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 (Delta C-new > 0 whereas Delta C-old < 0). We applied this method to calcareous grassland exposed to 600 mu L CO2 L-1 for 6 years. The CO2 used for atmospheric enrichment was depleted in C-13 relative to the background atmosphere, and this distinct isotopic signature was used to quantify net soil C-new fluxes under elevated CO2. Using C-13/C-12 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 approximate to 90 g C-new m(-2) yr(-1) in elevated CO2. Accounting for C-old-losses, this figure was reduced to approximate to 30 g C m(-2) yr(-1) at elevated CO2; the elevated CO2-effect on net C sequestration was in the range of approximate to 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.