Reaction Velocity A e Ea RT where A preexponential factor or y intercept Ea activation energy of the substrate R universal gas constant T temperature o K Boone et al 2003 Nature 396570572 ID: 476405
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Slide1Slide2Slide3
Arrhenius Kinetics
Reaction Velocity = A e
-
Ea/RT where,
A = pre-exponential factor, or y-interceptEa = activation energy of the substrateR = universal gas constantT = temperature, oKSlide4
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.Slide5
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
EnergySlide6
Craine
et al (2010) Nature Geoscience 3:854-857
Widespread coupling between the rate and temperature sensitivity of organic matter
decay
High Ea decomposes slowly
Low
Ea
decomposes rapidly
R20 = microbial respiration rate @ 20 oCSlide7
Issue of substrate supply
Low to highSlide8
What processes affect heterotrophic respiration?
activation energy of the substrate (e.g.,
Craine
et al. 2010) soil temperature & moisture (e.g., Lloyd & Taylor 1994
substrate supply
(e.g., Davidson and
Janssens
2006) O2 concentration (e.g., Skopp
et al. 1990)
C-use efficiency (e.g., Allison et al. 2010)
sorption – desorption dynamics (e.g.,
Hinsinger
2001)
substrate
supply
O
2
concentration
max. rate
of reaction
double
Michaelis-MentonfunctionSlide9
What processes affect heterotrophic respiration?
activation energy of the substrate
(e.g.,
Craine et al. 2010)
soil temperature
& moisture
(e.g., Lloyd & Taylor 1994
substrate supply (e.g., Davidson and Janssens 2006) O2 concentration (e.g.,
Skopp
et al. 1990)
C-use efficiency (e.g., Allison et al. 2010)
sorption – desorption dynamics (e.g.,
Hinsinger
2001)
substrate
supply
O
2
concentration
max. rate
of reaction
doubleMichaelis-Menton
function
Arrhenius
function
reaction rate increases with TSlide10
A simple test: predicting exoenzyme activity [Davidson et al.
2011
]
known substrate concentrations
constant temperature during incubation
36.5
o
C
27.5
oC4.5
o
C
12.7
o
C
23.7
o
C
aaaaa
50
40
30
20
10
0
reaction velocity
(
μmol
hr
-1
)
0.04
0.03
0.02
0.01
0
reaction velocity
(
μmol
hr
-1
)
substrate concentration [
Sx
]
0 20000 40000 60000 80000 100000 120000
0 20000 40000 60000 80000 100000
substrate concentration [
Sx
]Slide11
A complex test: predicting heterotrophic respiration in a trenching expt.
at the Harvard Forest [Davidson et
al. 2011]
substrate concentration @ reaction site
O
2
concentration @ reaction site
Sx
total
=soil C content
p
= solubility fraction
D
liq
=
difussivity
in water
Θ
= soil moisture
a
=air filled
porosity
BD=bulk density
PD=particle density
D
gas
=diffusivity of O
2
in airSlide12
A complex test: predicting heterotrophic respiration in a trenching expt.
at the Harvard Forest [Davidson et al.
2011
]
observations
model
model
w
/seasonality
by allowing variation
in
α
Sx
of
Vmax
Sx
=
α
Sx
X
e
-EaSx/RTSlide13Slide14Slide15
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).Slide16
Tarnocai
et al. 2009 Global Biogeochemical Cycles 0-30cm
191
Total=3224x1015gC
Circumarctic permafrost region 0-100cm 496 ~32% of global total 0-300cm 1024Slide17
Microbial C-use Efficiency:
an emerging topic in terrestrial biogeochemistry
Figure Source:
Schimel
and Weintraub (2003) Soil Biology and BiochemistrySlide18
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 SystemSlide19
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 uptake
not temperature sensitive
low efficiency
indirect uptake via extracellular decomposition
temperature sensitiveEfficiency decreases with increasing temperature& molecular complexity [
Ea
]
30% decrease
60% decrease
Frey et al. (2013) Nature Climate Change 3:395-398
The temperature response of soil microbial efficiency and its feedback to
climateSlide20
Frey et al. (2013) Nature Climate Change 3:395-398
The temperature response of soil microbial efficiency and its feedback to
climate
Two years following there is little change in the microbial CUE of phenol in warmed compared to control plots
18 years following experimental warming, phenol CUE “acclimates” in treatment relative to control plots* shifts in microbial physiology
* shifts in microbial community compositionSlide21
Allison et al (2010) Nature Geoscience 3:336 – 340
Soil
-carbon response to warming dependent on microbial
physiology
Model simulates temperature sensitivity of microbial [growth, CUE] and exoenzyme activitySlide22
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 o
C
]
Warming + varying CUE
CUE declines 0.31 to 0.23Warming + constant CUE CUE remains at 0.31Warming + acclimation thermal acclimation of microbial respiration
[
evolutionary adaptation, community
shifts
and
physiological
changes]
simulated by reducing T
sensitivity
of CUEAllison 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 s microbial enzyme prod./ respiration biomass activity SOC small transient
large transient
30%
long term - substrate limitation via SOC depletion
intermediate
15%
transientSlide23