Molecular analysis of arbuscular mycorrhizal fungi colonising a semi-natural grassland along a fertilisation gradient

The community of arbuscular mycorrhizal fungi (AMF) colonizing the roots of Festuca pratensis and Achillea millefolium was characterized in a Swedish pasture at different times, along a gradient of fertilization. The small subunit ribosomal RNA gene was subjected to PCR and denaturing gradient gel electrophoresis (DGGE), sequencing and phylogenetic analysis. The sequences found in this study clustered in 10 discrete sequence groups, seven belonging to Glomus, two to Scutellospora and one to Diversispora. A negative correlation was observed between soil mineral nitrogen and the number of AMF sequence groups in the roots. The frequency of occurrence of AMF in roots decreased dramatically between June and September. No plant-host specificity could be detected.     

Warmer winters increase the rhizosphere carbon flow to mycorrhizal fungi more than to other microorganisms in a temperate grassland

A decisive set of steps in the terrestrial carbon (C) cycle is the fixation of atmospheric C by plants and the subsequent C‐transfer to rhizosphere microorganisms. With climate change winters are expected to become milder in temperate ecosystems. Although the rate and pathways of rhizosphere C input to soil could be impacted by milder winters, the responses remain unknown. To address this knowledge‐gap, a winter‐warming experiment was established in a seminatural temperate grassland to follow the C flow from atmosphere, via the plants, to different groups of soil microorganisms. In situ ¹³CO₂ pulse labelling was used to track C into signature fatty acids of microorganisms. The winter warming did not result in any changes in biomass of any of the groups of microorganisms. However, the C flow from plants to arbuscular mycorrhizal (AM) fungi, increased substantially by winter warming. Saprotrophic fungi also received large amounts of plant‐derived C—indicating a higher importance for the turnover of rhizosphere C than biomass estimates would suggest—still, this C flow was unaffected by winter warming. AM fungi was the only microbial group positively affected by winter warming—the group with the closest connection to plants. Winter warming resulted in higher plant productivity earlier in the season, and this aboveground change likely induced plant nutrient limitation in warmed plots, thus stimulating the plant dependence on, and C allocation to, belowground nutrient acquisition. The preferential C allocation to AM fungi was at the expense of C flow to other microbial groups, which were unaffected by warming. Our findings imply that warmer winters may shift rhizosphere C‐fluxes to become more AM fungal‐dominated. Surprisingly, the stimulated rhizosphere C flow was matched by increased microbial turnover, leading to no accumulation of soil microbial biomass.