Re-visiting a long-term Free Air CO2 Enrichment (FACE) experiment in a Danish heathland/grassland ecosystem (CLIMAITE) reveals highly dynamic soil carbon

The feedback of the terrestrial carbon cycle to global climate change is among the largest uncertainties in climate change research. To test the potential ecosystem effects of future climate scenarios, a field-scale FACE (Free Air CO₂ Enrichment) experiment combined with increased temperatures and extended summer drought was performed in the period 2005–2013 on a temperate heathland/grassland ecosystem in Denmark (the CLIMAITE project). A major finding from the original experiment was that the soil carbon pool increased by approximately 20% under elevated CO₂ over the 8 years of the study*.

The FACE treatment was in effect also an in situ labeling experiment because the added CO₂ was depleted for ¹³C （¹³CO_2FACE=-29‰）compared to ambient atmospheric CO₂（¹³CO_2AIR=-8‰）. Therefore, the isotopic signal of the remaining soil carbon can be used to investigate the turnover of soil carbon during the time since the end of the original study.

During the growing season in 2020, seven years after the CO₂ fumigation experiment was terminated, soil samples were extracted in all plots using the same sampling strategy as in previous samplings. Interestingly, the direct soil C pool measurements showed that the extra soil carbon, which was stored during the eight years with elevated CO₂ had been lost again over the course of the following seven years. The isotopic composition of the different soil layers had also changed back towards the values measured in control plots, although still being slightly more depleted for ¹³C. Still, the convergence of the isotopic composition in the different treatments confirms the trend observed from the direct C pool measurements and also hints that a part of the more recalcitrant carbon taken up during the elevated CO2 experiment is still there while most of the labile/less recalcitrant carbon has been decomposed and reemitted to the atmosphere. The results show that the soil carbon pool in the ecosystem is extremely dynamic and may change fast in response to changes in major ecosystem drivers, and in particular is highly sensitive to the atmospheric CO₂ concentration.

*Dietzen CA, Larsen KS, Ambus P, Michelsen A, Arndal MF, Beier C, Reinsch S, Schmidt IK (2019) Accumulation of soil carbon under elevated CO₂ unaffected by warming and drought. Global Change Biology, 25: 2970–2977. doi: 10.1111/gcb.14699.