Short-term utilization of carbon by the soil microbial community under future climatic conditions in a temperate heathland

Abstract An in-situ 13 C pulse-labeling experiment was carried out in a temperate heath/grassland to study the impacts of elevated CO 2 concentration (510 ppm), prolonged summer droughts (annual exclusion of 7.6 ± 0.8%) and increased temperature (∼1 °C) on belowground carbon (C) utilization. Recently assimilated C ( 13 C from the pulse-label) was traced into roots, soil and microbial biomass 1, 2 and 8 days after pulse-labeling. The importance of the microbial community in C utilization was investigated using 13 C enrichment patterns in different microbial functional groups on the basis of phospholipid fatty acid (PLFA) biomarker profiles. Climate treatments did not affect microbial abundance in soil or rhizosphere fractions in terms of total PLFA-C concentration. Elevated CO 2 significantly reduced the abundance of gram-negative bacteria (17:0cy), but did not affect the abundance of decomposers (fungi and actinomycetes) in rhizosphere fractions. Drought favored the bacterial community in rhizosphere fractions whereas increased temperature reduced the abundance of gram-negative bacteria (19:0cy) and changed the actinomycetes community (10Me16:0, 10Me18:0). Fastest and highest utilization of recently assimilated C was observed in rhizosphere associated gram-negative bacteria followed by gram-positive bacteria. Utilization of recently assimilated C by rhizosphere associated actinomycetes and fungi was relatively low, but much more pronounced in the soil. The utilization of recently assimilated C by the microbial community was faster under elevated CO 2 conditions compared to ambient. We conclude that changing climatic conditions will affect C utilization by the soil microbial community but might not drastically change the terrestrial C balance.