Mycelial biomass growth stage at death determines fungal necromass decay dynamics

Fungal necromass is increasingly recognized as a major component of soil organic matter, and identifying the factors that govern its formation is critical for understanding and predicting the global carbon cycle. Among these factors, the biochemical composition of mycelial residues at senescence, particularly melanin content, has been consistently identified as a key determinant of the fraction of fungal necromass that persists in soils. However, even non-melanized mycelial residues exhibit a recalcitrant fraction that resists microbial decomposition, and the reasons for this persistence are not well understood. To address this gap, we asked whether the growth stage at which a single non-melanized fungal species dies governs the decay of its necromass in soil. Using Neurospora crassa , we produced seven necromass types that ranged from early exponential growth to prolonged starvation and decomposed them in forest soil. Necromass derived from biomass experiencing net growth at the time of harvest decomposed up to ten times faster than necromass from starved cultures, which were undergoing biomass loss. By the end of decomposition, only about 10% of necromass from early-growth-stage biomass remained, while nearly 65% of necromass from starved biomass persisted. Differences in mycelial biochemical traits, particularly C:N ratio and the degree of branching of glucans, which varied with fungal growth stage at death, explained variation in both decay rates and the size of the persistent fractions. Our findings suggest that the growth stage of fungi at death is a key factor driving fungal necromass decay profiles, with potentially large consequences for the contribution of fungal necromass to soil organic matter stocks. • Fungal biomass with net growth decays 10× faster than starved biomass. • The persistent fraction of necromass varies depending on the fungal growth stage • Starved fungal biomass produces a larger recalcitrant fraction of necromass • Fungal C:N ratio predicts necromass decay rate • Fungal glucan branching degree predicts necromass persistence