Shifting Fungal Guild Abundances are Associated with Altered Temperate Forest Soil Carbon Stocks

Despite the importance of fungi to forest carbon (C) cycling and increasing calls to include microbial interactions in ecosystem models, how shifting fungal guild abundances impact soil C stocks remains poorly quantified, particularly in mineral soils where most C is stored. Additionally, a greater understanding of how fungal interguild interactions affect belowground litter decomposition is needed to more fully characterize soil C dynamics. To address these knowledge gaps, we conducted a multi-year soil trenching experiment in two temperate Pinus strobus stands in Minnesota, USA. We found that after two years, trenching increased ectomycorrhizal fungal relative abundance while decreasing saprotrophic fungal relative abundance (decreased ectomycorrhizal/saprotrophic ratio) and concurrently decreased soil C stocks by 10%. The decreased C stocks were primarily due to changes in particulate organic matter and were largely constrained to the top 5 cm of the soil. Trenching also stimulated both root and fungal litter decomposition in surface soils. Together, these results support the often proposed but rarely tested hypothesis that shifting fungal guild abundances promote soil C accumulation. However, they also suggest this effect may be most relevant for short-term C storage in upper soil layers.