Application of Floating Beds Constructed with Woodchips for Nitrate Removal and Plant Growth in Wetlands

Abstract

Constructed wetlands and constructed floating wetlands are widely used for nitrogen (N) removal from surface water to combat eutrophication in freshwaters. Two main N removal pathways in freshwaters are plant biomass N uptake and denitrification, i.e. transformation of nitrate (NO₃-) to nitrous oxide (N₂O) or nitrogen gas (N₂) by different microbes possessing nirK, nirS, nosZI, and nosZII genes. In this study, we tested woodchips-based floating beds (WFBs) as a nature-based and environment-friendly method to remove nitrate-nitrogen (NO₃-N) from water. Moreover, we tested whether WFBs could support the growth of three selected plant species and the abundance of microbes on plant roots and woodchips as a proxy for WFBs’ denitrification potential. We conducted a greenhouse experiment for 90 days and measured NO₃-N removal rates from water in WFBs mesocosms during five sampling occasions. Plant biomass production, biomass N uptake, and plant morphology related to N uptake and abundance of denitrifying organisms were measured at the end of the experiment. NO₃-N removal rates were 29.17 ± 11.07, 28.18 ± 12.62, 25.28 ± 9.90, and 22.16 ± 7.79 mg L^–1 d^–1 m^–2 (mean ± standard deviation) in Glyceria maxima, Juncus effusus, Filipendula ulmaria, and unplanted WFBs treatments, respectively for whole experimental period. N content in above- and belowground biomass of studied species ranged between 0.98 – 1.15 and 1.09 – 1.28 (% dry weight), respectively. Plant relative biomass production was 215 ± 61, 67 ± 18, and 7 ± 17 (% dry weight) for G. maxima, J. effusus and F. ulmaria, respectively. Denitrifiers were detected both on plant roots and woodchips, indicating WFBs’ denitrification potential. Our study highlights that WFBs could be applied to enhance NO₃-N removal from surface water through plant biomass uptake and denitrification processes. Future studies should consider the long-term in situ application of WFBs for NO₃-N removal from water.