Elevated Carbon Dioxide Effects on Nitrogen Dynamics in Grasses, with Emphasis on Rhizosphere Processes

Abstract

Three perennial grass species, perennial ryegrass (Lolium perenne L.), colonial bentgrass (Agrostis capillaris L.), and sheep fescue (Festuca ovina L.), were grown at two CO2 concentrations (350 and 700 μL L−1) and under two N regimes: one with a minor addition of 8 kg N ha−1 and one with an addition of ≈278 kg N ha−1, both labeled with 15N. The effects of elevated CO2 on 15N and N uptake and dynamics in the plant–soil systems were determined after 32 and 55 d, with close attention to the rhizosphere. Total N uptake by the plants was not affected by elevated CO2, compared with ambient CO2, independent of N treatment and grass species. A clear decrease from 1.77 at ambient CO2 to 1.25 at elevated CO2 was observed in the shoot/root (S/R) ratio of N, resulting from a significant decrease of the N concentration in shoots, and an unchanged root N concentration. At 700 μL L−1 CO2, N concentration in the shoots decreased from 12.9 to 9.9 g kg−1, even at the low N supply, whereas the slight decrease in root N concentration for plants grown at elevated CO2 (7.9 vs. 7.3 g kg−1) was not significantly different. The relative increase of 15N found in the rhizosphere soil microbial biomass (SMB) and the rhizosphere soil residue under elevated CO2 was too small to affect plant growth, even in the low N treatment. The total amount of 15N recovered in the plants was not affected by the CO2 treatment. Although at the second harvest slightly more 15N was found in the plants than at the first harvest, probably due to turnover of the SMB, no interaction with CO2 was observed. This shows that the fertilizer 15N had not been immobilized to a larger extent or for a longer time by the SMB at elevated CO2 than under ambient CO2, even independent of N level and grass species. No evidence was found that under elevated CO2 substantial amounts of N had been immobilized by the SMB, nor that mineralization of native soil organic matter (SOM) had been stimulated by an increased supply of substrate to the SMB. We conclude that elevated CO2 has the potential to induce significant changes in plant N nutrition, modifying N allocation and tissue quality within perennial grasses, but that these effects appear to be independent of the SMB.