HortScience (Jul 2023)
Runoff Water Quality from Different Urban Agricultural Systems Using Common Nutrient Management Practices
Abstract
In recent decades, as global population has continued to increase, so has the demand for food (Ackerman et al. 2014; Opitz et al. 2016). This demand is only projected to rise as not only the population increases, but also the percentage of inhabitants in urban areas increases as well (Lin et al. 2015; Opitz et al. 2016). This situation has led to many communities experiencing food insecurity, primarily in urban areas throughout the globe (Ackerman et al. 2014; Lin et al. 2015). It has been widely documented that low-income and disadvantaged communities have less access to nutritionally dense foods, and these areas of reduced access are often called food deserts (Lin et al. 2015; Opitz et al. 2016). Food insecurity and food deserts are now among the most pressing issues in US cities (Meenar and Hoover 2012). To address this rise in food demand, especially for nutritionally adequate food, various forms of urban agriculture have risen in popularity. Additional motivations, such as the desire for locally grown food, the fact that culturally important foods may not be available in grocery stores, the need to reduce inconveniences related to supply chain issues such as those seen during the coronavirus disease 2019 pandemic, and the environmental and health benefits of urban agriculture, are also contributing to its growth (Gunia 2020; Kuta 2021; Lin et al. 2015; McDougall et al. 2019; McGril 2021; Mok et al. 2014; Van Tuijl et al. 2018). Urban agriculture has many definitions, but can be described simply as the process of growing food crops, or ornamental and medicinal plants—and even raising livestock—within cities and towns (Goldstein et al. 2016; Lin et al. 2015; Opitz et al. 2016). Some of the more common types of urban agriculture include community gardens, backyard gardens, and rooftop gardens, which are also referred to as green roofs (Mok et al. 2014). Although hydroponic and other indoor systems are available, outdoor soil or soil-based media production methods remain among the most common forms of urban agriculture (Mok et al. 2014). Furthermore, controlled environment agriculture and vertical farming require a huge initial investment and energy, making these methods less sustainable and suitable for small-scale farmers in an urban setting than outdoor vegetable production such as raised beds or green roofs (Barbosa et al. 2015; McDougall et al. 2019). It was estimated that urban agriculture could fulfill ∼15% to 20% of the global food supply (Knizhnik 2012; Lin et al. 2015), reaching as high as 90% of local vegetable, egg, and milk needs, and 70% of poultry needs in some cities (Nugent 2002). Since the early 1990s, urban agriculture in the United States has grown by more than 30%, primarily in underserved communities (Lin et al. 2015). Urban agriculture could provide 7% to 8% of the current vegetable consumption in Oakland, CA, USA (McClintock et al. 2013) and 15% of the food supply in Sydney, Australia (McDougall et al. 2020), when unoccupied urban land areas are used for food production. Urban agriculture has also gained popularity in the past two decades because of greater public awareness and concern for carbon footprints. Environmental benefits of urban agriculture include supporting native biodiversity by providing food and habitat resources, mitigating air pollution and urban heat island effects, providing stormwater management, and lowering energy use required for food transport (Ackerman et al. 2014; Lin et al. 2015; Mok et al. 2014). Benefits of urban agriculture can also include community building, mitigation of childhood obesity and malnutrition, improved mental and physical health, and educational benefits to students (Bahamonde 2019; Colman 2017; Ghose and Pettygrove 2014; Lin et al. 2015; Meenar and Hoover 2012; Monroe 2015; Nogeire-McRae et al. 2018; Ohly et al. 2016; van Averbeke 2007). In particular, creating urban farms in low-income communities can revitalize these communities by promoting social cohesion and improving economic well-being (Angotti 2015). Benefits of urban agriculture to the environment have been widely documented; however, there is also the potential for some negative effects associated with the increased practice of urban agriculture. One of the most significant environmental concerns with any agricultural system is the transport of excess nutrients and other agriculture-associated contaminants into waterways (Berka et al. 2001; Hart et al. 2004; King and Torbert 2007; Kleinman et al. 2011). The use of conventional or manufactured fertilizer has been attributed to increased rates of nutrient runoff, especially when applied before periods of increased precipitation (King and Torbert 2007). The same applies to outdoor urban agriculture (Bachman et al. 2016; Lusk et al. 2020), especially in areas where management is switching from no or low fertilizer use to greater fertilizer use for crop production (Bachman et al. 2016; Janke et al. 2017; Spence et al. 2012), and in areas such as parking lots that lack surrounding vegetation (Hale et al. 2015; Shetty et al. 2019). In addition, the use and overuse of nutrient sources has been shown to contribute to nutrient runoff from both green roofs (Czemiel Berndtsson 2010; Mitchell et al. 2017; Toland et al. 2012) and ground-level systems (Cameira et al. 2014; Dewaelheyns et al. 2013; Huang et al. 2006; Salomon et al. 2020; Shrestha et al. 2020; Small et al. 2019; Wielemaker et al. 2018, 2019). These substances can alter and affect the water quality of runoff negatively, which can lead to the impairment or degradation of nearby aquatic systems as well as potential health hazards (Berndtsson et al. 2009). Nitrogen (N) and phosphorus (P) are the nutrients commonly found in fertilizers most associated with increased aquatic plant or algal growth and eutrophication risks (Anderson et al. 2002; Conley et al. 2009; Correll 1998; Smith and Schindler 2009). Nitrogen, which supports protein synthesis, and P, which is needed for DNA, RNA, and energy transfer, are needed by both terrestrial and aquatic plants (Conley et al. 2009). In excess, however, the presence of N and P can accelerate the growth of aquatic plants and harmful cyanobacteria (Conley et al. 2009). Anthropogenic eutrophication and dead zones are the number-one problem facing aquatic ecosystems globally, and can affect all types of aquatic systems (Kleinman et al. 2011; Smith and Schindler 2009). Nitrogen in reactive forms such as ammonium (NH4+) and nitrate (NH3−) can cause soil acidification if excess fertilizer is applied, which can lead to leaching of aluminum and other toxic metals into waterways (Chadwick and Chen 2002). Important physicochemical properties of water, such as pH, electrical conductivity, color, and turbidity, help to explain the quality of runoff water and its possible effect on aquatic life cycles (Rameshkumar et al. 2019; Whittinghill et al. 2016). Use of different growing media, sources of fertilizer, and crop management practices may have an effect on these criteria. A pH that is too high or too low can kill many aquatic species, affect hatching and survival rates, and stress the entire aquatic animal system (Freda 1987). Most aquatic animals prefer a pH of 6 to 9 (Collier et al. 1990). Changing pH levels (high or low) may also facilitate the solubilization of numerous harmful heavy metals, hence increasing the risk of absorption by aquatic organisms (Gensemer et al. 2018). Electrical conductivity is a measure of the salinity of water. Increasing the salt level in freshwater aquatic systems may increase the cost of water treatments for human consumption, reduce freshwater diversity, alter ecosystem function, and, ultimately, affect socioeconomic well-being by altering the goods and services of the freshwater aquatic system (Cañedo-Argüelles et al. 2016). Water color, measured on the platinum/cobalt (Pt/Co) scale, is usually used to analyze the pollution level in wastewater. Water with a yellow tint has more watercolor on the Pt/Co scale and is considered more polluted. In general, such color in water is a result of the humic and fulvic fractions of dissolved organic compounds (Bennett and Drikas 1993). Turbidity is a water-quality parameter that measures the optical clarity of water (Davies-Colley and Smith 2001). Increased turbidity means less solar radiation penetration. High turbidity influences aquatic life through a reduction in photosynthesis and dissolved oxygen, affects movement resulting from poor visualization, and kills fish directly or reduces their growth (Sader 2017). Turbidity could also be a good factor to predict aquatic life diversity and richness (Figueroa-Pico et al. 2020). There has been extensive research over many decades on the effects of conventional farming practices on excess nutrient input and pesticide contamination, and the associated impacts of these contaminants on the water quality of various aquatic ecosystems (Berka et al. 2001; Elrashidi et al. 2005; Gaudreau et al. 2002; Hart et al. 2004; Heathwaite et al. 1998; King and Torbert 2007; Kleinman et al. 2011; Liu et al. 2014; McLeod and Hegg 1984). Until recently, there was a lack of research on monitoring these same effects from commonly used urban agricultural systems at ground level. Research has focused more on the use of green roof systems as opposed to soil-based urban agriculture systems used at ground level (Beck et al. 2011; Czemiel Berndtsson 2010; Emilsson et al. 2007; Getter and Rowe 2006; Kok et al. 2013; Toland et al. 2012; Whittinghill et al. 2016). At ground-level urban sites, where use of soil fertility testing and nutrient application recordkeeping can be limited (Small et al. 2019; Whittinghill and Sarr, 2021; Wielemaker et al. 2019; Witzling et al. 2011), overapplication of nutrients is especially common. This has been tied to nutrient buildup in the soil (Abdulkadir et al. 2013; Cameira et al. 2014; de Barros Sylvestre et al. 2019; Dewaelheyns et al. 2013; Huang et al. 2006; Salomon et al. 2020; Shrestha et al. 2020; Small et al. 2019; Witzling et al. 2011) and an increase in the nutrient concentration of runoff water (Huang et al. 2006; Jackson et al. 1994; Shrestha et al. 2020). Excessive nutrient losses can also be attributed to a preference for compost as a nutrient source (Cameira et al. 2014; Dewaelheyns et al. 2013; Metson and Bennett 2015; Small et al. 2019; Wielemaker et al. 2019); the lower fertilizer nutrient equivalencies for composts the year of application, with continued nutrient release in subsequent years; and a difference in the availability of nutrients from compost (Maltris-Landry et al. 2016; Mikkelsen and Hartz 2008; Wielemaker et al. 2019). It has been suggested that these nutrient inefficiencies in urban agriculture could represent a significant component of the global P budget, if urban agriculture were scaled up to its full potential (Small et al. 2019). It is, therefore, imperative to understand more fully the effects of nutrient runoff from ground-level urban agriculture systems on water quality. Our research compared water-quality variables from four different nutrient sources applied to raised beds and container gardens. The container gardens used in this experiment were constructed from small plastic wading pools. This growing system has been used on rooftops (Hell’s Kitchen Farm Project 2020) and has received increased social media attention (Michaels 2021; Pinterest 2021). These small plastic pools are readily available during the growing season and can be purchased at a fairly low cost. When comparing cost per area, small plastic pools cost about $12 for 1 m2 of planting area, compared with almost $70 for raised beds with sides (Durham et al. 2018), ∼$40 to $80 for nursery pots (Hummert International 2021), and as much as $127 for other commercially available planters (Lowe’s 2021). This makes the pool growing system a low-cost option suitable for urban areas without soil or where soil is contaminated and considered unsafe for growing. However, this growing system has received very little scientific attention, especially in terms of how it may affect yields and nutrient leaching when compared with raised beds and other in-ground production systems. Including the small plastic pool growing system in our research is a first step in addressing this knowledge gap.
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