Modeling the relationship between sorption and residence times
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Modeling the relationship between sorption and residence times. Melanie A. Mayes ([email protected]), Sindhu Jagadamma ([email protected]), W. Mac Post ([email protected]), Joshua Frerichs ([email protected]), and Gangsheng Wang ([email protected])

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Modeling the relationship between sorption and residence times

Melanie A. Mayes ([email protected]), Sindhu Jagadamma ([email protected]), W. Mac Post ([email protected]), Joshua Frerichs ([email protected]), and Gangsheng Wang ([email protected])

Oak Ridge National Laboratory, Oak Ridge, TN, USA

Overview

Preliminary Results

Experimental Methods

Conceptual Model

Rationale:

Sorption and Residence times:

Sorption Results:

53 µm

Particulate

Mineral associated

Glucose

Starch

Vanillic acid

Stearic acid

after Lawrence et al., 2009

Soil C = 1500 Pg

(1) Dissolved organic C pool (DO14C) uptake by microbes

(2) POC decomposition (CO2 , 14CO2 )

(3) MAOC decomposition (CO2 , 14CO2 )

(7) Sorption and (6) Desorption (Keq)

(4) Microbial growth, (5) Maintenance respiration, (8) Microbial turnover

ENZ = exoenzymes (MAOC, POC)

Sorption isotherms fitted with Langmuir equation to determine maximum sorption capacity (Qmax) (Bolster and Hornberger, 2007)

Surface sorption > subsurface sorption, surprising because mineral concentration is higher in subsurface

Surface/Subsurface pattern may indicate biodegradation surface samples

  • Current models lack mechanisms of sorption to soil minerals and for microbial degradation

  • Difficulties of current models in predicting soil response to environmental changes

All experiments conducted at 20ºC, soil pH, 1-100 mg C L-1

Sorption experiments at 1:60, 8h equilibrium

Moisture in incubation experiments at field capacity, 50 g MAOC, 10 g POC

Microbial biomass (MB14C, MBC) via chloroform fumigation

14C activity via liquid scintillation counting

  • Research Questions:

  • Does protective sorption of dissolved organic compounds (DOC) exist?

  • How does sorption-protection differ for soil DOC (sugars, lipids, complex sugars, aromatics)?

  • How can sorptive-protection and microbial degradation be accounted for in soil C models?

Incubation Results:

Model Parameterization

Developed a database based on extensive literature review

Soil, microbe data from 172 sources (~900 obs)

Substrate-specific enzyme parameters from 100 sources (~300 obs)

Michaelis-Menten kinetics :

Where v = reaction rate, Vmax= max. specific enzyme activity, Km = half-saturation constant, S = substrate concentration

  • Activation Energy (Ea):

  • Hydrolase substrates for cellulose

    • β-glucosidase (BG)

    • cellobiohydrolase(CBH)

    • endo-glucanase (EG)

  • Oxidase substrates for lignin

    • peroxidase (PER)

    • phenol oxidase (POX)

  • Arrhenius equation where k = rate constant, a = frequency, R = gas constant, T = temperature

  • Hypotheses:

  • Sorptive protection enhanced in mineral-associated organic C (MAOC) over particulate organic C (POC)

  • DOC that bonds strongly to minerals (e.g., aromatics, lipids) are more resistant than compounds forming weak bonds (e.g., sugars)

Temperate 3 Subsurface, with Glucose addition at 100 mg C /L

CO2 production in POC and bulk soils is much higher than in MAOC

Indicates effective sorptive protection and/or lack of microbial activity in MAOC

Coupling Experiments and Modeling:

Model implemented in MATLAB to link with Community Land Model

Model parameterized and validated using literature data

Apply model to experimental data on global soils

Maximum specific activity (Vmax) of Enzymes

Model Results:

T = 5, 21, 30, 40, 60C

Half saturation constant (Km) of Enzymes

SOC pools equilibrate in 30 yr

Enzyme pools, MAOC_Q (sorbed MAOC), DOC, MBC equilibrate in 10 yr

Equilibrium pool sizes used as model initialization in further simulations

References:

Bolster, C.H., and G.M. Hornberger. 2007. On the use of linearized Langmuir equations. Soil Sci. Soc. Am. J. 71:1796-1806.

Lawrence, C.L.; Neff, J.C.; Schimel, J.S. Does adding microbial mechanisms of decomposition improve soil organic matter models? A comparison of four models using data from a pulsed rewetting experiment. Soil Biol. Biochem.41, 1923 (2009).

Soils provided by: Julie Jastrow1, Yuri Zinn2, Ann Russell3, Christian Giardina4, Stan Wullschleger5, Guðrún Gísladóttir6

ACKNOWLEDGEMENTS: The work is funded through ORNL’s Laboratory Directors Research and Development Program (LDRD). ORNL is managed by the University of Tennessee-Battelle, LLC, under contract DE-AC05-00OR22725 with the US DOE.


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