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

Abstract

Three perennial grass species, Lolium perenne L., Agrostis capillaris L. and Festuca ovina L., were homogeneously labelled in phytotrons with ¹⁴CO₂ at two CO₂ concentrations (350 and 700 μl 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 CO₂ 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.