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

Abstract The effect of elevated CO 2 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 14 CO 2 at ambient and elevated CO 2 and two nitrogen regimes (low and high). After harvesting the plants, the soil was incubated to monitor total respiration and decomposition of 14 C-labeled rhizodeposits. Total and shoot biomass increased at high N but were not affected by CO 2 . Root biomass was not affected by either N or CO 2 treatments. Total 14 C uptake and shoot- 14 C increased upon adding N and elevating CO 2 but the N effect was strongest. Total 14 C uptake per unit shoot mass decreased with N, but increased with CO 2 . Root- 14 C content was not significantly affected by the N or CO 2 treatment. Total soil- 14 C slightly increased at elevated CO 2 whereas microbial 14 C 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 CO 2 treatment. Variation in 14 C distribution within each treatment was small compared to variation in total 14 C amounts in each plant–soil compartment. Initially, 14 C respiration from rhizodeposits correlated well with root- 14 C, total soil- 14 C, soil solution- 14 C and microbial 14 C, at harvest time and was increased by elevated CO 2 . By the end of the incubation, however, decomposition of labeled organic matter was not affected by the treatments whereas total (= 12 C+ 14 C) respiration was lowest for the elevated-CO 2 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 14 C 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 CO 2 .