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Urban vegetable gardens provide an opportunity to recycle nutrients from food waste back into the human food system through the application of compost. However, a reliance on compost for soil fertility can lead to excess phosphorus (P) inputs that can build up in garden soil and potentially be exported via leachate or runoff. We report the results of a 7‐year experiment in a campus research garden in which replicated raised‐bed garden plots received manure‐based compost or municipal compost that was applied at a higher rate targeted to meet crop nitrogen demand or a lower rate targeted to meet crop P demand. Control plots received either no soil inputs or targeted synthetic fertilizer. Higher input treatments for both types of composts showed steadily increasing concentrations of soil plant‐available P, with a corresponding increase in leachate phosphate concentration. For both higher input compost treatments, approximately 30% of P added as compost was recovered in harvested crops over the 7‐year period, compared to >88% in the lower input compost treatments. In both high‐ and low‐input manure compost treatments, export of P as leachate accounted for approximately 10% of total P input, compared to 4% for the municipal compost. Over the 7‐year study period, P exported as leachate ranged from 0.8 g P/m²in the no‐input treatments to 6.5 g P/m²in the higher input manure compost treatments. These results show that tradeoffs are not inevitable as targeted compost applications can lead to high yield and low leachate export.

 

Among the ecosystem services provided by urban greenspace are the retention and infiltration of stormwater, which decreases urban flooding, and enhanced evapotranspiration, which helps mitigate urban heat island effects. Some types of urban greenspace, such as rain gardens and green roofs, are intentionally designed to enhance these hydrologic functions. Urban gardens, while primarily designed for food production and aesthetic benefits, may have similar hydrologic function, due to high levels of soil organic matter that promote infiltration and water holding capacity. We quantified leachate and soil moisture from experimental urban garden plots receiving various soil amendments (high and low levels of manure and municipal compost, synthetic fertilizer, and no inputs) over three years. Soil moisture varied across treatments, with highest mean levels observed in plots receiving manure compost, and lowest in plots receiving synthetic fertilizer. Soil amendment treatments explained little of the variation in weekly leachate volume, but among treatments, high municipal compost and synthetic fertilizer had lowest leachate volumes, and high and low manure compost had slightly higher mean leachate volumes. We used these data to parameterize a simple mass balance hydrologic model, focusing on high input municipal compost and no compost garden plots, as well as reference turfgrass plots. We ran the model for three growing seasons under ambient precipitation and three elevated precipitation scenarios. Garden plots received 12–16% greater total water inputs compared to turfgrass plots because of irrigation, but leachate totals were 20–30% lower for garden plots across climate scenarios, due to elevated evapotranspiration, which was 50–60% higher in garden plots. Within each climate scenario, difference between garden plots which received high levels of municipal compost and garden plots which received no additional compost were small relative to differences between garden plots and turfgrass. Taken together, these results indicate that garden soil amendments can influence water retention, and the high-water retention, infiltration, and evapotranspiration potential of garden soils relative to turfgrass indicates that hydrologic ecosystem services may be an underappreciated benefit of urban gardens.

 

Urban sustainability initiatives often encompass such goals as increasing local food production, closing nutrient loops through recycling organic waste, and reducing water pollution. However, there are potential tradeoffs among these desired outcomes that may constrain progress. For example, expansion of urban agriculture for food production may create hotspots of nutrient pollution if nutrient recycling is inefficient. We used gardener and urban farmer survey data from the Twin Cities Metropolitan Area (Minnesota, USA) to characterize phosphorus (P) and nitrogen (N) inputs and harvest in order to determine nutrient use efficiencies, and measured soil P concentrations at a subset of these sites to test whether excess soil P was common. All survey respondents (n = 142) reported using some form of soil amendment, with plant-based composts being the most common. Median application rates were 300 kg P/ha and 1400 kg N/ha. Median nutrient use efficiencies were low (2.5% for P, 5.0% for N) and there was only a weak positive relationship between P and N inputs and P and N harvested in crop biomass. Garden soils had a median Bray P value of 80 ppm, showing a buildup of plant-available P far exceeding recommended levels. Our results show that urban gardens are characterized by high nutrient inputs and inefficient conversion of these nutrients into crops, leading to buildup and potential loss of P and N from garden soils. Although urban gardens make up only 0.1% of land area in the Twin Cities, compost application to these urban gardens still constitutes one of the largest inputs of P to the watershed. In order to maximize desired outcomes from the expansion of urban agriculture (UA), it will be necessary to target soil amendments based on soil nutrient levels and crop nutrient demand.

 

An experiment was conducted from 2017-2023 at the University of St. Thomas research garden (Saint Paul, MN) to determine rates of nutrient recycling and loss from compost applied to urban gardens. Thirty-two 4 m2 study plots received one of six different soil amendment treatments, with four different crops growing on each plot. Meteorological data includes hourly measurements of rainfall, solar radiation, temperature and relative humidity, and wind speed and direction, from June 2017-October 2023. Hourly soil moisture measurements were recorded at depths of 10 cm, 20 cm, and 30 cm, from June-December 2021, June-October 2022, and June-October 2023. Annual crop harvest totals from each subplot are reported for 2017-2023. Leachate was collected from lysimeters installed in each of the 132 subplots weekly from June-October of each year (2017-2023), recording total volume. Leachate subsamples were analyzed for NO3-N, NH4-N, and PO4-P. Soil samples were collected at the beginning and end of the growing season in 2017, and every two weeks during the growing season from 2018-2023, and analyzed for pH, organic matter, Bray-1 extractable P, available K, nitrate, and ammonium, at the University of Minnesota Analytical Research Laboratory.