Studies On Biodiversity-High Efficient Nitrogen Removal In Constructed Wetlands

Constructed wetlands (CWs) are a natural alternative to technical methods of wastewatertreatment, and have been widely applied for wastewater purification. Wastewater from differentsources has different NO₃^-/NH₄⁺ ratios. Plant species may have different preferences between NO₃^-and NH₄⁺, so species should be carefully selected according to the inorganic N composition ofwastewater. Considering the importance of plants in CWs, this paper studied various respondings ofsix commonly used plants in CWs to different NO₃^-/NH₄⁺ ratios and analyzed how the plants ofdifferent families responded to the N forms. Besides, considering the importance of microorganismsin the substrate in CWs, we also compared plant growth performances under sand and solutionculture supplied with different NO₃^-/NH₄⁺ ratios.The shoot, root and total dry matter (DM) of Lolium perenne, Canna indica and Coixlacryma-jobi were significantly higher under NO₃^-/NH₄⁺ ratios 75/25 and 50/50 than those under100%NO₃^- and 100%NH₄⁺ treatments. Morever, compared with 100%NO₃^-, shoot and root DM ofthose plants were much lower under 100%NH₄⁺ treatment. For Acorus calamus, shoot and root DMshowed the same trend, with NO₃^-/NH₄⁺ ratios 50/50 the highest and the 100%NO₃^- lowest. However,this study didn't find growth inhibitions for Acorus calamus under 100%NH₄⁺ treatment. For Irispseudacorus and Reineckia cornea, both shoot and root DM linearly declined with the increasingproportion of NH₄⁺. There are positive correlations between shoot DM and shoot N accumulationand between root DM and root N accumulation for all species. But N concentration was notcorrelated with N accumulation in shoot or root for most species except Acorus calamus. So plantDM should be a main factor contributing to their N removal abilities in CWs.After comprehensive analysis of the related N form studies, we found that most plants got moreDM under NO₃^- condition than NH₄⁺ condition. Solanaceae, Cucurbitaceae, Asteraceae, Fabaceae,Chenopodiaceae, Brassicaceae, Salicaceae, Rosaceae, Euphorbiaceae, Urticaceae, Araceae, Iridaceae,Cannaceae, Labiatae and some Gramineae prefer NO₃^-, while Alliaceae, Ericaceae, Pinaceae,Fagaceae, Cyperaceae, Proteaceae, Taxaceae, Myrtaceae, Theaceae, Balsaminaceae and someGramineae prefer NH₄⁺. For Gramineae, some plants exhibit preference for NO₃^-, e.g. Loliumperenne, Coix lacryma-jobi and Triticum aestivum and some plants prefer NH₄⁺, e.g. Oryza salivaand Glyceria maxima. Six plants of Mosla showed various respondings to different NO₃^-/NH₄⁺ ratios.Only Mosla pauciflora survived in sole NH₄⁺ treatment, other five plants died before harvest time,showing preference for NO₃^-. So the relationship between plant taxa and N form is not obvious, andecological adaptive functional groups play greater roles.Concerning the N removal abilities of different plant species, Lolium perenne got the most Naccumulation in both shoot and root under NO₃^-/NH₄⁺ ratios of 50/50 and 75/25, while NO₃^-/NH₄⁺ratio of 0/100 and 100/0 the lowest, which is similar with Canna indica, Acorus calamus and Coixlacryma-jobi. N accumulation in both shoot and root of Iris pseudacorus gradually declined with theincreasing proportion of NH₄⁺. Under the NO₃^-/NH₄⁺ ratio of 0/100, N accumulation in leaf and rootof Reineckia cornea is significantly lower than the other four treatments containing NO₃^-.Considering the importance of microorganisms in the substrate in CWs, we also compared plantgrowth performances under sand and solution culture supplied with different NO₃^-/NH₄⁺ ratios.Compared with sole NH₄⁺ treatment, leaf, stem, root DM and absolute growth rate (AGR) of Cannaindica and Coix lacryma-jobi were significantly higher under the mixed NO₃^-/NH₄⁺ ratio of 50/50.Root/shoot (R/S) was higher for both species grown in sand than that in nutrient solution.Furthermore, we found that the reduced gap of AGR between NO₃^-/NH₄⁺ ratios 0/100 and 50/50 was smaller in sand than that in nutrient solution. Compared with solution culture, plants grown in sandshowed slighter growth depressions, suggesting that microorganisms in sand played important rolesin the N transformation.The whole study analyzed both growth status and N removal abilities of several commonly usedplants in CWs supplied with different ratios, aiming to select and allocate plant species andbiodiversity groups that are highly efficient in removing N. We also analyzed how plants of differentfamilies responded to different N forms and have reached a primary classification, establishingtheoretical basis for species structure optimization in CWs and thus give implications for improvingN removal efficiency. Morever, we found that plants' preference and tolerance for NO₃^- or NH₄⁺ wasnot depend on plant taxa, but mainly related to the ecological functional groups. The study laid asolid theoretical foundation for the research about biodiversity-ecosystem function relationship.