Abstract
Elevated CO 2 concentrations generally stimulate grassland productivity, but herbaceous plants have only a limited capacity to sequester extra carbon (C) in biomass. However, increased primary productivity under elevated CO 2 could result in increased transfer of C into soils where it could be stored for prolonged periods and exercise a negative feedback on the rise in atmospheric CO 2 . Measuring soil C sequestration directly is notoriously difficult for a number of methodological reasons. Here, we present a method that combines C isotope labelling with soil C cycle modelling to partition net soil sequestration into changes in new C fixed over the experimental duration (C new ) and pre‐experimental C (C old ). This partitioning is advantageous because the C new accumulates whereas C old is lost in the course of time (ΔC new >0 whereas ΔC old We applied this method to calcareous grassland exposed to 600 μL CO 2 L −1 for 6 years. The CO 2 used for atmospheric enrichment was depleted in 13 C relative to the background atmosphere, and this distinct isotopic signature was used to quantify net soil C new fluxes under elevated CO 2 . Using 13 C/ 12 C mass balance and inverse modelling, the Rothamsted model ‘RothC’ predicted gross soil C new inputs under elevated CO 2 and the decomposition of C old . The modelled soil C pools and fluxes were in good agreement with experimental data. C isotope data indicated a net sequestration of ≈90 g C new m −2 yr −1 in elevated CO 2 . Accounting for C old ‐losses, this figure was reduced to ≈30 g C m −2 yr −1 at elevated CO 2 ; the elevated CO 2 ‐effect on net C sequestration was in the range of≈10 g C m −2 yr −1 . A sensitivity and error analysis suggests that the modelled data are relatively robust. However, elevated CO 2 ‐specific mechanisms may necessitate a separate parameterization at ambient and elevated CO 2 ; these include increased soil moisture due to reduced leaf conductance, soil disaggregation as a consequence of increased soil moisture, and priming effects. These effects could accelerate decomposition of C old in elevated CO 2 so that the CO 2 enrichment effect may be zero or even negative. Overall, our findings suggest that the C sequestration potential of this grassland under elevated CO 2 is rather limited.