Soil and plant specific effects on bacterial community composition in the rhizosphere

Eubacterial community structures in the plant rhizosphere were examined with respect to plant species, soil type, and root zone location. Three plant species (chickpea, rape and Sudan grass) were grown in intact cores of three California soils (a sandy soil, a sandy loam, and a clay) and were provided with a complete fertilizer solution with or without nitrogen supplied as ammonium nitrate. After 7.5 weeks, the plants were harvested and DNA was extracted from soil adhering to the root tips and from mature root zones at the sites of lateral root emergence. Eubacterial community structures were examined by PCR-DGGE of 16S rDNA to determine the relative abundance and species diversity. While both soil type and nitrogen fertilization affected plant growth, canonical correspondence analyses showed that nitrogen had no significant effect on eubacterial community structures. Eubacterial species diversity was higher in the mature root zones than at the root tips in the sandy soil and the clay but not in the loamy sand. Monte Carlo permutation tests indicated that plant species, root zone and soil type as well as the interactions between these variables had significant effects on community structure. The bacterial rhizosphere community of chickpea was influenced primarily by soil type, whereas root zone was less important. In contrast to chickpea, the community in the rhizosphere of rape and Sudan grass was more affected by the root zone than the soil type. In the sandy soil and the loamy sand, the eubacterial rhizosphere community structure was more affected by the root zone than the plant species and the three plant species had distinct communities. In the clay however, the root zone was less important than the plant species and the rhizosphere communities of chickpea differed from those of rape and Sudan grass. It is concluded that the bacterial community composition in the rhizosphere is affected by a complex interaction between soil type, plant species and root zone location.