Elevated CO2 enhances below-ground C allocation in three perennial grass species at different levels of N availability

Three perennial grass species, Lolium perenne L., Agrostis capillaris L. and Festuca ovina L., were homogeneously labelled in phytotrons with (CO2)-C-14 at two CO2 concentrations (350 and 700 mu l l(-1)). Plants were grown under two nitrogen regimes: one with a minor addition of 8 kg N ha(-1), the other with an addition of 278 kg N ha2(-1). Carbon allocation over the different compartments of the plant/soil systems was measured: shoots, roots, rhizosphere soil (soil solution, microbial biomass and soil residue), and bulk soil. Elevated CO2 increased total net C-14 recovery in all species by 14%, and significantly enhanced the below-ground C-14 allocation by 26%, this enhancement was 24%, 39% and 21%, for root, rhizosphere soil and bulk soil, respectively. Within the rhizosphere soil, the C-14 amounts in the soil solution (+ 69 %) and soil residue (+ 49 %) increased significantly. Total microbial biomass-C in the rhizosphere soil was also increased (15%) by the elevated CO2 treatment, but only in proportion to the increased root mass. No interactions were observed between the elevated CO2 and N treatments. The N treatment increased total net C-14 recovery by more than 300% and C-14 was preferentially allocated to the shoots, leading to a significant increase in shoot-to-root ratio. However, N fertilization also increased(+ 111 %)the absolute amount of C-14 in soil. The three species behaved differently, but no interactions were observed between CO2 treatment and plant species. These results show that elevated CO2 induces an increased C input into soil for all three grass species at both N levels. However, the highest absolute amounts were found in the soils of the fastest growing species and at the highest N level.