Soil contamination in nearby natural areas mirrors that in urban greenspaces worldwide

Soil contamination poses a significant threat to the health and sustainability of ecosystems globally. This issue is particularly relevant to the United Nations Sustainable Development Goals, impacting areas such as good health and wellbeing, sustainable ecosystems and cities, and climate change regulation. Environmental stress from soil contamination, whether natural or anthropogenic, directly affects biodiversity and ecosystem functions, compromising ecosystem resilience to climate change and disasters. Urban soil contamination also negatively affects human health through various pathways, including contaminated drinking water and vapor intrusion of organic contaminants. Urban greenspaces, while serving as important recreational areas, are also exposed to contaminants from vehicle emissions, industrial processes, and poor waste management. It's generally assumed that urban greenspaces are more contaminated than adjacent natural ecosystems due to their proximity to human activity. However, contaminants like metal(loid)s, pesticides, microplastics, and antibiotic resistance genes (ARGs) can disperse through air, water runoff, and uncontrolled waste disposal, impacting surrounding natural areas. Some contaminants, such as high levels of metal(loid)s and ARGs, can also have natural origins and affect managed ecosystems. While previous studies have shown contaminant dispersion at local and regional scales, they often focus on single contaminants, limiting our understanding of the overall impact of multiple contaminants on both urban and natural ecosystems. A comprehensive global study is needed to assess the distribution and extent of human-driven soil contamination, considering both human and natural factors. Understanding the responses of soil functional activities, particularly soil microbes, to these contaminants is vital to predict the effects on ecosystem services. Soil microbes play critical roles in ecosystem function and respond rapidly to contamination, making them suitable indicators of both natural and anthropogenic contamination.

The existing literature demonstrates that soil contamination is a significant environmental issue, affecting human health and ecosystem services. Several studies have shown the dispersion of specific contaminants, such as metal(loid)s, pesticides, and microplastics, at local and regional scales. However, these studies often lack a global perspective and rarely compare multiple contaminants simultaneously in urban and natural ecosystems. The impact of diverse anthropogenic activities on soil quality has been highlighted in research. Studies on urban areas show that various sources, including vehicle emissions, industrial discharge, and agricultural practices, contribute significantly to soil contamination. These contaminants can then spread beyond urban areas, impacting nearby natural ecosystems through processes like atmospheric transport and surface runoff. Additionally, the literature emphasizes the importance of soil microbes in ecosystem functioning and their sensitivity to environmental stressors. This makes them a valuable indicator of soil health and potential responses to contaminants. However, the literature lacks a comprehensive overview of the interconnectedness of various contaminants and the resulting impact on microbial communities across a diverse range of ecosystems globally.

This study conducted a global standardized field survey of soil contamination, collecting surface soil samples from 56 paired urban greenspaces and adjacent natural areas across six continents. The paired design enabled a direct comparison of urban and natural areas while considering biogeographic and macroclimatic patterns. The study investigated four main categories of soil contaminants: eight heavy metals and metalloids (As, Cd, Cr, Cu, Pb, Hg, Ni, and Zn), 46 pesticide residues, microplastics (characterized by shape and polymer type), and 285 ARGs. While acknowledging that some contaminants have natural origins, the study explored the associations between natural factors (climate, plants, and soil properties) and human factors (population size and density, HDI, GDP, and urban greenspace management practices) to understand the extent of anthropogenic influence. The study also analyzed the link between co-occurrence of multiple soil contaminants and microbial functional attributes (genes related to stress resistance, nutrient cycling, and pathogenesis). To assess the global spread of contaminants, the study compared soil contaminant levels in urban greenspaces with those in three remote Antarctic ecosystems. The sampling involved establishing a 30m x 30m plot at each site and collecting three composite soil samples from different sub-plots to account for spatial heterogeneity. A total of 336 composite soil samples were collected, with a subset (64 for microplastics, 54 for pesticides) analyzed due to resource constraints. Environmental factors (climate, plant cover, population density, GDP, HDI, management practices) were also recorded. Soil chemical properties (pH, total carbon, nitrogen, phosphorus) were determined, and contaminants were analyzed using various techniques: ICP-OES and CVAFS for metal(loid)s, HPLC-MS/MS for pesticides, density separation and Raman spectroscopy for microplastics, and HT-qPCR for ARGs. Metagenomic analysis was conducted on a subset of samples to assess microbial functional traits. Statistical analyses, including response ratios, nested PERMANOVA, structural equation modeling (SEM), linear mixed-effects models, Spearman correlation analysis, and ordinary least squares linear regressions, were performed to analyze the data. The multi-contamination index was created by averaging standardized values of the four contaminant categories.

The study found remarkably similar levels of multiple soil contaminants in urban greenspaces and adjacent natural areas across continents. This similarity held true across all contaminant categories: metal(loid)s, pesticides, microplastics, and ARGs, even though levels of individual contaminants varied across locations. Significant correlations were found among different contaminant types. While urban greenspaces showed higher Hg, Zn, Cu, and Cr levels than natural areas, both showed similar levels of Pb, Ni, Cd, and As. Pesticide residues were widely detected in both ecosystems. Microplastics, predominantly polypropylene and polyester fibers, were ubiquitous and equally present in both, mirroring findings in remote Antarctica. ARGs were prevalent in both urban and natural areas; however, urban greenspaces exhibited greater ARG diversity. Human-associated factors, particularly population density, strongly correlated with the distribution of multiple soil contaminants. Population density was the most significant predictor for microplastics. A negative association was found between city wealth (GDP and HDI) and soil microplastics and metal(loid)s. Soil fertilization was a key factor influencing the abundance of pesticides and ARGs. The study also found strong associations between soil contaminant levels and microbial functional traits. Higher metal(loid) concentrations in urban greenspaces correlated with increased proportions of genes associated with metal(loid) resistance and multidrug resistance. A higher multi-contamination index was linked to changes in genes involved in stress resistance, nutrient cycling (negative correlation with P metabolism), and pathogenesis (positive correlation with Listeria pathogenicity and multidrug resistance). Pesticide levels were negatively correlated with genes involved in nutrient cycling and basic metabolisms. These findings confirm that the presence of multiple soil contaminants above certain thresholds has significant effects on the functional genes involved in soil health.

The findings directly address the research question by demonstrating the striking similarity in soil contamination between urban greenspaces and nearby natural areas on a global scale. This highlights the significant impact of human activities, extending far beyond the immediate urban environment. The strong correlation between human factors (especially population density) and soil contamination underscores the need for strategies to mitigate human-induced pollution. The observation of similar contaminant profiles in remote locations like Antarctica points to the global scale and long-range transport of contaminants through atmospheric deposition and potentially other vectors. The impact on soil microbial communities, affecting functions vital for ecosystem health, emphasizes the broader ecological implications of widespread soil contamination. These findings are relevant to various fields, including environmental science, ecology, public health, and urban planning. They provide a compelling case for integrating soil health into broader environmental and urban management strategies, emphasizing the interconnectedness of urban and natural systems.

This study presents a comprehensive global assessment of soil contamination in urban and natural areas. It confirms that human activities result in significant and widespread soil contamination, impacting both urban and natural ecosystems. This contamination affects microbial communities, altering essential ecosystem functions. Future research could explore other contaminant types, assess the bioavailability of contaminants, and investigate the long-term effects of contamination on ecosystem services. Implementing effective waste management practices, improving urban planning, and promoting sustainable land management are crucial steps in mitigating soil contamination and protecting both human and environmental health.

While this study represents a substantial advancement in understanding global soil contamination patterns, some limitations exist. The unequal sample sizes across different contaminant categories (due to analytical constraints) could influence the statistical power of certain analyses. The study may not encompass the full spectrum of all possible contaminants. While correlations are identified between contaminant levels and microbial functions, it's essential to conduct further studies to establish definitive causal relationships. The reliance on existing socio-economic data might not capture the finer-scale variations in human activities and their specific contributions to soil contamination. The study focuses primarily on surface soil, potentially overlooking contamination in deeper soil layers. Finally, the study highlights correlations, and additional experiments are needed to establish causality between contamination and microbial functionality.