Nature manuscript 2003-08-08564
Title: Similar response of labile and resistant soil organic matter pools to
changes in temperature
Authors: Changming Fang, Pete Smith, John B Moncrieff and Jo U Smith
Supplementary Methods
1. Calculation of mean and relative respiration rate
During soil incubation, there was a significant decline in respiration rate with time,
probably due to the depletion of labile substrates. For each round of temperature
change (min-max-min, or 20 ºC-min-max-20 ºC, taking about 9 days), soil respiration
was measured at different temperatures. The mean respiration rate at a given
temperature was calculated from measured rates in order to minimise the time effect
on soil respiration rate and errors in estimated Q10 value. These mean respiration rates
were later used to fit an exponential model and calculate Q10 value. Supplementary
Figure 1 is an example to show how soil respiration changes with temperature and
time.
For comparison between soil samples, mean respiration rates were normalized against
the rate at 10 °C for each sample. Mean respiration rates were first fitted with an
exponential model:
R  a  ebT
(1)
where R is respiration rate, a and b are fitted parameters, respectively. Respiration rate
at 10 ºC, R10, is then calculated as:
R10  a  e10b
(2)
Relative respiration rate was calculated against R10:
Rrelative  R / R10
(3)
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Nature manuscript 2003-08-08564
2. Determination of Q10
By definition, the value of Q10 is the factor by which the respiration rate differs for a
temperature interval of 10 ºC:
Q10 
RT 10
RT
(4)
where RT and RT 10 are respiration rates at temperature of T and T+101-2 .
Q10 values derived from different models (e.g. exponential, Arrhenius and linear
models) may be different, either by magnitude, or with respect to temperature. With
the exponential model (Eq.1), the Q10 value is conceptually constant with temperature.
In other models, Q10 varies with temperature in different ways1-3.
Combining equation 1 and 4, Q10 can be estimated as:
Q10  e10b
(5)
or
ln Q10  ln
R 10

a T
(6)
For each soil sample and each round of temperature change, mean respiration rates at
different temperatures were fitted with equation 1 to estimate the Q10 value.
3. Contributions of SOM pools to the total SOM decomposition
SOM is a complex of C components with different decomposabilities. It is commonly
understood that SOM decomposition / soil basal respiration is driven by a small
portion of the labile component4-6. When labile C is depleted, basal respiration rate
drops rapidly. Despite the fact that resistant C comprises the majority of SOM in most
soils, it may only contribute a minor part to the total soil basal respiration due to its
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Nature manuscript 2003-08-08564
slow turnover rate. In most current models, the decomposition of each pool is
simulated by a first order kinetics with respect to the C concentration in the pool4-5.
It is difficult to partition soil basal respiration to different C components as there is no
effective way to partition SOM. To estimate the possible contribution of resistant C
component to the total SOM decomposition, we assumed that resistant component is
80% of TOC in our soil samples (mineral soil at 0-10 and 20-30 cm) at the beginning
of incubation. This ratio should be less than that reported for field soils or simulated at
equilibrium by current models5. We also assumed that the turnover rate constant for
resistant C is 0.02 year-1 (a rate constant used for humus in the RothC model). This
rate constant is similar to or is lower than the resistant pool in most models7-8. Despite
an assumed residence time of the passive pool in some models of more than one
thousand years5, the resistant components of soil organic matter as a whole typically
have a residence time of 20 to 50 years9.
The decomposition rate of the resistant component and its relative importance to
measured soil basal respiration are shown in Supplementary Figure 2. For the sample
at 0-10 cm depth, resistant C contributed 7% at the beginning and about 27% at later
stages of the incubation. Corresponding values for the sample at 20-30 cm were 14%
and 53% respectively. On average, the contribution of resistant C to SOM
decomposition increased from about 10% at the beginning to about 40% later in the
incubation.
4. Variation in Q10 value due to changes in SOM composition
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Nature manuscript 2003-08-08564
Supposing that decompositions of resistant and labile C have different temperature
dependences, the temperature sensitivity of total soil respiration can be described as:
Q10, total  RlabQ10, lab  RresQ10, res
(7)
where Q10,total, Q10,lab and Q10,res are Q10 values for total SOM, labile and resistant C
decomposition; Rlab and Rres are the percentage contributions of labile and resistant C
to the total SOM decomposition, respectively.
Q10,total was estimated with measured soil basal respiration rate at different
temperatures as described above. At the beginning of the incubation, average Q10,total
was 2.07 (0.021). At the end of incubation (around day 100), Q10,total was 2.14
(0.16). We assumed Q10,lab = 2.0 for analysing the contribution of Q10,res to Q10,total.
The magnitude of Q10,lab does not matter here, as comparison is made on a relative
scale. Supplementary Figure 3 shows the contribution of different Q10,res to Q10,total. As
a function of incubation time, Q10,total should gradually decrease if the decomposition
of resistant C has a significantly smaller Q10 than labile C. The results presented in
here do not support the hypothesis that the temperature dependence of resistant C
decomposition is significantly less than that of labile C pool. Resistant components of
SOM appear to have a similar response to global warming as do labile C pools.
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