Potentially Toxic Substances and Associated Risks in Soils Affected by Wildfires: A Review

The presence of toxic substances is one of the major causes of degradation of soil quality. Wildfires, besides affecting various chemical, physical, and biological soil properties, produce a mixture of potentially toxic substances which can reach the soil and water bodies and cause harm to these media. This review intends to summarise the current knowledge on the generation by wildfires of potentially toxic substances, their effects on soil organisms, and other associated risks, addressing the effects of fire on metal mobilisation, the pyrolytic production of potentially toxic compounds, and the detoxifying effect of charcoal. Numerous studies ascertained inhibitory effects of ash on seed germination and seedling growth as well as its toxicity to soil and aquatic organisms. Abundant publications addressed the mobilisation of heavy metals and trace elements by fire, including analyses of total concentrations, speciation, availability, and risk of exportation to water bodies. Many publications studied the presence of polycyclic aromatic hydrocarbons (PAH) and other organic pollutants in soils after fire, their composition, decline over time, the risk of contamination of surface and ground waters, and their toxicity to plants, soil, and water organisms. Finally, the review addresses the possible detoxifying role of charcoal in soils affected by fire.

Mapping Risks Associated with Soil Copper Contamination Using Availability and Bio-Availability Proxies at the European Scale

Soil contamination by trace elements like copper (Cu) can affect soil functioning. Environmental policies with guidelines and soil survey measurements still refer to the total contents of Cu in soils. However, Cu contents in soil solution or free Cu contents have been shown to be better proxies of risks of Cu mobility or (bio)availability for soil organisms. Several empirical equations have been defined at the local scale to predict the amounts of Cu in soil solution based on both total soil Cu contents and main soil parameters involved in the soil/solution partitioning. Nevertheless, despite the relevance for risk assessment, these equations are not applied at a large spatial scale due to difficulties to perform changes from local to regional. To progress in this challenge, we collected several empirical equations from literature and selected those allowing estimation of the amount of Cu in solution, used as a proxy of available Cu, from the knowledge of both total soil Cu contents and soil parameters. We did the same for the estimation of free Cu in solution, used as a proxy of bioavailable Cu These equations were used to provide European maps of (bio-)available Cu based on the one of total soil Cu over Europe. Results allowed comparing the maps of available and bio-available Cu at the European scale. This was done with respective median values of each form of Cu to identify specific areas of risks linked to these two proxies. Higher discrepancies were highlighted between the map of bioavailable Cu and the map of soil total Cu compared to the Cu available map. Such results can be used to assess environmental-related issues for land use planning.

The Selective Effects of Environmental Change on the Functional Diversity of Soil Decomposers

Whether decomposition can be affected by the biodiversity of soil organisms is an important question. Biodiversity is commonly expressed through indices that are based on species richness and abundances. Soil processes tend to saturate at low levels of species richness. A component of biodiversity is functional diversity, and we have shown that the absence of the influence of species richness on decomposition switched into a positive relationship between fauna diversity and decomposition when we expressed biodiversity in terms of interspecific functional dissimilarity. Communities with functionally dissimilar species are characterized by complementary resource use and facilitative interactions among species. It is suggested that the effects of environmental changes on ecosystem functions such as decomposition can be better understood if we have more knowledge about the selective effect of these changes on specific facets of soil biodiversity, such as functional diversity.

Super-Small Predators in Soils: Who Are They and What Do They Do?

There are millions of species living in soils. Most of this biodiversity is made up of bacteria and fungi, tiny organisms that make up what is called the soil microbiome. The size and composition of the soil microbiome is mainly controlled by two groups of predators: protists and nematodes. Protists are tiny single-celled organisms, while nematodes are tiny worms and the most numerous animals on Earth. Protists and nematodes together weight more than all the other animals on Earth! Protists and nematodes keep the soil microbiome in balance, which helps plants to grow and keeps soils functioning properly. Without protist and nematode soil predators, the functions and services provided by soils would change so much that it could even affect the Earth’s climate. So, let us not forget the importance of these tiny soil organisms!

Pyrolyzed Substrates Induce Aromatic Compound Metabolism in the Post-fire Fungus, Pyronema domesticum

Wildfires represent a fundamental and profound disturbance in many ecosystems, and their frequency and severity are increasing in many regions of the world. Fire affects soil by removing carbon in the form of CO2 and transforming remaining surface carbon into pyrolyzed organic matter (PyOM). Fires also generate substantial necromass at depths where the heat kills soil organisms but does not catalyze the formation of PyOM. Pyronema species strongly dominate soil fungal communities within weeks to months after fire. However, the carbon pool (i.e., necromass or PyOM) that fuels their rise in abundance is unknown. We used a Pyronema domesticum isolate from the catastrophic 2013 Rim Fire (CA, United States) to ask whether P. domesticum is capable of metabolizing PyOM. Pyronema domesticum grew readily on agar media where the sole carbon source was PyOM (specifically, pine wood PyOM produced at 750°C). Using RNAseq, we investigated the response of P. domesticum to PyOM and observed a comprehensive induction of genes involved in the metabolism and mineralization of aromatic compounds, typical of those found in PyOM. Lastly, we used 13C-labeled 750°C PyOM to demonstrate that P. domesticum is capable of mineralizing PyOM to CO2. Collectively, our results indicate a robust potential for P. domesticum to liberate carbon from PyOM in post-fire ecosystems and return it to the bioavailable carbon pool.

Potential for Interspecies Toxicity Estimation in Soil Invertebrates

Interspecies correlation estimation (ICE) models are linear regressions that predict toxicity to a species with few data using a known toxicity value in a surrogate species. ICE models are well established for estimating toxicity to fish and aquatic invertebrates but have not been generally developed or applied to soil organisms. To facilitate the development of ICE models for soil invertebrates, a database of single chemical toxicity values was compiled from knowledgebases and reports that included 853 records encompassing 192 chemicals and 12 species. Most toxicity data for single chemicals tested in soil media were for species of earthworms, with only limited data for other species and taxa. ICE models were developed for eleven separate species pairs as least squares log-linear regressions of acute toxicity values of the same chemicals tested in both the surrogate and predicted species of soil organisms. Model uncertainty was assessed using leave one out cross-validation as the fold difference between a predicted and measured toxicity value. ICE models showed high accuracy within order (e.g., earthworm to earthworm), but less prediction accuracy in the two across-taxa models (Arthropoda to Annelida and the inverse). This study provides a proof-of-concept demonstration that ICE models can be developed for soil invertebrates.

Cessation of grazing causes biodiversity loss and homogenization of soil food webs

There is widespread concern that cessation of grazing in historically grazed ecosystems is causing biotic homogenization and biodiversity loss. Here, we used 12 montane grassland sites along an 800-km north-south gradient across the United Kingdom, to test whether cessation of grazing affects local ɑ- and β-diversity of belowground food webs. We show that cessation of grazing leads to strongly decreased ɑ-diversity of both soil microbial and faunal diversity. In contrast, the β-diversity varied between groups of soil organisms. While soil microbial communities exhibited increased homogenization after cessation of grazing, we observed decreased homogenization for soil fauna after cessation of grazing. Overall, our results indicate that grazer exclusion from historically grazed montane grasslands has far-ranging consequences for the diversity and composition of belowground food webs, and underscore the importance of grazers for maintaining the diversity of belowground communities, which play a central role in ecosystem functioning.

Soil Biodiversity as a Key Sponsor of Regenerative Agriculture

Increasing knowledge and literacy around soil biodiversity is essential to discover and implement biological solutions for the discouraging challenges people face in agriculture and human wellbeing. Therefore, this review was done to get an insight into the awareness and understanding of the contribution of soil biodiversity to regenerative agriculture. The review was done by referring to the latest different research findings; reports, working guidelines, as well as knowledge shared from different soil biodiversity conferences and webinar discussion points. The review disclosed that to meet the increasing demand for food for the ever-increasing global population and the 2030 sustainable development goals, regenerating the already degraded lands through regenerative agriculture principles and practices is vitally important. The findings and report documents showed that soil biodiversity facilitates the regenerative agriculture system as soil organisms are using as soil health improvement machines, a remediates for soil and water pollution, a fertilizer, pesticide, as a means of carbon sink, and used in the pharmaceutical industry to discover new drugs and vaccines for animal and human health. Moreover, the meta-analysis publicized that the consideration and use of soil biodiversity in the regenerative agriculture system have promising results although little is known about the role of those soil organisms in the ecosystem due to the presence of knowledge gap and complexity of relationships in the soil system. Therefore, furthermore, attention should be given to the discoveries of soil biodiversity to use them as a natured based solution for regenerative agriculture in the 21st century and to meet the 2030 sustainable development goals.

Disentangling nematode and arbuscular mycorrhizal fungal community effect on the growth of range-expanding Centaurea stoebe in original and new range soil

Aims Numerous organisms show range expansions in response to current climate change. Differences in expansion rates, such as between plants and soil biota, may lead to altered interactions in the new compared to the original range. While plant-soil interactions influence plant performance and stress tolerance, the roles of specific soil organisms driving these responses remain unknown. Methods We manipulated the abundances of nematodes and arbuscular mycorrhizal fungi (AMF), collected from original and new range soils, and examined their effects on the biomass of range-expanding Centaurea stoebe and native Centaurea jacea. In the first approach, nematode and AMF communities were extracted from field soils, and inoculated to sterilized soil. In the second approach, the abundance of soil organisms in soil inocula was reduced by wet sieving; at first, plants were grown to condition the soil, and then plant-soil feedback was determined under ambient and drought conditions. Results The origin of soil communities did not influence the biomass production of range-expanding or native plant species, neither by addition nor by (partial) removal. However, after conditioning and under drought, range expanding C. stoebe produced more biomass with soil communities from the original range while C. jacea, native to both ranges, produced more biomass with new range soil communities. Conclusions We show that nematode and AMF communities from original and new range have similar effect on the growth of range expanding C. stoebe. Our results highlight that the effect of soil communities on plant growth increases after soil conditioning and under drought stress.