Carbon dynamics in a temperate grassland soil after 9 years exposure to elevated CO 2 (Swiss FACE)

Abstract

Elevated pCO 2 increases the net primary production, C/N ratio, and C input to the soil and hence provides opportunities to sequester CO 2–C in soils to mitigate anthropogenic CO 2. The Swiss 9 y grassland FACE (free air carbon-dioxide enrichment) experiment enabled us to explore the potential of elevated pCO 2 (60 Pa), plant species ( Lolium perenne L. and Trifolium repens L.) and nitrogen fertilization (140 and 540 kg ha −1 y −1) on carbon sequestration and mineralization by a temperate grassland soil. Use of 13C in combination with respired CO 2 enabled the identification of the origins of active fractions of soil organic carbon. Elevated pCO 2 had no significant effect on total soil carbon, and total soil carbon was also independent of plant species and nitrogen fertilization. However, new (FACE-derived depleted 13C) input of carbon into the soil in the elevated pCO 2 treatments was dependent on nitrogen fertilization and plant species. New carbon input into the top 15 cm of soil from L. perennne high nitrogen (LPH), L. perenne low nitrogen (LPL) and T. repens low nitrogen (TRL) treatments during the 9 y elevated pCO 2 experiment was 9.3±2.0, 12.1±1.8 and 6.8±2.7 Mg C ha −1, respectively. Fractions of FACE-derived carbon in less protected soil particles >53 μm in size were higher than in <53 μm particles. In addition, elevated pCO 2 increased CO 2 emission over the 118 d incubation by 55, 61 and 13% from undisturbed soil from LPH, LPL and TRL treatments, respectively; but only by 13, 36, and 18%, respectively, from disturbed soil (without roots). Higher input of new carbon led to increased decomposition of older soil organic matter (priming effect), which was driven by the quantity (mainly roots) of newly input carbon ( L. perenne) as well as the quality of old soil carbon (e.g. higher recalcitrance in T. repens). Based on these results, the potential of well managed and established temperate grassland soils to sequester carbon under continued increasing concentrations of atmospheric CO 2 appears to be rather limited.