Harvesting Plants in Constructed Wetlands to Increase Biomass Production and Na+ and Cl− Removal from Recycled Greenhouse Nutrient Solution

Abstract

Recycled greenhouse nutrient solution requires treatment to prevent increased concentrations of both Na+ and Cl− ions from damaging crops or impacting the environment subsequent to its discharge. Constructed wetlands (CW) planted with species capable of hyper-accumulating Na+ and Cl− may be one viable treatment option. To mitigate the unwanted ions from CW, plant material may need to be harvested and removed. Research suggests that multiple harvests throughout the growing season can maximize the CW phytoremediation potential. To determine the ideal frequency of CW plant harvesting, an 18-week, outdoor microcosm experiment was conducted in which three wetland plant species, Juncus torreyi Coville. (Torrey’s rush), Schoenoplectus tabernaemontani (C.C. Gmel.) Palla (softstem bulrush), and Typha latifolia L. (broad leaf cattail), were subjected to the following harvesting treatments: (i) one harvest, (ii) two harvests, and (iii) three harvests. The total amounts of Na+ and Cl− accumulated in the aboveground dry biomass per square meter of plant growth area were calculated. Treatments with the highest Na+ accumulation were as follows: T. latifolia with three harvests, 24.7 g m−2, T. latifolia with two harvests, 16.2 g m−2, J. torreyi with two harvests, 12.7 g m−2. Treatments with the highest Cl− accumulation were as follows: J. torreyi with two harvests, 111.3 g m−2, T. latifolia with three harvests, 94.8 g m−2, T. latifolia with two harvests, 81.4 g m−2. Harvesting, whether two or three times, increased the Na+ and Cl− accumulated by T. latifolia and J. torreyi but did not influence the Na+ and Cl− accumulation or growth of S. tabernaemontani. However, the average Na+ and Cl− removal efficiencies of the all treatments were low, between 1–5 % for Na+ and 7–15 % for Cl−, suggesting that phytodesalinization may not be the best option for Na+ and Cl− treatment.