The physical structure of soil: Determinant and consequence of trophic interactions

Trophic interactions play a vital role in soil functioning and are increasingly considered as important drivers of the soil microbiome and biogeochemical cycles. In the last decade, novel tools to decipher the structure of soil food webs have provided unprecedent advance in describing complex trophic interactions. Yet, the major challenge remains to understand the drivers of the trophic interactions. Evidence suggests that small scale soil physical structure may offer a unifying framework for understanding the nature and patterns of trophic interactions in soils. Here, we review the current knowledge of how restrictions on soil organisms' ability to sense and access food resources/prey inherent to soil physical structure essentially shape trophic interactions. We focus primarily on organisms unable to deform the soil and create pores themselves, such as bacteria, fungi, protists, nematodes and microarthropods, and consider pore geometry, connectivity and hydration status as main descriptors of the soil physical structure. We point that the soil physical structure appears to mostly limit the sensing and accessibility to food resources/prey, with negative effects on bottom up controls. The main mechanisms are (i) the reduced transport of sensing molecules, notably volatiles, through the soil matrix and (ii) the wide presence of refuges leading to pore size segregation of consumer/predators and food sources/prey in pores of contrasting size. In addition, variations in the connectivity of the soil pores and the water film is suggested as a central aspect driving encounter probability between consumers/predator and food source/prey and hence locally decrease or increase top-down controls. Constraints imposed by the soil physical structure on trophic interactions are thought to be major drivers of the soil diversity and local community assemblage, notably by favoring a variety of adaptations to feed in this dark labyrinth (food specialists/flexible/generalists) and by limiting competitive exclusion through limited encounter probability of consumers. We conclude with possible future ways for an interdisciplinary and more quantitative research merging soil physics and soil food web ecology.