A reduced fraction of plant N derived from atmospheric N (%Ndfa) and reduced rhizobial nifH gene numbers indicate a lower capacity for nitrogen fixation in nodules of white clover exposed to long-term CO2 enrichment

[摘要] Using the δ¹⁵N natural abundance method, we found that the fraction ofnitrogen derived from atmospheric N (%Ndfa) in field-grown white clover(Trifolium repens L.) plants was significantly lower (72.0% vs.89.8%, p = 0.047 in a grassland exposed to elevated CO₂ for 13 yrusingfree air carbon dioxide enrichment (FACE). Twelve months later weconducted an experiment to investigate the reasons behind the reduced Nfixation. We took cuttings from white clover plants growing in the FACE andestablished individual plants in a glasshouse using soil from the appropriateambient or elevated CO₂ treatments. The established plants were thentransplanted back into their "rings of origin" and sampled over a 6-weekperiod. We used molecular ecological analyses targeting nifH genes andtranscripts of rhizobia in symbiosis with white clover (Trifoliumrepens L.) to understand the potential mechanisms. Shoot biomass wassignificantly lower in eCO₂, but there was no difference in nodule numberor mass per plant. The numbers of nifH genes and gene transcripts per nodulewere significantly reduced under eCO₂, but the ratio of gene totranscript number and the strains of rhizobia present were the same in bothtreatments.We conclude that the capacity for biological nitrogen fixation was reduced byeCO₂ in white clover and was related to the reduced rhizobia numbers innodules. We discuss the finding of reduced gene number in relation to factorscontrolling bacteroid DNA amount, which may imply an influence of nitrogen aswell as phosphorus.

[学科分类] 地球化学与岩石