Nitrogen fertilization reduces the standing biomass, abundance, and size of <i>Cenococcum</i> sclerotia: a ubiquitous but rarely quantified ectomycorrhizal soil carbon pool

Summary Unlike most ectomycorrhizal (EM) fungi, Cenococcum geophilum is a prolific producer of sclerotia, which represent a large and persistent, yet rarely quantified pool of EM fungal biomass and carbon in soils. How biomass of these asexual propagules is impacted by global change factors, such as anthropogenic nitrogen (N) deposition, remains unquantified. This study examined the effects of long‐term experimental N fertilization on the standing biomass, abundance, and size of C. geophilum sclerotia in an oak ( Quercus spp.) savanna ecosystem at Cedar Creek Ecosystem Science Reserve in Minnesota, USA. Standing sclerotia biomass in the control treatment averaged 192 g m −2 (95% CI = 136–267 g m −2 ) and declined sharply under N enrichment, by 44% (95% CI = −53–79%) and 66% (95% CI = 39–82%) in the low N (5.4 g N m −2 yr −1 ) and high N (17 g N m −2 yr −1 ) treatments, respectively. Sclerotia abundance also declined under both fertilization levels by 58% (95% CI: 8–81%) and 62% (95% CI: 12–84%), while sclerotia diameter was significantly reduced only under high N. Given their high carbon content, melanization, and long persistence, the observed declines in C. geophilum sclerotia ( c. 84–127 g m −2 ) represent substantial losses from belowground carbon (C) pools. These findings indicate that chronic N deposition suppresses the formation of a functionally important and recalcitrant fungal structure, likely impacting soil C storage and mycorrhizal functional diversity.