Soil Hotspots Research Articles

Anthrax in the South of Russia

Objective: analysis of the current epizootological and epidemiological situation on anthrax and characteristic of stationary hazardous for this infection areas (SHAs) in the South of Russia.Materials and methods: we used data on SHAs and anthrax burials from the Rospotrebnadzor Directorates for the subjects of the Southern (SFD) and North Caucasus (NCFD) federal districts, regional Veterinary Directorates, official data from Rospotrebnadzor and Rosselkhoznadzor on morbidity in humans and livestock.Results: anthrax is almost ubiquitous in the regions of the SFD and NCFD. According to data updated in 2023, in the South of Russia, 3845 SHAs were taken into account, in which 9815 outbreaks were registered. In 1999-2023 92 SHAs showed activity, 23 of which are new. There are 350 anthrax burials located in the SHA’s territories. Over 25 years, there were 104 outbreaks affecting at least 269 livestock and 134 people, with 4 deaths. Human infection occurred mainly during the forced slaughter of sick cattle, unvaccinated against anthrax, through contact with contaminated meat.Conclusion: the epidemiological and epidemiological situation on anthrax in the South of Russia continues to remain unstable. The problem is due to the presence of a significant number of soil hotspots against the backdrop of incomplete implementation of preventive measures. Stabilization of the anthrax situation can only be achieved by implementing a full range of preventive and anti-epidemic measures in the format of interdepartmental interaction.

Primary carbon sources and self-induced metabolic landscapes shape community structure in soil bacterial hotspots

Direct observations of the complex and highly dynamic metabolic landscapes that affect the structure and functioning of bacterial communities in natural soil are limited by soil opacity and pore space complexity. In this study, we employ community metabolic network models to predict how the emerging bacterial community composition alters its structure and composition as a function of the primary carbon source in both well-mixed and spatially explicit systems. We provide systematic, experimental validation using a synthetic community comprised of four bacterial species grown on prescribed carbon sources and quantify their abundance using qPCR. Results suggest that community members may benefit from trophic interactions or suffer from increased competitive stress depending on the carbon source. The modeling is expanded to consider interactions in soil-like spatial context. Results show emergence of distinct bacterial community compositions as a function of primary carbon sources typical to soil hotspots (carbohydrates or organic acids). The ability to link genetic information with bacterial community trophic interactions in representative soil environments is a critical first step towards attaining predictability of soil ecological functioning.

Heavy metal load and effects on biochemical properties in urban soils of a medium-sized city, Ancona, Italy

Urban soils are often mixed with extraneous materials and show a high spatial variability that determine great differences from their agricultural or natural counterparts. The soils of 18 localities of a medium-sized city (Ancona, Italy) were analysed for their main physicochemical and biological properties, and for chromium (Cr), copper (Cu), cobalt (Co), lead (Pb), nickel (Ni), zinc (Zn), and mercury (Hg) total content, distribution among particle-size fractions, and extractability. Because of the absence of thresholds defining a hot spot for heavy metal pollution in urban soils, we defined a “threshold of attention” (ToA) for each heavy metal aiming to bring out hot spot soils where it is more impellent to intervene to mitigate or avoid potential environmental concerns. In several city locations, the soil displayed sub-alkaline pH, large contents of clay-size particles, and higher TOC, total N, and available P with respect to the surrounding rural areas, joined with high contents of total heavy metals, but low availability. The C biomass, basal respiration, qCO2, and enzyme activities were compared to that detected in the near rural soils, and results suggested that heavy metals content has not substantially compromised the soil ecological services. We conclude that ToA can be considered as a valuable tool to highlight soil hot spots especially for cities with a long material history and, for a proper risk assessment in urban soils, we suggest considering the content of available heavy metals (rather than the total content) and soil functions.

The microplastisphere: Biodegradable microplastics addition alters soil microbial community structure and function

Plastics accumulating in the environment, especially microplastics (defined as particles <5 mm), can lead to a range of problems and potential loss of ecosystem services. Polyhydroxyalkanoates (PHAs) are biodegradable plastics used in mulch films, and in packaging material to minimize plastic waste and to reduce soil pollution. Little is known, however, about the effect of microbioplastics on soil-plant interactions, especially soil microbial community structure and functioning in agroecosystems. For the first time, we combined zymography (to localize enzyme activity hotspots) with substrate-induced growth respiration to investigate the effect of PHAs addition on soil microbial community structure, growth, and exoenzyme kinetics in the microplastisphere (i.e. interface between soil and microplastic particles) compared to the rhizosphere and bulk soil. We used a common PHAs biopolymer, poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and showed that PHBV was readily used by the microbial community as a source of carbon (C) resulting in an increased specific microbial growth rate and a more active microbial biomass in the microplastisphere in comparison to the bulk soil. Higher β-glucosidase and leucine aminopeptidase activities (0.6–5.0 times higher Vmax) and lower enzyme affinities (1.5–2.0 times higher Km) were also detected in the microplastisphere relative to the rhizosphere. Furthermore, the PHBV addition changed the soil bacterial community at different taxonomical levels and increased the alpha diversity, as well as the relative abundance of Acidobacteria and Verrucomicrobia phyla, compared to the untreated soils. Overall, PHBV addition created soil hotspots where C and nutrient turnover is greatly enhanced, mainly driven by the accelerated microbial biomass and activity. In conclusion, microbioplastics have the potential to alter soil ecological functioning and biogeochemical cycling (e.g., SOM decomposition).

How “hot” are hotspots: Statistically localizing the high-activity areas on soil and rhizosphere images

The topic of microbial hotspots in soil requires not only visualizing their spatial distribution and biochemical analyses, but also statistical approaches to identify these hotspots and separate them from the surrounding activities (background). We hypothesized that each hotspot type (e.g. enzyme activities in the rhizosphere, root exudation, localization of herbicide accumulation) is a result of local process driven by biotic and/or abiotic factors, and the process rates in the hotspots are much faster than those in the soil background. We further hypothesized that the background and hotspot activities in soil belong to different statistical distributions. Consequently, hotspot determination should be based on statistical separation of activities significantly higher than the background. We analyzed for the statistical distributions of grey values on three groups of published images: 1) 14C images of carbon input by roots into the rhizosphere, 2) 14C glyphosate accumulation in the plant, and 3) zymogram of leucine aminopeptidase activity in rooted soil. The two Gaussian distributions were fit (the first representing the background, the second the hotspots) to the distribution of grey values in the images, the parameters (means and standard deviations, SD) of the fitted distributions were calculated, and the background was removed. Thus, we identified hotspots as areas outside of the Mean+2SD image intensity (corresponding to the upper ~ 2.5% of activity, being over 97.5% of background values) and finally, visualized images of solely hotspot locations. Finally, these results were compared with previously used decisions on hotspot intensity thresholding (i.e. Top-25% and 17 standard thresholding approaches in ImageJ) and discussed the advantages of the Mean+2SD as well as Mean+3SD approaches. These advantages include: i) simple unification of the thresholding approach for several imaging methods with various principles of activity distribution, ii) identification of hotspots with various activity levels, iii) analysis of “time-specific” hotspots in temporal sequences of images. Compared with 17 standard thresholding methods, we concluded that objectively elucidating and separating the hotspots should be based on statistical distribution analysis, e.g. using the Mean+2SD or Mean+3SD approaches. This simple Mean+2SD approach delivered suitable results for three groups of images and so, helps to understand the processes responsible for the highest activities and elucidate hotspots.

The Long-Term Effect of Ongoing Spruce Decay on Carbon Exchange in Taiga Forests

The long-term effect of spruce stand decay on the CO2 balance was studied in the taiga of Valdai (Russia, Novgorod oblast). The CO2 emission from soil, the respiration of coarse wood debris, the total respiration, and the net fluxes of CO2 and water vapor were evaluated. In areas of dead spruce forest affected by xylophages and saprotrophs, as well as by windfall, a noticeable decrease in specific gross respiration is absent. This is related to the fact that reduced CO2 respiration of living trees is compensated by emission from coarse wood debris and a sharp (3- to 3.5-fold) and long-term (no less than six years long) rise in soil respiration (hotspots) under dead-standing spruce trees. The contribution to carbon emission from soil hotspots per unit area in dead spruce stands under dry standing trees is greater than the role of decomposition of coarse wood debris (14.4 and 9%, respectively). The observation period is characterized by a significant decrease in the annual net CO2 sink in the area studied: from –300 to –95 g C m–2 year–1, which is followed by reduced evapotranspiration (0.0116 g H2O m–2 s–1 in May–October 2018 as compared to 0.0142 g H2O m–2 s–1 in the same period of 2010). We assume that the observed decrease in carbon uptake is related both to the reduction in the primary spruce productivity and to an increase in gross respiration from larger areas of decomposed spruce trees. The calculations show that if the area of the dead spruce stands is further increased to 27%, the resulting net balance of the territory will switch to a CO2 source for the atmosphere. This should be taken into account for remote forecasting.

Thermal desorption as ahigh removal remediation technique for soils contaminated with per- and polyfluoroalkyl substances (PFASs).

Soils contaminated with per- and polyfluoroalkyl substances (PFASs) are an important source for impacting drinking water delivery systems and surface water bodies world-wide, posing an urgent risk to human health and environmental quality. However, few treatment techniques have been tested for PFAS-contaminated soil hotspots. This study investigated the possibility of thermal desorption as a possible technique to remediate soils contaminated with multiple PFASs. Two fortified soils (∑9PFAS ≈ 4 mg kg-1) and one field-contaminated soil (∑9PFAS ≈ 0.025 mg kg-1) were subjected to a 75-min thermal treatment at temperatures ranging from 150 to 550°C. Soil concentrations of PFASs showed a significant decrease at 350°C, with the ∑9PFAS concentration decreasing by, on average, 43% and 79% in the fortified and field contaminated soils, respectively. At 450°C, >99% of PFASs were removed from the fortified soils, while at 550°C the fraction removed ranged between 71 and 99% for the field contaminated soil. In the field contaminated soil, PFAS classes with functional groups of sulfonates (PFSAs) and sulfonamides (FOSAs) showed higher removal than the perfluoroalkyl carboxylates (PFCAs). Thus thermal desorption has the potential to remove a wide variety of PFASs from soil, although more studies are needed to investigate the cost-effectiveness, creation of transformation products, and air-phase vacuum filtration techniques.

An iron-dependent burst of hydroxyl radicals stimulates straw decomposition and CO2 emission from soil hotspots: Consequences of Fenton or Fenton-like reactions

Plant residues in soil create temporal and spatial hotspots of extremely high microbial activities leading to very intensive greenhouse gas (GHG) fluxes that challenge our mechanistic understanding and predictive power. Using a series of well-controlled soil microcosm experiments, we examine how abiotic processes (e.g., iron reduction-oxidation cycling) at residue/soil interfaces contribute to hotspot dynamics. We quantify for the first time the contributions of microbially-initiated Fenton reactions, which produce strongly oxidizing hydroxyl radicals (HO), to organic matter solubilization and mineralization in hotspots 0–3 mm from the litter surface. The concentrations of ferrous iron (Fe2+), hydrogen peroxide (H2O2) and HO were 2.1–3.0, 3.0–9.0 and 2.6–2.8 times higher, respectively, at the straw-soil interface than in the bulk soil. Thus, iron minerals, especially in concert with microorganisms, produce a burst of hydroxyl radicals that explain extremely high GHG fluxes from soil hotspots. Our findings highlight how Fe minerals and microorganisms synergistically influence global carbon cycling. These synergistic effects should be accounted for mechanistic understanding of processes in soil hotspots.

Soil protist communities in burrowing and casting hotspots of different earthworm species

Biotic interactions in the soil food web have been increasingly recognized as crucial for ecosystem functioning, however there are still knowledge gaps in terms of the associations between macrofauna and microorganisms, particularly in soil hotspot microsites. A manipulative study was performed to investigate the protist community in the drilosphere and casts of three earthworm species (epigeic Amynthas cortices, anecic Amynthas hupeiensis and endogeic Drawida gisti) and a control soil without earthworms. Results showed that earthworm presence significantly increased the relative abundance of parasitic protists (p < 0.05). The diversity of protists within drilosphere and casts depended on earthworm species, with D. gisti and A. hupeiensis increasing diversity and A. cortices decreasing protist diversity (p < 0.05). Structural equation modelling indicated that protist abundance and diversity in the drilosphere were regulated indirectly via neutralized soil pH and accelerated C and N mineralization, while in casts were affected directly through earthworm gut processing and indirectly through elevated water content. In conclusion, the marked effects on soil protist community mediated by earthworms varied from distinct earthworm species and microsites. The present study suggests the necessity to consider interactions between protist and earthworms in evaluating the effects of soil fauna on soil biota and the related ecosystem processes.

Physical constraints for respiration in microbial hotspots in soil and their importance for denitrification

Abstract. Soil denitrification is the most important terrestrial process returning reactive nitrogen to the atmosphere, but remains poorly understood. In upland soils, denitrification occurs in hotspots of enhanced microbial activity, even under well-aerated conditions, and causes harmful emissions of nitric (NO) and nitrous oxide (N2O). The timing and magnitude of such emissions are difficult to predict due to the delicate balance of oxygen (O2) consumption and diffusion in soil. To study how spatial distribution of hotspots affects O2 exchange and denitrification, we embedded microbial hotspots composed of porous glass beads saturated with growing cultures of either Agrobacterium tumefaciens (a denitrifier lacking N2O reductase) or Paracoccus denitrificans (a “complete” denitrifier) in different architectures (random vs. layered) in sterile sand that was adjusted to different water saturations (30 %, 60 %, 90 %). Gas kinetics (O2, CO2, NO, N2O and N2) were measured at high temporal resolution in batch mode. Air connectivity, air distance and air tortuosity were determined by X-ray tomography after the experiment. The hotspot architecture exerted strong control on microbial growth and timing of denitrification at low and intermediate saturations, because the separation distance between the microbial hotspots governed local oxygen supply. Electron flow diverted to denitrification in anoxic hotspot centers was low (2 %–7 %) but increased markedly (17 %–27 %) at high water saturation. X-ray analysis revealed that the air phase around most of the hotspots remained connected to the headspace even at 90 % saturation, suggesting that the threshold response of denitrification to soil moisture could be ascribed to increasing tortuosity of air-filled pores and the distance from the saturated hotspots to these air-filled pores. Our findings suggest that denitrification and its gaseous product stoichiometry depend not only on the amount of microbial hotspots in aerated soil, but also on their spatial distribution. We demonstrate that combining measurements of microbial activity with quantitative analysis of diffusion lengths using X-ray tomography provides unprecedented insights into physical constraints regulating soil microbial respiration in general and denitrification in particular. This paves the way to using observable soil structural attributes to predict denitrification and to parameterize models. Further experiments with natural soil structure, carbon substrates and microbial communities are required to devise and parametrize denitrification models explicit for microbial hotspots.

Stabilization of per- and polyfluoroalkyl substances (PFASs) with colloidal activated carbon (PlumeStop®) as a function of soil clay and organic matter content

The global problem of contamination of drinking water sources and the aquatic environment with per- and polyfluoroalkyl substances (PFASs) originating from highly contaminated soils is addressed in this study. For the first time, a colloidal activated carbon (AC) product (PlumeStop®) was systematically assessed for PFASs stabilization in soil. Colloidal (particle size 0.1–1.1 μm) AC has the advantage that field application is non-intrusive, comprising injection under high pressure in situ at PFAS-contaminated soil hotspots. In the assessment, 10 different soil mixtures with gradually increasing organic carbon and clay fractions were spiked with 18 different PFASs of varying perfluorocarbon chain length and four different functional groups and aged for one year. Equilibrium leaching tests showed that the ability of colloidal AC to increase sorption of PFASs to soil was highly dependent on PFAS perfluorocarbon chain length. The best treatment efficiency was observed for perfluorocarbon chain lengths 6–7 at which colloidal AC resulted in sorption of 81%, 85%, and 86% for perfluorooctanoate (PFOA), 6:2 fluorotelomer sulfonate (6:2 FTSA) and perfluorohexane sulfonate, (PFHxS), respectively. Sorption of individual PFASs decreased significantly (p < 0.05) with increasing organic carbon content in soil treated with colloidal AC indicating stearic hindrance of the ACs pore structure. On the other hand, the sorption of the majority of PFASs increased significantly (p < 0.05) with increasing clay content in colloidal AC-treated soil, which can be explained by increase in surface area that colloidal AC can sorb to. Overall, the results indicate that the colloidal AC product tested can be useful in remediation approaches for certain PFASs under specific field conditions and PFAS contamination.

Dairy soil bacterial responses to nitrogen application in simulated Italian ryegrass and white clover pasture

Through clearing and use of fertilizer and legumes, areas of southwestern Australia's unique coastal sand plains can support relatively low-cost dairies. However, the ancient, highly weathered nature of the soils in this region makes the dairies susceptible to a range of threats, including nutrient leaching and erosion. Despite this, Western Australian dairy cows typically produce up to 5,500 L of milk per head annually supported by inorganic nitrogen (N) fertilizer (commonly 50:50 urea and ammonium sulfate) at rates up to <320 kg of N/ha per year. Where hotspots exist (up to 2,000 kg of N/ha per year), total N exceeds pasture requirements. We investigated plant and soil bacteria responses to N fertilizer rates consistent with Australian legislated production practices on dairy farms for pure and mixed swards of white clover (Trifolium repens) and Italian ryegrass (Lolium multiflorum) in a long-term pasture experiment in controlled glasshouse conditions. Although the soil bacterial community structure at phylum level was similar for white clover and Italian ryegrass, relative abundances of specific subgroups of bacteria differed among plant species according to the N fertilizer regimen. Marked increases in relative abundance of some bacterial phyla and subphyla indicated potential inhibition of N cycling, especially for N hotspots in soil. Ammonium concentration in soil was less correlated with dominance of some N-cycling bacterial phyla than was nitrate concentration. Changes in bacterial community structure related to altered nutrient cycling highlight the potential for considering this area of research in policy assessment frameworks related to nutrient loads in dairy soils, especially for N.

Spatial variation and soil nitrogen potential hotspots in a mixed land cover catchment on the Chinese Loess Plateau

Soil nitrogen (N) is critical to ecosystem services and environmental quality. Hotspots of soil N in areas with high soil moisture have been widely studied, however, their spatial distribution and their linkage with soil N variation have seldom been examined at a catchment scale in areas with low soil water content. We investigated the spatial variation of soil N and its hotspots in a mixed land cover catchment on the Chinese Loess Plateau and used multiple statistical methods to evaluate the effects of the critical environmental factors on soil N variation and potential hotspots. The results demonstrated that land cover, soil moisture, elevation, plan curvature and flow accumulation were the dominant factors affecting the spatial variation of soil nitrate (NN), while land cover and slope aspect were the most important factors impacting the spatial distribution of soil ammonium (AN) and total nitrogen (TN). In the studied catchment, the forestland, gully land and grassland were found to be the potential hotspots of soil NN, AN and TN accumulation, respectively. We concluded that land cover and slope aspect could be proxies to determine the potential hotspots of soil N at the catchment scale. Overall, land cover was the most important factor that resulted in the spatial variations of soil N. The findings may help us to better understand the environmental factors affecting soil N hotspots and their spatial variation at the catchment scale in terrestrial ecosystems.

Gene mobility in microbiomes of the mycosphere and mycorrhizosphere -role of plasmids and bacteriophages.

Microbial activity in soil, including horizontal gene transfer (HGT), occurs in soil hot spots and at "hot moments". Given their capacities to explore soil for nutrients, soil fungi (associated or not with plant roots) can act as (1) selectors of myco(rrhizo)sphere-adapted organisms and (2) accelerators of HGT processes across the cell populations that are locally present. This minireview critically examines our current understanding of the drivers of gene mobility in the myco(rrhizo)sphere. We place a special focus on the role of two major groups of gene mobility agents, i.e. plasmids and bacteriophages. With respect to plasmids, there is mounting evidence that broad-host-range (IncP-1β and PromA group) plasmids are prominent drivers of gene mobility across mycosphere inhabitants. A role of IncP-1β plasmids in Fe uptake processes has been revealed. Moreover, a screening of typical mycosphere-inhabiting Paraburkholderia spp. revealed carriage of integrated plasmids, next to prophages, that presumably confer fitness enhancements. In particular, functions involved in biofilm formation and nutrient uptake were thus identified. The potential of the respective gene mobility agents to promote the movement of such genes is critically examined.

More than Meets the Dye: Evaluating Preferential Flow Paths as Microbial Hotspots

Core Ideas O2–sensitive optodes located microbial hotspots along artificial preferential flow pathways. O2 depletion served as a reference for microbial activity. O2 depletion locations show preferential flow paths as dynamic biogeochemical entities. Preferential flow paths have been suggested to function as microbial hotspots. This idea is speculative, based on biomass counts rather than assessments of the activity of the microbial population. In this study we used O2–sensitive optodes, or optical sensor devices, to observe O2 depletion by soil microbes as a reference for microbial activity along three artificially constructed preferential flow path geometries. The flow paths were constructed using contrasting fine (210–297 μm) and coarse (595–841 μm) sand textures in the geometries of (i) coarse sand in the center surrounded by fine sand, (ii) half coarse sand on the left and half fine sand on the right, and (iii) fine sand in the center surrounded by coarse sand. A soil slurry, containing soil microbes and glucose as an added source of C, was flowed through the sands to create nutrient gradients due to nonuniform flow. Results suggest that O2 depletion is greatest along the boundary between preferential flow paths (coarse sand) and the bulk matrix (fine sand). While the results offer insight into the locations of microbial activity, the exact positioning remains unclear. Our work suggests that preferential flow plays a key role in the spatial distribution of microbial hotspots in soil and demonstrates the nexus between soil physical and microbial processes. We show that O2–sensitive optodes were effective in monitoring O2 depletion due to microbial activity along the paths. To address issues related to C cycling under the changing environment, biophysical processes in flow paths must be better understood.

Hotspots and hot moments of amino acid N in soil: Real-time insights using continuous microdialysis sampling

Protein hotspots in soil, such as those associated with decaying soil fauna or plant litter, may produce ephemeral patches of disproportionately high soil nutrients. These hotspots may occur at the macro- and microscale in close proximity to plant roots, however, the likely concentration of soluble products produced in these hotspots remains poorly understood. To address this, we buried two contrasting biomass residues in soil, namely earthworm (Lumbricus terrestris) and clover (Trifolium repens). Their transformation to amino acids, NH4+ and NO3− were monitored continually over 6 days using microdialysis. All treatments showed greater soluble nitrogen (N) concentrations compared to the unamended controls. The highest concentrations of both amino acids (12.9 mM after 12 h) and NH4+ (45.3 mM after 6 h) were generated in the vicinity of decomposing earthworm. In comparison, dried clover residues yielded 2.7 mM of amino acids at 6 h. After 12 h, amino acid and NH4+ concentrations in both earthworm and dried clover treatments showed a steep decline, returning close to background levels (<20 μM). Through the use of microdialysis we are able to show that soil nutrient hotspots may provide nearby roots with concentrations of amino acids and NH4+ several orders of magnitude higher than found in the bulk soil solution.

Identifying heavy metal pollution hot spots in soil-rice systems: A case study in South of Yangtze River Delta, China

The soil-rice system in China is subjected to increasing concentrations of heavy metals (HMs) which derived from various sources. It is very critical to investigate the concentrations, spatial characteristics and hot spots of HMs content in the soil-rice system. This study presents work completed on 915 soil-rice sample pairs collected from South of Yangtze River Delta, China. These samples were evaluated for HM concentrations. Ordinary Kriging and the Getis-Ord index were used to explore spatial distributions and pollution hot spots. Averaged HMs content in soil is shown to be Zn > Cr > Pb > Cu > Ni > As > Hg > Cd, and concentrations in rice arrange as Zn > Cu > Cr > Ni > As > Cd > Pb > Hg. Compared with Chinese maximum permissible limits, mean content of all HMs in farmland soil are at safe levels and averaged content of all HMs in rice were also at safe levels except As and Ni. Ni was most polluted HM in soil Most of and showed relatively high content in farmland soil in southeastern part. As and Ni are the most polluted in rice, with highest content distributed in the northwestern and southern area, respectively. The majority of HMs pollution hot spots in soil clustered in the central area. Pollution hot spots of Ni and As in rice are mainly concentrated in the central part and southeastern part, correspondingly. Our results found a weak link between content and spatial pattern of pollution status of HMs in soil and rice. The results are anticipated to contribute to more efficient and accurate control of HMs pollution in soil-rice system, and assist decision-makers achieve a balance between cost and regulation of HM pollution.

Soil zymography as a powerful tool for exploring hotspots and substrate limitation in undisturbed subsoil

It is widely accepted that soil microorganisms are not evenly distributed but are often concentrated in spatially segregated hotspots that are characterized by higher substrate availability compared to the surrounding bulk soil. However, microorganisms outside of hotspots may be in a dormant or inactive state, since they have depleted all available substrates within their vicinity. So far, the knowledge about the spatial distribution and dynamics of microbial activity in subsoil is very scarce, since most available data has been acquired from either homogenized soil samples or as bulk signals from undisturbed soil cores. In this study, we introduced a new incubation approach combining soil zymography and substrate addition on undisturbed soil core surfaces. We mapped three extracellular enzymes (β-glucosidase, chitinase and acid phosphatase) on a subsoil sample from 60 cm depth and analyzed their activity-patterns using different geostatistical and spatial analyses. After initial enzyme mapping, the soil was homogenously sprayed with 14C glucose as model substrate and incubated for 14 days. Soil zymography was suitable for detecting hotspots in undisturbed soil, making up a proportion of 2.4% on average of the total area. Consequently, microbial-driven biogeochemical processes can be expected to be limited to small areas in this subsoil, while the major part of the soil volume is not contributing. Glucose additions considerably increased enzyme activities up to 900% in initial non-hotspots, while the effect was far lower in initial hotspots. These results show that microorganisms in the subsoil outside of hotspots can be activated and release enzymes when substrate is supplied. Thus, dormant or inactive microorganisms outside of hotspots are able to contribute to SOC mineralization when substrate limitation is overcome, thus most likely inducing positive priming effects. Our results clearly demonstrate the benefits of combining enzyme mapping with substrate additions on undisturbed soil to gain new insights about microbial hotspots and C-cycling in subsoils using spatial analyses. In contrast to traditional incubation experiments, this method gives high spatial information about microbial activity, allowing a more differentiated interpretation of incubation results.

Is There Anybody Out There? Substrate Availability Controls Microbial Activity outside of Hotspots in Subsoils

Soil organic carbon (SOC) turnover in subsoils is assumed to be limited to spatially restricted microsites where fresh substrate inputs occur. Vice versa, the growth and activity of microorganisms outside of such hotspots may be limited by easily available substrates. The apparent long-term stability of subsoil organic carbon could thus be a result of microbial inactivity in these vast “cold regions” outside of hotspots. The aim of this study was to obtain realistic data about the in situ distribution of microbial hotspots in deep soil using soil zymography for three extracellular enzymes on undisturbed soil slices sampled from 0 to 161 cm depth. The results showed that most enzyme-driven turnover processes were concentrated to small portions of &lt;1 to 10% of the subsoil volume, while enzymes in the major part of subsoils were barely active. In a second step, soil slices were homogenously sprayed with 14C glucose, incubated for 2 weeks and again analyzed with soil zymography. After glucose application, enzyme activities greatly increased in non-hotspot areas, thus confirming that substrate availability limits microbial activity in most of the subsoil volume. This implies that substrate limitation is a controlling factor for SOC stability in subsoils, suggesting that SOC in non-hotspots is persisting over long time periods until substrate becomes available and increases microbial activity.

Spatiotemporal variation and exposure risk to human health of potential toxic elements in suburban vegetable soils of a megacity, SW China, 2012-2016.

Trace element contamination in soils of vegetable fields can threat public health. Seven potential toxic elements (As, Cd, Cr, Cu, Ni, Pb, and Zn) in suburban vegetable soils of Chengdu city, Southwest China, in 2012 and 2016, were analyzed to identify their sources with the spatiotemporal variation and assess their contamination and health risk for residents. The results showed that the concentrations of soil elements did not increase significantly in 2016 compared with that in 2012, whereas their spatial distributions altered markedly. The hot spots of soil As, Cd, and Pb as well as Cu and Zn in 2016 revealed the anthropogenic sources including agricultural activities, industrial emissions, road dust with heavy traffic, and open burning of solid waste. The apparent spatial difference of anthropogenic elements was related to the layout of land use surrounding the vegetable field. The contamination of soil elements decreased in the order of Cd > As ≈ Zn > Cu ≈ Pb > Cr ≈ Ni in 2012 and Cd > Zn > As ≈ Cu ≈ Pb > Cr ≈ Ni in 2016, and the vegetable soils were slightly to moderately contaminated by these elements through integrated contamination index. The sites affected by thetrace elements did not increase in 2016 than in 2012, whereas the sites with relatively high contamination increased markedly. The non-carcinogenic risk of trace elements was generally acceptable, and children showed higher health risk than adults. The As carcinogenic risk for children varied between 5.48 × 10-5 and 1.59 × 10-4 in 2012 and between 4.40 × 10-5 and 1.82 × 10-4 in 2016, and the sites above acceptable levels (> 10-4) reached 60.6% and 48.5% in 2012 and 2016, respectively. The health risk of As in the vegetable soils should be paid more attention due to its high toxicity.