A decade of free-air CO2 enrichment increased the carbon throughput in a grass-clover ecosystem but did not drastically change carbon allocation patterns

Abstract

The response of the soil carbon cycle to increasing atmospheric CO2 concentration has far reaching consequences for the ecosystem carbon balance under future climatic conditions. We report on work carried out in the Swiss free-air CO2 enrichment (FACE) experiment, where we used in situ 13CO2 labelling to determine whether elevated CO2 (+230 μL L-1) concentration changes the fate of recently assimilated carbon in the soil microbial community. Elevated CO2 (eCO2) concentration had an overall positive effect on microbial abundance (P < 0·001) with the gram-negative bacteria showing significantly increased quantities. Gram-negative bacteria and saprotrophic fungi tended to utilize a higher amount of recently assimilated carbon under eCO2. Arbuscular mycorrhizal fungi (AMF) utilized plant-assimilated carbon within 1 day after the 13CO2 pulse and 13C uptake patterns in AMF suggest that carbon transfer is faster under eCO2 concentration than under ambient CO2 (aCO2). Additionally, the respiration of recently assimilated carbon was significantly higher under eCO2 than aCO2 concentration. Our data suggest that elevated atmospheric CO2 concentration accelerated and increased the utilization of recently assimilated carbon by the microbial community without changing the microbial community composition drastically. We conclude that a higher standing soil microbial biomass under eCO2 concentration was the key cause for the higher carbon flow through the plant-soil system. Carbon utilization by microbial functional groups was only little affected by a decade of CO2 enrichment. © 2013 The Authors. Functional Ecology © 2013 British Ecological Society.