Carbon allocation and decomposition of root-derived organic matter in a plant–soil system of Calluna vulgaris as affected by elevated CO2

The effect of elevated CO2 on C allocation in plant and soil was assessed using soil cores planted with 1-y-old heather (Calluna vulgaris (L.) Hull). Plants were pulse-labeled with 14CO2 at ambient and elevated CO2 and two nitrogen regimes (low and high). After harvesting the plants, the soil was incubated to monitor total respiration and decomposition of 14C-labeled rhizodeposits. Total and shoot biomass increased at high N but were not affected by CO2. Root biomass was not affected by either N or CO2 treatments. Total 14C uptake and shoot-14C increased upon adding N and elevating CO2 but the N effect was strongest. Total 14C uptake per unit shoot mass decreased with N, but increased with CO2. Root-14C content was not significantly affected by the N or CO2 treatment. Total soil-14C slightly increased at elevated CO2 whereas microbial 14C increased due to high N. C allocation to shoots increased at the expense of roots, soil and respiration at high N but was not affected by the CO2 treatment. Variation in 14C distribution within each treatment was small compared to variation in total 14C amounts in each plant–soil compartment. Initially, 14C respiration from rhizodeposits correlated well with root-14C, total soil-14C, soil solution-14C and microbial 14C, at harvest time and was increased by elevated CO2. By the end of the incubation, however, decomposition of labeled organic matter was not affected by the treatments whereas total (=12C+14C) respiration was lowest for the elevated-CO2 soils. We speculate that initially, respiration is dominated by decomposition of fresh root exudates whereas in the longer term, respiration originates from decomposition of more recalcitrant root material that had been formed during the entire experiment. The increased net 14C uptake and unchanged distribution pattern, combined with an increased decomposition of easily-decomposable compounds and a decreased decomposition of more recalcitrant root-derived material indicated a small sink function of a Calluna plant–soil system under elevated CO2.