Use of stable nitrogen isotopes to track plant uptake of nitrogen in a nature-based treatment system

Abstract

In nature-based treatment systems, such as constructed wetlands, plant uptake of nutrients can be a significant removal pathway. Current methods for quantifying plant uptake of nitrogen in constructed wetlands, which often involve harvesting biomass and assuming that all nitrogen stored in plants was derived from wastewater, are inappropriate in pilot- and full-scale systems where other sources of nitrogen are available. To improve our understanding of nitrogen cycling in constructed wetlands, we developed a new method to quantify plant uptake of nitrogen by using stable isotopes and a mixing model to distinguish between nitrogen sources. We applied this new method to a pilot-scale horizontal levee system (i.e., a subsurface constructed wetland) over a two-year monitoring period, during which 14% of nitrogen in plants was wastewater-derived on average and the remaining plant nitrogen was obtained from the soil. Analysis of nitrogen isotopes indicated substantial spatial variability in the wetland: 82% of nitrogen in plants within the first 2 m of the slope came from wastewater while less than 12% of plant nitrogen in the remainder of the wetland originated from wastewater. By combining these source contributions with remote-sensing derived total biomass measurements, we calculated that 150 kg N (95% CI = 50 kg N, 330 kg N) was taken up and retained by plants during the two-year monitoring period, which corresponded to approximately 8% of nitrogen removed in the wetland. Nitrogen uptake followed seasonal trends, increased as plants matured, and varied based on design parameters (e.g., plant types), suggesting that design decisions can impact this removal pathway. This new method can help inform efforts to understand nitrogen cycling and optimize the design of nature-based nutrient control systems.

Fate of Dissolved Nitrogen in a Horizontal Levee: Seasonal Fluctuations in Nitrate Removal Processes

“…Porewater, influent and effluent water, soils, and plant tissues were collected at locations delineated in Figure S1 of the Supporting Information using methods described previously. , Porewater was collected quarterly along the wetland slope to track changes in chemical indicators. Porewater samples were collected at depths ranging from 0.6 to 0.9 m using stainless-steel PushPoint porewater samplers (MHE Products, East Tawas, MI, USA).…”

“…Sample processing and analytical methods were described previously. , Briefly, anions and cations were analyzed by ion chromatography, , and non-purgeable organic carbon, dissolved inorganic carbon, and total dissolved nitrogen were measured using a Shimadzu TOC-V/CSH analyzer with an attached TN-1 unit (Shimadzu Scientific Instruments, Columbia, MD) . Dissolved metals (i.e., Mn (aq) and Fe (aq) ) were analyzed by inductively coupled plasma mass spectrometry (ICP–MS) .…”

“…Despite these promising findings, the pathways for nitrogen removal in horizontal levees were not fully characterized in past research. Microbial processes in the subsurface were presumed to be important because isotopic analyses indicated that plant uptake was only responsible for about 10% of wastewater-nitrogen removal . However, the combination of the quantified removal pathways and canonical denitrification did not explain the lack of seasonal variations in dissolved nitrogen removal.…”

“…However, the combination of the quantified removal pathways and canonical denitrification did not explain the lack of seasonal variations in dissolved nitrogen removal. Plant senescence and lower water temperatures during winter led to lower nitrogen uptake rates by plants and should have resulted in lower rates of heterotrophic denitrification, , but overall nitrogen removal rates did not exhibit seasonal variations …”

Enabling Water Reuse by Treatment of Reverse Osmosis Concentrate: The Promise of Constructed Wetlands

“…Where wintertime treatment is necessary, subsurface systems that are insulated from air temperatures may continue to provide treatment year-round, despite wastewater temperatures as low as 10 °C. , For instance, a pilot-scale subsurface-flow constructed wetland treating municipal wastewater effluent maintained water temperatures above approximately 10 °C in the winter and exhibited little seasonal variation in nitrate removal, which occurred primarily via denitrification. , Similarly, COD removal was maintained at temperatures as low as 4 °C in planted microcosms simulating subsurface wetlands . In contrast, nitrification activity has been found to decrease in subsurface wetlands in the winter, and in some cases, denitrification rates have slowed in subsurface wetlands at temperatures below 15 °C .…”