Experimental Methods

1. Modeling the relationship between sorption and residence times Melanie A. Mayes (mayesma@ornl.gov), Sindhu Jagadamma (jagadammas@ornl.gov), W. Mac Post (wmpostiii@ornl.gov), Joshua Frerichs (frerichsj@ornl.gov), and Gangsheng Wang (wangg@ornl.gov) 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.