Community shifts and carbon translocation within metabolically-active rhizosphere microorganisms in grasslands under elevated CO₂

Abstract. The aim of this study was to identify the microbial communities that are actively involved in the assimilation of rhizosphere-C and are most sensitive in their activity to elevated atmospheric CO₂ in a temperate semi-natural low-input grassland ecosystem. For this, we analyzed ¹³C signatures in microbial biomarker phospholipid fatty acids (PLFA) from an in-situ ¹³CO₂ pulse-labeling experiment in the Giessen Free Air Carbon dioxide Enrichment grasslands (GiFACE, Germany) exposed to ambient and elevated (i.e. 50% above ambient) CO₂ concentrations. Short-term ¹³C PLFA measurements at 3 h and 10 h after the pulse-labeling revealed very little to no ¹³C enrichment after 3 h in biomarker PLFAs and a much greater incorporation of new plant-C into fungal compared to bacterial PLFAs after 10 h. After a period of 11 months following the pulse-labeling experiment, the ¹³C enrichment of fungal PLFAs was still largely present but had decreased, while bacterial PLFAs were much more enriched in ¹³C compared to a few hours after the pulse-labeling. These results imply that new rhizodeposit-C is rapidly processed by fungal communities and only much later by the bacterial communities, which we attributed to either a fungal-mediated translocation of rhizosphere-C from the fungal to bacterial biomass or a preferential bacterial use of dead root or fungal necromass materials as C source over the direct utilization of fresh root-exudate C in these N-limited grassland ecosystems. Elevated CO₂ caused an increase in the proportional ¹³C enrichment (relative to the universal biomarker 16:0) of the arbuscular mycorrhizal fungal biomarker PLFA 16:1ω5 and one gram-positive bacterial biomarker PLFA i16:0, but a decrease in the proportional ¹³C enrichment of 18:1ω9c, a commonly used though questionable fungal biomarker PLFA. This suggests enhanced fungal rhizodeposit-C assimilation only by arbuscular mycorrhizal fungal species under elevated CO₂.