Fungal guilds are evenly distributed along a vertical spruce forest soil profile while individual fungi show pronounced niche partitioning

Saprotrophic and ectomycorrhizal (EcM) forest fungi decompose organic matter and mobilize nutrients for host plants, respectively. Competition between the two guilds may cause the so-called Gadgil effect, i.e., decreased litter decomposition rates resulting in increased carbon storage in soil. The Gadgil effect was supposed to even affect global climate, highlighting the necessity to understand fungal distribution and interactions in soil. Searching for evidence of competition between saprotrophic and mycorrhizal fungi, we analyzed the distribution of fungi along a well-stratified vertical spruce forest soil profile in two seasons, i.e., autumn and the following spring. The different soil strata (i.e., two mineral horizons and two organic layers) underneath the litter layer were colonized by distinct fungal communities, which included roughly consistent proportions of all fungal guilds and phyla at each time. However, the community composition changed quantitatively between the sampling dates. Along the vertical soil profile, it differed mostly between the organic layers and the mineral soil, which is supposed to be due to differences in the predominant energy sources (i.e., aboveground litter and rhizodeposition, respectively). Network analyses revealed co-occurrences (i.e., positive correlations of individual abundances) to outweigh mutual exclusions (i.e., negative correlations) between individual fungi in each soil stratum and season. This also applied for interactions between saprotrophic and EcM fungi. Network analyses therefore provided no indications for a possible Gadgil effect. However, considering individual nutrient use efficiencies might refine insights from network analyses in future studies and facilitate linking community dynamics to ecosystem processes.