Three perennial grass species, Lolium perenne L., Agrottis capillaris L. and Festuca uvina L., were homogeneously labelled in phytotrons with 14CO2 at two CO2 concentrations (350 and 700μl 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 ha−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 CO., increased total net 14C recovery in all species by 14%, and significantly enhanced the below-ground 14C allocation by 26%, this enhancement was 24%, 39% and 21 % for root, rhizosphere soil and bulk soil, respectively. Within the rhizosphere soil, the 14C 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 14C recovery by more than 300% and 14C 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 14C 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.
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