Nitrogen transformations in constructed wetlands: A closer look at plant-soil interactions using chemical imaging

Constructed wetlands have long been used for domestic wastewater treatment. Despite the widespread application of constructed wetlands for wastewater remediation, they are still regarded as a black box in terms of the complex biogeochemical processes occurring internally, particularly with respect to plant-soil (and nitrogen) interactions. Additionally, many critical processes pertaining to nitrogen transformations in constructed wetlands are thought to occur in microzones within the rhizosphere, highlighting the need for studies with sub-cm spatial resolution. In this study we coupled nitrogen porewater measurements with chemical imaging to determine spatio-temporal patterns in porewater O-2 and pH to assess the extent of plant-induced changes in soil redox dynamics that influence nitrogen biogeochemical cycling during dosed application of nitrogen-rich artificial waste-water. Planar optode imaging revealed extensive O-2 fluxes to otherwise anoxic sediment via radial oxygen loss (ROL) from Typha latifolia roots. The contribution of photosynthetic O-2 from this plant species was minimal as a strong oxic signal persisted in darkness (diel cycles). NH4+ and NOx- removal were strongly correlated with the extent of oxic and anoxic areas, a function largely attributed to the presence of plants and the associated enhanced microbial communities supported. The distribution of nitrogen species within the Typha rhizosphere exhibited reproducible trends as a function of distance from roots, with concentrations highest close to roots (1-5 mm from root surface) and subsequently decreasing at greater distances. Microscale spatio-temporal redox heterogeneity within the rhizosphere due to ROL imposed by plants promoted nitrogen removal likely by stimulating the coupling between nitrification and denitrification in these systems. Collectively, this study highlights the profound importance of plants in exerting controls on soil conditions and nitrogen cycling in constructed wetland systems. With careful considerations, constructed wetlands designed to promote wetland plants' functions may enhance nitrogen removal and mitigate nitrogen pollution. Published by Elsevier B.V.