Nursery and greenhouse crop production often results in high concentrations of nutrients within production runoff. Effluent nutrient concentrations can range from 0.1 to 387 mg·L−1 nitrate-nitrogen (NO3-N), 0.9 to 47 mg·L−1 ammoniacal-nitrogen (NH4-N), and 0.01 to 306 mg·L−1 total P (Dole et al., 1994; Prystay and Lo, 2001; Roseth and Haarstad, 2010; White, 2013; Wilson et al., 2010). FTWs effectively remediate both N and P using a buoyant floating surface planted with macrophyte species (Tanner and Headley, 2011; White and Cousins, 2013). In a review of FTW systems, Pavlineri et al. (2017) identified more than 42 FTW experiments to date evaluating FTW performance for a variety of parameters. The pH at which the majority of those experiments were conducted was neutral or near neutral, between 6.2 and 7.4. However, the pH of nursery and greenhouse runoff is much more variable.
Argo et al. (1997) conducted a geographical analysis of irrigation water applied to greenhouse operations across the United States and Canada and found that the pH of water applied as irrigation ranged from 2.7 to 11.3, and that the alkalinity of water applied as irrigation ranged from 0 to 1120 mg·L−1 calcium carbonate (CaCO3). The overall mean pH of all water samples was 7.0, with a median value of 7.1. Of the samples, 44% had a pH between 5 and 7, but 53% had a pH >7 (Argo et al., 1997). Although the pH of applied irrigation water does not directly translate to the pH of greenhouse or nursery runoff, few studies have characterized the quality of irrigation runoff on a nationwide basis. Chen et al. (2003) reported that runoff water had a higher pH than irrigation water (7.6 in well water vs. 9.7 in captured water). Changes in pH largely depend on geography, planting substrate and amendments, and irrigation system design, among other factors, including alkalinity. Alkalinity is a measure of the buffering capacity of water; when it is high, it can increase pH (Kuehny and Morales, 1998).
For the growth of greenhouse crops using a soilless substrate, the general consensus is that the ideal pH range is 5 to 7 (Argo et al., 1997; Chen et al., 2003). However, assessments of plant growth in aquatic systems, such as the conditions for plants grown in FTWs, are lacking. Research related to pH and crop growth in hydroponic or aquaponic systems may be most closely aligned with conditions in FTWs. Microbial nitrification of NH4+ to nitrite (NO2−) and NO2− to NO3− is optimized at pH 8.5. Plant nutrient uptake for many crop species is optimized with a pH near 6.0; therefore, the pH in aquaponic systems is managed near 7.0 (Wortman, 2015). Zou et al. (2016) determined that a pH of 6.0 was optimal for plant growth and N utilization efficiency in aquaponics, but it resulted in increased nitrous oxide (N2O) emissions due to high denitrification. Solution pH further impacts P availability and the forms in which P exists. Dissociation of phosphoric acid (H3PO4) to dihydrogen phosphate (H2PO4−) and then to hydrogen phosphate (HPO42−) occurs at pH 2.1 and 7.2, respectively (Schachtman et al., 1998). Plants can only absorb P as the free orthophosphate ions H2PO4− and HPO42− (Becquer et al., 2014). Therefore, the rate of P uptake is directly related to the pH of the solution (White, 2012). Knowledge of pH effects is important for managing nutrient remediation and uptake by FTW systems treating runoff from greenhouse and nursery operations.
Previous research in a variety of disciplines (forest ecology, wetland ecology, hydroponics, etc.) has suggested that plant growth and nutrient uptake vary by species, cultivar, and the characteristics of the system (Härdtle et al., 2004; Wagner et al., 2016; Wortman, 2015). Furthermore, some plant species directly influence their growing conditions through root-induced pH changes. These changes of pH in the rhizosphere are a long-documented chemical interaction, but they mostly result from root–soil interactions. Roots can substantially alter their rhizosphere pH by releasing hydrogen (H+) or hydroxide (OH−) ions, cation–anion exchange balance, organic anion release, root exudation and respiration, and redox-coupled processes (Hinsinger et al., 2003). Numerous authors have shown that the processes by which rhizosphere change occurs depend largely on nutritional limitations within the environment (Bertrand et al., 1999; Grinsted et al., 1982; Imas et al., 1997; Neumann and Römheld, 1999).
This study was conducted to determine how pH impacts the N and P remediation efficacies of three species of plants and to identify any root-induced pH changes by the three different species of plants. Visual MINTEQ 3.1, an equilibrium-based computer model for the calculation of chemical speciation and solubility of dissolved mineral phases in aqueous solution (Gustafsson, 2012), was used to simulate the speciation and activity of key nutrients in an aqueous solution as a function of pH.
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