Changes in non-symbiotic nitrogen-fixing bacteria inhabiting rhizosphere soils of an invasive plant Ageratina adenophora

This study evaluated the soil characteristics and nitrogen-fixing bacterial (NFB) communities inhabiting the rhizosphere of an invasive plant, Ageratina adenophora, in Southwest of China. In a large geographic range, heavily invaded (HI) soils generally had higher nutrient levels compared with lightly invaded (LI) and non-invaded (NI) ones. The NFB number and species diversity of HI soils were significantly greater than those of NI ones. A total of 416 NFB isolates were divided into 47 operational taxonomic units (OTUs). They were phylogenetically categorized into Proteobacteria (63.9%), Actinobacteria (32.2%), Firmicutes (1.9%) and Bacteroidetes (1.9%). At a lower taxonomic level, these isolates were grouped into 20 genera, among which, the dominant isolates were Arthrobacter, Mitsuaria, Burkholderia, Sinorhizobium, Pseudomonas and Rhizobium. The phylogenetical structures of NFB communities obtained from HI, LI and NI soils were slightly different, with their phylogenetic structure similarity indices (theta(YC)) being less than 0.8. However, the percentages of isolates represented by the shared species between any two communities were more than 80%, and in most cases, nearly 100%. There were 16 OTUs shared by these three communities (HI, NI and LI), representing 351 isolates (84.38% of total isolates). These facts indicated that numerically dominant species among HI, LI and NI soils were highly similar. In conclusion, A. adenophora invasion significantly increased the NFB number and diversity of soils, but did not change their phylogenetic structure of core community. (C) 2011 Elsevier B.V. All rights reserved.