Carbon-13 input and turn-over in a pasture soil exposed to long-term elevated atmospheric CO2

The impact of elevated CO2 and N-fertilization on soil C-cycling in Lolium perenne and Trifolium repens pastures were investigated under Free Air Carb on dioxide Enrichment (FACE) conditions. For six years, swards were exposed to ambient or elevated CO2 (35 and 60 Pa pCO(2)) and received a low and hi gh rate of N fertilizer. The CO2 added in the FACE plots was depleted in C- 13 compared to ambient (Delta - 40 parts per thousand) thus the C inputs co uld be quantified. On average, 57% of the C associated with the sand fraction of the soil was 'new' C. Smaller proportions of the C associated with the silt (18%) and cl ay fractions (14%) were derived from FACE. Only a small fraction of the tot al C pool below 10 cm depth was sequestered during the FACE experiment. The annual net input of C in the FACE soil (0-10 cm) was estimated at 4.6 /- 2.2 and 6.3 +/- 3.6 (95% confidence interval) Mg ha(-1) for T. repens an d L. perenne, respectively. The maximum amount of labile C in the T. repens sward was estimated at 8.3 +/- 1.6 Mg ha(-1) and 7.1 +/- 1.0 Mg ha(-1) in the L. perenne sward. Mean residence time (MRT) for newly sequestered soil C was estimated at 1.8 years in the T. repens plots and 1.1 years for L. pe renne. An average of 18% of total soil C in the 0-10 cm depth in the T. rep ens sward and 24% in the L. perenne sward was derived from FACE after 6 yea rs exposure. The majority of the change in soil delta(13)C occurred in the first three years of the experiment. No treatment effects on total soil C w ere detected. The fraction of FACE-derived C in the L. perenne sward was larger than in t he T. repens sward. This suggests a priming effect in the L. perenne sward which led to increased losses of the old C. Although the rate of C cycling was affected by species and elevated CO2, the soil in this intensively mana ged grassland ecosystem did not become a sink for additional new C.