Investigating potential hydrological ecosystem services in urban gardens through soil amendment experiments and hydrologic models

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.

Optimisation of a Numerical Model to Simulate the Dispersion and Chemical Transformations Within the Oxides of Nitrogen/Ozone System as Traffic Pollution Enters an Urban Greenspace

Urban greenspace has many health benefits, including cleaner air than the surrounding streets. In this study, a detailed exercise has been conducted to measure concentrations of NO/NO2/NOx and O3 within an urban greenspace, the University of Birmingham campus, using continuous analysers, as well as transects of NO2 measured with diffusion tubes. Concentrations have been simulated using the ADMS-Roads model which has been optimised initially using NOx concentrations for traffic emissions on surrounding roads, background concentrations, and meteorological data considering four candidate sites. Optimisation for prediction of NO2 shows the critical importance of the NO2:NOx ratio in traffic emissions, for which a derivation from atmospheric measurements is consistent with a value derived from optimisation of the model fit to roadside data. After optimisation, the model gives an excellent fit to continuous data measured at roadside. Comparison of model predictions with transects of NO2 across the greenspace also show generally good model performance. The incorporation of dry deposition processes for the nitrogen oxides into the model leads to a reduction of less than 1% in predicted concentrations, leading to the conclusion that the cleaner air within urban greenspace is primarily the result of dispersion rather than deposition processes.