Dark septate root endophytic fungi increase growth of Scots pine seedlings under elevated CO₂ through enhanced nitrogen use efficiency

Although increasing concentrations of atmospheric CO₂ are predicted to have substantial impacts on plant growth and functioning of ecosystems, there is insufficient understanding of the responses of belowground processes to such increases. We investigated the effects of different dark septate root endophytic (DSE) fungi on growth and nutrient acquisition by Pinus sylvestris seedlings under conditions of N limitation and at ambient and elevated CO₂ (350 or 700 μ1 CO₂ l⁻¹). Each seedling was inoculated with one of the following species: Phialocephala fortinii (two strains), Cadophora finlandica, Chloridium paucisporum, Scytalidium vaccinii, Meliniomyces variabilis and M. vraolstadiae. The trial lasted 125 days. During the final 27 days, the seedlings were labeled with ¹⁴CO₂ and ¹⁵NH ₄ ⁺ . We measured extraradical hyphal length, internal colonization, plant biomass, ¹⁴C allocation, and plant N and ¹⁵N content. Under elevated CO₂, the biomass of seedlings inoculated with DSE fungi was on average 17% higher than in control seedlings. Simultaneously, below-ground respiration doubled or trebled, and as a consequence carbon use efficiency by the DSE fungi significantly decreased. Shoot N concentration decreased on average by 57% under elevated CO₂ and was lowest in seedlings inoculated with S. vaccinii. Carbon gain by the seedlings despite reduced shoot N concentration indicates that DSE fungi increase plant nutrient use efficiency and are therefore more beneficial to the plant under elevated CO₂.