Root-soil contact defines rhizosphere characteristics but not early plant growth
The interface of root and soil is an essential factor for plant life, which, in interaction with microorganisms, regulates plant nutrient and water uptake and thus determines plant performance.  We investigate here the importance of root-soil contact for nutrient uptake and plant performance and for the establishment and composition of the bacterial community in and on the root.        We manipulated the root-soil contact by growing wildtype maize plants (WT) for four weeks in bulk soil (complete soil contact) and in the presence of pores (lower root-soil contact). The same treatments were applied to a maize mutant (rth3), which is deficient in root hair elongation and thus results in even lower root-soil contact, when grown in soil with pores. We measured plant parameters like plant height, dry shoot and root weight, and plant nutrient concentration. We used fluorescent microscopy to detect bacteria on the root surface, amplicon sequencing to analyse bacterial community composition, and qPCR for analysing the abundance of bacteria and archaea and bacterial nitrifiers and denitrifiers.   We did not detect differences in plant performance and plant nutrient concentration when comparing WT plants grown in bulk soil or with pores. However, the abundance of bacteria was increased with increased soil contact and the community was more divers regarding the bacteria on the root. The endophytes showed no response regarding the root-soil contact. 70% of the variation within the bacterial community of the WT was explained by the degree of root-soil contact. Also, abundance of bacteria involved in the nitrogen cycle was pronounced when more root-soil contact occurred. Moreover, the absence of root hairs in rth3 plants decreased root-soil contact, but rth3 plants grown in bulk soil showed similar plant and bacterial responses as WT plants growing in soil with pores. When the root-soil contact was reduced even further, by growing rth3 plants in soil with pores, the bacterial response was even more pronounced, but still no difference in plant responses was observed compared to WT.  Thus root-soil contact was critical for bacterial community abundance and diversity at the root surface. However, in our experiment, this did not result in lower plant performance and nutrient uptake, potentially due to the short growth period.