The importance of root-soil contact in the soil-rhizosphere-plant continuum

At the root-soil interface several important processes take place like nutrient and water uptake and the interaction with microorganisms. Rhizosphere establishment at this interface is hypothesized to be affected by root-soil contact level and being modulated by physicochemical properties of the soil interface, with implications for plant performance.

To investigate the importance of root-soil contact, we modified root-soil contact levels and properties. We grew maize plants in the presence of artificial pores (abiogenic pores), artificial pores incubated with earthworms (biogenic pores) or without pores under greenhouse and field conditions. Plant performance and rhizosphere traits were analyzed, including bacterial abundance and community composition, with a special focus on nitrogen cycling bacteria as indicators for rhizosphere nutrient cycling.

Using endoscopy, we observed that roots in pores grew in part without any contact to soil. Fluorescence microscopy and quantitative PCR (qPCR) revealed that root surface colonization was strongly impaired without root-soil contact. Amplicon sequencing (16S rRNA, ITS1) showed that reduced contact had also consequences for the composition of the rhizosphere microbiota, especially the bacterial community. It was furthermore essential for nitrogen cycling in the rhizosphere, as nitrifying bacteria were more abundant in the rhizosphere with full contact to the soil according to qPCR (amoA). However, root-soil contact had no effects on early plant performance in the greenhouse experiments. By comparison, in the field experiment covering also later growth stages, reduced root -soil contact due to the presence of abiogenic pores resulted in growth deficits compared to the control, which were fully compensated when the interface between root and soil was enriched in nutrients due to the activity of the soil fauna in biogenic pores.

Our findings underline the importance of root-soil contact for rhizosphere processes and plant performance.