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PurposeSoil cracking is a common natural phenomenon. The existence of soil cracks has significant effects on the engineering properties of clayey soil, and can cause significant problems in geotechnical, geological, and environmental aspects. Understanding of the potential mechanisms of soil cracking is essential in assessments of potential damages to earthen infrastructures.Materials and methodWe review the past research efforts devoted to the experimental investigations and applications of fracture mechanics in soil cracking, attempting to provide a better understanding of the formation mechanism of desiccation cracking with a perspective of fracture mechanics.Results and discussionThis review analyzes the influence of soil cracking on soil engineering properties and the significance of soil cracking phenomena. Past and current formulations of soil fracture criteria and their experimental investigation are discussed. This review reveals the factors that affect the mechanisms of soil fracture can be divided into two groups, namely soil intrinsic properties and test-related factors. The applications of fracture mechanics in soil cracking are also discussed with particular focus on soil fracture models that are separately based on linear elastic fracture mechanics (LEFM), elastic plastic fracture mechanics (EPFM), and numerical simulations of soil cracking based on fracture mechanics. Some challenges and prospects of the applications of fracture mechanics in soil cracking are presented.ConclusionsFracture mechanics is a significant method to explain soil crack initiation and propagation. It is expected that researchers can gain better understanding of the range of fracture mechanics applications in soil cracking, and seek improvements and extensions of existing models through this review.

PurposeIn recent years, microplastic (MP) contamination has raised enormous concern. However, data on the influence of solid waste treatment systems on MP pollution around agricultural soil are lacking. This study investigated the distribution and characteristics of MPs in agricultural soil surrounding a solid waste treatment center in southeastern China.Materials and methodsFifty-seven agricultural topsoil samples around the solid waste treatment center were collected. The samples were pretreated by drying, flotation separation using NaCl solution, and digestion by H2O2. The abundance and morphological characteristics of MPs were determined by a microscope, followed by Raman spectroscopy analysis identified polymer types and SEM–EDS analysis observed surface morphology and the type of metals accumulated on the MPs.Results and discussionSoil MPs’ abundance ranged from 280 to 2360 items/kg, while a higher abundance of MPs was distributed in the downwind area. The < 1-mm MPs were dominant, and white fragment MPs were widely found. Polyethylene (52.86%) and polypropylene (27.14%) were the most common. Moreover, SEM–EDS images illustrated that MPs were significantly weathered and showed the uneven distribution of metal(loid) elements on the surface, implying that MPs may migrate as heavy metal vectors to threaten agroecosystem safety.ConclusionsThis study reveals the distribution and characteristics of MPs in agricultural soil surrounding a solid waste treatment center in southeastern China, as well as the potential source of soil MPs, and provides systematic data for further research on MP pollution in agricultural soil.

PurposeMicroorganisms are an essential component of riverine ecosystems. However, the assembly mechanisms of riverine microbial sub-communities with different rarity are poorly understood. Thus, this study aimed to examine the assembly mechanisms of microbial communities and the sub-communities with different rarity in soils, sediments, and water along the Nu River.Materials and methodsWater, sediment, and riparian soil samples were collected from 20 sites along the Nu River. The microbial abundance and diversity were examined via real-time PCR and Illumina amplicon high-throughput sequencing, respectively. The assembly mechanisms of microbial communities and the sub-communities with different rarity were determined based on iCAMP which is a recently developed phylogenetic bin-based null model analysis method.Results and discussionThe results showed that microbial abundance, diversity, and community profiles significantly differed among different habitats. The microbial community assembly was mainly driven by homogeneous selection, dispersal limitation, and drift. With an increase in rarity, the contribution of drift and homogenizing dispersal to the assembly of microbial sub-communities significantly increased, while the contribution of homogeneous selection showed a reverse trend. This study also showed that the contribution of stochastic and deterministic processes to the microbial community assembly was significantly correlated with geographic distance and environmental factors, respectively.ConclusionsThese findings suggest that the assembly mechanisms of riverine microbial sub-communities with different rarity are distinct, improving our understanding of riverine microbial diversity maintenance mechanisms.

Abstract PurposeThe objective of this study was to investigate the effects of application of biochar and microbial inoculants on the bioavailability of phosphorus and potassium, tomato growth, and the bacterial community in greenhouse soil.Materials and methodsThe experiment was conducted in a greenhouse with tomato mono-cropped for 21 years at Yongqing County of Hebei Province from November 2018 to June 2019. The treatments included conventional fertilization control (CF), 2 t/ha of biochar application (B, manufactured from apricot shell), 75 L/ha of microbial inoculants application (M, containing effective strains of Bacillus megaterium and Paenibacillus mucilaginosus), the mixture of microbial inoculants and biochar application (BM).ResultsThe results showed that the application of 75 L/ha microbial inoculants in greenhouse tomato could increase the yields of tomato by 23.41%, vitamin C (Vc) and soluble sugar concentrations by 14.41% and 13.62%, respectively. The microbial inoculants combined with 2 t/ha biochar enhanced the effects of microbial inoculants on the growth promotion of tomatoes. The application of microbial inoculants combined with biochar increased the P and K accumulation in tomato plants by 28.72–57.14% and 19.53–29.03%, respectively, during the whole growing stage. Moreover, the application of microbial inoculants significantly increased the relative abundance of Bacillus, Paenibacillus, and Flavobacterium and decreased the relative abundance of Acidobacterium.ConclusionsThe application of microbial inoculants improved the bioavailability of phosphorus and potassium and tomato growth by altering the composition of soil bacterial community. These results show the potential of co-application of biochar and microbial inoculants as a potential tool to sustain longer-term production of monoculture vegetable systems in greenhouses.

PurposeSoil heavy metal distribution is a key issue in environmental and health research. In recent years, an increasing number of anthropogenic disturbances have been identified on the Qinghai-Tibetan Plateau, leading to increased heavy metal contamination of soils. Therefore, this study investigated the distribution, sources, ecological risk, and prediction of heavy metals in soils on the Qinghai-Tibetan Plateau.Material and methodsHere, for each of the seven heavy metals (i.e., Cr, Ni, Cu, Zn, As, Cd, and Pb) investigated in the soil of the Qinghai-Tibet Plateau, we collected soil concentration values from the literature and carried out field sampling to generate new data. We then assessed the pollution characteristics and sources of soil heavy metals on the Qinghai-Tibet Plateau using geostatistics, positive matrix factorization model (PMF), disjunctive kriging, and mapped the pollution risk probability distribution of each heavy metal element.Results and discussionSpatial autocorrelation was found for all seven heavy metals in the soils. Under different land-use types, the average concentrations of Cr, As, and Cd were highest in the water areas; Ni levels were highest in woodland; and Cu, Zn, and Pb concentrations were highest in residential, industrial, and mining lands. Using analysis by the PMF model and correlation analysis, four heavy metal sources were identified, including industrial sources and atmospheric deposition (Cd), traffic emissions (Cr, Zn, and Zn), natural sources (Ni and As), and mining activities (Cr and Pb). The pollution risk probabilities of Cr, Ni, Cu, Zn, As, and Pb were low in most areas of the Qinghai-Tibet Plateau, whereas the pollution risk probability of Cd was high over 29% of the area of the Qinghai-Tibet Plateau.ConclusionThe results highlighted no potential contamination for Cr, local potential contamination for Ni, Cu, Zn, As and Pb, while Cd exhibits more extensive contamination in the soils of the Qinghai-Tibet Plateau. Cd reduction should therefore be considered an important component of the strategy, policy, and action plan for soil pollution management on the Qinghai-Tibet Plateau.

PurposeThe increasing soil microplastics (MPs) pollution and the dry–wet alternation caused by climate change occur widely. Emerging studies have shown their negative impacts on the structure of soil bacteria and plant physiology. However, there is limited research on the effects of MPs exposure under different water regimes, and the influence processes are inconclusive. Therefore, the impacts of MPs under different water regimes on the antioxidant activity of vegetable and the soil bacterial community were studied.Materials and methodsA continuous water supply and a dry–wet cycle were established to compare the effects of two types of MPs (polypropylene (PP); polyethylene terephthalate (PET)) on lettuce (Lactuca sativa L. var. longifolia) growth and soil bacterial community, determined by antioxidant enzyme activity analysis, and by high-throughput sequencing analysis, respectively.ResultsThe PET influenced the lettuce growth and bacterial diversity largely compared to PP. The dry–wet cycle enhanced the effects of MPs on the activity of the antioxidant enzymes in lettuce, while it reduced lettuce biomass and root activity. Proteobacteria and Actinobacteria were the dominant soil bacteria under all treatments. The addition of MPs increased the relative abundance of nutrient-related bacteria such as Proteobacteria. MPs addition increased intragroup differences in soil bacterial diversity, and the dry–wet cycle reduced intergroup differences caused by MPs. ConclusionThe polymer type of MPs influenced the antioxidant activity of lettuce roots and leaves. The dry–wet cycle intensified the exposure risk of MPs by enhancing the stimulation of MPs on the antioxidant activity of lettuce. The dry–wet cycle significantly reduced the MPs-induced differences in bacterial communities. In short, the dry–wet cycle might alter the ecological risks in plant-soil ecosystem by shifting soil bacterial lineages related to nutrient cycling and metabolic function under MPs exposure.

PurposeWith the increased availability of nanoparticle-based products, their releases to soil are undoubtedly inevitable. Among the nanoparticle-based products, potential risks of metal oxide nanoparticles (MO-ENPs) have attracted increasing concerns. However, their effects on soil and soil microorganisms remain largely unknown.Materials and methodsIn this study, four metal oxide nanoparticles, i.e., zinc oxide nanoparticles (nZnO), titanium dioxide nanoparticles (nTiO2), cerium dioxide nanoparticles (nCeO2), and magnetite nanoparticles (nFe3O4), were enrolled to evaluate their impact on soil enzyme activities (invertase, urease, catalase, and phosphatase) and bacterial communities in two typical soils from northeast China (black soil and saline-alkali soil). The community structure and size were analyzed using pyrosequencing and real-time polymerase chain reaction (RT-PCR). The soils were exposed to metal oxide nanoparticles at 0.5, 1.0, and 2.0 mg g−1 for 15 and 30 days.Results and discussionIn general, nZnO had a stronger effect on soil enzymatic activities than nTiO2, nCeO2, and nFe3O4, and saline-alkali soil was more susceptible to metal oxide nanoparticles than black soil. In RT-PCR analysis, a significant decrease (41.66, 36.34, and 47.99%, respectively) on total bacteria population was only observed in saline-alkali soil treated by 0.5, 1.0, and 2.0 mg g−1 nZnO. Meanwhile, pyrosequencing analysis revealed that the samples of saline-alkali soil treated with nZnO showed high variance in their bacterial community composition, e.g., Bacilli, Alphaproteobacteria, and Gammaproteobacteria class.ConclusionsThe results suggested that metal oxide nanoparticle incubation could influence soil enzyme activities and change soil bacterial community. Moreover, the soil type was a key component dictating the effect of metal oxide nanoparticles on the bacterial community composition and size. These findings are of great help towards building a comprehensive understanding of the potential environmental risks of metal oxide nanoparticles.

PurposeGlobal warming and drying are important environmental issues. Our study aimed to investigate how warming and precipitation changes affect soil nematode communities in an Inner Mongolian desert steppe for 10 years.Materials and methodsSoil nematodes were extracted by the Baermann funnel method. Changes in the nematode communities under artificial warming and precipitation conditions were assayed by analyzing their abundance and ecological indices.Results and discussionSoil nematode abundance decreased significantly by 37.47% under artificial warming; however, there was no significant effect of warming on the nematode community diversity. As for precipitation experiment, the decreased precipitation eliminated some of non-dominant nematode genera, such as Pratylenchus, Helicotylenchus, and Aphelenchus. It caused not only a significant decrease (37.65%) in soil nematode abundance but also a more structured food web and shorter food chain. However, nematode faunal analysis indicated that the soil nematode community was more resistant to drought. Both soil nematode abundance and community diversity increased significantly as increase of precipitation. In particular, the abundance of plant parasitic nematodes increased by 46.69%, which may due to the increase in total nitrogen content in soil. Nematode faunal analysis showed that increased precipitation improved soil environment for the nematodes, and increased food web connectivity and food chain length. However, bacterivorous nematode abundance decreased by 74.39%, and the decomposition pathway of the nematode community had switched from the bacterial channel to the fungal channel.ConclusionsIn the Inner Mongolian steppe, both climate drying and warming had negative impacts on soil nematode abundance; however, only drying affected nematode community diversity and food web structure and slowly changed nematode ecological functions. Increased precipitation may aid soil nematode community recovery.

Purpose Identifying the sources of heavy metals in soils is a crucial issue for soil remediation and management. Most regions in China have been undergoing a rapid industrialization and urbanization since the last three decades. However, there is little information available on the spatial distribution of heavy metals in soils experiencing a rapid transition from agricultural-based to industrial-based economy. To resolve this problem and to provide references on similar regions, we carried out an investigation on heavy metals in soils in Ju country to identify potential sources and to map their spatial distributions. Materials and methods A total of 646 samples including 511 samples in topsoils (0–20 cm, regular grid of 2 × 2 km 2 ) and 135 samples in subsoils (150–200 cm, regular grid of 4 × 4 km 2 ) were collected in Ju country, and the total contents of Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, V, and Zn were determined. Enrichment factor method and multivariate analyses (correlation analysis, principal component analysis, and cluster analysis) were applied to identify the sources of ten heavy metals in topsoils. Additionally, ordinary kriging was used to map the spatial distributions of heavy metals concentration in topsoils. Results and discussion The overall levels of all heavy metals did not show high values, but the enrichment factor results suggested that Hg, Cd, Cu, Pb, and Zn in topsoils showed significant accumulation. Ten heavy metals can be grouped into three groups. Co, Cr, Mn, Ni, and V were associated with parent material and seemed to originate from a natural source. Cd, Cu, Pb, and Zn seemed to be related to the combination of parent material and anthropic inputs. Hg as an isolated element was dominated by atmospheric deposition inputs related to various human activities. Distribution maps derived by ordinary kriging suggested that Cd, Cu, Hg, Pb, and Zn were linked to the western part, corresponding well to the surroundings of urban areas located in the western part of Ju country. Conclusions Hg, Cd, Cu, Pb, and Zn were the main metals affected by human inputs in Ju country, and the high risk resulting from human influence was mainly shown around urban areas, consistent with the spatial distribution of industrial and traffic sites. Agricultural practices did not have significant influence on the spatial distribution of metals. The combination of multivariate analysis and geostatistics was found to be an effective approach to identify the origins and spatial distribution of heavy metals in soils.

PurposeOrganic treatments may improve soil nutrient availability and ecological functions. This study aimed to determine the effect of biogas slurry (BS) on soil properties, yields, and bacterial community activity and to examine the main environmental factors impacting bacterial compositions in the ecosystem.Materials and methodsA 3-year field experiment was conducted in yellow soil under a rice-rape rotation to understand the effect of BS originating from anaerobically digested pig waste on soil chemical and bacterial compositions.Results and discussionCompared with an inorganic nutrient treatment and a control, adding BS at a moderate dose (BS6, 165.1 t ha−1) positively affected the rice and rape yields, soil fertility, and bacterial diversity. The BS was more suitable than chemical fertilizer for maintaining agricultural soil sustainability, especially the BS6 and BS7 treatments (165.1 and 182.1 t ha−1, respectively), by improving nutrient content, increasing soil pH, and promoting soil crumb structure formation. The relative abundance of Actinobacteria in BS6 was decreased by 37.9% compared with the untreated soil and 27.7% compared with the fertilizer treatment, while the relative abundance of Nitrospirae in BS6 increased by 41.2% and 43.5% compared with these treatments, respectively. Redundancy analyses (RDA) showed that pH, rape yield, cation exchange capacity (CEC), and total nitrogen (TN) were significantly correlated with soil bacterial community composition and explained 56.0%, 46.4%, 43.2%, and 34.9% of the total community variability, respectively. The soil bacterial diversity depended mainly on soil pH, and crop rotation played an important role in changing the bacterial community.ConclusionsThe soil bacterial composition was clearly altered after 3 years of BS treatments. These increases in bacterial diversity could be of ecological significance in maintaining soil fertility and functionality.