| The relationship between biodiversity and ecosystem functioning is the major scientific issue of the present day in ecology. The theory of biodiversity effects have been developed from nitrogen-limited grasslands, but is still facing challenges when it was extended to nitrogen-rich ecosystems from nitrogen-limited grasslands. Because only plant uptake was the most crucial process in nitrogen-limited grasslands, biodiversity effects are mainly focusing on the uptake. However, the substrate processes was active in addition to the uptake in nitrogen-rich ecosystems, at this time biodiversity effects may be caused by multi-processes (For example, under high N level, denitrification contributes a considerable portion to the depletion together with plant uptake), thus the mechanism of biodiversity effects are more complicated. Therefore, the better understanding of the mechanism of combined effects is urgent needed.Water pollution and global warming are two major environmental problems concerned by international society now. The increasing greenhouse gas (GHG) concentration in the atmosphere is closely related to the increasing volume of wastewater discharged. The volume of wastewater generated by industrial, agriculture and domestic sources has increased dramatically with urbanization, population, economic development and improved living conditions. Traditional wastewater treatment has been regarded as a large emission source of GHG, mainly carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Therefore, we need to find the effective technology with high nitrogen removal capacity and low GHG emission.Constructed wetlands (CWs) are unique wastewater treatment technology that have been designed and constructed to strengthen the natural processes involving soils, plants and their associated microorganisms. Plants play an important role in the process of wastewater purification. Recently, biodiversity and ecosystem function research have been extended to nitrogen-rich CWs, showing that plant diversity improved pollutant removal efficiencies. However, to the best of our knowledge, the effects of plant diversity on GHG emission have not been reported so far.To investigate the effect of plant diversity on the GHG emission, and analyze the correlation between nitrogen removal efficiency and GHG emission, we have established4plant species richness levels (each level containing1,2,3and4species, respectively) and15communities by using180microcosms. Based on the nitrogen content of domestic wastewater, the supplied nitrogen level of the sand substrate and hydroponic microcosms is443g N m-2y-1(7times higher than local nitrogen fertilizer used in agriculture). Strengthen theoretical study through analyzing diversity effects, and then strengthen the structure of CWs and optimize the ecosystem function in practical application. The main results and findings are as follows:1. Plant species richness increased N2O emissions.(1) In sand systems, the N2O emission was positively correlated with plant species richness and ranged averagely from980.9to1848.5μg N2O m-2d-1. The proportional deviation index DMax(test whether there is over-emission of N2O in hybrid systems) increased with plant species richness and was greater than zero, showing that niche differentiation between species (resource-use complementarity) is a mechanism to increase N2O emission.(2) In hydroponic systems, the N2O emission was positively correlated with plant species richness and ranged averagely from27.1to115.4μg N2O m-2d-1. The average DMax was greater than zero across species richness levels, showing that complementarity occurred. The N2O emission in sand systems was higher than that in hydroponic systems. This may be related to sand substrate by providing accessible surface area for microbial attachment, thus promoted N2O production.2. Plant species richness increased CH4emissions. Plant species richness increased N2O emissions.(1) In sand systems, the CH4emission was positively correlated with plant species richness and ranged averagely from2.41to4.30mg CH4m-2d-1. The average DMax (test whether there is over-emission of CH4in hybrid systems) in the microcosms planted with3and4species was greater than zero, showing that complementarity occurred. Besides, organic matter content in substrate increased with plant species richness, which supplied methanogens with more available carbon, thus increasing CH4production. However, the effluent TOC concentration was low, and did not change with species richness, showing species richness would not increase the carbon pollution.(2) In hydroponic systems, the CH4emission was positively correlated with plant species richness and ranged averagely from1.63to4.07mg CH4m-2d-1. The proportional deviation index DMax increased with plant species richness, showing that niche differentiation between species (resource-use complementarity) is a mechanism to increase CH4emission. The CH4emission in sand systems was higher than that in hydroponic systems. This may be related to sand substrate by providing accessible surface area for microbial attachment, thus promoted CH4production.3. Plant species richness decreased net global warming potential consisting of CH4+N2O+CO2.(1) In sand systems, species richness has a positive effect on CH4+N2O emission. Similarly, species richness also has a positive effect on carbon fixation of plant. Assumed aboveground used for bioenergy to replace fossil fuels, the net global warming potential, consisting of CH4+N2O emission and carbon fixation of plant, decreased with plant species richness and ranged averagely from-7.4to-3.5g CO2-eq m-2d-1. The proportional deviation index DMax (test whether there is over-emission of CH4+N2O in hybrid systems) confirmed complementarity occurred in hybrid systems, showing complementarity is an important mechanism to increase CH4+N2O emission. Similarly, DMax (test whether there is over-fixation of carbon in hybrid systems) increased with plant species richness and was greater than zero, showing that niche differentiation between species is a mechanism to increase carbon fixation of plant. Further, we found complementarity and sampling effect are two mechanisms to increase carbon fixation of plant by calculating the selection and complementary effect of aboveground biomass.(2) In hydroponic systems, species richness has a positive effect on CH4+N2O emission and carbon fixation of plant. However, the net global warming potential, consisting of CH4+N2O emission and carbon fixation of plant, decreased with plant species richness and ranged averagely from-7.0to-5.2g CO2-eq m-2d-1. The average DMax was greater than zero across species richness levels, showing that complementarity occurred. The DMax of CH4+N2O confirmed complementarity occurred in hybrid systems, showing complementarity is an important mechanism to increase CH4+N2O emission. However, the average DMax of plant carbon fixation was lower than zero and did not change with species richness, showing no complementarity between species. Further, we found selection is a mechanism to increase carbon fixation of plant by calculating the selection and complementary effect of aboveground biomass. The net global warming potential in sand systems was higher than that in hydroponic systems. This may be related to sand substrate by providing accessible surface area for microbial attachment, thus promoted CH4production. On the one hand, the CH4+N2O emission in sand systems was higher than that in hydroponic systems. On the other hand, the carbon fixation of plant in sand systems was lower than that in hydroponic systems.4. Plant species richness not only improved nitrogen removal, but also increased the GHG emission per unit nitrogen removal.(1) In sand systems, the NO3--N and TIN concentration in effluents decreased with the species richness and ranged averagely from53.1to16.7mg L-1and from54.8to17.1mg L-1, showing species richness improved inorganic nitrogen removal efficiency. The average DMax was greater than zero across species richness levels, showing that resource-use complementarity is an important mechanism to result in under-depletion. Effect analysis on plant N pool and biomass showed that plant uptake is an important way to increased nitrogen removal. However, denitrification decreased with the species richness, showing denitrification had negative effect on nitrogen removal across species richness. On the level of species richness, the contribution rate of plant and denitrification on inorganic nitrogen removal were36%-76%and22%-60%, respectively. Substrate nitrogen retention has a small contribution to the nitrogen removal, less than5%.(2) In hydroponic systems, the NO3--N and TIN concentration in effluents decreased with the species richness and ranged averagely from14.3to2.3mg L-1and from14.8to2.7mg L-1. The average DMax was greater than zero across species richness levels, showing that resource-use complementarity is an important mechanism to result in under-depletion. Effect analysis on plant N pool and biomass showed that plant uptake is an important way to increased nitrogen removal. However, denitrification decreased with the species richness, showing denitrification had negative effect on nitrogen removal across species richness. On the level of species richness, the contribution rate of plant and denitrification on inorganic nitrogen removal were44%-66%and34%-56%, respectively. Due to complementarity, the N2O emission per unit nitrogen removal (the ratio of the N2O-N emission to the N03--N removal) and no-CO2GHG emission per unit nitrogen removal (the ratio of the CH4+N2O emission to the NO3--N removal) increased with the species richness. The NO3--N content in effluents in sand systems was higher than that in hydroponic systems. This is due to the plant N pool in hydroponic systems was higher than that in sand systems.5. Plant species identity affected net global warming potential.(1) In sand systems, the presence of O. hookeri within a given plant community decreased the net global warming potential, but the presence of R. japonicas, P. arundinacea and R. carnea had no influence on the net global warming potential. Further, species identity had no influence on the N2O or CH4emissions, showing O. hookeri decreased the net global warming potential by increasing the carbon fixation of plant.(2) In hydroponic systems, the presence of O. hookeri decreased the net global warming potential; the presence of R. carnea increased the net global warming potential; but the presence of R. japonicas or P. arundinacea did not influence the net global warming potential. Further, the presence of P. arundinacea increased the N2O and CH4emissions, but the presence of R. japonicas increased the CH4emissions. These show that O. hookeri decreased the net global warming potential by increasing the carbon fixation of plant; R. carnea increased the net global warming potential due to the lower carbon fixation of plant; but the increased GHG emissions offset by the increased carbon fixation when the R. japonicas or P. arundinacea present, therefore, the presence of R. japonicas or P. arundinacea had no influence on the net global warming potential on the whole.6. Plant species identity and community affected nitrogen removal efficiency.(1) In sand systems, the presence of R. japonicas decreased the NO3--N concentration in effluents; the presence of O. hookeri increased the NO3--N concentration; but the presence of R. carnea or P. arundinacea did not influence the NO3--N concentration in effluents. The NO3--N concentration in hybrid systems is between R. japonicas monoculture and O. hookeri monoculture.(2) In hydroponic systems, the presence of R. japonicas affected the decrease in the NO3--N concentration. However, the presence of O. hookeri, P. arundinacea or R. carnea did not influence NO3--N removal. Overall hybrid improved NO3--N removal capability.Above all, plant species richness has positive effect on N2O and CH4emission, but has negative effect on the net global warming potential in constructed wetland microcosms. This means that if we make full use of aboveground for bioenergy, plant species richness cannot increase GHG emissions but reduce the net global warming potential of the systems. Besides, species richness also has positive effect on nitrogen removal. DMax showed that complementarity is the mechanism to strengthen the positive effect of the systems. |
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