Unraveling soil multi-trophic interactions: perspectives from size-dependent predation theory

Protists and nematodes play a crucial role in maintaining biodiversity in soil ecosystems. Through selective feeding preferences, they prevent the dominance of individual species, fostering a diverse microbial community. This, in turn, enhances ecosystem resilience. Nonetheless, a critical uncertainty lies in the extent of size-dependent predation by soil protists and nematodes on active and total prokaryotic communities in soil. This lack of understanding hampers our ability to accurately assess the impact of these consumers on microbial dynamics and ecosystem functioning. Based on a soil microcosm experiment, the compositions of active and total prokaryotic, protistan, and nematode communities, as well as abundances of nitrogen-cycling genes, were assessed by PCR-amplicon sequencing and real-time PCR, respectively. Overall, we found that both small-sized protists and nematodes had a broader feeding preference compared to large-sized protists and nematodes. In the DNA-based multitrophic network, small-sized nematodes interacted more with Actinobacteriota and Proteobacteria, while in the RNA-based network, they favored associations with Crenarchaeota, Firmicutes, and Proteobacteria. Similarly, both small and large phagotrophic protists in the DNA network showed strong connections with Proteobacteria, Actinobacteriota, and Bacteroidota, whereas in the RNA network, their interactions were more pronounced with Crenarchaeota, Proteobacteria, and Firmicutes. Furthermore, small-sized protists and nematodes had more significant correlations with N-cycling gene abundances compared to large-sized protists and nematodes based on the Mantel test. Consequently, this study elucidates the feeding preferences of protists and nematodes towards both the active and total soil prokaryotic communities, providing novel insights into the size-dependent predation theory within the interactions of multitrophic soil organisms.