Arrhenius KineticsReaction Velocity = A e-Ea/RT where, A = pre-exponential factor, or y-interceptEa = activation energy of the substrateR = universal gas constantT = temperature, oK

Boone et al (2003) Nature 396:570-572.Figure 1: Season time course of soil respiration. Figure 2. Relationship between soil temperature and rate of soil respiration. The different experimental treatments reflect modifications to the rate of substrate supply.

In the above energy diagram for the reaction A B we have the following features:

1.Overall, the reaction is energetically favorable. In other words, the product, B, is at a lower energy level than the reactant, A. Energetically, the reaction will proceed with a net release of energy (i.e. goes downhill energetically as it goes from A B)2. However, for the reaction to proceed, there is an activation energy barrier that molecule A will have to overcome.

Molecules of A will have to acquire enough energy to overcome Ea in order for the reaction to proceed. This energy will come from the kinetic energy associated with molecular collisions

Ener

gy

Craine et al (2010) Nature Geoscience 3:854-857Widespread coupling between the rate and temperature sensitivity of organic matter decay

High Ea decomposes slowly

Low Ea decomposes rapidly

R20 = microbial respiration rate @ 20 oC

Issue of substrate supply

Low to

high

What processes affect heterotrophic respiration?

(1) activation energy of the substrate (e.g., Craine et al. 2010)(2) soil temperature & moisture (e.g., Lloyd & Taylor

1994(3) substrate supply (e.g., Davidson

and Janssens 2006)(4) O2 concentration (e.g.,

Skopp et al. 1990)(5) C-use efficiency (e.g., Allison

et al. 2010)(6) sorption – desorption dynamics (e.g., Hinsinger 2001)

substratesupply

O2

concentrationmax. rateof reaction

doubleMichaelis-Menton

function

What processes affect heterotrophic respiration?

(1) activation energy of the substrate (e.g., Craine et al. 2010)(2) soil temperature & moisture (e.g., Lloyd & Taylor

1994(3) substrate supply (e.g., Davidson

and Janssens 2006)(4) O2 concentration (e.g.,

Skopp et al. 1990)(5) C-use efficiency (e.g., Allison

et al. 2010)(6) sorption – desorption dynamics (e.g., Hinsinger 2001)

substratesupply

O2

concentrationmax. rateof reaction

doubleMichaelis-Menton

function

Arrheniusfunction

reaction rate increases with T

A simple test: predicting exoenzyme activity [Davidson et al. 2011]1. known substrate concentrations2. constant temperature during incubation

36.5 oC

27.5 oC

4.5 oC12.7 oC23.7 oC

aaaaa

50

40

30

20

10

0

reaction velocity

(μmol hr-1)

0.04

0.03

0.02

0.01

0reaction velocity

(μmol hr-1)

substrate concentration [Sx]

0 20000 40000 60000 80000 100000 120000 0 20000 40000 60000 80000 100000

substrate concentration [Sx]

A complex test: predicting heterotrophic respiration in a trenching expt.at the Harvard Forest [Davidson et al. 2011]

substrate concentration @ reaction site O2 concentration @ reaction site

Sxtotal=soil C content

p = solubility fraction

Dliq =difussivity in

waterΘ = soil

moisture

a =air filled porosity

BD=bulk density

PD=particle density

Dgas=diffusivity of O2 in air

A complex test: predicting heterotrophic respiration in a trenching expt.at the Harvard Forest [Davidson et al. 2011]

observations

model

model w/seasonalityby allowing variationin αSx of

VmaxSx= αSx X e-EaSx/RT

Wieder et al. 2013 Nature Climate Change 3:909-912 Global soil carbon projections are improved by modelling microbial processesdoi:10.1038/nclimate1951

Observations, global total = 1,259 Pg C. b, CLM4cn, global total = 691 Pg C (spatial correlation with observations (r) = 0.55, model-weighted root mean square error (r.m.s.e) = 7.1 kg C m−2). c, DAYCENT, global total = 939 Pg C (r = 0.53, r.m.s.e = 7.6). d, The CLM microbial model, global total = 1,310 Pg C (r = 0.71, r.m.s.e = 5.3).

Tarnocai et al. 2009 Global Biogeochemical Cycles 0-30cm 191 Total=3224x1015gCCircumarctic permafrost region

0-100cm 496 ~32% of global total

0-300cm 1024

Microbial C-use Efficiency: an emerging topic in terrestrial biogeochemistry

Figure Source: Schimel and Weintraub (2003) Soil Biology and Biochemistry

Microbial C-use Efficiency: an emerging topic in terrestrial biogeochemistry

Melillo et al (2003) Science 13:2173-2176 Soil Warming and Carbon-Cycle Feedbacks to the Climate System

Figure 1. Soil samples were collected from control plots at two soil warming studies at the Harvard Forest LTER site, amended with one of four substrates (glucose, glutamic acid, oxalic acid or phenol) and incubated at 5, 15 or 25 °C. Error bars represent one standard error.

direct uptake, not temperature sensitivehigh efficiency

direct uptakenot temperature sensitivelow efficiency

indirect uptake via extracellular decompositiontemperature sensitiveEfficiency decreases with increasing temperature& molecular complexity [Ea]

30% decrease

60% decrease

Frey et al. (2013) Nature Climate Change 3:395-398The temperature response of soil microbial efficiency and its feedback to climate

Frey et al. (2013) Nature Climate Change 3:395-398The temperature response of soil microbial efficiency and its feedback to climate

a. Two years following there is little change in the microbial CUE of phenol in warmed compared to control plots

b. 18 years following experimental warming, phenol CUE “acclimates” in treatment relative to control plots

* shifts in microbial physiology* shifts in microbial community composition

Allison et al (2010) Nature Geoscience 3:336 – 340Soil-carbon response to warming dependent on microbial physiology

Model simulates temperature sensitivity of microbial [growth, CUE] and exoenzyme activity

CUE w/T when respiration more sensitive to T than biomass production

Soil studies suggest CUE declines by at least 0.016 oC-1

Model Simulation [+5 oC]Warming + varying CUE

CUE declines 0.31 to 0.23

Warming + constant CUECUE remains at 0.31

Warming + acclimationthermal acclimation of microbial

respiration [evolutionary adaptation, community shifts and physiological changes]

simulated by reducing Tsensitivity of CUE

Allison et al (2010) Nature Geoscience 3:336 – 340Soil-carbon response to warming dependent on microbial physiology

M O D E L D Y N A M I C Smicrobial enzyme

prod./respiration biomass activity

SOCsmall transient

large transient 30%

long term - substrate limitation via SOC depletion

intermediate 15%

transient