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

Abstract

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.