Development of specific rhizosphere bacterial communities in relation to plant species, nutrition and soil type

Rhizosphere microbial communities are important for plant nutrition and plant health. Using the culture-independent method of PCR-DGGE of 16S rDNA for community analyses, we conducted several experiments to investigate the importance of pH, soil type, soil amendment, nutritional status of the plant, plant species and plant age on the structure of the bacterial community in the rhizosphere. At the same time, we assessed the spatial variability of bacterial communities in different root zone locations. Our results showed that the bacterial community structure is influenced by soil pH and type of P fertilization. In a short-term experiment ( 15 - 22 days) with cucumber and barley growing in a N deficient or a P deficient soil, the bacterial community structure in the rhizosphere was affected by soil type and fertilization but not by plant species. In a 7.5-week experiment with three plant species ( chickpea, canola, Sudan grass) growing in three different soils ( a sand, a loam and a clay), the complex interactions between soil and plant effects on the rhizosphere community were apparent. In the sand and the loam, the three plant species had distinct rhizosphere communities while in the clay soil the rhizosphere community structures of canola and Sudan grass were similar and differed from those of chickpea. In all soils, the rhizosphere community structures of the root tip were different from those in the mature root zone. In white lupin, the bacterial community structure of the non-cluster roots differed from those of the cluster roots. As plants matured, different cluster root age classes ( young, mature, old) had distinct rhizosphere communities. We conclude that many different factors will contribute to shaping the species composition in the rhizosphere, but that the plant itself exerts a highly selective effect that is at least as great as that of the soil. Root exudate amount and composition are the key drivers for the differences in community structure observed in this study.