Microbial nitrogen transformation in constructed wetlands treating contaminated groundwater
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Pathways of ammonium (NH4 +) removal were investigated using the stable isotope approach in constructed wetlands (CWs). We investigated and compared several types of CWs: planted horizontal subsurface flow (HSSF), unplanted HSSF, and floating plant root mat (FPRM), including spatial and seasonal variations. Plant presence was the key factor influencing efficiency of NH4 + removal in all CWs, what was illustrated by lower NH4 +-N removal by the unplanted HSSF CW in comparison with planted CWs. No statistically significant differences in NH4 + removal efficiencies between seasons were detected. Even though plant uptake accounted for 32–100 % of NH4 + removal during spring and summer in planted CWs, throughout the year, most of NH4 + was removed via simultaneous nitrification-denitrification, what was clearly shown by linear increase of δ15N-NH4 + with decrease of loads along the flow path and absence of nitrate (NO3 −) accumulation. Average yearly enrichment factor for nitrification was −7.9 ‰ for planted HSSF CW and −5.8 ‰ for FPRM. Lack of enrichment for δ15N-NO3 − implied that other processes, such as nitrification and mineralization were superimposed on denitrification and makes the stable isotope approach unsuitable for the estimation of denitrification in the systems obtaining NH4 + rich inflow water.
KeywordsNitrogen Ammonium Constructed wetland Nitrification Denitrification Isotope fractionation
This research was completed within the framework of the Marie Curie Initial Training Network ADVOCATE—Advancing sustainable in situ remediation for contaminated land and groundwater, funded by the European Commission, Marie Curie Actions Project No. 265063, SAFIRA project and the Helmholtz Interdisciplinary Graduate School for Environmental Research (HIGRADE). We are also grateful to Martina Neuber of the stable isotope laboratory Halle/Salle for conducting isotope analyses of the samples. Thank is also addressed to Dang Thi Kim Anh for her valuable assistance in the laboratory and field.
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