BayCEER Workshop 2026

Using stable isotopes to trace the fate of fungal necromass in soil

Soil fungi actively decompose organic matter (OM) and provide OM as necromass. Most plants live in symbiosis with mycorrhizal fungi (MF), with plants providing carbon (C) to MF, emphasizing the fungal role in soil C cycling. Although fungal necromass often exceeds bacterial and plant residues, we lack knowledge of its fate in the soil. We carried out three experiments using stable isotopes of C and nitrogen (N) to understand fungal necromass decomposition, recycling, and stabilization. First, we quantified the contribution of fungal necromass to OM fractions in forests dominated by arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) trees using Bayesian mixing models informed by ¹³C and ¹⁵N natural abundance data. Second, we investigated the formation of mineral-associated OM (MAOM) with ¹³C-enriched AM-fungal necromass in mesocosms with grassland soils of varying texture. Lastly, we conducted a microcosm experiment with different ECM fungal species growing on fungal necromass and leaf litter, using an isotopically enriched (¹³C, ¹⁵N) medium to analyze OM recycling. We found that fungal necromass was the main source of MAOM. In AM forest systems, saprotrophic fungi contributed more, whereas MF contributed more in ECM systems, indicating that ECM fungi suppress saprotrophs. Despite its high contribution to MAOM, fungal necromass is assumed to be quickly decomposed. Indeed, the mesocosm experiment revealed both rapid decomposition and formation of MAOM with fungal necromass. Surprisingly, the latter was mostly unstable and negatively correlated with the soil clay content, both potentially related to organo-organo associations on mineral surfaces. Finally, our microcosm study showed that ECM fungi recycled more N and C from fungal necromass than from leaf litter. Collectively, the use of natural-abundance stable isotopes and stable-isotope labeling revealed a high importance of fungal necromass in both the stabilization as MAOM and as a source for C and N recycling.