Soil fungal communities show little response to CO2 enrichment but significant change over time in a 42-month litter decomposition experiment in mature oak woodland

Soil fungal communities play central roles in decomposition. Rising atmospheric CO₂ levels  may impact these communities as additional carbon becomes available for allocation to soil fungi, with potential repercussions for soil carbon and nutrient cycling via decomposition.

To explore this, soil fungal communities were analysed from a long-term litter decomposition experiment at the Birmingham Institute of Forest Research Free Air Carbon dioxide Enrichment Facility (BIFoR FACE), a unique experiment in England in which patches of mature oak-dominated woodland have been exposed to elevated [CO₂] (+150 μmol/mol) throughout each growing season since 2017. Litterbags of three different mesh sizes (1 μm, 41 μm, 2 mm) and two litter types (oak roots, leaves) were buried under elevated and ambient [CO₂] at BIFoR FACE in November 2020. Each consisted of an inner mesh bag containing the litter within an outer mesh bag containing soil. Soil from within the litterbags collected at three timepoints (March 2021, February 2022, May 2024) was used for ITS metabarcoding and the resulting data were analysed in conjunction with soil chemistry data from the experiment.

Timepoint was found to be the dominant factor structuring fungal communities. Across all mesh sizes, soil from May 2024 showed significantly higher relative abundances of ectomycorrhizal fungi and lower relative abundances of saprotrophs relative to the earlier timepoints, and a concurrent increase in Basidiomycota at the expense of Ascomycota. In parallel with these fungal community shifts, soil C:N returned in May 2024 to levels similar to those of March 2021 (mean 12.9 ± 0.1 SE at both timepoints), having fallen to a minimum in February 2022 (mean 10.8 ± 0.1 SE). The later increase in soil C:N was driven primarily by reduced total soil nitrogen; this may reflect a decline in available N contributing to increased ectomycorrhizal abundance, which in turn led to further N losses through ectomycorrhizal N mining . Having accounted for the effect of timepoint, however, neither saprotrophic nor ectomycorrhizal relative abundances were related to litter mass loss. CO₂ enrichment had little impact on soil fungal OTU richness or guild relative abundances and no taxa were differentially abundant between ambient and elevated [CO₂]. However, CO₂ enrichment was found to be significantly associated with fungal beta diversity (alongside timepoint, mesh size, litter type, dissolved organic carbon, pH, and soil moisture).

These results demonstrate clear patterns of fungal community change as decomposition progresses. These patterns were largely unaffected by CO₂ enrichment, despite the fact that decomposition rates have been found to differ between ambient and elevated CO₂ in the same experiment. The most notable change was an increased relative abundance of ectomycorrhizal taxa by the final timepoint, likely related to declining N levels.