The influence of soil warming on organic carbon sequestration of arbuscular mycorrhizal fungi in a sub-arctic grassland

A substantial portion of grassland photosynthates is allocated belowground to arbuscular mycorrhizal fungi (AMF), but controversy remains about whether this carbon (C) contributes to soil organic carbon (SOC) under warming. The goal of this study was to investigate how AMF biomass and C sequestered by AMF (C_New) are influenced by soil warming. We estimated the AMF biomass and C_New, assumed to be mostly AMF necromass, in mycelial ingrowth bags buried for 1, 2, or 3 years in soil under warming (~+0.5–16.4 °C). The AMF biomass had a positive, curvilinear response to warming gradients after one year of burial. About 107 g C m⁻² of C_New accumulated over the three years and ~12% of this C was from glomalin-related soil protein. Modelling suggested the production rate of AMF biomass was 153 g C m⁻² yr⁻¹ with a rapid (36–75 days) turnover while AMF necromass turnover was much slower (1.4 ± 0.2 yr⁻¹). Warming duration (7–9 years vs. > 50 years) did not have significant influence on AMF biomass or C_New (P > 0.05). Our results suggest that AMF are more tolerant to increases in temperature than other microbes or fine roots. The dramatic loss of soil C and stable soil aggregates under warming found earlier at this site were not attributed to a decrease in AMF biomass or C_New. Despite a low AMF standing biomass, its contribution to SOC may be substantial.