Special section on soil and human health - An editorial

Humans have long recognized the links between soil and human health, but movement towards the scientific study of these links is a relatively new undertaking (Brevik & Pereg, 2017; Oliver & Gregory, 2015). This may be due in part to the fact that the relations between soil and human health are complex and that many people worldwide do not eat locally produced food. Because the relations are often obscure there is a lack of widespread public interest in the topic, also undoubtedly contributed to by the relative lack of research in this area (Brevik et al., this issue). There is already much that we know regarding soil and human health. Soil supplies nutrients for the human diet through the soil–plant system, leads to exposures to pathogens and various chemical species both organic and inorganic, purifies water, provides medicines, influences climate and affects human health in many other ways, both direct and indirect. Several recent reviews have highlighted these links (e.g. Cakmak & Kutman, 2018; Li et al., 2018; Oliver & Gregory, 2015; Steffan et al., 2018; Stockamp & Thompson, 2016; Wall et al., 2015; Xie et al., 2018). Nevertheless, many questions still remain. This special section of the European Journal of Soil Science (EJSS) was undertaken in an effort to promote better understanding of this important topic. The special section had its origin in Soil and Human Health sessions organized by Lily Pereg and Eric Brevik at recent professional meetings. The first was at the 2017 European Geosciences Union meeting in Vienna, Austria (organized with assistance from Johan Six), and the second was at the 2018 World Congress of Soil Science in Rio de Janeiro, Brazil. Authors who presented in either session were given the opportunity to submit full manuscripts to EJSS with the organizers as guest editors (L. Pereg being the editorial lead), and most of the papers in this special section began in one of these sessions. The special section includes two original research papers, two review papers and a position paper. Martínez-Martínez et al. (this issue) investigated the suitability of phytostabilization aided by soil-building techniques to reclaim a mining area with soil of very low productivity that was contaminated with a number of heavy metals and metalloids that could have potentially negative effects on human health. The techniques investigated improved several aspects of soil health and reduced the availability of contaminants by as much as 99%. In addition, most of the plant species took up small amounts of metal(loid)s or concentrated those that they did take up in their root systems. Martínez-Martínez et al. (2019) concluded that their amendments were successful in transforming the low-productivity soil into one that would support a thriving soil ecological system, which in turn enhanced phytostabilization of the contaminants at the study site. This would, in turn, provide benefits to human health. Oliveira et al. (this issue) were interested in the possibility that the application of poultry litter to soil as an organic amendment might increase the populations of colistin-resistant bacteria in soil. This is a concern because colistin is currently the antibiotic of last resort in the treatment of multidrug-resistant bacteria, but it is also commonly used in poultry feed to prevent infection and promote growth. To test their concern, Oliveira et al. (2019) compared the abundance of the mcr-1 gene (the colistin resistance gene) in soil amended with uncomposted poultry litter and used for intensive vegetable production to its abundance in nearby areas with native vegetation in Brazil. They found the mcr-1 gene in all the locations studied, both those amended with poultry litter and natural soils. Although the smallest mcr-1 count was found at one of the natural vegetation sites, large counts occurred at both amended and natural sites. Oliveira et al. (2019) concluded that it is important for us to gain a better understanding of the behaviour and persistence of antibiotic-resistant genes in the soil environment. It is also the first paper to report on the presence of the colistin resistance gene in Brazil's soil. Winter & Pereg (this issue) reviewed what we know about soil, mycotoxins and their links back to human health. Aspergillus spp. fungi have the ability to produce secondary metabolites known as mycotoxins, which can reach toxic levels in human food products. Many aspects of our food production system can create conditions that lead to the presence of mycotoxins in food products, including animal feed practices, storage of food products and the regulatory system. Our food production starts mainly in the soil and mycotoxins can be introduced into food products from soil. Therefore, understanding the soil ecology of Aspergillus spp. and soil–fungal–plant interactions is an important part of understanding the threat that mycotoxins pose to human health. The Winter & Pereg (2019) review analyses each stage in this relationship. Palmer et al. (this issue) reviewed clostridial pathogens found in agricultural soil. Clostridium is a genus of bacteria that causes a number of human diseases including botulism, gas gangrene and tetanus. Palmer et al. (2019) investigated the distribution and abundance of clostridial pathogens in soil as well as environmental and soil factors that might influence the bacteria's behaviour. Clostridial pathogens have been documented in soils worldwide, and the amendment of soil with manure and organic wastes was identified as a major route of bacteria introduction to the soil. High soil water content was identified as a factor that probably enhanced bacterial survival, but better understanding of the organisms' soil ecology was suggested as a needed future research direction. The incorporation of modern molecular and sequencing techniques into future studies was also recommended. Palmer et al. (2019) concluded that understanding clostridial pathogen amount, distribution and behaviour in soil was important because agricultural soil is the first control point in the food contamination pathway. The position paper by Brevik et al. (this issue) argues that we need to find ways to connect people to soil as one avenue to improving human health. They go through three primary avenues by which this might be achieved, using the concepts of terroir, soil health and soil security. In each case, strengths and weaknesses of each approach are covered. For example, a strength of terroir and soil health is that they both already have populations that accept them, compared to soil security, which is a relatively new idea. However, there are already concerns that moving the terroir concept to other food products is diluting it, and soil–health relationships are not a product with a location of production such as a variety of wine, coffee, and so on. An advantage of soil security over soil health is that soil security is broader, and it attempts to tap into the widespread acceptance of concepts such as food security, energy security and water security. A major benefit of using soil health is that it is already accepted by many non-scientists, including farmers and policy makers. Brevik et al. (2019) conclude by suggesting that social marketing campaigns focused on soil health or soil security, or both, are a potential route to connect people to soil, and they give examples of some nascent soil health social marketing campaigns that have already started. Unfortunately, Lily Pereg did not live to see this project completed. She was murdered during a trip to Argentina in January 2019 as we were working on the final aspects of this special section. Lily was a joy to be around, always smiling and friendly with kind words for all (Figure 1). She made meaningful contributions to soil science in many ways, including her published research, editing for journals (including guest editor of this special section), volunteer leadership for professional organizations, and her teaching and mentoring of both her students and colleagues. Lily was always willing to share her knowledge and lend a helping hand. She trained as a microbiologist and brought her interests to bear on issues related to soil science, including nitrogen fixation and assimilation and the ways that soil microorganisms could improve crop growth. But her interests went far beyond what one might call classical soil microbiology. Lily was actively working on areas such as soil and human health, including its historical aspects, and she was actively discussing plans to investigate how soil intersected with Australian Aboriginal and North American Native American cultures. She was very interested in the inter- and transdisciplinary aspects of soil science, and also worked in aquatic microbial ecology (which had links to soil microbial ecology). Together with her work on this EJSS special section, Winter & Pereg (2019) will, sadly, be one of Lily's final contributions to our field. She should still be contributing to so many projects that she had in the planning stages with friends and colleagues around the world, and we all miss her!