The Study On Rhizosphere Facilitation Of Elsholtzia Splendens Nakia On Commelina Communis L. Under Cu Stress

Soil heavy metal (HM) contamination represents one of the worldwideenvironmental problems. HMs in the soil mainly come from anthropogenic activitiessuch as mining and smelting, dustfall, sewage irrigation, fertilizer and pesticideapplication, and industrial production, etc. It is reported that mining alone has generatedabout1,500,000ha of wasteland in China and the number is increasing at a rate of46,700ha per year. The remediation of HM contaminated soils includes physical,chemical and biological methods. Phytoremediation, which cleanups contaminantsthrough transportation, absorption and transformation using plants received extensiveconcerns from ecologists and environmentalists. Phytoremediation is a promisingapproach as it is an in situ, low cost and environmental friendly method compared tophysical and chemical counterparts. Despite its advantages, the prospect ofphytoremediation is limited by relatively long cycle and population level vegetationrestoration, which results in low plant diversity and ecosystem stability.Plant-plant interactions shape plant community structure, maintain biodiversity,drive community succession and ultimately influence ecosystem functioning. For a longtime, plant competition is the focus of classical ecological theories and dominatesecological mainstream. However, facilitation, a positive interaction, received extensiveconcerns in the past two decades. Increasing evidence indicates that facilitation, whichmostly occurs in severe environments or under high consumer pressure, promotesestablishment, growth and recruitment of target plants, and reduces seedling mortality.We found that Elsholtzia splendens Nakai was a pioneering and predominant species atthe early stage of secondary succession on the Cu mine spoils. Other species such asCommelina communis L. appeared gradually and mostly distributed in the proximity of E.spelendens. We therefore hypothesized that E. splendens could facilitate the co-occurringneighbors as nurse plant through modifying microhabitat. The performance of targetspecies, Commelina communis L. either coexisted with E. splendens or grown in opengaps was compared along a Cu gradient. In this study, we conducted a field experiment to test this hypothesis and our specific objectives were (1) to verify whether facilitationdoes exist between E. splendens and the coexisting species;(2) to uncover the underlyingmechanisms of facilitation, and (3) to discuss its significances in community levelvegetation restoration and community succession in Cu mine spoils. The main resultsobtained were showed as below.1. The amelioration on soil heavy metal concentrations or plant heavy metal uptakesby E. splendens is slightly, and no consistent and significant changes could be found inmost soil physiochemical properties. E. splendens, with small canopy is not a goodtemperature buffer; however, can significantly increase soil humidity in coexistencesubplots.2. E. splendens increased basal respiration and enzyme activities significantly.Therefore, the facilitaiton between E. splendens and C. communis might occur indirectly.3. E. splendens significantly increased Shannon weiner index and richness at low Cuconcentration. Sequencing and phylogenetic analysis data indicated that the sequencefragments belonged to Acidobacteria, Proteobacteria, Actinobacteria, Firmicutes,Cyanobacteria, Bacteroidetes, Chloroflexi and TM7phylum, in which Acidobacteria,Ktedonobacteria, Proteobacteria and Flavobacteria were dominant groups. Redundancyanalysis indicated rhizosphere soil bacterial community composition was mainlyinfluenced by Cu concentrations and nutrient conditions.4. Both negative and positive interactions were detected and the outcomes highlydepended on estimators of plant performance, the algorithm used and Cu levels as well.The facilitation based community level restoration would accelerate the process ofphytoremediation, increase plant diversity of the ecosystem. Our study investigated thefacilitation between Cu tolerant plants under Cu stress and enriched the theoretical andempirical methodology of ecological remediation, which would provide examples forHM contaminated soils.