Fungal necromass composition highlights the ecological significance of microbial death pathways in soil

Microbial activity drives soil carbon mineralization, while microbial necromass along with other residues contributes to the stable soil organic carbon pool. Still, precise quantification and characterization of microbial residues remains methodologically challenging in complex soil systems, requiring controlled microbial experiments. We have recently presented the conceptual framework of microbial death pathways in soil, where we hypothesized that different agents of death would lead to varying chemical properties of microbial necromass, with consequences for the fate of microbial necromass carbon in soil.

In the studies presented here, we have now tested these hypotheses experimentally and analysed fungal mycelial residues exposed to diverse agents of death. We investigated the composition of mycelial residues by (i) microscopic live/dead staining, (ii) measurements of carbon, nitrogen and melanin contents, (iii) Raman spectroscopy and (iv) Nanoscale Secondary Ion Mass Spectrometer (NanoSIMS). Using fungal isolates in a controlled experimental design, we found that heat or fungicide exposure led to rapid hyphal death with less chemical transformation of necromass compared to biomass. By contrast, starvation or senescence (ageing of hyphae) allowed mycelia to internally recycle cytosolic components, leading to residues reduced in cytosolic compounds and characterized by wider C:N ratios and increased melanin contents. A litterbag experiment in soil showed that mycelia resembling the chemical properties of biomass are mineralized more rapidly than chemically altered fungal necromass.

We further tested the impact of nitrogen availability on residue formation. Necromass nitrogen contents affect mineralization rates, but also stabilization due to preferential binding of nitrogen-rich compounds to mineral surfaces. Here, fungal residues from nitrogen depleted media showed wide C:N ratios (50-90), resulting from internal recycling of cytosolic compounds but also differential cell wall composition (as indicated by Raman spectroscopy and NanoSIMS analyses). Interestingly, independent of medium nitrogen supply, fungal residues in contact with mineral surfaces (goethite) were strongly nitrogen enriched, indicating preferential binding of nitrogen-rich compounds independent of overall mycelial C:N ratios.

In conclusion, specific microbial death pathways may alter the composition of microbial residues in soil, with consequences for carbon mineralization and stabilization processes. These results further highlight the interaction of carbon and nitrogen cycling via microbial turnover and stabilization, mechanisms that must be integrated in future conceptual and experimental approaches.