Globally, plants face a change in diet imposed by rising atmospheric CO₂ which often causes increased carbon allocation to below-ground sinks, particularly to fine root production and turnover. However, these pronounced responses have principally been reported for expanding systems while late successional systems have rarely been addressed. Here, we aimed to determine whether elevated CO₂ affects fine root and microbial biomass under mature deciduous trees (80-100 yrs) exposed to long-term free air CO₂ enrichment (FACE) in a near-natural central European forest. Fine root biomass was assessed using soil cores and root ingrowth cores while the chloroform-fumigation extraction (CFE) method was applied to estimate microbial C and N.

Results/Conclusions

After 6 years of CO₂ enrichment, both, soil cores and ingrowth-cores installed for 2 seasons yielded similar and (marginally) significant biomass reductions (~30%) under elevated CO₂ for total (live + dead) and live fine roots in the diameter classes ≤ 2 mm and ≤ 1 mm. Dead fine roots (≤ 2 mm and ≤ 1 mm) obtained from soil cores showed significantly lower biomass (~65%) under elevated CO₂. However, dead fine root biomass derived from ingrowth cores did not differ significantly in response to elevated CO₂.

In order to assign tree fine roots to individual tree species, additional soil cores were taken in close vicinity to tree trunks in year 7 of CO₂ enrichment. Regardless of diameter and vitality, fine roots did not differ significantly in biomass between elevated and ambient CO₂. Soil microbial C and N (C_mic and N_mic) increased by 14 and 13% (marginally significant for C_mic), respectively, in response to elevated CO₂. The CO₂ enrichment led to significantly higher soil moisture, which can affect nutrient availability and might thus account for the negative or lacking growth response of fine roots. In this context, the greater microbial biomass suggests increased carbon input into the soil via turnover and/or root exudates. Our findings and other published work indicate that fine root growth in late successional systems may rather be suppressed instead of being stimulated by elevated CO₂ and altered soil moisture regimes offer a likely explanation.