Effect of arbuscular mycorrhizal and bacterial inocula on nitrate concentration in mesocosms simulating a wastewater treatment system relying on phytodepuration
High nitrogen concentration in wastewaters requires treatments to prevent the risks of eutrophication in rivers, lakes and coastal waters. The use of constructed wetlands is one of the possible approaches to lower nitrate concentration in wastewaters. Beyond supporting the growth of the bacteria operating denitrification, plants can directly take up nitrogen. Since plant roots interact with a number of soil microorganisms, in the present work we report the monitoring of nitrate concentration in macrocosms with four different levels of added nitrate (0, 30, 60 and 90 mg l−1), using Phragmites australis, inoculated with bacteria or arbuscular mycorrhizal fungi, to assess whether the use of such inocula could improve wastewater denitrification. Higher potassium nitrate concentration increased plant growth and inoculation with arbuscular mycorrhizal fungi or bacteria resulted in larger plants with more developed root systems. In the case of plants inoculated with arbuscular mycorrhizal fungi, a faster decrease of nitrate concentration was observed, while the N%/C% ratio of the plants of the different treatments remained similar. At 90 mg l−1 of added nitrate, only mycorrhizal plants were able to decrease nitrate concentration to the limits prescribed by the Italian law. These data suggest that mycorrhizal and microbial inoculation can be an additional tool to improve the efficiency of denitrification in the treatment of wastewaters via constructed wetlands.
KeywordsConstructed wetland Arbuscular mycorrhizae Denitrification Plant growth-promoting bacteria Phytoremediation Phragmites australis
This work was part of the project “Sviluppo di sistemi di abbattimento dell’inquinamento tramite lagunaggio” and partly funded by AMIAS (Azienda Multiservizi Idrici e Ambientali Scrivia) and by Cassa di Risparmio di Alessandria.
The authors wish to thank Mybasol s.r.l. (Alessandria, Italy) for the support during the experimental work and Dr. Elisa Gamalero for kindly providing Pseudomonas fluorescens PF7 and critically reading the manuscript.
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