Summary Elevation of atmospheric CO 2 concentration is predicted to increase net primary production, which could lead to additional C sequestration in terrestrial ecosystems. Soil C input was determined under ambient and Free Atmospheric Carbon dioxide Enrichment (FACE) conditions for Lolium perenne L. and Trifolium repens L. grown for four years in a sandy‐loam soil. The 13 C content of the soil organic matter C had been increased by 5‰ compared to the native soil by prior cropping to corn ( Zea mays ) for > 20 years. Both species received low or high amounts of N fertilizer in separate plots. The total accumulated above‐ground biomass produced by L. perenne during the 4‐year period was strongly dependent on the amount of N fertilizer applied but did not respond to increased CO 2 . In contrast, the total accumulated above‐ground biomass of T. repens doubled under elevated CO 2 but remained independent of N fertilizer rate. The C:N ratio of above‐ground biomass for both species increased under elevated CO 2 whereas only the C:N ratio of L. perenne roots increased under elevated CO 2 . Root biomass of L. perenne doubled under elevated CO 2 and again under high N fertilization. Total soil C was unaffected by CO 2 treatment but dependent on species. After 4 years and for both crops, the fraction of new C ( F ‐value) under ambient conditions was higher ( P = 0.076) than under FACE conditions: 0.43 vs. 0.38. Soil under L. perenne showed an increase in total soil organic matter whereas N fertilization or elevated CO 2 had no effect on total soil organic matter content for both systems. The net amount of C sequestered in 4 years was unaffected by the CO 2 concentration (overall average of 8.5 g C kg −1 soil). There was a significant species effect and more new C was sequestered under highly fertilized L. perenne . The amount of new C sequestered in the soil was primarily dependent on plant species and the response of root biomass to CO 2 and N fertilization. Therefore, in this FACE study net soil C sequestration was largely depended on how the species responded to N rather than to elevated CO 2 .
Summary An increase in concentration of atmospheric CO 2 is one major factor influencing global climate change. Among the consequences of such an increase is the stimulation of plant growth and productivity. Below‐ground microbial processes are also likely to be affected indirectly by rising atmospheric CO 2 levels, through increased root growth and rhizodeposition rates. Because changes in microbial community composition might have an impact on symbiotic interactions with plants, the response of root nodule symbionts to elevated atmospheric CO 2 was investigated. In this study we determined the genetic structure of 120 Rhizobium leguminosarum bv. trifolii isolates from white clover plants exposed to ambient (350 μmol mol −1 ) or elevated (600 μmol mol −1 ) atmospheric CO 2 concentrations in the Swiss FACE (Free‐Air‐Carbon‐Dioxide‐Enrichment) facility. Polymerase Chain Reaction (PCR) fingerprinting of genomic DNA showed that the isolates from plants grown under elevated CO 2 were genetically different from those isolates obtained from plants grown under ambient conditions. Moreover, there was a 17% increase in nodule occupancy under conditions of elevated atmospheric CO 2 when strains of R. leguminosarum bv. trifolii isolated from plots exposed to CO 2 enrichment were evaluated for their ability to compete for nodulation with those strains isolated from ambient conditions. These results indicate that a shift in the community composition of R. leguminosarum bv. trifolii occurred as a result of an increased atmospheric CO 2 concentration, and that elevated atmospheric CO 2 affects the competitive ability of root nodule symbionts, most likely leading to a selection of these particular strains to nodulate white clover.
The aim of this study was to test the effect of oxygen partial pressure as a possible limiting factor of nitrogen fixation following defoliation. The response of nitrogenase activity (C2H2-reduction) of defoliated and undefoliated white and red clover plants (Trifolium repens L. and Trifolium pratense L.) to either 19 kPa oxygen or 55 kPa oxygen was investigated. Prior to defoliation, white clover plants were grown for five weeks in a growth chamber, and red clover plants for 7 or 11 weeks in a glasshouse. The results included measurements of 16N2-uptake. Increasing oxygen partial pressure from 19 to 55 kPa severely restricted nitrogenase activity of undefoliated white clover plants; however, 2 h after complete defoliation, the same treatment caused a significant increase. A fivefold increase in nitrogenase activity upon exposure to the elevated oxygen partial pressure was found at the end of a 24 h period. This beneficial effect decreased gradually from 1 to 5 d after defoliation. The response of recently defoliated red clover plants to 55 kPa oxygen partial pressure was similar to that of white clover, independently of plant age. The gradual recovery of nitrogenase activity during three weeks of regrowth was associated with a simultaneous change in the response to increased oxygen partial pressure, leading again to the response of undefoliated plants. These data suggested that lack of oxygen at the site of nitrogen fixation, resulting from a dramatic increase in oxygen-diffusion resistance, is the main factor limiting nitrogenase activity following defoliation.
Alfalfa (Medicago sativa L.) seeds and roots can create complex rhizosphere effects by releasing flavonoids that induce nodulation (nod) genes in Rhizobium meliloti. Previous reports identified luteolin and 4,4′-dihydroxy-2′-methoxychalcone as strong inducers that are released from seeds and roots, respectively, and 4′,7-dihydroxyflavone and 4′,7-dihydroxyflavanone as weaker inducers which are exuded by roots. As a first step toward identifying flavonoid interactions that may occur in the rhizosphere, combinations of these molecules were tested for transcriptional effects on a nodABC-lacZ fusion in R. meliloti. At low concentrations (e.g. 8.4 nanomolar), interactions of the three nod gene inducers from root exudate were additive. When the strong inducers 4,4′-dihydroxy-2′-methoxychalcone and luteolin were present separately at higher concentrations (e.g. 21 nanomolar), their effect could be decreased significantly by the weaker inducers 4′,7-dihydroxyflavone and 4′,7-dihydroxyflavanone. In contrast, when low concentrations of luteolin from seed rinses and 4,4′-dihydroxy-2′-methoxychalcone from root exudate were present together, they produced synergistic increases in nod gene transcription. Tests with mixtures of the three nod gene inducers from root exudate indicated that alfalfa seedlings might easily decrease the strong inductive effect of the chalcone by releasing modest amounts of the weaker inducers. In addition, mixtures of luteolin and the nod gene inducers in root exudate suggested that interactions between nod gene inducers from seeds and roots may create a zone highly favorable to root nodule formation near the top of the primary root.
