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Elevated atmospheric CO2 concentration and climate change may substantially alter soil carbon (C) dynamics, which in turn may impact future climate through feedback cycles. However, only very few field experiments worldwide have combined elevated CO2 (eCO2) with both warming and changes in precipitation in order to study the potential combined effects of changes in these fundamental drivers of C cycling in ecosystems. We exposed a temperate heath/grassland to eCO2, warming, and drought, in all combinations for 8 years. At the end of the study, soil C stocks were on average 0.927 kg C/m2 higher across all treatment combinations with eCO2 compared to ambient CO2 treatments (equal to an increase of 0.120 ± 0.043 kg C m−2 year−1), and showed no sign of slowed accumulation over time. However, if observed pretreatment differences in soil C are taken into account, the annual rate of increase caused by eCO2 may be as high as 0.177 ± 0.070 kg C m−2 year−1. Furthermore, the response to eCO2 was not affected by simultaneous exposure to warming and drought. The robust increase in soil C under eCO2 observed here, even when combined with other climate change factors, suggests that there is continued and strong potential for enhanced soil carbon sequestration in some ecosystems to mitigate increasing atmospheric CO2 concentrations under future climate conditions. The feedback between land C and climate remains one of the largest sources of uncertainty in future climate projections, yet experimental data under simulated future climate, and especially including combined changes, are still scarce. Globally coordinated and distributed experiments with long‐term measurements of changes in soil C in response to the three major climate change‐related global changes, eCO2, warming, and changes in precipitation patterns, are, therefore, urgently needed. [ABSTRACT FROM AUTHOR]

Soil respiration (Rs) is an important component of ecosystem carbon balance and accurate quantification of the diurnal and seasonal variation of Rs is crucial for correct interpretation of the response of Rs to biotic and abiotic factors, as well as for estimating annual soil CO2 efflux rates. In this study, we measured Rs hourly for one year by automated closed chambers in a temperate Danish beech forest. The data showed a clear diurnal pattern of Rs across all seasons with higher rates during night-time than during day-time. However, further analysis showed a clear negative relationship between flux rates and friction velocity (u∗) above the canopy, suggesting that Rs was overestimated at low atmospheric turbulence throughout the year due to non-steady state conditions during measurements. Filtering out data at low u∗ values removed or even inverted the observed diurnal pattern, such that the highest effluxes were now observed during day-time, and also led to a substantial decrease in the estimated annual soil CO2 efflux. By installing fans to produce continuous turbulent mixing of air around the soil chambers, we tested the hypothesis that overestimation of soil CO2 effluxes during low u∗ can be eliminated if proper mixing of air is ensured, and indeed the use of fans removed the overestimation of Rs rates during low u∗. Artificial turbulent air mixing may thus provide a method to overcome the problems of using closed chamber gas exchange measurement techniques during naturally occurring low atmospheric turbulence conditions. Other possible effects from using fans during soil CO2 efflux measurements are discussed. In conclusion, periods with low atmospheric turbulence may provide a significant source of error in Rs rates estimated by the use of closed chamber techniques and erroneous data must be filtered out to obtain unbiased diurnal patterns, accurate relationships to biotic and abiotic factors, and before estimating Rs fluxes over longer time scales. [ABSTRACT FROM AUTHOR]

Elevated atmospheric CO2 concentration (eCO2) and climate change may significantly alter soil carbon (C) dynamics and thus feedback to future climate. However, only very few field experiments world-wide have combined eCO2 with both warming and changes in precipitation in order to study the potential combined effects of changes in these fundamental drivers of carbon cycling in ecosystems. We exposed a temperate heath/grassland to eCO2, warming, and drought, in all combinations for 8 years. Soil C under ambient CO2 remained constant over time, whereas soil C stocks increased by 0.167 kg C m-2 yr-1 on average across all treatment combinations with eCO2 and showed no sign of slowed accumulation over time. Further, the response to eCO2 was not affected by simultaneous exposure to warming and drought. The robust increase in soil C under eCO2 observed here, even when combined with other climate change factors, suggests that there is continued and strong potential for enhanced soil carbon sequestration in some ecosystems to mitigate increasing atmospheric CO2 concentrations under future climate conditions. The feedback between land C and climate remains one of the largest sources of uncertainty in future climate projections, yet experimental data under simulated future climate, and especially including combined changes, are still scarce. Globally coordinated and distributed experiments with long-term measurements of changes in soil C in response to the three major climate change-related global changes, eCO2, warming, and changes in precipitation patterns, are therefore urgently needed. [ABSTRACT FROM AUTHOR]

• Extra stored soil carbon was lost again seven years after FACE termination. • Increased soil carbon during FACE stimulated the decomposition of old soil carbon. • Large transfer of nitrogen from deeper soil layers upon increasing plant demand. • Soil carbon and nitrogen pools in this ecosystem are highly dynamic. The response of soil carbon to global climate change remains one of the largest uncertainties for future climate projection. In this study, we re-sampled the soil in a long-term, field-scale, multi-factorial climate experiment, CLIMAITE (Free Air CO 2 Enrichment (FACE), warming and drought in all combinations in a Danish heathland ecosystem) in 2020, seven years after the experiment was terminated. We aimed to study the dynamics of the soil carbon after the cessation of long-term multi-factorial climate manipulation, with special attention to the fate of the additional soil carbon (19% increase) that was sequestered in plots exposed to elevated CO 2 concentrations (eCO 2). Soil carbon pools in former eCO 2 plots, as well as in drought and warming plots, had normalized again by 2020. However, the difference in soil isotopic composition between ambient and former eCO 2 plots remained, indicating similar loss fractions from older and newer soil carbon pools in the eCO 2 plots as well as stimulation of the decomposition of old soil carbon via priming. Throughout the study period, soil nitrogen dynamics tracked the changes in soil carbon, suggesting that nitrogen from deeper soil layers was transported upwards to meet increasing plant demand during eCO 2 but was lost again from the topsoil after termination of the FACE treatment. Our findings show that the soil carbon and nitrogen pools in this ecosystem are highly dynamic and may respond strongly and rapidly to changes in major ecosystem drivers, and that revisiting climate experiments after the cessation of treatments may provide valuable insights into the dynamics, stability and resilience of major element pools in ecosystems. [ABSTRACT FROM AUTHOR]