EGU2020-10075, updated on 12 Jun 2020
https://doi.org/10.5194/egusphere-egu2020-10075
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Radiocarbon incubations of archived soils: insights into drying/rewetting effects and constraining soil C models

Jeffrey Beem Miller1, Marion Schrumpf1, Georg Guggenberger2, and Susan Trumbore1,3
Jeffrey Beem Miller et al.
  • 1Max Planck Institute for Biogeochemistry, Biogeochemical Processes, Germany
  • 2Leibniz Universität Hannover, Institut für Bodenkunde, Germany
  • 3Department of Earth System Science, University of California, Irvine, USA

Radiocarbon measurements of heterotrophically respired C (∆14C-CO2) in laboratory soil incubations provide information about the age and source of microbially-available soil organic matter. However, due to the influence of “bomb” radiocarbon (from nuclear weapons testing in the mid-20th century), measurements of 14C at a single time point can yield multiple solutions when modeling soil C cycling rates. Measuring ∆14C-CO2 on archived soils would provide additional time points to assess which solution is appropriate. We had two hypotheses regarding the effect of archiving on ∆14C-CO2: 1) long-term storage does not affect ∆14C-CO2, and 2) drying and rewetting effects on ∆14C-CO2 are limited to CO2 released immediately following rewetting, without significant effects on CO2 released after respiration rates equilibrate.

To address the first hypothesis, sample splits of soils collected at nine grassland and 21 forest sites (n=30) between 2004 and 2011 (for which ∆14C-CO2 had been previously measured) were incubated again in 2018 after undergoing air-drying and storage. The difference in ∆14C-CO2 measured before and after archiving was significant (p < 0.05); however, in line with our hypothesis, the number of years archived was not a significant predictor of the difference in a regression analysis.

To test the second hypothesis we first collected and analyzed ∆14C-CO2 following the “pre-incubation” period, i.e. the period immediately following rewetting, as well as after the equilibrium respiration period for the subset of samples (six grassland, six forest) for which we had data on the original pre-incubation period. In this subset we observed different responses in forest versus grassland soils in the equilibrium respiration period: ∆14C-CO2 decreased from the original value by 12.7 (±4.5) per mille in forests (p = 0.08), but increased by 22.2 (±6.7) per mille in grasslands (p < 0.05) (errors are twice the standard error of the mean difference). In contrast to our second hypothesis the ∆14C of the CO2 released immediately following rewetting was not significantly different from the ∆14C of the CO2 respired under equilibrium respiration conditions, despite the much higher rate of respiration following rewetting. A final incubation experiment comparing freshly collected soils that were dried but not archived was conducted to distinguish conclusively between rewetting and storage effects, but we are still awaiting the data.

In conclusion, the drying/rewetting effect appears to drive the differences between ∆14C-CO2 measured in incubations before and after archiving, rather than duration of storage. The radiocarbon incubation technique for archived samples is promising: the 12 to 22 per mille differences observed are not insignificant, but in many cases should be within the range of acceptable error in a modeling context. The wider implication of our results is that drying and rewetting soils appears to mobilize a different pool of soil organic matter than would otherwise be available to microbes, an effect that persists throughout an incubation and affects grassland and forest soils differently. This effect applies to radiocarbon incubations in general and warrants further investigation.

How to cite: Beem Miller, J., Schrumpf, M., Guggenberger, G., and Trumbore, S.: Radiocarbon incubations of archived soils: insights into drying/rewetting effects and constraining soil C models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10075, https://doi.org/10.5194/egusphere-egu2020-10075, 2020

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Display material version 2 – uploaded on 04 May 2020
Added specification of water holding capacity for incubation experiments
  • CC1: Comment on EGU2020-10075, Albert C. Brangarí, 08 May 2020

    Thanks for this nice presentation and very interesting project. This radiocarbon technique seems a very powerful tool!! I haven't understood much what was the set-up of the sampling. I mean, are the samples at day 4 (for instance) an integrated measure of the gas accumulated from 0 to 4?? I am also intrigued by what would be the results in soils exposed to partial drying or different intensities of drought (previous to rewetting).

    • AC1: Reply to CC1, Jeffrey Beem Miller, 08 May 2020

      Hi Albert,

      Yes, the samples at day 4 are an integrated measure of gas accumulated over that period. I measured CO2 concentrations daily for the majority of the samples, but as I was using data from an experiment conducted in 2011 by a colleague, and they only measured the integrated CO2 accumulated over the first 4 days, so I showed the data at the coarser resolution for the purpose of comparison.

      For the 2011 treatment samples (air-drying + storage treatment) that I made daily measurements on, the grassland soils showed a very strong immediate peak in respiration (~24 hrs), but the forest samples peaked much later (~96 hrs), and at a lower peak rate. In contrast, the air-dry treatment for both forest and grassland soils from the 2019 sampling campaign showed similar trends in both the timing (~36 hrs) and magnitude of peak respiration.

      I am also intrigued by the possible effect of the degree of rewetting. Something that would be interesting to probe further!

      Thanks for taking a look at the display. Best, Jeff

Display material version 1 – uploaded on 04 May 2020, no comments