As per 2022 National Water Quality Assessment study, agricultural activities are among the major sources of surface water pollution in U.S. It is the leading cause of impairment to rivers and streams, the second leading source of pollution for wetlands, and the third leading source of pollution to lakes (US EPA 2015). Excessive use of commercial fertilizers in agricultural production systems results in substantial fertilizer runoff, making fertilizers the primary pollutant originating from agricultural practices (Chen et al. 2018; Jabbar & Grote, 2019; Kourgialas et al. 2017). Among fertilizers, nitrogen (N) and phosphorus (P) are major plant nutrients that can limit plant growth therefore, they are applied often applied excessively to ensure optimal plant growth and performance. However, this excessive application frequently results in runoff, leading to water pollution. (Mouri et al. 2011; Yu et al. 2015). Typically, only 2–10% of applied nitrogen accumulates in crops, while the remaining portion can leach into water, potentially affecting the health of plants, animals, and humans (Fields 2004; Sengupta et al. 2015). Nitrogen runoff contributes to eutrophication, leading to depleted oxygen levels in water, which can be fatal to aquatic life. Additionally, nitrogen runoff causes harmful aquatic algal blooms, which produce neurotoxins capable of affecting both animals and humans (Baron et al. 2013; Chislock et al. 2013; US EPA 2013). Leaching a substantial amount of N into underground aquifers results in groundwater pollution. These underground aquifers vital drinking water source in the U.S., hence such pollution has the potential to severely restrict access to clean drinking water (Foley et al. 2012). Harmful algal blooms are present across all 50 states of the United States, with notable occurrences concentrated in the Chesapeake Bay, Lake Erie and the Gulf of Mexico. Therefore, in an effort to protect both surface and groundwater from nitrate (NO3-) pollution, the U.S. Environmental Protection Agency (EPA) has imposed regulations limiting the discharge of water containing nitrate-nitrogen (NO3–N) concentrations exceeding 10 mg/L or 45 mg/L when expressed as (NO3-) (U.S. EPA 1986).
There are various ways to remediate NO3- laden water resulting from N-related runoff (Ali et al. 2013; Kafle et al. 2022). One easy and low-cost way to remediate NO3- is through bioretention. Bioretention is a pollution remediation technique that uses soil, plants, and microbes to clean contaminated water before it discharges. This technique is frequently utilized to eliminate NO3- from water. Bioretention involves multiple degradation processes like adsorption, ion exchange, photodegradation, and microbial transformation, making it effective for removing nitrates from water. In a study conducted by Li et al. (2021), it was demonstrated that employing a bioretention system effectively removed 72.9–100% of NO3–N from wetlands. Constructed wetlands are modified bioretention systems that extensively employ plants to remove contaminants and are equally or even more effective at removing NO3- than traditional wastewater treatment methods. The removal of contaminants from water using wetland plants is called phytoremediation (Fox et al. 2008; Ojoawo et al. 2015; Su et al. 2019). The use of a constructed wetland to remove N from irrigation return flow generated from a commercial nursery indicated that NO3–N remediation efficiency averaged 94.7% during spring and fall and 70.7% during winter (Taylor et al. 2006). Research by McMaine et al. (2020) demonstrated a NO3- removal rate exceeding 78% from irrigation return flow generated from nurseries and greenhouses using constructed wetlands.
A study by Henderson et al. (2007) concluded that vegetated mesocosms, artificial setups resembling natural wetlands in water dynamics and ecological functions but designed for research purposes, removed 63 to77% more N from constructed wetlands than non-vegetated mesocosms. The efficacy of phytoremediation, among many other factors, also depends on plant type, target contaminant, and concentration of the contaminant in water (Kirk & Kronzucker 2005). In a study by Jamshidi et al. (2014), the wetland plants Phragmites sp. and Typha sp., removed 79% and 77% of N and 21% and 14% of P, respectively. Wetland plants such as Typha latifolia, Juncus effusus (soft rush), Iris pseudacorus (common reed), and Phragmites australis are commonly employed for N removal in wetlands. Several other wetland species such as Sagittaria lancifolia (bultongue arrowhead), Pontederia cordata (pickerelweed), Canna × generalis ‘australia’, Scripus Validus (bulrush), and Cyperus haspan (dwarf papyrus) have demonstrated effectiveness in N removal (Chen et al. 2009; Read et al. 2008; Srivastava et al. 2008; Stearman et al. 2003). In a study by Wang et al. (2002), wetland plants Polygonum amphibium (sharp dock) and Eichhornia crassipes (water hyacinth) had N uptake of 63.6 and 42.8 mg of NO3–N per kg plant dry weight, respectively. For Lemma minor (duckweed), Oenathe javanica (water dropwort), and Lepironia articulata (calamus), N uptake ranged from 20–30 g per kg of plant dry weight. In a study by Kirumba et al. (2015), Pontederia cordata (pickerelweed) reduced water NO3- concentration in wetlands by 50–70%. Thus, a wide range of wetland plants have the potential to remove NO3- from runoff water with various efficacy rates. Floating treatment wetlands (FTW) function similarly to constructed wetlands yet they differ in their permanence (White 2021). While constructed wetlands are typically permanent to semi-permanent structures, FTW systems offer flexibility. They can be effortlessly installed and removed at various sites and across different time frames. Additionally, in FTW, entire plants can be easily harvested, facilitating the relocation of biomass between locations to optimize efficiency. This convenient plant removal feature of FTW also helps prevent the release of nutrients back into the water if entire biomass are removed (Chen et al. 2009).
Even after 50 years of the Clean Water Act in U.S., almost half of rivers and streams in U.S. are impaired. In Utah, 152,691 acres of lakes and ponds are classified as impaired (Larsen 2022). Nitrogen is one of the key sources of water impairment (Manuel 2014) and phytoremediation employing FTW is one of the efficient way to remove N from water. Helianthus maximiliani (maximilian sunflower) and Asclepias speciosa (showy milkweed) are common wetland plants grown throughout the United States, including Utah. These plants are non-invasive, aesthetically attractive, and thrive in wetlands. However, their applicability for FTW and their efficiency in removing nitrates is still unknown.
Nutrient absorption by plants depends on root temperature and, for FTW that is water temperature (Reddy & De Busk, 1985), as root zone temperature affects photosynthesis and growth. In a study by Ali et al. (1996), NO3- transport from roots to leaves decreased as root temperature was reduced from 68⁰ to 54⁰ F. Similarly, increase in root temperature using TGRooZ, a device that generates a temperature gradient, showed positive effect on shoot growth (González-García et al., 2022). Therefore, by controlling and maintaining root zone temperature in FTW, it may be possible to enhance plant growth and nitrate uptake. Therefore, the objective of our research was to 1) to assess the suitability of growing Helianthus maximiliani and Asclepias speciosa in a FTW system, 2) to evaluate the NO3- uptake capacity of these plants under three distinct N water concentrations, and 3) to determine the impact of water temperature on NO3- uptake.