Estimating soil carbon sequestration under elevated CO2 by combining carbon isotope labelling with soil carbon cycle modelling

Elevated CO₂ 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 CO₂ 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 CO₂. 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<0). We applied this method to calcareous grassland exposed to 600 μL CO₂ L^?1 for 6 years. The CO₂ used for atmospheric enrichment was depleted in ¹³C relative to the background atmosphere, and this distinct isotopic signature was used to quantify net soil C_new fluxes under elevated CO₂. Using ¹³C/¹²C mass balance and inverse modelling, the Rothamsted model ‘RothC’ predicted gross soil C_new inputs under elevated CO₂ 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 CO₂. Accounting for C_old‐losses, this figure was reduced to ≈30 g C m^?2 yr^?1 at elevated CO₂; the elevated CO₂‐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 CO₂‐specific mechanisms may necessitate a separate parameterization at ambient and elevated CO₂; 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 CO₂ so that the CO₂ enrichment effect may be zero or even negative. Overall, our findings suggest that the C sequestration potential of this grassland under elevated CO₂ is rather limited.