Effects Of Carbon Nanotube Disturbance On Biodiversity-ecosystem Functioning Relationships

The relationship between biodiversity and ecosystem functioning(BEF)has been a frontier issue in ecology in recent decades.Recently,increasing attention has been paid to the effects of disturbances(e.g.,nitrogen addition and warming)on the BEF relationship.With the increasing use and disposal of engineering nanomaterials(e.g.,carbon nanotubes,CNTs),the wasted nanomaterials have become a new kind of disturbance to the ecosystem by affecting organisms and ecosystem processes and functions.However,the existing studies mainly focused on the effect(toxicity)of CNTs on the performance of single species,especially crops.How the CNTs affect wild plants,structure,and BEF relationship in ecosystems with multiple coexisting species remains unknown.Constructed wetlands(CWs)is a kind of widely used wastewater treatment systems.Due to their controllable experiment conditions,CWs have become a good platform for the BEF study.The wasted CNTs entering the wastewaters initially sink in the wetland ecosystems.Therefore,CWs are an ideal experimental platform to explore the effect of emerging disturbance of CNTs on BEF relationship.This study set up a biodiversity control experimental system including 108 independent CW microcosms.These microcosms were divided into two groups,in which one was treated with CNTs and the other was the control.Four common wetland plants were selected to set up three richness gradients(1,3,4)and 9 species combinations.This study aims to explore the effects of CNTs disturbance on plant biomass production and interspecific competition,the relationship between species diversity and ecosystem productivity,nitrogen processes,and nitrogen functions.The main results and conclusions are as follows.(1)The disturbance of CNTs induced an asymmetric effect on biomass production among different plant species.In monocultures,CNTs decreased the biomass production of Phragmites australis by 26% compared with the controls but did not affect those of the other three species.In mixtures,the effect of carbon nanotubes was reversed.CNTs did not affect the biomass production of P.australis,but decreased the biomass production of Acorus calamus,Echinodorus amazonicus,and Pontederia cordata in mixtures by 15%,24%,and 17%,respectively.The asymmetric response of species changed the interspecific competition.After the disturbance of CNTs,the competitive advantage(relative yield)of the original superior competitor,P.australis,was enhanced,while the competitive abilities of the other three species were weakened.These exacerbated the asymmetry of interspecific competition.In ecosystems without CNTs,there were two superior competitors,P.australis and P.cordata,while only one superior competitor(P.australis)left after the disturbance of CNTs.(2)The disturbance of CNTs reduced the productivity of CW ecosystem by 12% on average.However,the positive effect of plant species richness on ecosystem productivity remained unchanged under the disturbance.The CNTs intensified competition and reduced the complementarity effect,but the increased competitive advantage of the most productive species slowed down the decline of productivity in mixtures.Finally,the decline of productivity in monocultures and mixtures were similar,and the relationship between species richness and productivity was robust to CNT disturbance.(3)CNTs disturbance altered the nitrogen processes in CW ecosystems.CNTs reduced plant nitrogen uptake by 11% but enhanced denitrification by 50%(proved by the mass balance analysis and stable isotope analysis).In ecosystems without CNTs,increasing plant species richness reduced denitrification,while the negative relationship between species richness and denitrification was flattened after CNTs disturbance.It is due to the increase in denitrification of mixtures(72%)was higher than that of monocultures(29%).However,the positive correlation between species richness and plant nitrogen uptake kept unchanged under CNTs disturbance.(4)CNTs disturbance increased nitrogen removal rate by 4% and triggered a positive species diversity effect on nitrogen removal in CWs.Species richness did not affect the total inorganic nitrogen removal in ecosystems without disturbance,and became increase total inorganic nitrogen removal after CNTs disturbance.The enhancement of denitrification in mixtures led to the increase in nitrate and total inorganic nitrogen removal in mixtures.In addition,an "over-removal effect" appeared in the combination of P.australis × A.calamus × E.amazonicus after the disturbance,that is to say,the total inorganic nitrogen removal rate of the combination was 3%higher than that of the monoculture with the highest removal rate.(5)CNTs disturbance altered the relationship between species richness and ammonia volatilization in CWs.In CWs without disturbance,species richness decreased ammonia volatilization by a cascading effect through increasing plant biomass production.However,species richness did not affect ammonia volatilization after CNTs disturbance.This was because the inhibitory effect of CNTs was so effective that the ammonia volatilization of monocultures and mixtures decreased to the same low level.This indicated another pattern of CNTs disturbance on the BEF relationship.That is to say,the CNTs had a large enough effect on ammonia volatilization reduction to uncouple the negative relationship between species richness and ammonia volatilization.(6)CNTs disturbance did not affect nitrous oxide emission of CWs.Species richness kept a negative effect on nitrous oxide emission under disturbance,due to the unchanged positive diversity-productivity relationship.However,CNTs disturbance triggered a negative relationship between species richness and nitrous oxide emission per unit of denitrification.It was because that nitrous oxide emission per unit of denitrification remained unchanged in monocultures,but decreased in mixtures.In addition,the species richness also triggered a negative diversity effect on methane emission.The negative effect of species richness on the health damage cost integrated with nitrous oxide and methane emissions remains unchanged under the disturbance of CNTs;at the same time,the mitigation effect of species richness on the health damage cost per unit of nitrogen removal was not affected by disturbance.It is indicated that CNTs not only triggered a positive effect of plant diversity on nitrogen removal,but also did not affect the mitigation effect of diversity on health damage cost of gaseous pollutants in CWs.This study firstly reported that CNTs,as a new type of disturbance,altered the interspecies relationship and biodiversity-ecosystem process and functioning relationship.In this study,on the one hand,the types of ecosystem disturbances were extended to engineering nanomaterials;on the other hand,the toxicity and safety risk of engineering nanomaterials were extended to their effects on the ecosystem structure and structure-function relationship.CNTs intensified interspecific competition and reshaped the role of biodiversity in maintaining or enhancing ecosystem functioning.This work provides new insights into understanding the changes in ecosystem structure and function under the emerging disturbance of engineering nanomaterials.To deal with the disturbance of emerging anthropogenic pollutants,further study on the ecosystem and regional scale is needed.