Evaluating trace elements in urban forest soils across three contrasting New England USA towns and cities by pXRF and mass spectrometry.

Urban forest soils perform important ecosystem services, such as the sequestration of potentially toxic elements, but some can serve as a source for human exposure if ingested or eroded by wind. Assessment of trace elements in urban soils through traditional methods is costly but portable X-ray fluorescence (pXRF) can allow for rapid screening for hazardous concentrations. Our objectives were (1) evaluate trace elements (As, Cd, Cr, Cu, Pb, and Zn) at 460 locations in and around urban forests across three town and cities in New England and across multiple land-uses, (2) evaluate accuracy and precision of pXRF compared with traditional digestion-inductively coupled plasma analyses, (3) determine if pXRF measurements are improved by processing the soil (oven drying, rock fragment removal, and organic matter (OM) removal). Our findings show that urban forest soils have extensive Pb accumulations (42 out of 460 with >200 mg/kg), while As and Zn exceeded regulatory limits in limited outlier samples. Urban forest soils adjacent to multi-family residences have higher Pb and Zn concentrations than recreational and open lots. Fortunately, Cd, Cr, and Cu were far below hazardous concentrations. Our comparisons suggest pXRF measurements of Cu, Pb, and Zn can be considered accurate while Cd and Cr may be screened by pXRF. However, As should not be considered qualitative nor quantitative due to 70% type 1 (false positive) error on samples. While the accuracy and precision were nominally improved through soil drying, removal of rock fragments, and removal of OM (<5% decrease in relative error), our results show processing soil samples is likely not needed.

Nutrient and toxic elements in soils and plants across 10 urban community gardens: Comparing pXRF and ICP-based soil measurements.

Urban community gardens are becoming increasingly important to rehabilitate developed lands and combat the lack of access to fresh produce. Portable X-ray fluorescence (pXRF) offers a rapid, cost-effective method for assessing the elemental composition of soils but needs further study to determine its efficacy in urban agriculture. The objectives of this study were to evaluate if pXRF measurements of macronutrients (Ca, K, P), micronutrients (Cu, Mn, Zn), and toxic elements (As, Pb) generate results comparable with traditional soil analyses and if the soil measurements correlate with plant tissue concentrations at 10 community gardens across the eastern United States. From field-condition analyses of soils by pXRF and pseudototal digestions, we observed that both methods provide agreeable estimates of concentrations for some elements (Mn, Cu, Zn, Pb) but not for macronutrients (Ca, K, P). We hypothesize that low accuracy in pXRF measurements and macronutrients within silicates caused the poor agreement between the methods. Sieved and dried soil pXRF concentrations were in strong agreement with field-condition pXRF concentrations, suggesting rock removal and drying did not improve measurements. Our results highlight that pXRF can be an accurate and effective tool for screening for Mn, Cu, Zn, and Pb. Some elements, such as Pb in fruits; Mn, Cu, and Zn in leaves; and Zn and Pb in roots, could be estimated by soil pXRF or inductively coupled plasma-based analyses. Macronutrients were poorly estimated for fruits, leaves, and roots. Instead of soil concentrations, identifying genus-specific and garden-specific factors may be important for generating plant uptake predictive models.