ELEVATED CO2 ENHANCES BELOW-GROUND-C ALLOCATION IN 3 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 wit h (CO2)-C-14 at two CO2 concentrations (350 and 700 mu l l(-1)). Plant s 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 w as measured: shoots, roots, rhizosphere soil (soil solution, microbial biomass and soil residue), and bulk soil. Elevated CO2 increased tota l 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. With in the rhizosphere soil, the C-14 amounts in the soil solution (+ 69 % ) and soil residue (+ 49 %) increased significantly. Total microbial b iomass-C in the rhizosphere soil was also increased (15%) by the eleva ted CO2 treatment, but only in proportion to the increased root mass. No interactions were observed between the elevated CO2 and N treatment s. 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 sig nificant increase in shoot-to-root ratio. However, N fertilization als o increased(+ 111 %)the absolute amount of C-14 in soil. The three spe cies behaved differently, but no interactions were observed between CO 2 treatment and plant species. These results show that elevated CO2 in duces an increased C input into soil for all three grass species at bo th N levels. However, the highest absolute amounts were found in the s oils of the fastest growing species and at the highest N level.