Fungal Networks Constrain Microfauna-Stimulated Nitrogen Mineralisation in a Soil Microcosm Experiment

Soil microfauna are recognised as key regulators of nitrogen (N) transformations, primarily through grazing and translocation mechanisms. The interactions between soil microorganisms and their microfaunal grazers play a crucial role in controlling N mineralisation and immobilisation processes. Despite the well-established role of bacterivore nematodes and other microbial grazers in enhancing N mineralisation, the extent to which these organisms contribute to overall nutrient cycling within fungal-dominated systems remains unclear. In a non-amended soil microcosm experiment, we investigated microorganisms-microfauna interaction using morphological observations, quantitative polymerase chain reaction and high-fthroughput sequencing. Our findings indicate that microbial grazing by microfauna did not enhance N mineralisation contrary to our hypothesis, despite an increase in bacterial grazers and bacterial abundance compared to the defaunated control. Instead, we observed a dominant fungal-driven N immobilisation process, as evidenced by the increased presence of saprophytic fungi, fungivore nematodes, and a high nematode channel index. The absolute abundance of fungal communities, particularly members of the Sordariomycetes class, further supports the hypothesis that fungi play a central role in regulating N transformations. These results challenge the conventional assumption that microfauna-driven bacterial turnover leads to enhanced N availability and highlight the significant role of fungal networks in N retention.