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:

(1)

where R is respiration rate, a and b are fitted parameters, respectively. Respiration rate at 10 ºC, R10, is then calculated as:

(2)

Relative respiration rate was calculated against R10:

(3)

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:

(4)

where and 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:

(5)

or

(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 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

Supposing that decompositions of resistant and labile C have different temperature dependences, the temperature sensitivity of total soil respiration can be described as:

(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.

References

1.  Winkler, J. P., Cherry, R. S. & Schlesinger, W. H. The Q10 relationship of microbial respiration in a temperate forest soil. Soil Biol. Biochem. 28, 1067-1072 (1996).

2.  Fang, C. & Moncrieff, J. B. The dependence of soil CO2 efflux on temperature. Soil Biol. Biochem. 33, 155-165 (2001).

3.  Lloyd, J. & Taylor, J. A. On the temperature dependence of soil respiration. Funct. Ecol. 8, 315-323 (1994).

4.  Parton, W. J., Schimel, D. S., Cole, C. V. & Ojima, D. S. Analysis of factors controlling soil organic matter levels in Great Plains grasslands. Soil Sci. Soc. Ame. J. l51, 1173-1179 (1987).

5.  Coleman, K. Jenkinson, D. S. RothC-26.3¾A model for the turnover of carbon in soil. In: Evaluation of Soil Organic Matter Models Using Existing Long-Term Datasets. Powlson, D. S., Smith, P. Smith, J. U. (Eds), NATO ASI Series I, Vol. 38, Springer-Verlag, Heidelberg, pp.237-246 (1996).

6.  Davidson, E. A., Trumbore, S. E. & Amundson, R. Soil warming and organic carbon content. Nature 408, 789-790 (2000).

7.  Mueller, T., Jensen, L. S., Hansen, S. & Nielsen, N. E. Simulating soil carbon and nitrogen dynamics with the soil-plant-atmosphere system model Daisy. In: Evaluation of Soil Organic Matter Models Using Existing Long-Term Datasets. Powlson, D. S., Smith, P. Smith, J. U. (Eds), NATO ASI Series I, Vol. 38, Springer-Verlag, Heidelberg, pp. 275-282 (1996).

8.  Fu, S., Cabrera, M. L., Coleman, D. C., Kisselle, K. W., Garrett, C. J., Hendrix, P. F. & Crossley, D. A. Jr Soil carbon dynamics of conventional tillage and no-till agroecosystems at Georgia Piedmont — HSB-C models. Ecol. Model. 131 229–248 (2000).

9.  Chapin, F. S. III., Matson, P. A. & Mooney, H. A. Principles of terrestrial ecosystem ecology. Springer-Verlag, New York, pp.151-175 (2002).

6