Vertical variation of a black soil's properties in response to freeze-thaw cycles and its links to shift of microbial community structure

Maize residue retention shapes soil microbial communities and co-occurrence networks upon freeze-thawing cycles

Maize residue retention is an effective agricultural practice for improving soil fertility in black soil region, where suffered from long freezing-thawing periods and intense freeze-thawing (FT) cycles. However, very few studies have examined the influence of maize residue retention on soil microbial communities under FT cycles. We investigated the response of soil microbial communities and co-occurrence networks to maize residue retention at different FT intensities over 12 cycles using a microcosm experiment conditioned in a temperature incubator. Our results indicated that maize residue retention induced dramatic shifts in soil archaeal, bacterial and fungal communities towards copiotroph-dominated communities. Maize residue retention consistently reduced soil fungal richness across all cycles, but this effect was weaker for archaea and bacteria. Normalized stochastic ratio analysis revealed that maize residue retention significantly enhanced the deterministic process of archaeal, bacterial and fungal communities. Although FT intensity significantly impacted soil respiration, it did not induce profound changes in soil microbial diversity and community composition. Co-occurrence network analysis revealed that maize residue retention simplified prokaryotic network, while did not impact fungal network complexity. The network robustness index suggested that maize residue retention enhanced the fungal network stability, but reduced prokaryotic network stability. Moreover, the fungal network in severe FT treatment harbored the most abundant keystone taxa, mainly being cold-adapted fungi. By identifying modules in networks, we observed that prokaryotic Module #1 and fungal Module #3 were enhanced by maize residue retention and contributed greatly to soil quality. Together, our results showed that maize residue retention exerted stronger influence on soil microbial communities and co-occurrence network patterns than FT intensity and highlighted the potential of microbial interactions in improving soil functionality.

Effects of biochar and compost on microbial community assembly and metabolic processes in glyphosate, imidacloprid and pyraclostrobin polluted soil under freezethaw cycles

Biochar and organic compost are widely used in agricultural soil remediation as soil immobilization agents. However, the effects of biochar and compost on microbial community assembly processes in polluted soil under freezingthawing need to be further clarified. Therefore, a freezethaw cycle experiment was conducted with glyphosate (herbicide), imidacloprid (insecticide) and pyraclostrobin (fungicide) polluted to understand the effect of biochar and compost on microbial community assembly and metabolic behavior. We found that biochar and compost could significantly promote the degradation of glyphosate, imidacloprid and pyraclostrobin in freezethaw soil decrease the half-life of the three pesticides. The addition of immobilization agents improved soil bacterial and fungal communities and promoted the transformation from homogeneous dispersal to homogeneous selection. For soil metabolism, the combined addition of biochar and compost alleviated the pollution of glyphosate, imidacloprid and imidacloprid to soil through up-regulation of metabolites (DEMs) in amino acid metabolism pathway and down-regulation of DEMs in fatty acid metabolism pathway. The structural equation modeling (SEM) results showed that soil pH and DOC were the main driving factors affecting microbial community assembly and metabolites. In summary, the combined addition of biochar and compost reduced the adverse effects of pesticides residues.

Effects of freeze-thaw cycles on soil greenhouse gas emissions: A systematic review

In the context of global warming, increasingly widespread and frequent freezing and thawing cycles (FTCs) will have profound effects on the biogeochemical cycling of soil carbon and nitrogen. FTCs can increase soil greenhouse gas (GHG) emissions by reducing the stability of soil aggregates, promoting the release of dissolved organic carbon, decreasing the number of microorganisms, inducing cell rupture, and releasing carbon and nitrogen nutrients for use by surviving microorganisms. However, the similarity and disparity of the mechanisms potentially contributing to changes in GHGs have not been systematically evaluated. The present study consolidates the most recent findings on the dynamics of soil carbon and nitrogen, as well as GHGs, in relation to FTCs. Additionally, it analyzes the impact of FTCs on soil GHGs in a systematic manner. In this study, particular emphasis is given to the following: (i) the reaction mechanism involved; (ii) variations in soil composition in different types of land (e.g., forest, peatland, farmland, and grassland); (iii) changes in soil structure in response to cycles of freezing temperatures; (iv) alterations in microbial biomass and community structure that may provide further insight into the fluctuations in GHGs after FTCs. The challenges identified included the extension of laboratory-scale research to ecosystem scales, the performance of in-depth investigation of the coupled effects of carbon, nitrogen, and water in the freeze-thaw process, and analysis of the effects of FTCs through the use of integrated research tools. The results of this study can provide a valuable point of reference for future experimental designs and scientific investigations and can also assist in the analysis of the attributes of GHG emissions from soil and the ecological consequences of the factors that influence these emissions in the context of global permafrost warming.

Microbial community structure and carbon transformation characteristics of different aggregates in black soil

Background

Previous research on whole-soil measurements has failed to explain the spatial distribution of soil carbon transformations, which is essential for a precise understanding of the microorganisms responsible for carbon transformations. The microorganisms involved in the transformation of soil carbon were investigated at the microscopic scale by combining 16S rDNA sequencing technology with particle-level soil classification.

Methods

In this experiment,16S rDNA sequencing analysis was used to evaluate the variations in the microbial community structure of different aggregates in no-tillage black soil. The prokaryotic microorganisms involved in carbon transformation were measured before and after the freezing and thawing of various aggregates in no-tillage black soil. Each sample was divided into six categories based on aggregate grain size: >5, 2-5, 1-2, 0.5-1, 0.25-0.5, <0.25 mm, and bulk soil.

Results

The relative abundance of Actinobacteria phylum in <0.25 mm aggregates was significantly higher compared to that in other aggregates. The Chao1 index, Shannon index, and phylogenetic diversity (PD) whole tree index of <0.25 mm aggregates were significantly smaller than those of in bulk soil and >5 mm aggregates. Orthogonal partial least-squares discrimination analysis showed that the microbial community composition of black soil aggregates was significantly different between <1 and >1 mm. The redundancy analysis (RDA) showed that the organic carbon conversion rate of 0.25-0.5 mm agglomerates had a significantly greater effect on their bacterial community structure. Moreover, humic acid conversion rates on aggregates <0.5 mm had a greater impact on community structure. The linear discriminant analysis effect size (LEfSe) analysis and RDA analysis were combined. Bradyrhizobium, Actinoplane, Streptomyces, Dactylosporangium, Yonghaparkia, Fleivirga, and Xiangella in <0.25 mm aggregates were positively correlated with soil organic carbon conversion rates. Blastococcus and Pseudarthrobacter were positively correlated with soil organic carbon conversion rates in 0.25-0.5 mm aggregates. In aggregates smaller than 1 mm, the higher the abundance of functional bacteria that contributed to the soil's ability to fix carbon and nitrogen.

Discussion

There were large differences in prokaryotic microbial community composition between <1 and >1 mm aggregates. The <1 mm aggregates play an important role in soil carbon transformation and carbon fixation. The 0.25-0.5 mm aggregates had the fastest organic carbon conversion rate and increased significantly more than the other aggregates. Some genus or species of Actinobacteria and Proteobacteria play a positive role in the carbon transformation of <1 mm aggregates. Such analyses may help to identify microbial partners that play an important role in carbon transformation at the micro scale of no-till black soils.

Comamonas-dominant microbial community in carbon poor aquitard sediments revealed by metagenomic-based growth rate investigation

The microbiological ecology of a low-nutrient shallow aquifer with high arsenic content in the Yinchuan Plain was investigated in this study. Amplicon sequencing data from five samples (depths: 1.5 m, 3.5 m, 11.2 m, 19.3 m, and 25.5 m) revealed diverse and adaptable microbial community. Among the microbial community, Comamonas was the most prominent, accounting for 10.52 % of the total. This genus displayed high growth rates, with a maximum growth rate of 12.06 d-1 and a corresponding doubling time of 1.38 days, as determined through an analysis of codon usage bias. Functional annotation of Metagenome-Assembled Genomes (MAGs) for samples at 1.5 m and 11.2 m depths revealed Comamonas' metabolic versatility, including various carbon pathways, assimilative sulfate reduction (ASR), and dissimilatory reduction to ammonium (DNRA). The TPM (Transcripts Per Kilobase of exon model per Million mapped reads) of MAGs at 11.2 m sample was 15.7 and 12.3. The presence of arsenic resistance genes in Comamonas aligns with sediment arsenic levels (65.8 mg/kg for 1.5 m depth, 32.8 mg/kg for 11.2 m depth). This study highlights the role of Comamonas as a 'generalist' bacteria in challenging oligotrophic sediments, emphasizing the significance of such organisms in community stability and ecological functions. ENVIRONMENTAL IMPLICATION: Low-biomass limits the microbial activity and biogeochemical study in oligotrophic environments, which is the typical condition for underground aquatic ecosystems. Facilitated by growth rate estimation, our research focuses on active functional microorganisms and their biogeochemical metabolic in oligotrophic aquifer sediments, revealing their impact on the environment and response to arsenic threats. Findings illuminate the metabolic advantage of a 'generalist life-style' in carbon-scarce environments and contribute to a broader understanding of bacterial ecosystems and environmental impacts in oligotrophic aquifer sediments worldwide.

Impact of polyamide microplastics on riparian sediment structures and Cd(II) adsorption: A comparison of natural exposure, dry-wet cycles, and freeze-thaw cycles

Microplastics (MPs) accumulation in sediments has posed a huge threat to freshwater ecosystems. However, it is still unclear the effect of MPs on riparian sediment structures and contaminant adsorption under different hydrological processes. In this study, three concentrations of polyamide (PA) MPs-treated sediments (0.1%, 1%, and 10%, w/w) were subjected to natural (NA) exposure, dry-wet (DW) cycles, and freeze-thaw (FT) cycles. The results indicated that PA MPs-added sediment increased the micro-aggregates by 10.1%-18.6% after FT cycles, leading to a decrease in aggregate stability. The pH, OM, and DOC of sediments were significantly increased in DW and FT treatments. In addition, the increasing concentration of PA MPs showed an obvious decrease in aromaticity, humification, and molecular weight of sediment DOM in FT treatments. Also, high level of MPs was more likely to inhibit the formation of humic-like substances and tryptophan-like proteins. For DW and FT cycles, 0.1% and 1% PA MPs-treated sediments slightly increased the adsorption capacity of Cd(II), which may be ascribed to the aging of MPs. Further correlation analysis found that DW and FT altered the link between DOM indicators, and aggregate stability was directly related to the changes in sediment organic carbon. Our findings revealed the ecological risk of MPs accumulating in riparian sediments under typical hydrological processes.

High level of winter warming aggravates soil carbon, nitrogen loss and changes greenhouse gas emission characteristics in seasonal freeze-thaw farmland soil

Changes in the soil environment caused by winter warming is affecting the carbon and nitrogen cycles of seasonal freeze-thaw farmland soil. A field experiment was conducted in a seasonal freeze-thaw farmland soil of northeast China to investigate the effects caused from different levels of warming (W1 + 1.77 °C, W2 + 0.69 °C and C + 0 °C) on soil carbon and nitrogen dynamics, microbial biomass and greenhouse gases fluxes. During the early and middle winter, the contents of all kinds of soil carbon and nitrogen (Ammonium, nitrate, total nitrogen, dissolved organic carbon, readily oxidizable organic carbon and soil organic carbon) tended to increase with the increase of warming level, while during the late winter, their contents under different temperature treatments roughly present trend of W2 ≥C > W1. Except for the late thawing period, warming increased the contents of soil microbial biomass carbon and nitrogen, during the late thawing period, with the increase of warming level, MBC and MBN decreased significantly. Warming would stimulate the release of greenhouse gases from soil. But due to the differences of soil environmental conditions in each period and soil nutrient dynamics under different treatments, which made the effects of different levels of warming on soil GHGs fluxes in different periods are different. Our study suggested that low-level warming improved the availability of soil carbon and nitrogen, increased the contents of microbial biomass and greenhouse gas emissions. However, although high-level winter warming showed a similar phenomenon in the early and middle winter to the low-level warming, during the late winter, high-level warming increased soil nutrients loss and broke the seasonal coupling relationship between crop nutrient acquisition and soil microbial nutrient supply, and even led to the adaptation of soil CO2 release to it. This is of great significance for exploring the carbon and nitrogen cycle mechanisms of global terrestrial ecosystem.

Effects of freeze-thaw dynamics and microplastics on the distribution of antibiotic resistance genes in soil aggregates

This is the first study investigating the effects of freeze-thaw (FT) and microplastics (MPs) on the distribution of antibiotic resistance genes (ARGs) in soil aggregates (i.e., soil basic constituent and functional unit) via microcosm experiments. The results showed that FT significantly increased the total relative abundance of target ARGs in different aggregates due to the increase in intI1 and ARG host bacteria. However, polyethylene MPs (PE-MPs) hindered the increase in ARG abundance caused by FT. The host bacteria carrying ARGs and intI1 varied with aggregate size, and the highest number of hosts was observed in micro-aggregates (<0.25 mm). FT and MPs altered host bacteria abundance by affecting aggregate physicochemical properties and bacterial community and enhanced multiple antibiotic resistance via vertical gene transfer. Although the dominant factors affecting ARGs varied with aggregate size, intI1 was a co-dominant factor in various-sized aggregates. Furthermore, other than ARGs, FT, PE-MPs, and their integration promoted the proliferation of human pathogenic bacteria in aggregates. These findings suggested that FT and its integration with MPs significantly affected ARG distribution in soil aggregates. They amplified antibiotic resistance environmental risks, contributing to a profound understanding of soil antibiotic resistance in the boreal region.

Different responses of CO2 and CH4 to freeze-thaw cycles in an alpine forest ecosystem in northwestern China

The frequency and intensity of freeze-thaw cycles (FTCs) at high latitudes and altitudes are expected to increase with climate change, with likely effects on soil carbon turnover and CO₂ and CH₄ fluxes. However, the effects of FTCs on CO₂ and CH₄ fluxes remain unexplored, especially in alpine forest ecosystems. Here, we conducted an incubation experiment using intact soil cores to explore the effects of FTCs on CO₂ and CH₄ fluxes. Since temperature and moisture are considered as potential factors affecting FTCs, two freezing temperatures and three moisture levels were included in the incubation conditions. Our results showed that FTCs significantly affected CO₂ and CH₄ fluxes, but their response patterns to FTCs were distinct. The FTCs promoted CO₂ emission during the soil thawing phase, but reduced CH₄ uptake during the freezing phase, resulting at times in the transition of the soil from a sink to a source of CH₄. Both freezing temperature and soil moisture had significant impacts on CO₂ (F_temperature = 185.54, P < 0.001; F_moisture = 117.47, P < 0.001) and CH₄ fluxes (F_temperature = 123.68, P < 0.001; F_moisture = 14.55, P < 0.001), and their interaction also had significant impacts on CO₂ (F_CO2 = 3.16, P < 0.05) and CH₄ fluxes (F_CH4 = 15.19, P < 0.001) during FTCs. The effect of microclimate composed by freezing temperature and soil moisture on CO₂ and CH₄ fluxes is a direct and dominant pathway (path coefficient ≥ 0.50). Substrate quality, and microbial properties also influenced CO₂ fluxes during FTCs with the effect of the substrate being greater than that of microbial properties; substrate was important but the role of microorganisms was insignificant for CH₄ fluxes. Our study revealed that the different responses of CO₂ and CH₄ to FTCs in soil cores from an alpine forest ecosystem could improve the understanding of soil carbon release during FTCs and support accurate assessment of the carbon balance.

Assessing the impact of seasonal freezing and thawing on the soil microbial quality in arid northwest China

In drylands, soil microorganisms play a vital role in restoring degraded soils, and soil microbiota is significantly affected by human activities and climate events, such as seasonal freezing and thawing. However, the response of soil microbes to freezing and thawing, as well as their properties in drylands agroecosystems, remains unknown. This study investigated the effects of seasonal freezing and thawing on soil fungal and bacterial communities, multifunctionality, and soil microbial quality in a dryland agroecosystem. It has been observed that seasonal freezing and thawing promoted nutrient releases such as total carbon and available phosphorus. After thawing, soil catalase and cellulase activities increased while acid phosphatase and urease activities and total nitrogen content at topsoil decreased. Soil microbial biomass carbon content at 0-40 cm depth was significantly reduced by 94.77 %. Importantly, freezing and thawing considerably shifted the composition of fungal groups, while the soil bacterial community exhibited more stress tolerance to freezing-thawing. Compared to pre-freezing, the relative abundance of dominant fungal phyla such as Basidiomycota and Mortierellomycota decreased. At the same time, Ascomycota increased after thawing, and the relative abundance of pathogenic fungus also increased. For dominant bacteria phylum, freezing and thawing increased the relative abundance of Proteobacteria and Gemmatimonadetes while Micrococcaceae declined. Freezing and thawing significantly increased bacterial diversity and evenness by 4.94 % and 4.19 %, respectively, but decreased fungal richness and diversity by 23.49 % and 14.91 %, respectively. The minimum and total data sets were used to evaluate soil quality and we found that freezing and thawing significantly negatively impacted soil multifunctionality and microbial quality. In summary, this study demonstrates that the seasonal freezing-thawing has a significant negative impact on soil microbial quality and multifunctionality, and accelerates soil degeneration in dryland agroecosystem.

N2O emission associated with shifts of bacterial communities in riparian wetland during the spring thawing periods

Soil freeze-thaw processes lead to high nitrous oxide (N₂O) emissions and exacerbate the greenhouse effect. The wetlands of the Inner Mongolia Plateau are in the pronounced seasonal freeze-thaw zone, but the effect of spring thaw on N₂O emissions and related microbial mechanisms is still unclear. We investigated the effects of different periods (freeze, freeze-thaw, and thaw) on soil bacterial community diversity and composition and greenhouse gas emissions during the spring freeze-thaw in the XiLin River riparian wetlands in China by amplicon sequencing and static dark box methods. The results showed that the freeze-thaw periods predominantly impact on the diversity and composition of the bacterial communities. The phyla composition of the soil bacteria communities of the three periods is similar in level, with Proteobacteria, Chloroflexi, Actinobacteria, and Acidobacteria dominating the microbial communities. The alpha-diversity of bacterial communities in different periods varies that the freezing period is higher than that of the freeze-thaw period (p < .05). Soil total carbon, soil water content, and microbial biomass carbon were the primary factors regulating the abundance and compositions of the bacterial communities during spring thawing periods. Based on functional predictions, the relative abundance of nitrification and denitrification genes was higher in the freezing period than in the thawing period, while the abundance was lowest in the freeze-thawing period. The correlation results found that N₂O emissions were significantly correlated with amoA and amoB in nitrification genes, indicating that nitrification may be the main process of N₂O production during spring thaw. This study reveals potential microbial mechanisms of N₂O emission during spring thaw and provides data support and theoretical basis for further insight into the mechanism of N₂O emission during spring thaw.

Responses of Nitrous Oxide Emissions and Bacterial Communities to Experimental Freeze–Thaw Cycles in Contrasting Soil Types

Nitrous oxide (N2O) pulse emissions are detected in soils subjected to freeze–thaw cycles in both laboratory and field experiments. However, the mechanisms underlying this phenomenon are poorly understood. In this study, a laboratory incubation experiment that included freeze–thaw cycles (FTC), freezing (F) and control (CK) treatments was performed on three typical Chinese upland soils, namely, fluvo-aquic soil (FS), black soil (BS) and loess soil (LS). A higher similarity in soil properties and bacterial community structure was discovered between FS and LS than between FS and BS or LS and BS, and the bacterial diversity of FS and LS was higher than that of BS. FTC significantly increased the denitrification potential and the proportion of N2O in the denitrification gas products in FS and LS but decreased the denitrification potential in BS. Accordingly, with the increasing number of freeze–thaw cycles, the bacterial community composition in the FTC treatments in FS and LS diverged from that in CK but changed little in BS. Taxa that responded to FTC or correlated with denitrification potential were identified. Taken together, our results demonstrated that the effects of FTC on N2O emissions are soil-type-dependent and that the shift in the microbial community structure may contribute to the elevated N2O emissions.

Does the repeated freezing and thawing of the aerobic layer affect the anaerobic release of N2O from SWIS?

Subsurface wastewater infiltration systems (SWIS) is an efficient, economical, and less temperature affected sewage treatment technology. Pollutants are removed by physical, chemical, and biological reactions such as filtration, adsorption, oxidation, and degradation. Under the conditions of limited carbon source and inactivation of nitrous oxide reductase, N₂O, an important greenhouse gas, is released from the anaerobic layers of SWIS. However, is N₂O release affected by repeated freeze-thawing of the upper aerobic layer? How does microbial population structure and denitrogenate activity in different profiles respond to the freeze-thaw cycle (FTC)? These questions have not yet been revealed. In this study, a SWIS simulator with in-situ regulation of FTC was first constructed. The re-distribution of N₂O, microbial composition and denitrogenate activity were analyzed in response to FTC. Furthermore, potential bio-markers were screened and identified. The results revealed that the release of N₂O was correlated with FTC, with the anaerobic layer being the main contributor throughout, accounting for 73.32-75.8 % of the total release. The limiting factor for N₂O emissions was the NO₃^--N concentration in the anaerobic zone, and there were no simple linear communications between total nitrogen and N₂O generations. High throughput sequencing results showed the main markers of SWIS were Proteobacteria, Gemmatimonadetes, Firmicutes, Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi and Nitrospirae, accounting for 97.4 %-98.1 % of the total relative abundance. A significant positive correlation between Firmicutes and anaerobic release of N₂O was observed, where Firmicutes abundance increased from 5 % to 21 % during the experimental cycle, while N₂O concentration increased from 2.65 mg·L^-1 to 18.88 mg·L^-1. The results indicated that Firmicutes was an important biomarker of N₂O release under freeze-thaw conditions.

Effects of autumn diurnal freeze–thaw cycles on soil bacteria and greenhouse gases in the permafrost regions

Understanding the impacts of diurnal freeze–thaw cycles (DFTCs) on soil microorganisms and greenhouse gas emissions is crucial for assessing soil carbon and nitrogen cycles in the alpine ecosystems. However, relevant studies in the permafrost regions in the Qinghai-Tibet Plateau (QTP) are still lacking. In this study, we used high-throughput pyrosequencing and static chamber-gas chromatogram to study the changes in topsoil bacteria and fluxes of greenhouse gases, including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), during autumn DFTCs in the permafrost regions of the Shule River headwaters on the western part of Qilian Mountains, northeast margin of the QTP. The results showed that the bacterial communities contained a total of 35 phyla, 88 classes, 128 orders, 153 families, 176 genera, and 113 species. The dominant phyla were Proteobacteria, Acidobacteria, Actinobacteria, Chloroflexi, and Gemmatimonadetes. Two DFTCs led to a trend of increasing bacterial diversity and significant changes in the relative abundance of 17 known bacteria at the family, genus, and species levels. These were predominantly influenced by soil temperature, water content, and salinity. In addition, CO2 flux significantly increased while CH4 flux distinctly decreased, and N2O flux tended to increase after two DFTCs, with soil bacteria being the primary affecting variable. This study can provide a scientific insight into the impact of climate change on biogeochemical cycles of the QTP.

Biochar addition regulates soil phosphorus fractions and improves release of available phosphorus under freezing-thawing cycles

Currently, the shortage of phosphorus resources is becoming more and more serious. In general, phosphorus fertilizer is poorly utilized in soil and tends to gradually accumulate. Freezing-thawing cycles (FT) are seasonal phenomenon occurring in high latitudes and altitudes regions, which have obvious influence on the form of phosphorus in soil. This study investigates the effect of biochar on soil physicochemical properties, phosphorus form and availability under FT and thermostatic incubation (TH) condition. Compared with treatment without biochar, 4 % biochar addition increased the soil pH value, electrical conductivity, organic matter and Olsen-P of soil by a maximum of 0.76, 285.55 μS/cm, 28.60 g/kg and 139.27 mg/kg, respectively. Moreover, according to Hedley-P classification results, under FT condition, the content of labile phosphorus pool is always higher than those under TH. FT may promote the conversion of phosphorus from other fractions to labile phosphorus pool. Redundancy analysis results show that biochar addition and FT can not only directly change the soil phosphorus pool, but also alter the soil physicochemical properties and microbial community, which further affect the adsorption and mineralization of phosphorus in soil. The results of this study will be devoted to understanding the changes in soil phosphorus fractions under the effects of biochar addition and FT, providing references for agricultural production in areas where FT occur.

Alfalfa modified the effects of degraded black soil cultivated land on the soil microbial community

Legume alfalfa (Medicago sativa L.) is extensively planted to reduce chemical fertilizer input to the soil and remedy damaged fields. The soil mechanism of these effects is potentially related to the variations in alfalfa-mediated interactions of the soil microbial community. To understand the impact of planting alfalfa on the soil microbial community in degraded black soil cultivated land, a 4-year experiment was conducted in degraded black soil cultivated land. We assessed soil parameters and characterized the functional and compositional diversity of the microbial community by amplicon sequencing that targeted the 16S rDNA gene of bacteria and ITS of fungi in four systems under corn cultivation at the Harbin corn demonstration base (Heilongjiang, China): multiyear corn planting (more than 30 years, MC1); 2 years of alfalfa-corn rotation (OC); 3 years of alfalfa planting (TA); and 4 years of alfalfa planting (FA). It was found out that alfalfa led to changes in the alpha diversity of soil bacteria rather than in fungi in the degraded arable land. The abundance of the bacterial groups Gemmatimonadetes, Actinobacteria, Planctomycetes, and Chloroflexi was increased in OC, while Proteobacteria and Acidobacteria and the fungal group Glomeromycota were increased in TA and FA. OC, TA, and FA significantly increased the pH level but reduced soil electrical conductivity, but they had no impact on soil available nitrogen and soil available potassium at the 0-15 cm soil depth. However, with the years of alfalfa planting, soil available nitrogen and soil available potassium were reduced at the 15-30 cm soil depth. OC, TA, and FA significantly reduced the soil available phosphorus and soil total phosphorus at the 15-30 cm soil depth. There was no significant impact made on soil total nitrogen. FA significantly reduced the soil organic matter at the 15-30 cm soil depth. Planting alfalfa in degraded black soil cultivated land can reduce the salt content of the soil, and the nutrient content of soil planted with alfalfa without fertilization was equivalent to that of degraded corn cultivated land with annual fertilization. Besides, alfalfa recruited and increased contained taxa with the capacity to improve soil nutrient utilization and inhibit the harmful influences of pathogens for subsequent crops. Meanwhile, the planting of alfalfa can modify soil conditions by promoting the proliferation of specific beneficial microbiota groups.

Key factors determining soil organic carbon changes after freeze-thaw cycles in a watershed located in northeast China

The transformation and migration process of soil organic carbon (SOC) could be changed during freeze-thaw cycles (FTCs) and may further affect the SOC distribution in the watershed. In this study, both field investigation and lab incubation combined with geostatistics were used to clarify how environmental factors influence the SOC heterogeneity in Mollisol after FTCs, from a watershed in northeast China. The results showed that after FTCs, SOC decreased in 68.5% of the total watershed area at 0-20 cm soil depth, and the mean value decreased by 7.4%. Spatial autocorrelation (Moran's І) decreased in the 0-5 cm and 10-20 cm soil depths after FTCs (P < 0.01), but did not change in 5-10 cm and 0-20 cm soil depths. SOC increased at the top slope positions, the watershed outlet, and the upper slope position of the intersection area between farmland and forestland. The SOC decrease was 25 times and 14 times greater at the 0-5 cm and 0-20 cm soil depths respectively in the forestland than in the farmland. The SOC decrease was significantly higher on the 6-8° slopes than on the 0-2° slopes in 0-20 cm soil depth. SOC increased in most areas of cross-slope tillage (CST), but decreased in most areas of downslope tillage. The increase of SOC (positive change) decreased with increasing soil depth under soybeans field, while the decrease of SOC (negative change) increased with increasing soil depth under corn. Topographical factors alone, and topographical factors combined with land use types all influenced the SOC change in this watershed. High levels of ferrous minerals tended to reduce the rate of SOC after FTCs. SOC change is positively correlated to soil bulk density, FTC frequency and soil moisture after FTCs at the soil depth of 0-30 cm. Equations based on soil properties before FTCs, topographical factors, sediment transport index, runoff intensity index, and tillage method can be used for coarsely predicting SOC distribution after FTCs (45% < R² < 78%, P < 0.01). Generally, SOC dynamics were mainly determined by topography, land use, and to tillage methods that possibly attributes to soil and water loss during FTCs. Both erosion caused by snowmelt runoff and vertical migration of SOC could be the key factors that changed the SOC spatial pattern. CST was beneficial to conserve SOC during FTCs, while forestland could reduce SOC loss by reducing snowmelt erosion.

Correlation Between Fe/S/As Speciation Transformation and Depth Distribution of Acidithiobacillus ferrooxidans and Acidiphilium acidophilum in Simulated Acidic Water Column

It is well known that speciation transformations of As(III) vs. As(V) in acid mine drainage (AMD) are mainly driven by microbially mediated redox reactions of Fe and S. However, these processes are rarely investigated. In this study, columns containing mine water were inoculated with two typical acidophilic Fe/S-oxidizing/reducing bacteria [the chemoautotrophic Acidithiobacillus (At.) ferrooxidans and the heterotrophic Acidiphilium (Aph.) acidophilum], and three typical energy substrates (Fe²⁺, S⁰, and glucose) and two concentrations of As(III) (2.0 and 4.5 mM) were added. The correlation between Fe/S/As speciation transformation and bacterial depth distribution at three different depths, i.e., 15, 55, and 105 cm from the top of the columns, was comparatively investigated. The results show that the cell growth at the top and in the middle of the columns was much more significantly inhibited by the additions of As(III) than at the bottom, where the cell growth was promoted even on days 24–44. At. ferrooxidans dominated over Aph. acidophilum in most samples collected from the three depths, but the elevated proportions of Aph. acidophilum were observed in the top and bottom column samples when 4.5 mM As(III) was added. Fe²⁺ bio-oxidation and Fe³⁺ reduction coupled to As(III) oxidation occurred for all three column depths. At the column top surfaces, jarosites were formed, and the addition of As(III) could lead to the formation of the amorphous FeAsO₄⋅2H₂O. Furthermore, the higher As(III) concentration could inhibit Fe²⁺ bio-oxidation and the formation of FeAsO₄⋅2H₂O and jarosites. S oxidation coupled to Fe³⁺ reduction occurred at the bottom of the columns, with the formations of FeAsO₄⋅2H₂O precipitate and S intermediates. The formed FeAsO₄⋅2H₂O and jarosites at the top and bottom of the columns could adsorb to and coprecipitate with As(III) and As(V), resulting in the transfer of As from solution to solid phases, thus further affecting As speciation transformation. The distribution difference of Fe/S energy substrates could apparently affect Fe/S/As speciation transformation and bacterial depth distribution between the top and bottom of the water columns. These findings are valuable for elucidating As fate and toxicity mediated by microbially driven Fe/S redox in AMD environments.

Assessment of benthic invertebrate diversity and river ecological status along an urbanized gradient using environmental DNA metabarcoding and a traditional survey method

Benthic invertebrate diversity is one of the most commonly used bioindicators for assessing aquatic ecosystem health in river systems. Although an increasing number of studies have focused on assessing benthic invertebrate diversity using environmental DNA metabarcoding and traditional survey methods, benthic invertebrate diversity and ecological status assessments performed across different landscapes within river systems have not been well documented. Here, the diversity and ecological status of benthic invertebrates and the influence of water quality on the invertebrate assemblage distribution along an urbanization gradient in rivers from the Jingjinji (JJJ) region, China, were investigated using eDNA metabarcoding and the traditional method. With the combination of the two methods, 395 benthic invertebrates from 6 phyla, 27 orders, 94 families, and 222 genera were identified. The species richness of the benthic invertebrate community in the mountain area was significantly higher than that in the urban and agricultural areas. Compared to the traditional results, eDNA metabarcoding obtained a significantly greater number of species from every sampling site (P = 0.000) and detected a notably higher abundance in Annelida (P = 0.000). Furthermore, the nonmetric multidimensional scaling (NMDS) and permutational multivariate analysis of variance (PERMANOVA) based on the Bray-Curtis dissimilarity index indicated that the benthic invertebrate communities from the different habitats were discriminated more accurately and easily using eDNA metabarcoding (P = 0.038) than with the traditional method (P = 0.829). Additionally, the assemblages identified by eDNA metabarcoding were more closely linked to water quality and could be realistically used to assess the ecological status of rivers. Our findings highlight that eDNA metabarcoding could represent a rapid and reliable method for estimating benthic invertebrate diversity and ecological status in river systems.

Freeze-thaw cycles promote vertical migration of metal oxide nanoparticles in soils

Understanding the migration of engineered nanoparticles (ENPs) in soil is of great significance for evaluating the potential risks of ENPs to ecosystem. So far, their migration under freeze-thaw cycles (FTCs) has not been investigated. This study explored the impacts of FTCs on the migration of three commonly used ENPs, copper oxide (CuO-NPs), cerium oxide (CeO₂-NPs), and zinc oxide (ZnO-NPs), in three types of soil. After 32 FTC cycles, the highest migration rate of ENPs was found in black soil due to its higher clay particle content. CeO₂-NPs with low surface charge exhibited the highest mobility among three ENPs, which migrated to 9-11 cm layer with the concentration of 42.1 mg/kg in the black soil column. ZnO-NPs were less influenced by FTCs as they were adsorbed onto sand grains due to electrostatic interaction, which migrated to 3-5 cm layer with the concentration of 25.2 mg/kg in the black soil. Higher moisture contents (50% and 100%) resulted in increased migration depth of the ENPs in all soils. Lower freezing temperature (-25 °C) caused fragmentation of large soil particles and produced more clay colloids. FTCs promoted the movement of moisture, which penetrated the soil and thus facilitated the movement of ENPs by increasing the contents and movement of clay colloids. This work reveals the migration behavior of ENPs in soils in freeze-thaw period and provides insights into the fate and environmental risk of nanomaterial at middle and high latitudes.

Lead and Chromium Immobilization Process Subjected to Different Freeze-Thaw Treatments in Soils of the Northeastern Qinghai-Tibet Plateau

The freeze-thaw cycle is one of the important processes that affected heavy metal behaviors in soil. However, information regarding the adsorption and desorption behavior of heavy metals in soils under different freeze-thaw conditions is relatively less. Therefore, different freeze-thaw conditions including unfrozen, 15 freeze-thaw cycles at 60% water content, and 15 freeze-thaw cycles at 100% water content were investigated. Then the adsorption and desorption behaviors of Pb and Cr in freeze-thaw soils were studied. Results showed the Pb and Cr adsorption amount mostly decreased with increasing water-soil ratio, and the soil performance of Pb and Cr adsorption at same water-soil ratios showed variation under different freeze-thaw conditions. The Pb isothermal adsorption was higher for most freeze-thaw treatments compared to the control. The soil performance of Cr isothermal adsorption showed variation under different freeze-thaw conditions. Most electrostatic binding of Pb and Cr were stronger under unfrozen and freeze-thaw conditions than unfrozen conditions. Most Pb and Cr adsorption kinetics patterns of freeze-thaw treated soils were rapid than unfrozen conditions. These results implied that freeze-thaw cycles could change the soil adsorption and desorption patterns of Pb and Cr. Therefore, further studies are urgently needed to investigate Pb and Cr immobilization mechanisms in soils during freeze-thaw cycles. Hence, these findings provided useful information on Pb and Cr immobilization process in soils that underwent freeze-thaw cycles to offer an additional insight into predicting Pb and Cr behaviors in cold and freezing environments.

Dock2 affects the host susceptibility to Citrobacter rodentium infection through regulating gut microbiota

BACKGROUND

Dysregulated gut microbiota is one of major pathogenic factors in the development of colitis. Dock2 acts as a guanine nucleotide exchange factor (GEF) and activates small G protein RAC1. Our previous study showed that, compared to wild type (WT) mice, Dock2^-/- mice were more susceptible to colitis induced by Citrobacter rodentium infection. However, it is not clear whether gut microbiota affects the host susceptibility to enteric bacterial infection in Dock2^-/- mice.

RESULTS

In this study, we demonstrated that Dock2 regulated the gut microbiota and affected the host susceptibility to C. rodentium infection by co-housing, fecal microbiota transfer and antibiotic treatment methods. Microbiota analysis by 16 S rRNA gene sequencing showed that Dock2 increased the abundance of prevotellaceae-NK3B31-group and Lactobacillus but decreased that of Helicobacter.

CONCLUSIONS

These results suggest that Dock2 regulates the composition of gut microbiota and affects the host susceptibility to C. rodentium infection.

Persistence of Salmonella Typhimurium in apple-pear (Pyrus bretschneideri Rehd.) orchard soils influenced by bacterial communities and soil properties

2In this study, we investigated the persistence of Salmonella Typhimurium in 26 soil samples from apple-pear orchards in Yanji, Longjing and Helong in northeastern China. The time to reach detection limit (ttds) of Salmonella Typhimurium in soils varied from 20 to 120 days. Redundancy analysis and variation partition analysis elucidated that bacterial communities, clay content, pH, electrical conductivity (EC) salinity, and NO₃^--N could explain more than 85% of overall variation of the persistence behaviors. Results of structural equation models and Mantel tests revealed that clay content and EC displayed both direct and indirect effect on ttds, while NO₃^--N and pH exhibited direct and indirect effect on the survival patterns, respectively. Furthermore, Actinobacteria, Acidobacteria and Deltaproteobacteria at class level showed highly close correlations with ttds. Our results revealed that certain biotic and abiotic factors could greatly contribute to the overall persistence of Salmonella in apple-pear orchard soils.

Effects of freeze-thaw cycles on the spatial distribution of soil total nitrogen using a geographically weighted regression kriging method

Freeze-thaw cycles (FTCs) change the soil physicochemical properties and biogeochemical cycles and possibly also change the spatial heterogeneity of soil total nitrogen (TN) in the watershed. In this study, 912 soil samples were collected at 0-5 cm, 5-10 cm and 10-20 cm soil depths in the autumn and the spring of next year after FTCs of 2016-2017 and 2017-2018 in a Mollisol watershed (1.86 km²) of northeast China. The field investigations combined with classical statistics and geographically weighted regression kriging (GWRK) were used to explore the spatiotemporal distribution of TN before and after FTCs. Terrain information (e.g., slope aspect) and land management (e.g., tillage method) was main covariates were used for GWRK. The results showed the following. (1) TN decreased by 3.7-5.7% after FTCs at 0-20 cm soil depths at the watershed scale, decreasing more than 60% of the total watershed area. (2) The spatial pattern of TN did not change in the field with slope aspects and tillage methods after FTCs, but it changed with slope steepness and land uses. (3) TN was mainly influenced by snowmelt erosion during FTCs. TN increased in parts of the top slope, at land use intersection, in gully banks and at the watershed outlet. (4) Simulation accuracy of GWRK was higher than ordinary kriging (OK) for predicted TN at 0-20 cm soil depths before and after FTCs. (5) Spatial distribution of soil TN after FTCs can be predicted (R² = 0.521, p < 0.0001) and validated (R² = 0.494, p < 0.0001) using the data before FTCs based on GWRK. Generally, to reduce N loss and increase farmland fertility after FTCs, conservational techniques, e.g., tillage and straw amendment, could be used, especially in the middle slope positions. Moreover, fertilization should be appropriately reduced in parts of the watershed after FTCs, especially on the top slope, land use intersection and watershed outlet.

Critical review on soil phosphorus migration and transformation under freezing-thawing cycles and typical regulatory measurements

Freezing-thawing period plays an important role in the soil nutrient cycling. The frequency of freezing-thawing cycles (FTCs) can directly affect the supply of effective soil nutrients, further influences the growth and development of crops. Phosphorus is one of the essential nutrients for crop growth, and almost no compounds in gas form in nature, which is non-renewable resources. In modern agricultural production, phosphorus required by plants is mainly from the soil, but the utilization rate of phosphorus fertilizer in soil is generally only 10%-25%. Therefore, it is of great significance to study phosphorus migration and transformation behavior of soil in the non-growth period and related interfacial processes for improving the utilization efficiency of phosphorus fertilizer, increasing crop yield, reducing excessive application of phosphorus fertilizer, and subsiding environmental pollution. This paper systematically concludes key interfacial process of soil phosphorus in freezing-thawing soil system and relative mechanisms describing migration and transformation behavior of soil phosphorus. Besides, it summarizes the mediating effects of widely used soil conditioner on phosphorus cycling. The results show that freezing- thawing will destroy the structure of the soil, causing phosphorus to migrate along with runoff, soil water and heat movement. It also affects the types of microorganisms, the activity of microbial communities and the oxidation-reduction reaction of related minerals, making the phosphorus in soil from an unstable form to an active form. Biochar and humic substances can improve the physical and chemical properties of the soil, and have favorable effects on soil during freezing-thawing period. This review has important significance for the rational utilization of existing phosphorus resources, the maintenance of soil phosphorus cycle balance and the sustainable development of agriculture, meanwhile, has guiding significance for the reasonable utilization of agricultural wastes.

Vertical distribution of microbial communities and their response to metal(loid)s along the vadose zone-aquifer sediments

AIMS

This study attempted to demonstrate the vertical shift in bacterial, archaeal and fungal communities along the vadose zone-aquifer sediments and their respective responses to environmental factors.

METHODS AND RESULTS

We collected samples from the vadose zone and three aquifer sediments along a 42·5 m bore of a typical agricultural land. The results showed that the bacterial community shifted greatly with depth. The classes of Actinobacteria (19·5%) and NC10 (11·0%) were abundant in the vadose zone while Alphaproteobacteria (22·3%) and Gammaproteobacteria (20·1%) were enriched in the aquifer. Archaeal and fungal communities were relatively more homogeneous with no significant trend as a function of depth. Process analyses further indicated that selection dominated in the bacterial community, whereas stochastic processes governed archaeal and fungal communities. Moreover environment-bacteria interaction analysis showed that metal(loid)s, especially alkali metal, had a closer correlation with the bacterial community than physicochemical variables.

CONCLUSIONS

Depth strongly affected bacterial rather than archaeal and fungal communities. Metal(loid)s prevailed over physicochemical variables in shaping the bacterial community in the vadose zone-aquifer continuum.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our study provides a new perspective on the structure of microbial communities from the vadose zone to the deep aquifers.

Comparative Proteomic Analysis of Adhesion/Invasion Related Proteins in Cronobacter sakazakii Based on Data-Independent Acquisition Coupled With LC-MS/MS

Cronobacter sakazakii is foodborne pathogen that causes serious illnesses such as necrotizing enterocolitis, meningitis and septicemia in infants. However, the virulence determinants and mechanisms of pathogenicity of these species remain unclear. In this study, multilocus sequence typing (MLST) was performed on 34 C. sakazakii strains and two strains with the same sequence type (ST) but distinct adhesion/invasion capabilities were selected for identification of differentially expressed proteins using data-independent acquisition (DIA) proteomic analysis. A total of 2,203 proteins were identified and quantified. Among these proteins, 210 exhibited differential expression patterns with abundance ratios ≥3 or ≤0.33 and P values ≤0.05. Among these 210 proteins, 67 were expressed higher, and 143 were expressed lower in C. sakazakii SAKA80220 (strongly adhesive/invasive strain) compared with C. sakazakii SAKA80221 (weakly adhesive/invasive strain). Based on a detailed analysis of the differentially expressed proteins, the highly expressed genes involved in flagellar assembly, lipopolysaccharide synthesis, LuxS/AI-2, energy metabolic pathways and iron-sulfur cluster may be associated with the adhesion/invasion capability of C. sakazakii. To verify the accuracy of the proteomic results, real-time qPCR was used to analyze the expression patterns of some genes at the transcriptional level, and consistent results were observed. This study, for the first time, used DIA proteomic to investigate potential adhesion/invasion related factors as a useful reference for further studies on the pathogenic mechanism of C. sakazakii.

Soil Microbial Community Response Differently to the Frequency and Strength of Freeze-Thaw Events in a Larix gmelinii Forest in the Daxing'an Mountains, China

Sustained climate warming increases the frequency and strength of soil freeze-thaw (FT) events, which strongly affect the properties of soil microbial communities. To explore the responses and mechanisms of the frequency and strength of freeze-thaw events on soil microbial communities, a lab-scale FT test was conducted on forest soil in permafrost region from the Daxing'an Mountains, China. The number of FT cycles (FTN) had a greater effect on microbial communities than FT temperature fluctuation (FTF). The FTN and FTF explained 20.9 and 10.8% of the variation in microbial community structure, respectively, and 22.9 and 11.6% of the variation in enzyme activities, respectively. The total and subgroup microbial biomass, the ratio of fungi to bacteria (F/B), and C- and N-hydrolyzing enzyme activities all decreased with an increase in FTN. Among microbial groups, arbuscular mycorrhizal fungi (AMF) were the most sensitive to FT events. Based on the changes of F/B and AMF, the reduction in soil carbon sequestration caused by frequent FT events can be explained from a perspective of microorganisms. Based on redundancy analysis and Mental Test, soil moisture, total organic carbon, and total nitrogen were the major factors affecting microorganisms in FT events. In the forest ecosystem, soil water and fertilizer were important factors to resist the damage of FT to microorganism, and sufficient water and fertilizer can lighten the damage of FT events to microorganisms. As a result of this study, the understanding of the responses of soil microorganisms to the variation in FT patterns caused by climate changes has increased, which will lead to better predictions of the effects of likely climate change on soil microorganisms.

Interaction between Fungal Communities, Soil Properties, and the Survival of Invading E. coli O157:H7 in Soils

Pathogens that invade into the soil cancontaminate food and water, andinfect animals and human beings. It is well documented that individual bacterial phyla are well correlated with the survival of E. coliO157 (EcO157), while the interaction betweenthe fungal communities and EcO157 survival remains largely unknown. In this study, soil samples from Tongliao, Siping, and Yanji in northeast China were collected and characterized. Total DNA was extracted for fungal and bacterial community characterization. EcO157 cells were spiked into the soils, and their survival behavior was investigated. Results showed that both fungal and bacterial communities were significantly correlated (p < 0.01) with the survival of EcO157 in soils, and the relative abundances of fungal groups (Dothideomycetes and Sordariomycetes) and some bacterial phyla (Acidobacteria, Firmicutes, gamma- and delta-Proteobacteria)weresignificantly correlated with ttds (p < 0.01). Soil pH, EC (electric conductance) salinity, and water-soluble nitrate nitrogen were significantly correlated with survival time (time to reach the detection limit, ttd) (p < 0.05). The structural equation model indicated that fungal communities could directly influence ttds, and soil properties could indirectly influence the ttds through fungal communities. The first log reduction time (δ) was mainly correlated with soil properties, while the shape parameter (p) was largely correlated with fungal communities. Our data indicated that both fungal and bacterial communities were closely correlated (p < 0.05)with the survival of EcO157 in soils, and different fungal and bacterial groups might play different roles. Fungal communities and bacterial communities explained 5.87% and 17.32% of the overall variation of survival parameters, respectively. Soil properties explained about one-third of the overall variation of survival parameters. These findings expand our current understanding of the environmental behavior of human pathogens in soils.

Lead immobilization processes in soils subjected to freeze-thaw cycles

Soil freeze-thaw cycles (FTCs) change the physical and chemical properties of soils; however, information is limited about the consequences for heavy metal sorption and desorption. Lead (Pb) sorption isotherms and successive desorption tests were measured for three soils from North China (Chestnut, Lou and Black), following multiple freeze-thaw cycles (0, 1, 3, 6 and 9 FTCs) of -5 °C for 12 h and then +5 °C for 12 h. Lead adsorption dominated the sorption processes for all soils, and sorption capacity increased with additional FTCs. The Freundlich affinity parameter of soils for Pb sorption (i.e. A; L^β mmol^1-β kg^-1), was linearly correlated with carbonate content for soils with multiple FTCs. The effects of FTCs on lead adsorption may be more dependent on carbonate and clay contents than organic matter (OM), cation exchange capacity (CEC) and amorphous iron content. Repeated FTCs increased the pH of soil solutions at applied Pb concentrations >1.4 mmol L^-1, which could facilitate formation of inner-sphere complexes of Pb in studied soils. Cation exchange, a weak association, could occupy specific adsorption sites with increasing Pb doses in soils and it can also be facilitated by FTCs. Our results demonstrate the great potential for increasing Pb immobilization with repeated FTCs, by facilitating the formation of both inner-sphere and outer-sphere complexes. Hence, these findings provide useful information on Pb immobilization in contaminated soils that undergo frequent FTCs and offer an additional insight into predicting Pb behavior in cold and freezing environments like the polar regions.

Plant Taxonomic Diversity Better Explains Soil Fungal and Bacterial Diversity than Functional Diversity in Restored Forest Ecosystems

Plant attributes have direct and indirect effects on soil microbes via plant inputs and plant-mediated soil changes. However, whether plant taxonomic and functional diversities can explain the soil microbial diversity of restored forest ecosystems remains elusive. Here, we tested the linkage between plant attributes and soil microbial communities in four restored forests (Acacia species, Eucalyptus species, mixed coniferous species, mixed native species). The trait-based approaches were applied for plant properties and high-throughput Illumina sequencing was applied for fungal and bacterial diversity. The total number of soil microbial operational taxonomic units (OTUs) varied among the four forests. The highest richness of fungal OTUs was found in the Acacia forest. However, bacterial OTUs were highest in the Eucalyptus forest. Species richness was positively and significantly related to fungal and bacterial richness. Plant taxonomic diversity (species richness and species diversity) explained more of the soil microbial diversity than the functional diversity and soil properties. Prediction of fungal richness was better than that of bacterial richness. In addition, root traits explained more variation than the leaf traits. Overall, plant taxonomic diversity played a more important role than plant functional diversity and soil properties in shaping the soil microbial diversity of the four forests.

The Variation of the Soil Bacterial and Fungal Community Is Linked to Land Use Types in Northeast China

From the west to the east across Northeast China, there are three major land use types, ranging from agricultural-pastoral interlaced land, crop land, and forest land. The soil microbial community of each land use type has been reported; however, a thorough comparison of the soil microbial ecology of soils from each land use type has not been made. In the current study, soil samples from agricultural-pastoral land, crop land, and an artificial economic forest were collected from Tongliao, Siping, and Yanji, respectively. The structure and composition of bacterial and fungal communities was investigated by a next generation sequencing protocol, and soil physicochemical properties were also determined. Pair-wise analysis showed some soil parameters were significantly different between agricultural-pastoral land and crop land or forest land, while those soil parameters shared more similarities in crop land and forest land soils. Principal coordinates analysis and dissimilarity analyses jointly indicated that bacterial and fungal communities from each sampling site were quite different. Canonical correspondence analysis and a partial Mantel test showed that the community structures of bacteria and fungi were mainly affected by clay, pH, water soluble organic carbon (WSOC), and total soluble nitrogen (TN). Co-occurrence network analysis and the associated topological features revealed that the network of the bacterial community was more complex than that of the fungal community. Clay, pH, WSOC, and NH4+-N were major drivers and pH and WSOC were major factors in shaping the network of the bacterial community and the fungal community, respectively. In brief, our results indicated that microbial diversity, co-occurrence network patterns, and their shaping factors differed greatly among soils of distinct land use types in Northeast China. Our data also provided insights into the sustainable use of soils under different land use types.

Application of seasonal freeze-thaw to pretreat raw material for accelerating green waste composting

The recalcitrance of green waste, caused by its high lignocellulose content, is a technical challenge for accelerating green waste composting. However, because lignocellulose degradation in litter (similar to green waste) can be promoted during the freeze-thaw season, and the composting is difficult to implement in this period (due to the low temperature); seasonal freeze-thaw was intended to be used as a pretreatment strategy for the existing technical challenge in the winter of cold regions. In this process, green waste was pretreated with seasonal freeze-thaw to enhance its lignocellulose degradation for subsequent composting. To verify this assumption, two strategies for the pretreatment were used: the green waste was either drenched or immersed in water during the freeze-thaw season, and the effects on subsequent composting were evaluated. The results demonstrated that both strategies can significantly promote the mineralization of TOC (total organic carbon, by 2.73%-8.01% compared with the control, the following comparisons were all based on the control), TN (total nitrogen, by 0.21%-0.52%), and lignocellulose (lignin degradation was promoted by 3.52%-3.73%, cellulose degradation was promoted by 13.23%-14.26%) during composting and that the synthesis of humus was also enhanced (by 19.19%-21.43%). Furthermore, since the loss of NH₄⁺N and NO₃^-N was significantly less in the drenched treatment than in the immersed treatment (by 9.15% for the loss of NH₄⁺N and 7.66% for the loss of NO₃^-N), drenching the green waste during the freeze-thaw season might be a better strategy than immersing for nitrogen conservation. An additional advantage of drenching compared to immersing is water conservation.

Effect of Soil Layer and Plant–Soil Interaction on Soil Microbial Diversity and Function after Canopy Gap Disturbance

Gaps by thinning can have different microclimatic environments compared to surrounding areas, depending on the size of the gap. In addition, gaps can play important roles in biological dynamics, nutrient cycling, and seedling regeneration. The impacts of gap size on soil microbial communities and enzyme activities in different soil layers in Chinese pine plantations are not well understood. Here, we created gaps of 45 m2 (small, G1), 100 m2 (medium, G2), and 190 m2 (large, G3) by thinning unhealthy trees in an aged (i.e., 50 years old) monoculture Chinese pine plantation in 2010. Soil samples were collected in 2015. The total, bacterial, Gram-positive (G+), and Gram-negative (G−) phospholipid fatty acid (PLFA) profiles were highest in medium gaps in both the organic and mineral layers. These indicesdecreased sharply as gap size increased to 190 m2, and each of the detected enzyme activities demonstrated the same trend. Under all the gap size managements, abundances of microbial PLFAs and enzyme activities in the organic layers were higher than in the mineral layers. The soil layer was found to have a stronger influence on soil microbial communities than gap size. Redundancy analysis (RDA) based on the three systems with different gap sizes showed that undergrowth coverage, diversity, soil total nitrogen (TN), total organic carbon (TOC), and available phosphorus (AT) significantly affected soil microbial communities. Our findings highlighted that the effect of gap size on soil microenvironment is valuable information for assessing soil fertility. Medium gaps (i.e., 100 m2) have higher microbial PLFAs, enzyme activity, and soil nutrient availability. These medium gaps are considered favorable for soil microbial communities and fertility studied in a Chinese pine plantation managed on the Loess Plateau.