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

Three perennial grass species, Lolium perenne L., Agrottis capillaris L. and Festuca uvina L., were homogeneously labelled in phytotrons with CO at two CO concentrations (350 and 700μl l ). Plants were grown under two nitrogen regimes: one with a minor addition of 8 kg N ha , the other with an addition of 278 kg N ha . 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 C recovery in all species by 14%, and significantly enhanced the below-ground C allocation by 26%, this enhancement was 24%, 39% and 21 % for root, rhizosphere soil and bulk soil, respectively. Within the rhizosphere soil, the C 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 CO treatment, but only in proportion to the increased root mass. No interactions were observed between the elevated CO , and N treatments. The N treatment increased total net C recovery by more than 300% and C 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 in soil. The three species behaved differently, but no interactions were observed between CO treatment and plant species. These results show that elevated CO 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.