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# SOM as a whole typically has a residence time of 20 to 50 years

By Loretta Anderson,2014-05-07 09:34
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SOM as a whole typically has a residence time of 20 to 50 years

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

value. These mean respiration rates 10temperature was calculated from measured rates in order to minimise the time effect

were later used to fit an exponential model and calculate Q value. Supplementary 10on soil respiration rate and errors in estimated Q

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:

bTR?a?e (1)

where R is respiration rate, a and b are fitted parameters, respectively. Respiration rate at 10 ºC, R, is then calculated as: 10

10bR?a?e (2) 10

Relative respiration rate was calculated against R: 10

R?R/R (3) relative10

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Nature manuscript 2003-08-08564

10

By definition, the value of Q is the factor by which the respiration rate differs for a 10

temperature interval of 10 ºC: 2. Determination of Q

R?10T (4) ?Q10RT

1-2where and are respiration rates at temperature of T and T+10 . RRTT?10

Q values derived from different models (e.g. exponential, Arrhenius and linear 10

models) may be different, either by magnitude, or with respect to temperature. With

the exponential model (Eq.1), the Q value is conceptually constant with temperature. 10

1-3In other models, Q varies with temperature in different ways. 10

Combining equation 1 and 4, Q can be estimated as: 10

10bQ?e (5) 10

or

R10??lnQln (6) 10aT

For each soil sample and each round of temperature change, mean respiration rates at

different temperatures were fitted with equation 1 to estimate the Q value. 10

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

4-5.

slow turnover rate. In most current models, the decomposition of each pool is

It is difficult to partition soil basal respiration to different C components as there is no simulated by a first order kinetics with respect to the C concentration in the pool

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

-1resistant C is 0.02 year (a rate constant used for humus in the RothC model). This

7-8rate constant is similar to or is lower than the resistant pool in most models. Despite

an assumed residence time of the passive pool in some models of more than one

5thousand years, the resistant components of soil organic matter as a whole typically

9have a residence time of 20 to 50 years.

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 Q

value due to changes in SOM composition 10

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Nature manuscript 2003-08-08564

Supposing that decompositions of resistant and labile C have different temperature Q?RQ?RQ (7) 10,totallab10,labres10,res

dependences, the temperature sensitivity of total soil respiration can be described as: where Q, Q and Q are Q values for total SOM, labile and resistant C 10,total10,lab10,res10

decomposition; R and R are the percentage contributions of labile and resistant C labres

to the total SOM decomposition, respectively.

Qwas estimated with measured soil basal respiration rate at different 10,total

temperatures as described above. At the beginning of the incubation, average Q 10,total

was 2.07 (?0.021). At the end of incubation (around day 100), Q was 2.14 10,total

(?0.16). We assumed Q = 2.0 for analysing the contribution of Q to Q. 10,lab10,res10,total

The magnitude of Q does not matter here, as comparison is made on a relative 10,lab

scale. Supplementary Figure 3 shows the contribution of different Q to Q. As 10,res10,total

a function of incubation time, Qshould gradually decrease if the decomposition 10,total

of resistant C has a significantly smaller Q than labile C. The results presented in 10

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.

References

1. Winkler, J. P., Cherry, R. S. & Schlesinger, W. H. The Q

relationship of 10

microbial respiration in a temperate forest soil. Soil Biol. Biochem. 28, 1067-

1072 (1996).

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Nature manuscript 2003-08-08564

efflux on temperature. 2

Soil Biol. Biochem. 33, 155-165 (2001).

2. Fang, C. & Moncrieff, J. B. The dependence of soil CO

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controlling soil organic matter levels in Great Plains grasslands. Soil Sci. Soc.

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5. Coleman, K. & Jenkinson, D. S. RothC-26.3?A model for the turnover of

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Nature manuscript 2003-08-08564 8. Fu, S., Cabrera, M. L., Coleman, D. C., Kisselle, K. W., Garrett, C. J., Hendrix,

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