Secondary Decomposers Meet Their Predators: Decomposition Stage and Substrate Quality Jointly Structure Microbial Brown Food Webs During Fungal Necromass Decay

Mycelial residues, also known as fungal necromass, represent a substantial fraction of soil organic matter (SOM) pools in terrestrial ecosystems worldwide. Although microbial decomposers are increasingly recognised as key drivers of fungal necromass carbon stock formation, the diversity and composition of their microbial predators—and the roles these predators play in mediating fungal necromass decomposition—have not been explored to date. To address this gap, we produced fungal necromass of varying biochemical quality from Hyaloscypha bicolor and decomposed it in forest topsoil in Minnesota, USA, to investigate how microbial decomposer (bacteria and fungi) and predator (protists and nematodes) communities differ between soil and necromass. We also examined whether microbial predators influence the abundance of fungal necromass decomposers and affect necromass decomposition rates. Over two sampling times (4 and 12 weeks), necromass exhibited rapid early mass loss followed by reduced decay, with a higher stabilised mass in high melanin necromass. Microbial abundances were higher in necromass than in surrounding soil, especially in low melanin necromass. Community composition of both decomposers and their predators differed between soil and necromass and shifted markedly with necromass quality and decomposition stage. Predator community composition was linked to bacterial and fungal abundances at both early and late stages of decay and was marginally associated with decomposition rates. We conclude that fungal necromass acts as a microbial ‘hotspot’ not only for decomposers but also for their predators. These findings highlight the importance of microbial predator–decomposer interactions to better understand the formation of fungal-derived SOM.