Effects of six years atmospheric CO2 enrichment on plant, soil, and soil microbial C of a calcareous grassland

Abstract

Stimulated plant production and often even larger stimulation of photosynthesis at elevated CO2 raise the possibility of increased C storage in plants and soils. We analysed ecosystem C partitioning and soil C fluxes in calcareous grassland exposed to elevated CO2 for 6 years. At elevated CO2, C pools increased in plants (+23%) and surface litter (+24%), but were not altered in microbes and soil organic matter. Soils were fractionated into particle size and density separates. The amount of low-density macroorganic C, an indicator of particulate soil C inputs from root litter, was not affected by elevated CO2. Incorporation of C fixed during the experiment (Cnew) was tracked by C isotopic analysis of soil fractions which were labelled due to 13C depletion of the commercial CO2 used for atmospheric enrichment. This data constrains estimates of C sequestration (absolute upper bound) and indicates where in soils potentially sequestered C is stored. Cnew entered soils at an initial rate of 210±42 g C m-2 year-1, but only 554±39 g Cnew m-2 were recovered after 6 years due to the low mean residence time of 1.8 years. Previous process-oriented measurements did not indicate increased plant-soil C fluxes at elevated CO2 in the same system (13C kinetics in soil microbes and fine roots after pulse labelling, and minirhizotron observations). Overall experimental evidence suggests that C storage under elevated CO2 occurred only in rapidly turned-over fractions such as plants and detritus, and that potential extra soil C inputs were rapidly re-mineralised. We argue that this inference does not conflict with the observed increases in photosynthetic fixation at elevated CO2, because these are not good predictors of plant growth and soil C fluxes for allometric reasons. C sequestration in this natural system may also be lower than suggested by plant biomass responses to elevated CO2 because C storage may be limited by stabilisation of Cnew in slowly turned-over soil fractions (a prerequisite for long-term storage) rather than by the magnitude of C inputs per se.