Soil remediation and the threat of recontamination in urban agroecosystems
Lead (Pb) soil contamination is arguably the greatest challenge facing urban food production because soil Pb has some potential for plant uptake and can become airborne as Pb-rich aerosols. The most common strategy for mitigating exposure to Pb is raised-bed gardening, but beds may become recontaminated through atmospheric deposition of aerosols. Two complementary studies are being conducted in Illinois with objectives to 1) quantify the rate of Pb recontamination of raised-bed gardens across the Chicago metro region, and 2) to compare soil properties of raised-bed and direct soil production systems. To address objective 1, surface soils from six experimental gardens along an urban to rural transect in Chicago are sampled two times per year for Pb extraction, and potential plant uptake of Pb is quantified once using ion exchange resin stakes. To address objective 2, we established direct soil and raised-bed gardens (with compost only or a 1:1 compost and soil mix) and manage each bed with either synthetic or organic inputs. Soil chemical (e.g., available nutrients), physical (e.g., infiltration rate), and biological (e.g., respiration) data are collected along with weed and crop yield data.
In the first study, bulk soil Pb and simulated plant uptake of Pb after one season were greatest at an urban farm characterized by aging housing stock and proximity to major roads. In contrast, simulated Pb uptake was lowest in the most rural garden. In the second study, relative to direct soil, compost-only raised beds increased organic matter 766%, cation exchange capacity 119%, total nitrogen 607%, phosphorus 122%, potassium 346%, sulfate 4,040%, magnesium 233%, and sodium 576%. Soil pH in compost-only beds was 7.9 compared to 7.2 in direct soil. Alkaline soil pH may help reduce plant availability of Pb in soil. Infiltration rate in the compost-only raised beds was 106 mm min-1 compared to 33 and 3 mm min-1 in the compost + soil raised beds and direct soil, respectively. Thus, combining soil and compost in raised-beds may be useful in retaining soil water and reducing leaching and runoff from urban agriculture. Yield of radish and spinach was 63 and 194% greater in raised beds compared to direct soil. Preliminary results from these studies suggest that a mix of compost and soil in raised-beds may provide the greatest balance of crop productivity and environmental stewardship. However, raised-beds alone are not sufficient for eliminating the threat of heavy metal exposure in urban agriculture.