• The microbial metabolism was limited by soil carbon (C) and phosphorus (P) under heavy metal stress.

Heavy metals can exist in soil for a long time and seriously affect soil quality. The coexistence of various heavy metal pollutants leads to biotoxicity and alters the activity of microorganisms. Soil microbial metabolism plays an important role in nutrient cycling and biochemical processes of soil ecosystem. However, the effects of heavy metal contamination on microbial metabolism in soil are still unclear. This study aims to reveal the responses of microbial metabolic limitation to heavy metals using extracellular enzyme stoichiometry, and further to evaluate the potential impacts of heavy metal pollution on soil nutrient cycle. The results showed that soil microbial metabolism reflected by the ecoenzymatic activities had a significant response to soil heavy metals pollution. The metabolism was limited by soil carbon (C) and phosphorus (P) under varied heavy metal levels, and the increase of heavy metal concentration significantly increased the microbial C limitation, while had no effect on microbial P limitation. Microorganisms may increase the energy investment in metabolism to resist heavy metal stress and thus induce C release. The results suggest that energy metabolism selected by microorganisms in response to long-term heavy metal stress could increase soil C release, which is not conducive to the soil C sequestration. Our study emphasizes that ecoenzymatic stoichiometry could be a promising methodology for evaluating the toxicity of heavy metal pollution and its ecological effects on nutrient cycling.

● Microplastics (MPs) increased activities of N and P hydrolases in paddy soil.

Microplastics provide a new ecological niche for microorganisms, and the accumulation levels of microplastics (MPs) in terrestrial ecosystems are higher than those in marine ecosystems. Here, we applied the zymography to investigate how MPs – polyethylene [PE], and polyvinyl chloride [PVC]) at two levels (0.01% and 1% soil weight) impacted the spatial distribution of soil hydrolases, nutrient availability, and rice growth in paddy soil. MPs increased the above-ground biomass by 13.0%–15.5% and decreased the below-ground biomass by 8.0%–15.1%. Addition of 0.01% and 1% MPs reduced soil NH₄⁺ content by 18.3%–63.2% and 52.2%–80.2%, respectively. The average activities of N- and P-hydrolases increased by 0.8%–4.8% and 1.9%–6.3% with addition of MPs, respectively. The nutrient uptake by rice plants and the enzyme activities in hotspots increased with MP content in soil. The accumulation of MPs in paddy soil could provide an ecological niche that facilitates microbial survival, alters the spatial distribution of soil hydrolases, and decreases nutrient availability.

Soil organic carbon (SOC) is the largest pool of carbon in terrestrial ecosystems and plays a crucial role in regulating atmospheric CO₂ concentrations. Identifying the essential relationship between soil bacterial communities and SOC concentration is complicated because of many factors, one of which is geography. We systematically re-analyzed 6102 high-quality bacterial samples in China to delineate the bacterial biogeographic distribution of bacterial communities and identify key species associated with SOC concentration at the continental scale. The type of land use was the principal driver of bacterial richness and diversity, and we used machine learning to calculate its influence on microbial composition and their co-occurrence relationship with SOC concentration. Cultivated land was much more complex than forest, grassland, wetland and wasteland, with high SOC concentrations tending to enrich bacteria such as Rubrobacter, Terrimonas and Sphingomona. SOC concentration was positively correlated with the amounts of soil total nitrogen and key bacteria Xanthobacteraceae, Streptomyces and Acidobacteria but was negatively correlated with soil pH, total phosphorus and Micrococcaceae. Our study combined the SOC pool with bacteria and indicated that specific bacteria may be key factors affecting SOC concentration, forcing us to think about microbial communities associated with climate change in a new way.

• Continuous chlorine treatment have no obvious effect on soil microbial community structure and composition.

Chlorine-containing disinfectants have been widely used around the world for the prevention and control of the COVID-19 pandemic. However, at present, little is known about the impact of residual chlorine on the soil micro-ecological environment. Herein, we treated an experimental soil-plant-microbiome microcosm system by continuous irrigation with a low concentration of chlorine-containing water, and then analyzed the influence on the soil microbial community using metagenomics. After 14-d continuous chlorine treatment, there were no significant lasting effect on soil microbial community diversity and composition either in the rhizosphere or in bulk soil. Although metabolic functions of the rhizosphere microbial community were affected slightly by continuous chlorine treatment, it recovered to the original status. The abundance of several resistance genes changed by 7 d and recovered by 14 d. According to our results, the chlorine residue resulting from daily disinfection may present a slight long-term effect on plant growth (shoot length and fresh weight) and soil micro-ecology. In general, our study assisted with environmental risk assessments relating to the application of chlorine-containing disinfectants and minimization of risks to the environment during disease control, such as COVID-19.

The mutual interdependence of plants and arbuscular mycorrhizal fungi (AMF) is important in carbon and mineral nutrient exchange. However, an understanding of how AMF community assemblies vary in different forests and the underlying factors regulating AMF diversity in native tropical forests is largely unknown. We explored the AMF community assembly and the underlying factors regulating AMF diversity in a young (YF) and an old-growth forest (OF) in a tropical area. The results showed that a total of 53 AMF phylogroups (virtual taxa, VTs) were detected, 38±1 in the OF and 34±1 in the YF through high-throughput sequencing of 18S rDNA, and AMF community composition was significantly different between the two forests. A structural equation model showed that the forest traits indirectly influenced AMF diversity via the plant community, soil properties and microbes, which explained 44.2% of the total observed variation in AMF diversity. Plant diversity and biomass were the strongest predictors of AMF diversity, indicating that AMF diversity was dominantly regulated by biotic factors at our study sites. Our study indicated that forest community traits have a predictable effect on the AMF community; plant community traits and soil properties are particularly important for determining AMF diversity in tropical forests.

•Ÿ Rice microbiota responded to lindane pollutant was studied spatiotemporally.

Soil-derived microbiota associated with plant roots are conducive to plant growth and stress resistance. However, the spatio-temporal dynamics of microbiota in response to organochlorine pollution during the unstable vegetative growth phase of rice is not well understood. In this study, we focused on the rice (Oryza sativa L.) microbiota across the bulk soil, rhizosphere and endosphere compartments during the vegetative growth phase in two different soils with and without lindane pollutant. The results showed that the factors of growth time, soil types and rhizo-compartment had significant influence on the microbial communities of rice, while lindane mostly stimulated the construction of endosphere microbiota at the vegetative phase. Active rice root-soil-microbe interactions induced an inhibition effect on lindane removal at the later vegetative growth phase in rice-growth-dependent anaerobic condition, likely due to the root oxygen loss and microbial mediated co-occurring competitive electron-consuming redox processes in soils. Each rhizo-compartment owned distinct microbial communities, and therefore, presented specific ecologically functional categories, while the moderate functional differences were also affected by plants species and residual pollution stress. This work revealed the underground micro-ecological process of microbiota and especially their potential linkage to the natural attenuation of residual organochlorine such as lindane.

• Soil phosphorus shaped both abundant and rare bacterial communities.

Uncovering the mechanisms underlying the diversity patterns of abundant and rare species is crucial for terrestrial biodiversity maintenance. However, the response of abundant and rare community assembly to ecological succession has not been explored, particularly considering soil profiles. Here 300 soil samples were collected from reforestation ecosystems from depths of up to 300 cm and horizontal distances of 30-90 cm from a tree. We revealed that soil phosphorus not only affected alpha diversity and community structure, but also mediated the balance of stochastic and deterministic processes for abundant and rare sub-communities, which exhibited contrasting assembly strategies. The abundant sub-community changed from variable selection to stochasticity with the increase of phosphorus, while the rare sub-community shifted from homogeneous selection to stochasticity. Dispersal limitation was the main assembly process in the abundant sub-community, while the rare sub-community was governed primarily by homogeneous selection. Moreover, the relative influence of deterministic processes increased with succession for both sub-communities. At the scale of a single tree, stochastic processes increased with soil depth in rare sub-community, while deterministic processes increased with the radius from a single tree in the abundant sub-community. Overall, our results highlight the importance of the soil phosphorus-driven assembly process in understanding the re-assembly and maintenance of soil bacterial diversity.

● Film mulching decreased soil organic C content in soil aggregates with 0.053–0.25 mm diameter.

Microplastic distribution is non-homogeneous in agricultural soil following plastic film degradation. However, the distribution of microplastics by shape and particle size in different soil aggregates remains unknown. To elucidate the distribution of microplastic shapes and particle sizes in soil aggregates with increasing years of film mulching, four paired fields with film mulching (FM) and no mulching (NM) were examined at 1, 5, 10, and 20 years after continuous mulching. An increase in soil aggregates of 0.053–0.25 mm diameter was observed; however, soil organic carbon content decreased after long-term FM. Microplastics primarily combined with 0.053–2 mm soil aggregates. Specifically, long-term FM was associated with dominance of film- and fiber-shaped microplastics in soil aggregates of 0.25–2 mm and 0.053–0.25 mm diameter, respectively. Fiber- and granule-shaped microplastics of 0.25–1 mm diameter primarily combined with 0.053–0.25 and 0.25–2 mm soil aggregates, respectively. Film-shaped microplastics of diameter > 1 mm and diameter 0.05–0.25 mm primarily combined with 0.25–2 mm soil aggregates. Therefore, distribution of microplastics in soil aggregates can be used to monitor soil health and quality, greatly enhancing our understanding of the risk posed by microplastics to the environment.

• Soil nematode samples can be quite turbid, which are not satisfactory for microscopy.

Soil nematodes are useful ecological indicators and can be extracted from soil by a variety of techniques. Because the extracted nematode samples (suspensions) can be quite turbid (i.e., they contain soil particles and organic particles in addition to nematodes), quantitative and taxonomic analyses of the nematodes by microscopy can be difficult. In this study, the following three methods for cleaning turbid suspensions obtained from Baermann funnels were assessed: repeated centrifugation at 692.5´g for 1 min, repeated settling at low-temperature (4°C) for 24 h, and a combination of low-temperature settling and centrifugation. Nematodes were extracted with Baermann funnels from soil samples collected from four land-use types (since land-use type can affect the turbidity of nematode suspensions), and the resulting suspensions were cleaned by the three methods before nematode abundance was assessed. As a control, samples (i.e., suspensions) were simply diluted with water, and nematodes were counted in the entire volume. The results showed that, within each land-use type, nematode abundance did not significantly differ between the control and the three cleaning methods. Averaged across all land-use types, however, the nematode recovery rate was slightly higher with repeated centrifugation than with the other two cleaning methods. Therefore, the proposed methods are sound for cleaning turbid nematode suspensions, and repeated centrifugation is the most efficient method.

• Resource-conservation practices are emerging for attaining sustainability in agriculture.

Besides contributing majorly in the growth of a country, agriculture is one of the severely affected sectors at present. Several modifications and adaptations are being made in agricultural practices to cope-up with the declining soil fertility and changing climate scenarios across the world. However, the development and adoption of a single agricultural practice may not help in the holistic mitigation of the impacts of climate change and may result in economic vulnerability to farmers. Therefore, it is high time to develop and recommend a group of agricultural practices i.e. package-based agriculture system having some compatibility for one another in the long term. In this article, a viewpoint has been given on some emergent agronomic practices adopted in the tropical agro-ecosystems which have potential to be developed as compatible agricultural package in combination. Moreover, we also emphasized on exploring some key indicators/environmental factors to assess the compatibility of different agronomic practices. For identifying the research transition from single to combined agricultural practices, a bibliometric analysis was performed by using conservation agriculture (CA), the system of rice intensification (SRI), organic agriculture and soil (biochar) amendment as the major agronomic practices being used for improving agro-ecological services such as improving nutrient cycling, soil fertility and crop productivity as well as climate change mitigation. The results revealed that scientific communities are now paying attention to exploring the role of combined agricultural practices for agro-ecological balance and climate change adaptation. Moreover, the limitations of the adoption of agronomic packages under different agro-climatic zones have also been highlighted. The recommendations of the study would further help the environmental decision-makers to develop potential measures for climate change mitigation without compromising the agro-ecological balance.

• Straw returning significantly affects silicon fraction transformation;

Returning crop straw into the soil is an important practice to balance biogenic and bioavailable silicon (Si) pool in paddy, which is crucial for the healthy growth of rice. However, owing to little knowledge about soil microbial communities responsible for straw degradation, how straw return affects Si bioavailability, its uptake, and rice yield remains elusive. Herein, we investigate the change of soil Si fractions and microbial community in a 39-year-old paddy field amended by a long-term straw return. Results show that rice straw return significantly increased soil bioavailable Si and rice yield from 29.9% to 61.6% and from 14.5% to 23.6%, respectively, when compared to NPK fertilization alone. Straw return significantly altered soil microbial community abundance. Acidobacteria was positively and significantly related to amorphous Si, while Rokubacteria at phylum level, Deltaproteobacteria, and Holophagae at class level was negatively and significantly related to organic matter adsorbed and Fe/Mn-oxide-combined Si in soils. Redundancy analysis of their correlations further demonstrated that Si status significantly explained 12% of soil bacterial community variation. These findings suggest that soil bacteria community and diversity interact with Si mobility by altering its transformation, thus resulting in the balance of various nutrient sources to drive biological Si cycle in agroecosystem.

• Biocrust succession alters diazotrophic diversity and community compositions.

The diazotrophic community in biological soil crusts (biocrusts) is the key supplier of nitrogen in dryland. To date, there is still limited information on how biocrust development influences the succession of diazotrophic community, and what are the most important factors mediating diazotrophic communities during biocrust succession. Using the high throughput nifH amplicon sequencing, the diazotrophs in soils at different developmental stages of biocrust were comparatively studied. The results evidenced the decrease of TOC/TN ratio and pH value with biocrust development. Nostoc and Scytonema were the most dominant diazotrophic genera at all biocrust stages, while Azospirillum and Bradyrhizobium were abundant only in bare soil. Diazotrophic co-occurrence networks tended to be less complex and less connected with biocrust succession. The soil TOC/TN ratio was the most dominant factor mediating diazotrophic diversity, community composition and assembly processes, while diazotrophic-diversity and NO3⁻-N/NH4⁺-N ratio were positively correlated with the nitrogenase activity during biocrust succession. This study provided novel understandings of nitrogen fixation and succession patterns of diazotrophic community, by showing the effects of biocrust succession on diazotrophic diversity, community composition, community assembly and co-occurrence networks, and recognizing TOC/TN ratio as the most dominant factor mediating diazotrophs during biocrust succession.

•Ÿ Five methods of soil HM pollution evaluation based on enzyme activity were reviewed

Soil enzyme activities have been suggested as suitable indicators for the evaluation of metal contamination because they are susceptible to microbial changes caused by heavy metal stress and are strictly related to soil nutrient cycles. However, there is a growing lack of recognition and summary of the historic advancements that use soil enzymology as the proposal of evaluation methods. Here, we review the most common methods of heavy metal pollution evaluation based on enzyme activities, which include single enzyme index, combined enzyme index, enzyme-based functional diversity index, microbiological stress index, and ecoenzymatic stoichiometry models. This review critically examines the advantages and disadvantages of these methods based on their execution complexity, performance, and ecological implications and gets a glimpse of avenues to come to improved future evaluation systems. Indices based on a single enzyme are variable and have no consistent response to soil heavy metals, and the following three composite indices are characterized by the loss of many critical microbial processes, which thus not conducive to reflect the effects of heavy metals on soil ecosystems. Considering the dexterity of ecoenzymatic stoichiometry methods in reflecting changes in soil functions under heavy metal stress, we propose that microbial metabolic limitations quantified by ecoenzymatic stoichiometry models could be promising indicators for enhancing the reality and acceptance of results and further improving the potential for actual utility in environmental decision-making.

● Occurrence of microplastics was widespread in long-term mulched farmland soils.

Soil contamination from film debris following the prolonged application of mulching film has emerged as a worldwide concern. However, the extent that mulching films contribute to soil microplastics, and the spatial distribution of soil contamination from film debris remain unclear. In this study, the cotton field in Xinjiang (China), which underwent film mulching for a prolonged period of 5−30 years, was selected as the research location. A total of 360 soil samples were collected, aiming to study the spatial distribution characteristics of mulching film debris pollution. The samples were extracted using the density flotation method combined with stereomicroscopic; the source, composition, abundance, and distribution characteristics of soil MPs were identified by the scanning electron microscopic, and Fourier transform infrared spectroscopic analyses. In soil mulched for a 30 year period, the abundance of microplastics across the studied soil depth (0−60 cm) was 78.51±2.57 n/(100 g). The μ-FTIR analyses revealed that the composition of the microplastics matched that of polyethylene materials. Therefore, plastic mulching could be inferred as a major contributor to microplastic pollution in agricultural lands. Overall, it is necessary to study the distribution characteristics of plastic film remaining for further study of plastic pollution in farmland soils.

● Bacterial richness declined but fungal richness increased under salinization.

Soil salinization is a typical environmental challenge in arid regions worldwide. Salinity stress increases plant convergent adaptations and facilitative interactions and thus destabilizes communities. Soil bacteria and fungi have smaller body mass than plants and are often efficient against soil salinization, but how the stability of bacterial and fungal communities change with a wide range of soil salinity gradient remains unclear. Here, we assessed the interactions within both bacterial and fungal communities along a soil salinity gradient in the Taklamakan desert to examine (i) whether the stability of bacterial and fungal communities decreased with soil salinity, and (ii) the stability of which community decreased more with soil salinity, bacteria or fungi. Our results showed that the species richness of soil fungi increased but that of soil bacteria decreased with increasing salinity in topsoils. Fungal communities became more stable under soil salinization, with increasing compartmentalization (i.e., modularity) and proportion of competitions (i.e., negative:positive cohesion) as salinity increased. Bacterial communities exhibited no changes in modularity with increasing salinity and smaller increases in negative:positive cohesion under soil salinization compared to fungal communities. Our results suggest that, by altering interspecific interactions, soil salinization increases the stability of fungal not bacterial communities in extreme environments.

• 10-year of CC was a cut-off point in separating soil bacterial community structures.

Soil-borne plant diseases cause major economic losses globally. This is partly because their epidemiology is difficult to predict in agricultural fields, where multiple environmental factors could determine disease outcomes. Here we used a combination of field sampling and direct experimentation to identify key abiotic and biotic soil properties that can predict the occurrence of bacterial wilt caused by pathogenic Ralstonia solanacearum. By analyzing 139 tomato rhizosphere soils samples isolated from six provinces in China, we first show a clear link between soil properties, pathogen density and plant health. Specifically, disease outcomes were positively associated with soil moisture, bacterial abundance and bacterial community composition. Based on soil properties alone, random forest machine learning algorithm could predict disease outcomes correctly in 75% of cases with soil moisture being the most significant predictor. The importance of soil moisture was validated causally in a controlled greenhouse experiment, where the highest disease incidence was observed at 60% of maximum water holding capacity. Together, our results show that local soil properties can predict disease occurrence across a wider agricultural landscape, and that management of soil moisture could potentially offer a straightforward method for reducing crop losses to R. solanacearum

• Polyester fibers increased aboveground biomass.

Microplastics are increasingly recognized as a factor of global change. By altering soil inherent properties and processes, ripple-on effects on plants and their symbionts can be expected. Additionally, interactions with other factors of global change, such as drought, can influence the effect of microplastics. We designed a greenhouse study to examine effects of polyester microfibers, arbuscular mycorrhizal (AM) fungi and drought on plant, microbial and soil responses. We found that polyester microfibers increased the aboveground biomass of Allium cepa under well-watered and drought conditions, but under drought conditions the AM fungal-only treatment reached the highest biomass. Colonization with AM fungi increased under microfiber contamination, however, plant biomass did not increase when both AM fungi and fibers were present. The mean weight diameter of soil aggregates increased with AM fungal inoculation overall but decreased when the system was contaminated with microfibers or drought stressed. Our study adds additional support to the mounting evidence that microplastic fibers in soil can affect the plant–soil system by promoting plant growth, and favoring key root symbionts, AM fungi. Although soil aggregation is usually positively influenced by plant roots and AM fungi, and microplastic promotes both, our results show that plastic still had a negative effect on soil aggregates. Even though there are concerns that microplastic might interact with other factors of global change, our study revealed no such effect for drought.

•No notable effect from long-term warming on activity of nutrient-acquiring enzymes.

Microbes play an important role in the carbon cycle and nutrient flow of the soil ecosystem. However, the response of microbial activities to long-term warming over decades is poorly understood. To determine how warming changes ecoenzyme activity and microbial nutrient limitation, we conducted a long-term, 21 years, experiment, on the Qinghai–Tibet Plateau. We selected typical grass- and shrub-covered plots, used fiberglass open-top chambers (OTCs) to raise the temperature, conducted soil sampling at different depths, studied the response of nutrient-acquiring enzyme activity and stoichiometry, and conducted vector analysis of stoichiometry. Our results showed that long-term warming did not have a notable effect on the activity of nutrient-acquiring enzymes or enzymatic stoichiometry. However, Spearman correlation analysis indicated a significant and positive correlation between ecoenzyme activity and the available nutrients and microbial biomass in soil. Vector analysis of stoichiometry showed phosphorus limitation for all soil microbes at different depths, regardless of whether the soil experienced warming. These changes in enzymatic stoichiometry and vector analysis suggested that microbial nutrient limitation was not alleviated substantially by long-term warming, and warming did not considerably affect the stratification of microbial nutrient limitation. Our research has also shown that long-term warming does not significantly change soil ecoenzyme activity and original microbial nutrient limitation at different soil depths within the OTUsʼ impact range. These results could help improve understanding of microbial thermal acclimation and response to future long-term global warming.

● Reduced oxygen increased microbial metabolic quotient (qCO₂).

Mangroves are one of the most ecologically sensitive ecosystems to global climate change, which have cascading impacts on soil carbon (C), nitrogen (N) and phosphorus (P) cycling. Moreover, mangroves are experiencing increasing N and P loadings and reduced oxygen availability due to intensified climate change and human activities. However, both direct and interactive effects of these perturbations on microbially mediated soil C, N and P cycling are poorly understood. Here, we simultaneously investigated the effects of N and P loadings and reduced oxygen on microbial biomass, microbial respiration, and extracellular enzyme activities (EEAs) in mangrove soils. We calculated the microbial metabolic quotient (qCO₂), which is regarded as a useful inverse metric of microbial C use efficiency (CUE). Our results show that reduced oxygen significantly increases both qCO₂ and microbial specific EEAs (enzyme activity per unit of microbial biomass) for C-, N- and P-acquisition regardless of N or P loadings. Furthermore, we found that qCO₂ positively correlated with microbial specific EEAs under reduced oxygen, whereas no clear relationship was detected under ambient oxygen. These results suggest that reduced oxygen increases microbial specific EEAs at the expense of increasing microbial respiration per unit biomass, indicating higher energy cost per unit enzyme production.

● Decay stages and meteorological factors affect leaf litter’s microbial community.

Litter microorganisms play a crucial role in the biological decomposition in forest ecosystems; however, the coupling effect of meteorological and substrate changes on it during the different stages of leaf decomposition in situ remains unclear. Hence, according to meteorological factors dynamics, a one-year field litter of Quercus wutaishanica in situ decomposition experiment was designed for four decay stages in a warm temperate forest. Microbial community composition was characterized using Illumina sequencing of fungal ITS and bacterial 16S genes. Bacterial (6.6) and fungal (3.6) Shannon indexes were the largest after 125 days’ litter decomposition (October). The relative abundance of Acidobacteria after 342 days and Bacteroidetes after 125 days were 3 and 24 times higher than after 31 days, respectively. Some non-dominant species (bacteria: Firmicutes, Planctomycotes, and Verrucomicrobia; fungi: Chytridiomycota and Glomeromomycota) may be absent or present at different decomposition stages due to litter properties or meteorological factors. Chemoheterotrophy and aerobic-chemoheterotrophy were the dominant bacterial functional groups, and the dominant fungal functional groups were saprotrophs, pathotrophs, and symbiotrophs. Precipitation and relative humidity significantly affected bacteria. Temperature, sunlight intensity, and net radiation significantly affected fungi. Besides, among the relative contributions of changes in bacterial and fungal community structure, leaf litter properties alone explained the variation of 5.51% and 10.63%. Microbial diversity and decay stage directly affected the litter mass-loss rate, with meteorological factors (precipitation, relative humidity, air temperature, and sunlight intensity) being indirect. Our findings highlight the importance of microbial diversity for leaf litter decomposition and the influence of meteorological factors.

● Soil processes affect metal chemical speciation and their biogeochemical activity.

From soil contamination and risk assessment perspectives, it is imperative to understand the ecological processes occurring in soils. Certain soil processes greatly affect chemical speciation of potentially toxic metals (PTMs), and thus also influence their biogeochemical activity. The current study analyzed chemical speciation of eight PTMs (Cd, Cr, Fe, Cu, Mn, Ni, Zn, and Pb) in upper and lower soil layers for 15 agronomic fields of Vehari-Pakistan using Visual Minteq software. The divalent forms of most PTMs (PTM²⁺) generally predominated in both soil layers (79.9% overall occurrence). However, chromium revealed a different pattern of chemical speciation (95.8% as CrOH⁺) compared to other PTMs. The mean percentage of all the PTMs²⁺ was slightly higher for the lower soil layer (81.3%) than in the upper layer (78.4%), the trend being same for all the PTMs, except Cr. This higher PTMs²⁺ percentage in lower soil layers than upper layers was due to lower content of organic matter and other anions such as Cl⁻ and HCO₃⁻. The mean percentage ages of all the PTMs²⁺ was similar among all the 15 agronomic fields, which was confirmed by strong Pearson correlation values (R² > 0.95). The PCA graph grouped all the agronomic fields and PTM²⁺ closely, except Cr²⁺ and Cu²⁺. This grouping confirmed the similar chemical speciation of PTMs, except Cu and Cr in studied fields.

● The abundance of N-cycling genes differently responded to NPK application.

Straw and manure are widely applied to agricultural systems, and greatly shape soil N-cycling microflora. However, we still lack a comprehensive understanding of how these organic materials structure soil N-cycling microbial communities. In this study, metagenomic analysis was performed to investigate the compositional variation in N-cycling microbial communities in a 30-year long-term experiment under five fertilization regimes: no fertilization (Control), chemical fertilization only (NPK), and NPK with wheat straw (NPK + HS), pig manure (NPK + PM), and cow manure (NPK + CM). Long-term NPK application differentially changed N-cycling gene abundance and greatly altered N-cycling microbial community structure. NPK + HS resulted in a similar pattern to NPK in terms of gene abundance and community structure. However, NPK + PM and NPK + CM significantly increased most genes and resulted in a community similar to that of the Control. Further analysis revealed that serious soil acidification caused by long-term NPK fertilization was a major factor for the variation in N-cycling microbial communities. The addition of alkaline manure, rather than wheat straw, stabilized the N-cycling microbial community structure presumably by alleviating soil acidification. These results revealed the strong impact of soil acidification on microbial N-cycling communities and illustrated the possibility of resolving nitrogen-related environmental problems by manipulating pH in acidified agricultural soils.

• Trace metal contamination in soils of 29 China’s mountains was investigated.

Trace metal contamination in soils is a threat with an uncertain limit to maintain planet safety, and the issue of trace metal contamination in mountain soils is still of low concerned. In this study, we assessed the contamination of six trace metals (Cd, Cr, Cu, Ni, Pb, and Zn) in mountain soils across China and deciphered the potential drivers of their spatial distribution. The results showed that concentrations of Cd and Pb decreased significantly with soil depth, and their concentrations were markedly higher in north-west, south, and south-west China than elsewhere. Among the metals, Cd was the priority for control with moderate to heavy contamination, followed by Pb, whereas the other metals did not show evident contamination. The altitudinal pattern and isotopic tracing revealed that the significant enrichment and marked contamination of Cd and Pb in surface soils were primarily attributed to deposition through long-range transboundary atmospheric transport and condensation. Ore mining, nonferrous smelting, and coal and fuel combustion were identified as primary anthropogenic sources of the Cd and Pb. Soil organic matter content, pH, and soil forming processes directly determined the accumulation of trace metals in the soils, and orographic effects, including local climate, vegetation composition, and canopy filtering, regulated the spatial distribution of the metals. This study highlights the significance of soil Cd contamination in mountains, which are considered of low concern, and suggests that long-term monitoring of trace metal contamination is necessary to improve biogeochemical models that evaluate the responses of the mountain critical zone to future human- and climate-induced environmental changes.