Belowground heathland responses after 2 years of combined warming, elevated CO2 and summer drought

Terrestrial ecosystems are exposed to atmospheric and climatic changes including increases in atmospheric CO2 concentration, temperature and alterations of precipitation patterns, which are predicted to continue with consequences for ecosystem services and functioning in the future. In a field scale experiment on temperate heathland, manipulation of precipitation and temperature was performed with retractable curtains, and atmospheric CO2 concentration was increased by FACE.

The combination of elevated CO2 and warming was expected to affect belowground processes additively, through increased belowground sequestration of labile carbohydrates due to elevated CO2 in combination with temperature increased process rates. Together, these changes might increase microbial activity and availability of plant nutrients.

Two years after the start of the experiment, belowground processes responded significantly to the treatments. In the combined temperature and CO2 treatment the dissolved organic nitrogen concentration decreased and the ammonium concentration increased, but this release of nutrients was not mirrored by plant parameters.

Microbial biomass carbon and microbial enrichment with 13C and 15N (1 year after 13C215N-glycine was injected into the soil) increased in warmed plots and in elevated CO2 plots, but not when these treatments were combined. Furthermore, drought led to an increase in Calluna biomass and total plant nitrogen pool.

The full combination of warming, elevated CO2 and periodic drought did not unambiguously express the ecosystem responses of single factors additively, which complicates predictions of ecosystem responses to multifactor climate change.