The United States greenhouse crop industry is a high-value sector. Containerized crops, including potted flowering plants, annual bedding plants, and garden crops, accounted for nearly 3.4 billion U.S. dollars of greenhouse crop sales in 2015 (U.S. Department of Agriculture, 2016). High production densities, plant quality requirements, and the use of soilless substrates with little or no available mineral nutrients contribute to the necessity for fertilization. Synthetic fertilizers account for nearly 50% of all N applications globally (West and Marland, 2002) and are the primary source of nutrients used in horticultural production. However, drawbacks of synthetic fertilizers, including environmental impacts associated with their production and use, have led to demand for sustainable alternatives (Carpenter et al., 1998; Pelletier et al., 2008).
Nonsynthetic and biologically based (bio-based) materials show potential (Burnett et al., 2016; McCabe et al., 2016; Schrader et al., 2013). Bio-based fertilizers are more sustainable and environmentally benign than synthetic fertilizers (Pelletier et al., 2008). Many bio-based sources have potential as fertilizers, including coproducts of plant processing, such as soy flour and vegetable oils; animal manures; fish emulsion; coproducts of wastewater treatment, like sludge and millorganite; and algae (Chaney et al., 1980; Mulbry et al., 2005; Nelson et al., 2010; Schrader et al., 2013). Of these sources, algae, specifically that sourced from wastewater treatment systems, has shown particular promise for providing an alternative to synthetic fertilizers for container-crop production (Mulbry et al., 2005, 2006; Solovchenko et al., 2016).
The potential benefits of using wastewater-grown algae to replace synthetic fertilizers are multifaceted. Along with most other bio-based fertilizers, algae supply plant nutrients in organic forms, as opposed to the inorganic forms in synthetic fertilizers (Mulbry et al., 2005). Organic forms of nitrogen must undergo biological mineralization before becoming soluble and thus have reduced mobility compared with their inorganic counterparts, leading to a lower likelihood of nutrient loss via leaching (Gaskell and Smith, 2007; Solovchenko et al., 2016). In addition, unlike synthetic fertilizers, the fixation of nutrients in algal biomass occurs through biological processes, avoiding the ecological impacts associated with synthetic fertilizer production (Boelee et al., 2011; Mulbry et al., 2005). Furthermore, through sequestration of plant nutrients from wastewater streams, wastewater-grown algae serves the dual purpose of wastewater treatment and nutrient recycling (Boelee et al., 2011; Cai et al., 2013; Mulbry et al., 2005). However, despite these advantages, low production capacity and cost efficiency, along with the lack of applied research on specific application effects, have limited the adoption of these materials in container-crop production.
Although algal biomass production is heavily studied, actual algal biomass production has remained limited (Christenson and Sims, 2011). Until recently, methods for generating significant quantities of algal biomass required high-energy inputs, and whereas low-energy alternatives like raceway ponds exist, they produce insufficient quantities of algae. Furthermore, harvesting algae from these systems is difficult and accounts for more than 20% of production cost (Davis et al., 2011). New systems using fixation of algae to biofilms, like rotating algal biofilm (RAB) systems, have mitigated many drawbacks and significantly increased the efficiency of algal biomass production by increasing the volume of algae produced, reducing energy inputs required, simplifying the harvesting processes, and using wastewater streams as nutrient sources (Gross et al., 2013, 2015).
There are few reports of research on wastewater-grown algae as a container-crop fertilizer. Mulbry et al. (2005, 2006) and Coppens et al. (2016) assessed algae grown during the treatment of animal waste effluents as fertilizer. Mulbry et al. (2005, 2006) grew seedlings of corn and cucumber (Cucumis sativa L.) with dried algal biomass, whereas Coppens et al. (2016) grew tomatoes with algae–bacteria flocs. Plants supplied with algae in these studies had increased growth and visual quality compared with unfertilized controls. However, there has been no work measuring the fertilizing effect of algae harvested from municipal wastewater treatment systems, nor as a fertilizer for containerized floricultural crops. Additional applied research with these materials therefore is warranted.
We evaluated the efficacy of wastewater-grown algae paste and pellets as fertilizers for containerized french marigolds, tomato, and sweet corn. These algal materials are easily incorporated into the substrates used in container-crop production, and we expected them to provide a slow release of nutrients for plant uptake. We hypothesized that both algae paste and algae pellets will serve as effective fertilizers, and that plants supplied with these materials would be similar in size and perceived health to those provided with synthetic fertilizers. Our specific objective was to compare the growth and perceived health of plants fertilized with wastewater-grown algal materials with those traits of plants fertilized with two commercially available fertilizers, a synthetic controlled-release fertilizer (CRF) and a bio-based wastewater treatment coproduct that supplied N in an organic form.
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