The effects of elevated CO₂concentrations on the root growth of Lolium perenne and Trifolium repens grown in a FACE* system

Abstract

Lolium perenne and Trifolium repens were grown in a Free Air CO2 Enrichment (FACE) system at elevated (600 }.imol mol"*) and ambient (340 [jmol mol"^) carbon dioxide concentrations during a whole growing season. Using a root ingrowth bag technique the extent to which CO2 enrichment influenced the growth of L, perenne and T. repens roots under two contrasting nutrient regimes was examined. Root ingrowth bags were inserted for a fixed time into the soil in order to trap roots. It was also possible to follow the mortality of roots in bags inserted for different time intervals. Rool ingrowth of both L. perenne and T. repens increased under elevated COj conditions. In L. perenne, root ingrowth decreased with increasing nutrient fertilizer level, but for T. repens the root ingrowth was not affected by the nutrient application rate. Besides biomass measurements, root length estimates were made for T, repens. These showed an increase under elevated CO2 concentrations. Root decomposition appeared to decrease under elevated CO2 concentrations. A possible explanation for this effect is the observed changes in tissue composition, such as the increase in the carbonmitrogen ratio in roots of L. perenne at elevated CO2 concentrations.

Influence of an Elevated Atmospheric CO 2 Content on Soil and Rhizosphere Bacterial Communities Beneath Lolium perenne and Trifolium repens under Field Conditions

“…In this study, we focused on L. perenne and T. repens mainly to compare our data with other studies performed on the FACE field experiment in Eschikon (Switzerland) [23,25,29,30,42,57,72,74]. Moreover, the plants have different rhizosphere structures.…”

“…In a prairie ecosystem such as the FACE facility in Eschikon, soil carbon input is mainly the result of dead plants and rhizodeposition. Jongen et al [30] measured in the FACE study site of Eschikon an increased C:N ratio of L. perenne roots grown in monoculture at elevated pCO 2 , while they detected no changes in the C:N ratio of clover roots. From the perspective of microbial numbers, Zak et al [70] suggested that an increased carbon input into the soil could be counterbalanced by an increased C:N ratio, leading to unchanged microbial numbers.…”

“…Indeed, because of the competitive inhibition by O 2 of the ribulose-1,5-bisphosphate carboxylase, which is responsible for CO 2 fixation during photosynthesis [7], the C 3 plant photosynthetic rate increases under a CO 2 -enriched atmosphere [28,47,51]. An increased root biomass under CO 2 enrichment has been shown by several authors [25,30,47,52,53].…”

Source‐sink relations in Lolium perenne L. as reflected by carbohydrate concentrations in leaves and pseudo‐stems during regrowth in a free air carbon dioxide enrichment (FACE) experiment*

“…Carbohydrate concentrations at elevated Pcon with low N supply were consistently higher than in the other three treatments; this coiToborates the suggestion of a N-dependent sink limitation in plants subjected to that treatment. Plants grown at low N supply had a lower above-ground dry matter yield (Hebeisen et al 1997) and their root mass was larger (Jongen et al 1995;Hebeisen et al 1997); these symptoms strongly suggest nutrient-limited growth and, as a consequence, a possible change in root:shoot allometiy. Such a Ndependent sink limitation was most pronounced in first fully grown leaves.…”

“…Therefore, elevated /?co2 '^''^ "o* appear to accelerate metabolic recovery from defoliation in spite of the obvious increase in carbohydrate availability. Instead, the enhanced carbohydrate availability at elevated Pco2 rnay have been matched by a higher sink detnand -possibly as a consequence of the respitatory needs for the maintenance and function of a larger root system (Jongen et al, 1995;Hebeisen et al 1997) as no evidence for an enhanced initial rate of regrowth was observed (F. Stadelmann & T. Hebeisen, unpublished results).…”