Plant and Soil

, Volume 233, Issue 2, pp 189–202

Effects of six years atmospheric CO2 enrichment on plant, soil, and soil microbial C of a calcareous grassland


  • Pascal A. Niklaus
    • Institute of Botany
  • Monika Wohlfender
    • Institute of Botany
  • Rolf Siegwolf
    • Paul Scherrer Institut
  • Christian Körner
    • Institute of Botany

DOI: 10.1023/A:1010389724977

Cite this article as:
Niklaus, P.A., Wohlfender, M., Siegwolf, R. et al. Plant and Soil (2001) 233: 189. doi:10.1023/A:1010389724977


Stimulated plant production and often even larger stimulation of photosynthesis at elevated CO2 raise the possibility of increased C storage in plants and soils. We analysed ecosystem C partitioning and soil C fluxes in calcareous grassland exposed to elevated CO2 for 6 years. At elevated CO2, C pools increased in plants (+23%) and surface litter (+24%), but were not altered in microbes and soil organic matter. Soils were fractionated into particle size and density separates. The amount of low-density macroorganic C, an indicator of particulate soil C inputs from root litter, was not affected by elevated CO2. Incorporation of C fixed during the experiment (Cnew) was tracked by C isotopic analysis of soil fractions which were labelled due to 13C depletion of the commercial CO2 used for atmospheric enrichment. This data constrains estimates of C sequestration (absolute upper bound) and indicates where in soils potentially sequestered C is stored. Cnew entered soils at an initial rate of 210±42 g C m−2 year−1, but only 554±39 g Cnew m−2 were recovered after 6 years due to the low mean residence time of 1.8 years. Previous process-oriented measurements did not indicate increased plant–soil C fluxes at elevated CO2 in the same system (13C kinetics in soil microbes and fine roots after pulse labelling, and minirhizotron observations). Overall experimental evidence suggests that C storage under elevated CO2 occurred only in rapidly turned-over fractions such as plants and detritus, and that potential extra soil C inputs were rapidly re-mineralised. We argue that this inference does not conflict with the observed increases in photosynthetic fixation at elevated CO2, because these are not good predictors of plant growth and soil C fluxes for allometric reasons. C sequestration in this natural system may also be lower than suggested by plant biomass responses to elevated CO2 because C storage may be limited by stabilisation of Cnew in slowly turned-over soil fractions (a prerequisite for long-term storage) rather than by the magnitude of C inputs per se.

13C labelling carbon isotope discrimination carbon sequestration earthworms soil physical fractionation

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© Kluwer Academic Publishers 2001