Relationship between soil heavy metal contamination and soil food web health in vacant lots slated for urban agriculture in two post-industrial cities

Abstract

Urban agriculture offers a means of obtaining healthy food while making use of vacant land in cities. However, soil contamination with heavy metals is a major concern for human health, food safety and soil food web health. In a previous study we reported on the human health risks of heavy metal contamination in soil in 43 vacant lots in two low-income neighborhoods, Hough and Weinland Park, located in two post-industrial cities, Cleveland and Columbus (Ohio, USA), respectively. In this study, we determined the relationship between heavy metal concentrations and the soil food web health using nematode community as a surrogate in the same lots. A general absence of higher trophic level omnivorous and predatory nematodes and an over-abundance of plant-parasitic nematodes indicated the disturbed nature of the urban soil food web. When compared to the USEPA’s Ecological Soil Screening levels (Eco SSLs) only Zn was established as a metal of concern for soil invertebrates which was also negatively associated with nematode Channel index in the multiple regression analysis. A combination of As, Cd, Cr, soil texture and organic matter were significant factors associated with nematode abundance and community indices in the regression analysis. Bacteria and fungus feeding nematodes in the Hough neighborhood were negatively correlated with As and positively with Cd and Cr; whereas plant parasitic nematodes were positively correlated with Cr, % silt and active carbon content, perhaps due to the influence of these heavy metals on the nematode food sources. In Weinland Park, neighborhood, channel index was negatively correlated to Cd and Zn concentration. Soil Pb and Zn concentrations that were elevated in these vacant lots beyond natural background concentrations did not show any significant associations with the nematode community; whereas the metals present within background concentration, i.e., As, Cd and Cr were correlated with the opportunistic bacteria and fungus feeding nematodes. Principal component analysis revealed different clustering of heavy metals, soil properties and nematode parameters in the two cities, indicating unique associations. Results showed that soil Cd and Cr concentrations were positively associated with the abundance of lower trophic level nematodes in both neighborhoods. Additionally, strong correlations between As, Cd, Cr, organic matter and soil texture influenced the nematode population densities and should be further explored.