Analysis of microbial diversity in the soil of a wooded riparian strip

This research is part of a project aimed at verifying the potential of a specifically assessed wooded riparian zone in removing excess of combined nitrogen from the Zero river flow for the reduction of nutrient input into Venice Lagoon. The experimental site was built on an area previously used for arable crops. 300 seedlings of the following plant species were then planted: Quercus robur, Alnus glutinosa, Acer campestre, Salix alba, Coryllus avellana, Salix triandra, Prunus padus, Crataegus monogyna, Fraxinus ornus, Frangula alnus. The buffer zone was organized with an irrigation ditch carrying water from the Zero river and drainage ditches, resulting in a sub-surface water flow running through the entire buffer strips. Soils were sampled in March, April, June and October 2008 from three different depths through the soil profile. The analysis of the bacterial communities was performed by combined approaches involving cultivation on complex solid media to determine colturable bacterial cells, the use of di-chromatic fluorescent stain and CTC+ assay to determine the viable and the metabolically active fractions of the community. Amplified Ribosomal DNA Restriction Analysis (ARDRA) and computer assisted electrophoretic ARDRA fingerprinting were performed to analyze microbial diversity through (a) the comparison between the microbial communities in the wooded riparian strip (internal) to that of soil external to the experimental site, (b) the fluctuation of the microbial communities with time by collecting samples at each season, (c) the characterization of bacterial populations colonizing the soil at different depths. The number of total living cells did not follow significant and important seasonal fluctuations, but epifluorescence counts showed greater variations when surface soils was compared to medium and deep samples. The fraction of culturable bacteria did not show any significant variation between internal and external sampling; the main difference observed was the number of metabolically working cells, higher for the internal soil. These results indicate that the plants play an important role in supporting microbial growth in soil by overcoming the main limiting factor, almost always represented by the availability of organic carbon. To characterize microbial diversity of the culturable fraction, several hundreds colonies were then isolated from the wooded riparian strip and from the external site, 16S rDNA was amplified and HinfI- and HpaII-restricted. A variety of different DNA fragment patterns was revealed. ARDRA analysis from the wooded riparian strip discerned high numbers of OTUs (Operational Taxonomic Units) in all the seasons, both from the internal and external soils. From the 500 colonies analysed 271 OTUs were obtained and 55 of those were found to be always present in all the samples. These preliminary results indicate that the bacterial community structure in the wooded riparian strip seems to be significantly affected by the presence of the plants, associated to a high combined nitrogen input coming from the river Zero. The community was affected not numerically, but in terms of microbial diversity found to be directly related to different soil layers and seasons. The above observations could contribute to understand the bacterial population dynamic of a agricultural soil when transformed in a wooded strip and to provide key indications for the management of a phytoremediation site.