Roots and fungi accelerate carbon and nitrogen cycling in forests exposed to elevated CO2

A common finding in multiple CO2 enrichment experiments in forests is the lack of soil carbon (C) accumulation owing to microbial priming of old soil organic matter (SOM). However, soil C losses may also result from the accelerated turnover of young microbial tissues that are rich in nitrogen (N) relative to bulk SOM. We measured root-induced changes in soil C dynamics in a pine forest exposed to elevated CO2 and N enrichment by combining stable isotope analyses, molecular characterisations of SOM and microbial assays. We find strong evidence that the accelerated turnover of root-derived C under elevated CO2 is sufficient in magnitude to offset increased belowground inputs. In addition, the C losses were associated with accelerated N cycling, suggesting that trees exposed to elevated CO2 not only enhance N availability by stimulating microbial decomposition of SOM via priming but also increase the rate at which N cycles through microbial pools.