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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

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

^a Research Institute for Agrobiology and Soil Fertility (AB), Dep. of Soil Chemistry and Soil Ecology, P.O. Box 14, 6700 AA Wageningen, The Netherlands

a.gorissen{at}ab.wag-ur.nl

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.

Abbreviations: HN, high N treatment • LN, low N treatment • SMB, soil microbial biomass • SOM, soil organic matter • S/R, shoot/root [ratio of N]