Carbon allocation in calcareous grassland under elevated CO2: a combined C-13 pulse-labelling/soil physical fractionation study

1. To test whether plant-soil C fluxes in natural grassland increase under elevated atmospheric CO2 concentration, intact calcareous grassland monolit hs exposed to ambient or elevated CO2 were pulse-labelled and the dynamics of the C-13 label followed throughout the rest of the growing season. 2, The experiment revealed no increased fluxes of C to soils at elevated CO 2. The only changes found were relatively small shifts towards increased C allocation to roots by the end of the growing season. This effect was proba bly because wetter soil under elevated CO2 prolonged the growing period. At elevated CO2 plant C pools increased below ground (+28%) at the end of the season, resulting in slightly increased root : shoot ratios. Plant C-13 po ols increased significantly below ground. There were no effects of CO2 enri chment on C-13 in soil microbes, fine roots or earthworms, 3. Elevated CO2 caused a shift in soil particle size distribution towards s maller aggregate sizes, but had no effect on the total C and C-13 content o f low- and high density soil fractions. 4, The absence of effects of CO2 on the labelling of soil microbial biomass , and of C and C-13 accumulation in low-density macro-organic fractions, su ggest that there is no significant effect of elevated CO2 on root exudation or turnover, agreeing with published labelling studies, but conflicting wi th CO2-exchange budgets.