The use of plant, bacterial and fungal resources in soil food webs of ecto- and arbuscular mycorrhiza-dominated deciduous forests

Soil food webs, driven by complex interactions among plants, microbes, and
invertebrates, are crucial for carbon and nutrient cycling in forest
ecosystems. Over the last decade, it has become evident that forests
dominated by arbuscular mycorrhizal fungi (AMF) or ectomycorrhizal fungi
(EMF) differ in their litter chemistry and microbial community
composition, leading to different carbon and nutrient cycling. Still, the
role of soil invertebrates in soil food webs of AMF- and EMF-dominated
forests remains undescribed. Here, we tested whether trophic positions
(TP) and basal resources of nine soil invertebrate groups (Araneae,
Chilopoda, Collembola, Diplopoda, Diptera, Isopoda, Lumbricina,
Mesostigmata, and Oribatida) are different in AMF- (maple and/or ash) and
EMF-dominated (beech and/or oak) deciduous forests. We addressed this
question using compound-specific stable isotope analysis of carbon and
nitrogen in amino acids (CSIA-AA) – a novel method that allows precise
estimation of the TP and basal resource use (bacteria, fungi, plants) of
consumers in field conditions. Plant- and microbial-derived resources of
soil invertebrates generally varied little between AMF and EMF-dominated
forests. We only found a slight tendency of soil invertebrates in
AMF-dominated forests relying more on fungal-derived resources (ca. 2%
difference) and more on bacteria-derived resources in EMF-dominated
systems (ca. 2% difference). Most of the variation in resource use was
explained by the taxonomic identity of invertebrates, independently of the
forest type and location. Mesofauna, particularly Collembola and
Oribatida, were key consumers of fungi (relative contribution 49-72.1%),
while macrofauna (except earthworms) relied more on plant resources
(relative contribution 42.1-58.9%), suggesting that different size
compartments of soil food webs predominantly use different basal
resources. Earthworms and Myriapoda (Chilopoda and Diplopoda) had the
highest proportions of bacteria-derived essential amino acids (relative
contribution 23.2-35.2%). TPs of taxa remained largely consistent across
mycorrhizal types (except for an elevated TP of Isopoda in EMF-dominated
forests), suggesting similar vertical organisation of soil food webs
across the studied forests. Overall, our study shows that TPs and basal
resource use of soil fauna vary little across temperate deciduous forests,
being mainly explained by the faunal taxonomic identity.