Objectives Moso bamboo invasion could impact varied aspects of ecosystem, including nitrogen cycle in forest ecosystem. Biological nitrogen fixation is an important process of the nitrogen cycle in forest ecosystem. To understand changes of soil nitrogen-fixing microbes in the process of moso bamboo invasion can provide theoretical bases for evaluating the impacts of bamboo invasion on the ecological function.

Methods Three moso bamboo invasion belts (Qinlongshan, Shimendong and Jinshanmen) in the Tianmu Natural Reserve Area were selected for this study, topsoil samples were collected respectively in mono moso bamboo forest, mixed forest of bamboo and original forest, and the original forest along the direction of moso bamboo invasion in all three invasion belts. Soil pH, soil organic carbon (SOC), available N, available P, readily available K, nitrate N and ammonium N were analyzed. Variations of soil nitrogen-fixing microbial community structure and abundance were measured by terminal restriction fragment length polymorphism (T-RFLP) and real-time quantitative PCR (qPCR) based on gene nifH.

Results After the invasion of moso bamboo, three changing types in soil chemical properties were observed in all three moso bamboo invasion belts, soil pH, SOC, available nitrogen and available phosphorus were generally increased, and readily available potassium was decreased, while there was no significant change for nitrate nitrogen. In general, soil nutrients were increased after the moso bamboo invasion. The abundance of nitrogen-fixing microbes was decreased after the bamboo invasion in general, a significant difference was only found in the Jinshanmen invasion belt (P < 0.05). The results of the T-RFLP of soil nitrogen-fixing microbes showed that T-RF 48 bp were dominant in all the three invasion belts. The T-RFs displayed different changing tendency in the process of moso bamboo invasion, some of them disappeared after the moso bamboo invasion, while some of them appeared, and the relative percentage of other T-RFs had no significant alteration. The Shannon and evenness indices of soil nitrogen-fixing microbial communities decreased after the moso bamboo invasion, while the Simpson index was in the opposite way, and a significant difference was only observed in the Shimendong. The MRPP results showed that there were significant alteration for soil nitrogen-fixing microbial community structure in the Shimendong and Jinshanmen (P < 0.05). RDA discovered that the first two axes explained less than 30% of environmental variables that were responsible for alteration of nitrogen-fixing microbial communities, and there was no accordant soil chemical properties significantly related to soil nitrogen-fixing microbes in the three belts. Moreover, the sample sites of the three invasion belts had no uniform distribution patterns in the RDA results.

Conclusions Soil nitrogen-fixing microbial communities in the three moso bamboo invasion belts shifted in different ways in the process of moso bamboo invasion although they are located on same natural reserve area, and parental rock and original vegetation are interacted responsible for the variations of soil nitrogen-fixing microbes. More investigation should be done in order to discover the effect of the moso bamboo invasion on soil nitrogen-fixing microbes by combination of more factors.