Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Moisture: Microbes are sensitive to moistureAs w goes below -1 Mpa, some groups die ofw = -1.5 MPa A few bacteria and aquatic fungi w = -4 Mpa Nitrifiers, Bacillusw = -10 MPa Many fungiw = - 40 Mpa Penicillium, Aspergillusoxygen: closely related to moisture O2 diffuses 10,000 times faster in air than water So, water saturated soils have poor O2 supply Go anaerobic Most decomposers are aerobicAnaerobiosis slows decomposition That’s one reason peat forms: cold & wet so large stores of Cremember: most plants keel over at w -1.5 MPaimage: http://gardenwise.files.wordpress.com/2010/05/wilting-plant.jpgIntegrated effect of moisture:H2O ContentDecomposition RateSubstratew limitedO2 limitedHow do you integrate those effects? Water filled pore space: what fraction of soil pores are filled with H2OWater filled pore spaceDecomposition Rate60%100% WFPS~ 60% WFPSMacropores are drainedMicropores are water-filledk defines “litter quality”: an aggregate, ill-defined measure of litter’s chemical nature & how fast it decomposes.litter qualitySingle k Assumes litter is a uniform material!Single k Assumes litter is a uniform material!But in reality, litter is a complex material.Metabolic material decomposes quicklyprotein, sugarsStructural material decomposes slowercellulose, lignin, cutinFastModerateSlowdC/dt = kfCf + kmCm + ksCslitter chemistry controlled by leaf chemistry leaf chemistry: cell walls: celluloselignin cytoplasm: proteinsugarsnucleic acids defenses: tannins, alkaloids“state factors” of litter quality:1. Size of molecule2. Types of chemical bonds3. Regularity of structure4. Nutrient concentrations5. Toxicitymolecular size glucose: lasts hours to dayscellulose: glucose polymer (7-15,000 glucoses) lasts weeks to monthscan cells take up the molecule?types of bonds: organic functional groupsGlucoseGlutamineHexadecaneConiferyl alchoholLots of –OH or –NH2 groups make molecules reactiveC-C bonds are intermediateAromatic rings are hard to break downHexadecaneregularityCelluloseLignin: lasts months to yearsNutrient concentration Microbial growthGlucose No NGlucose + NIn which will glucose decompose faster?chemical compositionQualea rosea0.75% NPlatonia insignis1.42% Nwhat is different about these leaves?protein contentnutrient concentrationFresh litter: More N more degradableSoil organic: More N less degradable matterWhy?Fresh litter: N in protein SOM: N in complex chemical formsToxicityCondensed tanninHydrolyzable tanninCapsaicinHow do you measure “litter quality”? Need an index for general modelsC/N Ratio: High ratio means slow decompositionImperfect: ignores lignin and 2º chemicalsLignin content: High value means slow decompositionImperfect: ignores N and 2º chemicalsLignin/N ratio: High value means slow decompositionNot bad. Only ignores 2º chemicalsHow do you measure “litter quality”?Each constituent has a different k value. Build an aggregate modelWhat are the limitations of this approach?Standard model structure:dC/dt = k * C * (Md *Td) k 1st order rate constant C Size of pool Md, Td Moisture and Temperature reducing functionsstandard model: limitationsAssumptions:1st order kinetics: Decomposition is regulated by C decomposers are C-limited No decomposer: All the biology and ecology of decomposers can be collapsed into the “k” constantSingle k value: Assumes litter is a uniform material1st order kinetics: Decomposition is regulated by Cµg CO2-C g-1 C d-1No evidence of running out of C or that C availability regulates decomposition.22- 35% of total soil C respiredtest of microbial N limitation in tussock soil N, not C, limits microbial activityMicrobial responses increase with NMicrobiology is “implicit”: contained within rate constants and response functionsModel: No Decomposer:dC/dt = k * C * (Md *Td)stoichiometry of players is constant, no nutrients!E n v i r o n m e n t D e c o m p o s i t i o nM i c r o b i a lc o m m u n i t yHow do microbes process litter?dC/dt = k * CTraditional models: litter falls apartLitter CO2How do you eat something bigger than your head?You cut it up first!Litter DOCEnzymesMicrobesEnzyme-based modelDecomposition = k * Litter * EnzCO2Enzyme productionMaintenanceGrowthextracellular enzymes are the RATE LIMITING STEP in decompositionWhat are the critical extracellular enzymes?Major classes: Carbohydrate degrading enzymes cellulase: exo and endo Lignin-degrading enzymes (made by fungi) phenol oxidase peroxidase N-acquiring enzymes protease chitinase (chitin and peptidoglycan)Who makes the extracellular enzymes?How many types of enzymes are needed?Who uses the products of enzyme action?How do we model this?A biotically complex decomposition model: 3 guildsMoorhead and Sinsabaugh. 2006. Ecological Monographs. 76: 151-174.Solubles OpportunistsCellulose DecomposersLignin
View Full Document