Invasive plants modify the composition and function of soil microbial communities

Background/Question/Methods

Invasive plants are so prominent that they are considered part of human induced global change. Dr. Joan Ehrenfeld’s lab performed pioneering work which demonstrated that invasive plants can dramatically alter soil chemistry and soil microbial communities. We have recently explored the feedback between invasive plants and microbes using the invasive plant autumn olive (Elaeagnus umbellata) as a model. Autumn olive is a nitrogen fixer and produces allelopathic compounds. Therefore we predicted that it will have a strong effect on microbial communities in soil. We conducted a field study that explored the effect of autumn olive on soil nitrogen cycling communities. Soil samples were collected in a spatially explicit manner (a rectangular grid) at the edges of three different invaded areas.  DNA from each sample was extracted and the diversity of  bacterial and archaeal ammonia oxidizers, and nitrite oxidizers was determined using PCR-DGGE. In addition, we performed a lab-based microcosm study which explored the allelopathic effects of autumn olive on soil microbial communities. Soils were collected from invaded and non-invaded areas and used to construct small plant-free microcosms.  The microcosms were maintained for 12 weeks and subjected to one of two watering regimes. Some microcosms were watered with an autumn olive leachate produced by soaking autumn olive leaves in water. The rest of the microcosms were watered with water. The activity of microbial communities in microcosms was monitored by measuring enzyme activities and the decomposition rate of birch sticks.

Results/Conclusions

By using a spatial sample design we were able to show that soil nitrogen cycling microbial communities in an autumn olive population were highly heterogeneous. Soil samples that were closer to autumn olive appeared to have more altered microbial communities. Trends in the differences between invaded and uninvaded areas were site-specific and more prominent for certain parameters of the microbial community (for example phosphatase activity).  In general, autumn olive leachate appeared to stimulate microbial enzyme activity and decomposition of woody material in microcosms, regardless of the source of soil (invaded versus uninvaded). For some microbial activities, the response of invaded soils to leachate was significantly higher than the response of uninvaded soils. These results suggest that microbial communities in soil might be adapting to the specific chemical background that autumn olive roots and litter provide.