Rhizosphere synergy: The role of endogeic earthworms in nutrient cycling, plant growth, and soil organic matter stabilization

Living plant roots play a crucial role in stabilizing soil organic matter (SOM), which in turn influences overall soil function in ecosystems. Sequestration of SOM is also mediated by earthworms, which facilitate the transformation of older SOM and fresh plant material into more stable forms. While this role is well documented for earthworms feeding on litter, the interaction between endogeic, soil-dwelling earthworms, root-derived inputs to the soil, such as rhizodeposition, and the rhizosphere microbiome is even less known. In an eight-week laboratory experiment, we investigated the interaction between the endogeic earthworm Aporrectodea caliginosa and the roots of maize (Zea mays) using rhizoboxes with soil differing from the maize in its ¹³C natural abundance to determine the fate of rhizodeposition, the effects on nutrient availability, microbial activity and SOM, especially the dynamics of particulate organic matter (POM) and mineral-associated organic matter (MAOM), as well as the formation of stable aggregates in the rhizosphere. We found that earthworms promote plant growth by accelerating nitrogen recycling and increasing nitrate availability, to a lesser extent phosphorus uptake by plants, and to a large extent phosphorus accumulation in the rhizosphere microbial community. Earthworm activities, which led to increased plant biomass and rhizodeposition, stimulated microbial processes in the rhizosphere, accelerated SOM turnover, improved aggregate stability and appeared to favour the formation of stable MAOM. These results underline the positive influence of the interaction between soil-dwelling earthworms, plant roots and microorganisms in the rhizosphere on SOM stabilization and nutrient cycling, and thus on overall soil quality.