Decomposing the decomposers: elucidating the drivers of fungal necromass carbon cycling in soils

Annually, gigatons of atmospheric carbon (C) enter terrestrial fungal biomass through two primary pathways: decomposition of plant residues and various plant-fungi symbioses, exchanging plant C for fungal nutrients. The fate of this fungal-C, especially post fungal senescence, remains unclear—whether it is largely emitted back as atmospheric CO2 or stabilized in soil as organic matter. Despite extensive research on primary decomposition (plant residue breakdown), the secondary decomposition (fungal necromass degradation) is less understood, while crucial for predicting and anticipating shifts in the global carbon cycle. Recent studies indicate that soil organic C significantly originates from fungal necromass, suggesting that mycelial residues are resistant to degradation. Additionally, soil microbes, including bacteria, protists, and fungi, are known to be involved in decomposing fungal necromass. Yet, the process linking microbial breakdown of fungal residues to soil organic matter stabilization is not fully elucidated. Our proposal aims to integrate microbial diversity modification methods, molecular ecology, microscopy, and stable isotope 13C tracing to explore how microbial degradation of fungal residues drives necromass-C stabilization within soil organic matter. We will employ dilution-to-extinction techniques to modify soil microbial community complexity. These modified microbial communities will be inoculated into two sterile systems: soil microcosms and microfluidic chips that simulate soil micro-environments. The former, inoculated with 13C labeled fungal necromass, allows tracking of necromass-C movement between the atmosphere and soil organic matter. The latter, through microscopy, enables direct observation of fungal residue decomposers. This research will identify key microbial actors in fungal residue degradation and how microbial diversity influences the formation of stable necromass-C in soils. The findings will significantly enhance biogeochemical soil cycle models by integrating two critical elements: fungal residue decomposition and the role of microbial communities in these degradation processes and necromass-C dynamics.