporation • 13C/18O: Scaling of gas exchange from leaf to globe • Fractionation and Discrimination processes • Derivation of 13C discrimination terminology • The cherry orchard: an isotopic case history

  3. 13C: Organic material photosynthetic pathway

  4. 13C: Real-time • “instantaneous • fractionation”: • Effects of VPD • Reveal the • real PEPC signal

  5. Mesophyll conductance

  6. Screening populations for drought tolerance: Brassicaceae

  7. Water sources for plants, evaporation: climate and land topology

  8. Scaling of gas exchange • from leaf to globe: • annual cycles

  9. Scaling of gas exchange from leaf to globe: daily cycles Use of Keeling plots (d vs 1/CO2) to reveal end Members in two source mixing systems

  10. Stable Isotopes Distribution and Natural Abundance

  11.  = [ R sample – R standard] .1000 • R standard • Where R = 45/44 mass to charge ratio for sample and standard, with d having units of part per thousand or per mille (‰)

  12. Stable Isotopes – Why does fractionation occur? • Kinetic Fractionation Factors ,  • Molecules containing the heavy isotope diffuse and react more slowly, and from stronger bonds which break less easily. • Kinetic discrimination occurs during diffusion: • CO2 in air  = 1.0044, or  = +4.40/00 • CO2/Rubisco,  = ~1.030, or  = +300/00 • HCO3/PEPC,  = 1.002, or  = +20/00

  13. Stable Isotopes – Why does fractionation occur? • Equilibrium fractionation Factors • Fractionation during thermodynamic equilibrium • For example 13C in CO2 to HCO3 or CO3,  = 0.992 or  = -8‰ • (ie (bi)carbonates are enriched in 13C relative to source CO2 • for example • 18O in H2O during evaporation,  = 1.082,  =820/00 • D in H2O during evaporation,  = 1.010,  =100/00 • ie residual water becomes enriched in 18O and D • during evaporation – (vapour is depleted); fractionation recurs during precipitation favouring heavier isotope (rainfall is enriched!)

  14. For source CO2 and plant organic material: • R = 13CO2/12CO2 = 45/44 • Rs = 0.01124, Ra = 0.01115, Rp = 0.01090 • a = (Ra - 1) = 0.008 or -80/00; p = (Rp –1) = -0.0302 or –30.2 0/00 • Rs Rs • Discrimination,  = a - p = (-0.008 + 0.0302) = 0.0229 • 1 + p ( 1 – 0.0302) •  = 0.0229 or 22.90/00 • representing the POSITIVE DISCRIMINATIONwhich has been brought about against 13C during C assimilation (so  approximates to (a - p) or 22.20/00!)

  15. d18O signals and evaporative enrichment Organic matter (+27‰) Leaf water enriched by evaporation in proportion to stomatal conductance Source water taken in vein (reflects soil water) Leaf water (+4‰) -7‰ 2 CO2 300-1000 H2O CO2 H2O exchange d18O signal in CO2: A proxy for leaf water d18O= –18 ‰ (depleted in 18O)

  16. Derivation of 13C discrimination terminology: In Susanne’s fair hand!!

  17. The molar abundance ratio of 13C/12C in plant organic material (Rp) is related to the rate of assimilation of 13CO2 and 12CO2 (A‘and A, respectively), such that A'/A = Rp. For 12C A = g (pa - pi) and 13C A' = g'(p'a - p'i).

  18. Knowing the fractionation factors for discrimination against 13CO2 during Diffusion: a, or 4.4‰, such that g/g' = (1 + a), And carboxylation: p’a/pa =Ra; p’i/pi = Ri b = 27‰ and Ri/Rp = (1 + b), where Ri and Rp are the molar 13C/12C isotope ratios for CO2 in the intercellular spaces and plant material, respectively),and Ra is the source air The terms for 13C (A' = g'(p'a - p'i)) can be substituted with equivalent expressions based on 12C, such that A.Rp = g/(1 + a) . (Rapa - Ripi),

  19. A.Rp = g/(1 + a) . (Rapa - Ripi), and by rearrangement,since a = 1 + D = Ra/Rp a = (1 + a)(pa - pi)/pa + (1 + b)pi/pa, and hence D = a + (b - a)pi/pa, or D = 4.4 + 22.6 pi/pa.

  20. . Starting from the assumption that: A = vc - F - R, where vc is the rate of Rubisco carboxylation, F photorespiration and R "dark" respiration in the light. By substituting for the effects on 13C, but including fractionation during the diffusion and respiratory processes, the following expression can now been refined to consist of: D = ab(pa - ps)/pa + a(ps - pst)/pa + a(pst - pw)/pa + (es + a1)(pw - pc)/pa + bpc/pa - (eR/k + fG*)/pa But is that a valid starting point???

  21. The cherry orchard: an isotopic case history A collaboration with HRI East Malling (Neil Hipps, ChrisAtkinson) with major contributions from Joel Dunn, Nick Betson and Gary Lanigan Funded Aims: to investigate the impact of canopy manipulation (crown reduction, crown thinning) on water use and cause of subsidence in isolated urban trees

  22. 10 9 8 d18O of leaf water 7 6 5 23 22 21 Discrimination, D13C 20 19 800 700 600 Boundary layer conductance, gb (mmol m-2 s-1) 500 400 300 200 Control Reduced Thinned Cherry

  23. 4 Cherry 3 Leaf nitrogen (%) 2 1 0 4 London Plane 3 2 Leaf nitrogen (%) 1 0 Control Reduced Thinned

  24. 12 25 10 8 24 d18O of organic material 6 23 4 2 22 0 20 19 d18O of leaf water 18 17 Discrimination, D13C Control Reduced Thinned London Plane

  25. 35 30 Thinned Control Reduced 25 20 Whole tree sap flux (litre/day) 15 10 5 0

  26. d18O vs. d13C CONCLUSIONS: • Changes in WUE (implied from d13C and gas exchange in well-stirred cuvette) at leaf level not always concurrent with that measured at canopy level (from sap flow and boundary layer conductance). • Since d18O of leaf water is dependent on VPD (stomatal and boundary layer conductance), therefore reflects canopy water status and correlates with whole tree sap flux measurements. • Canopy manipulation alters radiation balance and coupling of entire canopy

  27. And finally, in the spirit of current global concerns: NETCARB and fellow delegates- Ask not what isotopes can do for you….. ask what you can do for isotopes!!

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