Bacterial Activity and Community Composition in the Rhizosphere. Influence of plant species, root age, nitrogen addition and mycorrhizal status

Abstract: Microbial activity in soil is generally assumed to be limited by carbon availability. The soil closest to roots, the rhizosphere, is therefore a zone of high microbial activity due to carbon containing substances released by the root. In this work short-term effects in the rhizosphere have been the subject of investigation. I have shown that the thymidine and leucine incorporation techniques are well suited to the study of the effect of roots on rhizosphere bacterial activity. Bacterial activity in the rhizosphere was found to be 2-4 times higher than in the bulk soil. The turnover time found for rhizosphere bacteria was in the same range as those reported earlier. Bacterial activity in the rhizosphere increased with time, both when barley seedlings were studied at different ages and when samples were taken along the root. The effect of nitrogen addition on the activity of rhizosphere bacteria varied with the nitrogen source added, and were related to the pH induced in the rhizosphere. Toxic substances inhibiting bacterial activity were released from leek roots during the extraction of bacteria due to root injury during the sampling procedure. The composition of the microbial community in the rhizosphere was investigated using three different techniques: direct soil-PLFA, cfu-PLFA and CLPP (Biolog). All three methods were found suitable for rhizosphere studies. Different plant species were found to influence the rhizosphere bacteria differently, concerning both activity and community composition. Arbuscular mycorrhizal (AM) colonisation also influences rhizosphere bacteria differently depending on plant species. Plant species had a greater effect on the bacterial community in the rhizosphere than AM colonisation. This work demonstrates that the direct measurement technique of thymidine and leucine incorporation has the potential to become a useful tool in studies of bacterial activity in the rhizosphere.