A Review of Nutrient Losses to Waters From Soil- and Ground-Based Urban Agriculture—More Nutrient Balances Than Measurements

Urban agriculture has a high potential to contribute to local circular economies, for instance by using nitrogen, phosphorus, and potassium in city organic waste streams as fertilizer inputs. However, inefficient use of waste-derived fertilizers could contribute to local water quality impairment related to nitrogen and phosphorus losses. Organic waste derived fertilizers are particularly challenging from a nutrient stoichiometry perspective, making over- and under-application of a particular nutrient likely. Where, and under what conditions, urban agriculture acts as a net positive for a circular nutrient economy vs. a nutrient water quality risk remains unclear. Here we review empirical peer-reviewed studies (2000–2021) on soil- and ground-based urban agriculture with a stated concern for nutrient losses to water. Of the 20 publications retained and reviewed (out of 241 screened), only seven measured losses to waters. There were four experimental studies, of which three measured nutrient leachate losses under different garden management practices. Of the 16 studies done in real-world conditions, only four quantified losses to water as leachate; average losses spanned 0.005 to 6.5 kg ha−1 for phosphorus, and 0.05 to 140 kg ha−1 for nitrogen. 13 of the 16 non-experimental studies provided data on nutrient inputs and harvested crop outputs, which could be used to calculate garden nutrient balances—an indicator of nutrient use efficiency. Although the value ranges were large, most studied gardens showed nutrient surpluses (inputs > crop harvest) for nitrogen and phosphorus (but not potassium); these surpluses were identified as a risk for losses to water. Contextual factors such as different access to fertilizers and knowledge, along with regulations and environmental factors can help explain the wide range of balance values and nutrient losses observed. Although a large surplus of inputs was often linked to increased leachate losses, it was not always the case in the limited number of studies we identified. Our review suggests that more field studies that measure losses to waters, and document contextual factors, are needed to determine how urban agriculture may contribute to a sustainable circular economy for all three nutrients without nutrient-related water quality impairment.

A framework for optimizing hydrologic performance of green roof media

One of the primary functions of green roofs in urban areas is to moderate rainwater runoff, and one of the major impediments to the survival of plants on an extensive green roof (EGR) is a lack of available water during dry periods. Runoff moderation and water storage are both influenced by the composition of the growing media. Here we present a framework for evaluating the hydrologic performance of EGR growing media and also provide hydrologic attribute data for several commonly used EGR media constituents. In this three-phase study, we: 1) measured hydrologic attributes of individual EGR media constituents, 2) predicted attributes of media mixtures using individual constituent data, and 3) tested the seven top-ranking mixtures to evaluate hydrologic performance. Hydrologic attributes included wet weight and water held at maximum retentive capacity, long-term water retention, and hydraulic conductivity. Because perlite was light in weight yet held the greatest amount of water both at its maximum retentive capacity and in the long term, media mixtures dominated by perlite were predicted to have the best overall hydrologic performance. Mixtures dominated by pumice were also predicted to perform relatively well but were heavier. Despite the slightly greater weight and slightly lower performance, pumice may be a preferred alternative to perlite because perlite is a processed constituent with greater estimated embodied energy. Results indicate that performance of mixtures can be adequately predicted using performance of individual constituents for wet weight, water held, and long-term water retention. Hydraulic conductivity was less predictable because the pore volume in mixtures can be unrelated to the pore volume of the individual constituents. The framework presented here can be used to evaluate the performance of other EGR media, and the media attribute data can be used in formulating EGR media mixtures for specific applications. In addition, the attribute data can serve as a benchmark for evaluating other EGR media. Our results underscore the need for standardization of methods for more effective comparisons of EGR substrates, and also reinforce the need to evaluate EGR components using real-world scenarios.