8. Below-ground soil responses

Plants are frequently observed to increase carbon allocation to below-ground sinks and to particularly accelerate fine-root turnover in response to elevated CO₂. While these strong below-ground responses have predominantly been observed in rapidly expanding systems, late successional plant communities have rarely been studied. We monitored fine root growth using ingrowth root cores, standing crop harvests and decomposition of roots using litter bags. Soil C dynamics were assessed by measuring fluxes of dissolved organic C, soil respiration and microbial biomass. Elevated CO₂ did not stimulate root growth of the treated vegetation nor did altered root decomposition occur. However, elevated CO₂ resulted in 20% more water extractable soil organic carbon and marginally higher soil respiration rates. Our data show that fine root growth may be much less stimulated by elevated CO₂ in late successional systems than in ecosystems with a rapidly expanding plant community biomass. The higher water extractable organic carbon measured at elevated CO₂, could have important implications for soil functioning by changing substrate availability for microorganisms or the potential for carbon loss from the system by leaching.