UC Riverside

Urban agricultural systems, like community and home gardens, may act as oases of biodiversity in cities dominated by impervious surfaces. They have also been shown to bridge gaps in food security and provide socio-cultural benefits. Despite these benefits, little research has been conducted that evaluates factors influencing garden plant biodiversity and ecosystem services (ES). Also less intensively researched are ecosystem disservices that gardens can contribute to, like gardener exposure to heavy metals. Urban soil can act as a sink for heavy metal contamination, which is mostly deposited through anthropogenic pollution. This dissertation addresses knowledge gaps about ES with two comprehensive surveys of garden biodiversity and ES production, one on community gardens in Los Angeles (LA), CA and one on home gardens across an urbanizing gradient in Beijing, China. It also addresses disservices and bioavailability of three heavy metals (lead, arsenic, and cadmium) through a soil survey and sequential analysis of heavy metals in LA gardens.

My main results indicate an overall shift in biodiversity from provisioning (food and medicinal production) to cultural (ornamental) services with increased gardener income and access to city resources (like grocery stores or markets) in both U.S. and Chinese gardens. This result supports a hierarchy of need, where gardeners preferentially plant species that support their most pressing needs, like food security. Urbanized regions in Beijing and immigrant-run gardens in Los Angeles also formed culturally distinct assemblages of edible species based on shared agricultural experiences. As the most common use for species was food, understanding metal bioavailability is important for accurate risk assessments. Lead, particularly in reducible form, increased the most with age of neighborhood, indicating oxidized lead paint buildup. Cd and As exchangeable fractions increased with proximity to road, indicating sources from air pollution. Finally, while Cd became less bioavailable with increased organic matter, reaction with organic humic acids released reducible As into the bioavailable fraction. These quantitative results can inform land managers about valued biodiversity and provisioning service from gardens in food insecure regions, as well as valuable information on how to predict metal accumulation hotspots and reduce plant uptake of metals.