Responses of ammonia-oxidizing microorganisms to biochar and compost amendments of heavy metals-polluted soil

Effects of nitrogen regulation on heavy metal phytoextraction efficiency (Leucaena leucocephala): Application of a nitrogen fertilizer and a fungal agent

Lead (Pb) and cadmium (Cd) have been identified as the primary contaminants in soil, posing potential health threats. This study aimed to examine the effects of applying a nitrogen fertilizer and a fungal agent Trichoderma harzianum J2 (nitrogen alone, fungi alone, and combined use) on the phytoremediation of soils co-contaminated with Pb and Cd. The growth of Leucaena leucocephala was monitored in the seedling, differentiation, and maturity stages to fully comprehend the remediation mechanisms. In the maturity stage, the biomass of L. leucocephala significantly increased by 18% and 29% under nitrogen-alone (NCK+) and fungal agent-alone treatments (J2), respectively, compared with the control in contaminated soil (CK+). The remediation factors of Pb and Cd with NCK+ treatment significantly increased by 50% and 125%, respectively, while those with J2 treatment increased by 73% and 145%, respectively. The partial least squares path model suggested that the nitrogen-related soil properties were prominent factors affecting phytoextraction compared with biotic factors (microbial diversity and plant growth). This model explained 2.56 of the variation in Cd concentration under J2 treatment, and 2.97 and 2.82 of the variation in Pb concentration under NCK+ and J2 treatments, respectively. The redundancy analysis showed that the samples under NCK+ and J2 treatments were clustered similarly in all growth stages. Also, Chytridiomycota, Mucoromucota, and Ciliophora were the key bioindicators for coping with heavy metals. Overall, a similar remediation mechanism allowed T. harzianum J2 to replace the nitrogen fertilizer to avoid secondary pollution. In addition, their combined use further increased the remediation efficiency.

Biochar and peat amendments affect nitrogen retention, microbial capacity and nitrogen cycling microbial communities in a metal and polycyclic aromatic hydrocarbon contaminated urban soil

Soil contaminants may restrict soil functions. A promising soil remediation method is amendment with biochar, which has the potential to both adsorb contaminants and improve soil health. However, effects of biochar amendment on soil-plant nitrogen (N) dynamics and N cycling microbial guilds in contaminated soils are still poorly understood. Here, a metal- and polycyclic aromatic hydrocarbon (PAH) contaminated soil was amended with either biochar (0, 3, 6 % w/w) and/or peat (0, 1.5, 3 % w/w) in a full-factorial design and sown with perennial ryegrass in an outdoor field trial. After three months, N and the stable isotopic ratio δ15N was measured in soil, roots and leaves, along with microbial responses. Aboveground grass biomass decreased by 30 % and leaf N content by 20 % with biochar, while peat alone had no effect. Peat in particular, but also biochar, stimulated the abundance of microorganisms (measured as 16S rRNA gene copy number) and basal respiration. Microbial substrate utilization (MicroResp™) was altered differentially, as peat increased respiration of all carbon sources, while for biochar, respiration of carboxylic acids increased, sugars decreased, and was unaffected for amino acids. Biochar increased the abundance of ammonia oxidizing archaea, while peat stimulated ammonia oxidizing bacteria, Nitrobacter-type nitrite oxidizers and comB-type complete ammonia oxidizers. Biochar and peat also increased nitrous oxide reducing communities (nosZI and nosZII), while peat alone or combined with biochar also increased abundance of nirK-type denitrifiers. However, biochar and peat lowered leaf δ15N by 2-4 ‰, indicating that processes causing gaseous N losses, like denitrification and ammonia volatilization, were reduced compared to the untreated contaminated soil, probably an effect of biotic N immobilization. Overall, this study shows that in addition to contaminant stabilization, amendment with biochar and peat can increase N retention while improving microbial capacity to perform important soil functions.

The spatiotemporal heterogeneity of fertosphere hotspots impacted by biochar addition and the implications for NH3 and N2O emissions

The fertosphere, as the interfaces between fertilizer granular and soil particles, represents a key hotspot for nitrogen transformation processes, particularly for ammonia (NH3) and nitrous oxide (N2O) emissions. Understanding the heterogeneity of the fertosphere, especially when incorporating organic amendments like biochars, is crucial for predicting NH3 and N2O emissions after soil fertilization. In this study, we investigated the effects of three types of biochar (pristine, aged, and acid-washed biochar) on heterogeneity of fertosphere induced by localized urea application. pH-specific planar optodes were employed to visualize pH gradients in fertosphere hotspots with high spatial and temporal resolution. In addition, we conducted thorough measurements of the gradient distribution of electric conductivity (EC), mineral N, aqueous NH3 in soil and enzyme activities relevant to nitrification. Concurrently, NH3 and N2O emissions from the soil were continuously monitored at a high temporal resolution. Initially, urea-induced fertosphere exhibited significant NH3 emissions, primarily attributed to the pH elevation resulting from urea hydrolysis. However, after 6 days, NH3 emissions subsided, and there was a notable sharp increase in N2O emissions. Importantly, compared to urea application alone, the inclusion of pristine biochar led to a delay in soil pH decline with a 19% rise in NH3 emission. Aged biochar, characterized by a higher content of oxygen functional groups, demonstrated increased NH4+/NH3 adsorption capacity and enhanced ammonia monooxygenase (AMO) activity in soil, resulting in an 18% reduction in NH3 emission. While a slight decrease of 5% in NH3 cumulative emission was observed in the acid-washed biochar treatment. Notably, biochar could potentially promote nitrification-derived N2O emissions due to the accumulation of NH3 oxidation products (NH2OH). These findings could contribute to refining N transformation models for fertilized soils, and optimizing N fertilizer application strategies.

Effect of composite organic amendment on Cd(II) ions stabilization and microbial activity under various ammonium sulfate levels

To attenuate the risk of Cadmium(Cd) contamination and the deterioration of soil quality caused by excessive nitrogen fertilizer application in greenhouse, a composite organic amendment (spend mushroom substrate and its biochar) was prepared to remedy Cd(II) ions contaminated soil (0.6 mg/kg) under different N fertilizer levels. The results showed that in the absence of a composite organic amendment, the soil pH decreased by 0.15 when the N level increased from 0.1 to 0.8 g N⋅kg-1. However, the pH increased by 0.86-0.91, the exchangeable Cd(II) ions content decreased by 26.0%-26.7%, the microbial biomass increased by 34.34%-164.46%, and the number of copies of the AOB gene increased by 13-20 times with the application of composite organic amendment and the increase of N level. Both Pearson correlation analysis and Mantel test demonstrated the reduction in Cd(II) ions availability, the restoration of soil properties and the increase in microbial biomass all contributed to the composite organic amendment, which is of importance for soil remediation under excessive N fertilizer.

Spatial distribution characteristics, risk assessment and management strategies of tailings ponds in China

The tailings ponds (TPS) stemming from mineral resource exploitation are becoming a global challenge due to their high hazards and pollution to the surrounding area. However, previous studies on China's tailings ponds have either focused on a single or few areas, or the number of tailings ponds varies greatly. A systematic assessment of the number, distribution characteristics, potential risks and management strategies of the tailings pond in China is lacking. This study obtained the latest list of tailings ponds in China up to the end of 2022 based on official information and assessed their spatial distribution characteristics, environmental risk and management strategies simultaneously. The results demonstrated that the distribution of TPS in China is relatively clustered and multiple factors affected the spatial distribution of TPS in China, which were concentrated in areas with low economic and population density, convenient transportation, and a developed water system. The risk assessment suggested that 1803 TPS had large or significant environmental risks, which were mainly distributed in Yunnan, Hunan, Shaanxi and Jiangxi provinces. To solve the problem of tailings ponds from the source, the key point of tailings pond management in China should be adjusted from the prevention of pollution or dam break accidents to the full resource utilization of tailings. In summary, this study will provide a scientific basis for the risk control of TPS and an innovative idea for the management of other solid waste.

Towards engineering mitigation of leaching of Cd and Pb in co-contaminated soils using metal oxide-based aerogel composites and biochar

Applications of metal-based nanomaterials for the remediation of heavy metal (HM) contaminated environments are of great importance. The ability of metal oxide-based carbon aerogel composite to immobilize HMs in multi-metal contaminated soils has not yet been investigated, particularly under acidic conditions. Herein, we investigate the performance of metal oxides (Sr0.7 Mn0.3 Co0.5 Fe0.5O3-δ)-based carbon aerogel composite (MO-CAg) compared with coconut coil fiber biochar (CCFB) and carbon aerogel (CAg) for Cd and Pb immobilization in contaminated soil. The MO-CAg, applied at 2% (w/w), significantly decreased Pb leaching by 67-75% and Cd by 60-65%, CAg decreased Cd by 54% and Pb by 46%, while biochar decreased Cd by 40-44% and Pb by 43%. The addition of MO-CAg altered Cd and Pb geochemical fractions by increasing their residual fraction, i.e., stabilized both metals compared to the control. This presents a comprehensive elaboration on the probable reaction interactions between the MO-Cag and heavy metals, including a combination of (co)precipitation, and reduction-oxidation as the predominant mechanisms of metal stabilization with MO-CAg. Moreover, MO-CAg increased Pb and Cd stabilization in soils by strengthening the bonding between metal oxides and Cd/Pb. By imbedding MO into the CAg, in MO-CAg, the immobilization of Cd(II) and Pb(II) occurred through inner-sphere complexation, while with CCFB and CAg metals, immobilization occurred through outer-sphere complexation. MO-CAg is a promising and highly efficient material that could be recommended for the remediation of Cd- and Pb-contaminated soils in subsequent studies.

Composition and assembly mechanisms of prokaryotic communities in wetlands, and their relationships with different vegetation and reclamation methods

Coastal wetlands are undergoing substantial transformations globally as a result of increased human activities. However, compared to other ecosystems, diversity and functional characteristics of microbial communities in reclaimed coastal wetlands are not well studied compared to other ecosystems. This is important because it is known that microorganisms can play a crucial role in biogeochemical cycling within coastal wetland ecosystems. Hence, this study utilized the high-throughput sequencing technique to investigate the structure and assembly processes of microbial communities in reclaimed coastal wetlands. The results revealed a substantial change in soil properties following coastal wetland reclamation. Remarkably, the reclaimed soil exhibited significantly lower pH, soil organic carbon (SOC), and total salinity (TS) values (p < 0.05). The dominant phyla included Proteobacteria, Chloroflexi, Bacteroidetes, Acidobacteria, and Planctomycetes among study sites. However, the relative abundance of Proteobacteria increased from un-reclaimed coastal wetlands to reclaimed ones. The Proteobacteria, Chloroflexi, and Acidobacteria showed higher relative abundance in vegetated soil compared to bare soil, while Bacteroidetes and Planctomycetes exhibited the opposite trend. Notably, vegetation types exerted the strongest influence on microbial diversity, surpassing the effects of soil types and depth (F = 34.49, p < 0.001; F = 25.49, p < 0.001; F = 3.173, p < 0.078, respectively). Stochastic assembly processes dominated in un-reclaimed soil, whereas deterministic processes governed the assembly in artificial sea embankment wetlands (SEW). The presence of Spartina alterniflora in all soil types (except SEW soils) indicated stochastic assembly, while Phragmites australis in reclaimed soils pointed toward deterministic microbial assembly. Furthermore, environmental factors such as pH, soil water content (SWC), SOC, total carbon (TC), total nitrogen (TN), total phosphorus (TP), NH4+-N, vegetation types, soil depth, and geographic distance exhibited significant effects on microbial beta diversity indices. Co-occurrence network analysis revealed a stronger association between taxa in SEW compared to land reclaimed from wetlands (LRW) and natural coastal wetlands (NCW). The bottom soil layer exhibited more complex network interactions than the topsoil layer. Besides soil parameters, reclamation and varieties of vegetation were also substantial factors influencing the composition, diversity, and assembly processes of microbial communities in coastal wetlands.

Insights on the assembly processes and drivers of soil microbial communities in different depth layers in an abandoned polymetallic mining district

Soil microbes, which play crucial roles in maintaining soil functions and restoring degraded lands, are impacted by heavy metal pollution. This study investigated the vertical distribution of bacterial communities along the soil profiles across four types of areas (heavy metal pollution level: tailings heap area > phytoremediation area > natural restoration area > original forest area) in an abandoned polymetallic mining district by 16S rRNA sequencing, and aimed to disentangle the assembly mechanisms and key drivers of the vertical variation in bacterial community structure. Bacterial diversity and composition were found to vary remarkably between the depth layers in all types of areas, with heterogeneous selection dominated the vertical distribution pattern of soil bacterial communities. Pearson correlation analysis and partial Mantel test revealed that soil nutrients mainly shaped the vertical distribution of bacterial microbiota along soil profiles in the original forest and natural restoration areas. Ni, As, and bioavailable As were the key drivers regulating the vertical variation of bacterial assemblages in the phytoremediation area, whereas Pb, pH, soil organic carbon, and available nitrogen were crucial drivers in the tailings heap area. These findings reveal the predominant assembly mechanisms and drivers governing the vertical distribution of soil bacterial microbiota and indicate the efficiency of phytoremediation and ecological restoration on ameliorating edaphic micro-ecosystems in heavy metal-contaminated areas.

Dichlorodiphenyltrichloroethane inhibits soil ammonia oxidation by altering ammonia-oxidizing archaeal and bacterial communities

Dichlorodiphenyltrichloroethane (DDT), a persistent organic pollutant, has known effects on natural microbes. However, its effects on soil ammonia-oxidizing microbes, significant contributors to soil ammoxidation, remain unexplored. To address this, we conducted a 30-day microcosm experiment to systematically study the effects of DDT contamination on soil ammonia oxidation and the communities of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). Our findings revealed that DDT inhibited soil ammonia oxidation in the early stage (0-6 days), but it gradually recovered after 16 days. The copy numbers of amoA gene of AOA decreased in all DDT-treated groups from 2 to 10 days, while that of AOB decreased from 2 to 6 days but increased from 6 to 10 days. DDT influenced the diversity and community composition of AOA but had no significant effect on AOB. Further, the dominant AOA communities comprised uncultured_ammonia-oxidizing_crenarchaeote and Nitrososphaera sp. JG1: while the abundance of the latter significantly and negatively correlated with NH 4+-N (P ≤ 0.001), DDT (0.001 < P ≤ 0.01), and DDD (0.01 < P ≤ 0.05) and positively correlated with NO3--N (P ≤ 0.001), that of the former significantly and positively correlated with DDT (P ≤ 0.001), DDD (P ≤ 0.001), and NH 4+-N (0.01 < P ≤ 0.05) and negatively correlated with NO3--N (P ≤ 0.001). Among AOB, the dominant group was the unclassified Nitrosomonadales in Proteobacteria, which showed significant negative correlation with NH 4+-N (0.01 < P ≤ 0.05) and significant positive correlation with NO3--N (0.001 < P ≤ 0.01). Notably, among AOB, only Nitrosospira sp. III7 exhibited significant negative correlations with DDE (0.001 < P ≤ 0.01), DDT (0.01 < P ≤ 0.05), and DDD (0.01 < P ≤ 0.05). These results indicate that DDT and its metabolites affect soil AOA and AOB, consequently affecting soil ammonia oxidation.

Response of soil microbial activities and ammonia oxidation potential to environmental factors in a typical antimony mining area

Mining, smelting and tailing deposition activities can cause metal(loid) contamination in surrounding soils, threatening ecosystems and human health. Microbial indicators are sensitive to environmental factors and have a crucial role in soil ecological risk assessment. Xikuangshan, the largest active antimony (Sb) mine in the world, was taken as the research area. The soil properties, metal(loid) contents and microbial characteristics were investigated and their internal response relationships were explored by multivariate statistical analysis. The assessment of the single pollution index and Nemerow synthetic pollution index (P_N) showed that the soils were mainly polluted by Sb, followed by Cd and As, in which sampling site S1 had a slight metal(loid) pollution and the other sampling sites suffered from severe synthetic metal(loid) pollution. The microbial characteristics were dissimilar among sampling points at different locations from the mining area according to hierarchical cluster analysis. The correlation analysis indicated that fluorescein diacetate hydrolase, acid phosphatase, soil basal respiration and microbial biomass carbon were negatively correlated with P_N, indicating their sensitivity to combined metal(loid) contamination; that dehydrogenase was positively correlated with pH; and that urease, potential ammonia oxidation and abundance of ammonia-oxidizing bacteria and archaea were correlated with N (nitrogen) contents. However, β-glucosidase activity had no significant correlations with physicochemical properties and metal(loid) contents. Principal components analysis suggested bioavailable Sb and pH were the dominant factors of soil environment in Xikuangshan Sb mining area. Our results can provide a theoretical basis for ecological risk assessment of contaminated soil.

Rhizosphere bacteria regulated arsenic bioavailability and accumulation in the soil-Chinese cabbage system

The accumulation of arsenic (As) in Chinese cabbage (Brassica rapa ssp. pekinensis) has recently been a source of concern for a potential risk to human health. It is unknown whether natural variations of As accumulation in different genotypes of Chinese cabbage are related to rhizobacterial characteristics. Experiments were conducted to investigate the mechanisms of rhizobacteria involving in As fates in a soil-Chinese cabbage system using various genotypes using high-throughput sequencing and quantitative PCR. There were significant differences in As accumulation in cabbage leaves between 32 genotypes, and genotypes of low-As-accumulation (LSA) and high-As-accumulation (HSA) were identified. The As concentrations in the shoots of LSA were 23.25 %, 24.19 %, 15.05 %, and 70.69 % lower than those of HSA in seedling stage (SS), rosette stage (RS), heading stage (HS), and mature stage (MS), respectively. Meanwhile, the relative abundances of phyla Patescibacteria (in RS), Acidobacteria and Rokubacteria (in HS) in the rhizosphere of LSA were 60.18 %, 28.19 %, and 45.38 % less than those of HSA, respectively. Additionally, both shoot-As and As translocation factor had significantly positive or negative correlations with the relative abundances of Rokubacteria or Actinobacteria. In LSA rhizosphere, the relative abundances of genera Flavobacterium (in SS), Ellin6055 and Sphingomonas (in HS) were 128.12 %, 83.69 % and 79.50 % higher than those of HSA, respectively. This demonstrated that rhizobacteria contribute to the accumulation and translocation of As in HSA and LSA. Furthermore, the gene copies of aioA and arsM in LSA rhizosphere were 25.54 % and 16.13 % higher than those of HSA, respectively, whereas the gene copies of arsC in LSA rhizosphere were 26.36 % less than those of HSA in MS, indicating that rhizobacteria are involved in As biotransformation in the soil. These results provide a comprehensive understanding of the relationship between characteristics of rhizobacterial communities and As variations in Chinese cabbage genotypes.

Effects of Long-Term Use of Organic Fertilizer with Different Dosages on Soil Improvement, Nitrogen Transformation, Tea Yield and Quality in Acidified Tea Plantations

In this study, sheep manure fertilizers with different dosages were used for five consecutive years to treat acidified tea plantation soils, and the effects of sheep manure fertilizer on soil pH value, nitrogen transformation, and tea yield and quality were analyzed. The results showed that soil pH value showed an increasing trend after a continuous use of sheep manure fertilizer from 2018 to 2022. After the use of low dosage of sheep manure fertilizer (6 t/hm²-15 t/hm²), tea yield, the content of tea quality indicators (tea polyphenols, theanine, amino acid, and caffeine) and soil ammonium nitrogen content, ammoniating bacteria number, ammoniating intensity, urease activity and protease activity showed increasing trends and were significantly and positively correlated to soil pH value, while the related indexes showed increasing and then decreasing trends after the use of high dosage of sheep manure fertilizer (18 t/hm²). Secondly, the nitrate nitrogen content, nitrifying bacteria number, nitrifying intensity, nitrate reductase activity, and nitrite reductase activity showed decreasing trends after the use of low dosage of sheep manure fertilizer and showed significant negative correlations with soil pH value, while the related indexes showed decreasing trends after the use of high dosage of sheep manure and then increased. The results of principal component and interaction analysis showed that the effects of sheep manure fertilizers with different dosages on tea yield and quality were mainly based on the transformation ability of ammonium nitrogen and nitrate nitrogen in the soil, and the strong transformation ability of ammonium nitrogen and the high ammonium nitrogen content in the soil were conducive to the improvement of tea yield and quality, and vice versa. The results of topsis comprehensive evaluation and analysis showed that the most influential effect on the fertilization effect was the ammonium nitrogen content in the soil and long-term treatment with 15 t/hm² of sheep manure fertilizer had the highest proximity to the best fertilization effect. This study provided an important practical basis for the remediation and fertilizer management in acidified tea plantation soils.

Biochar application for greenhouse gas mitigation, contaminants immobilization and soil fertility enhancement: A state-of-the-art review

Rising global temperature, pollution load, and energy crises are serious problems, recently facing the world. Scientists around the world are ambitious to find eco-friendly and cost-effective routes for resolving these problems. Biochar has emerged as an agent for environmental remediation and has proven to be the effective sorbent to inorganic and organic pollutants in water and soil. Endowed with unique attributes such as porous structure, larger specific surface area (SSA), abundant surface functional groups, better cation exchange capacity (CEC), strong adsorption capacity, high environmental stability, embedded minerals, and micronutrients, biochar is presented as a promising material for environmental management, reduction in greenhouse gases (GHGs) emissions, soil management, and soil fertility enhancement. Therefore, the current review covers the influence of key factors (pyrolysis temperature, retention time, gas flow rate, and reactor design) on the production yield and property of biochar. Furthermore, this review emphasizes the diverse application of biochar such as waste management, construction material, adsorptive removal of petroleum and oil from aqueous media, immobilization of contaminants, carbon sequestration, and their role in climate change mitigation, soil conditioner, along with opportunities and challenges. Finally, this review discusses the evaluation of biochar standardization by different international agencies and their economic perspective.

DNA extraction leads to bias in bacterial quantification by qPCR

Quantitative PCR (qPCR) has become a widely used technique for bacterial quantification. The affordability, ease of experimental design, reproducibility, and robustness of qPCR experiments contribute to its success. The establishment of guidelines for minimum information for publication of qPCR experiments, now more than 10 years ago, aimed to mitigate the publication of contradictory data. Unfortunately, there are still a significant number of recent research articles that do not consider the main pitfalls of qPCR for quantification of biological samples, which undoubtedly leads to biased experimental conclusions. qPCR experiments have two main issues that need to be properly tackled: those related to the extraction and purification of genomic DNA and those related to the thermal amplification process. This mini-review provides an updated literature survey that critically analyzes the following key aspects of bacterial quantification by qPCR: (i) the normalization of qPCR results by using exogenous controls, (ii) the construction of adequate calibration curves, and (iii) the determination of qPCR reaction efficiency. It is primarily focused on original papers published last year, where qPCR was applied to quantify bacterial species in different types of biological samples, including multi-species biofilms, human fluids, and water and soil samples. KEY POINTS: • qPCR is a widely used technique used for absolute bacterial quantification. • Recently published papers lack proper qPCR methodologies. • Not including proper qPCR controls significantly affect experimental conclusions.

Soil microbial community assembly model in response to heavy metal pollution

Heavy metal pollution affected the stability and function of soil ecosystem. The impact of heavy metals on soil microbial community and the interaction of microbial community has been widely studied, but little was known about the response of community assembly to the heavy metal pollution. In this study, we collected 30 soil samples from non (CON), moderately (CL) and severely (CH) contaminated fields. The prokaryotic community was studied using high-throughput Illumina sequencing of 16s rRNA gene amplicons, and community assembly were quantified using phylogenetic-bin-based null approach (iCAMP). Results showed that diversity and composition of both bacterial and archaeal community changed significantly in response to heavy metal pollution. The microbial community assembly tended to be more deterministic with the increase of heavy metal concentration. Among the assembly processes, the relative importance of homogeneous selection (deterministic process) increased significantly (increased by 16.2%), and the relative importance of drift and dispersal limitation (stochastic process) decreased significantly (decreased by 11.4% and 5.4%, respectively). The determinacy of bacterial and archaeal community assembly also increased with heavy metal stress, but the assembly models were different. The deterministic proportion of microorganisms tolerant to heavy metals, such as Thiobacillus, Euryarchaeota and Crenarchaeota (clustered in bin 32, bin59 and bin60, respectively) increased, while the stochastic proportion of microorganisms sensitive to heavy metals, such as Koribacteraceae (clustered in bin23) increased. Therefore, the heavy metal stress made the prokaryotic community be deterministic, however, the effects on the assembly process of different microbial groups differed obviously.

Biochar-microorganism interactions for organic pollutant remediation: Challenges and perspectives

Numerous harmful chemicals are introduced every year in the environment through anthropogenic and geological activities raising global concerns of their ecotoxicological effects and decontamination strategies. Biochar technology has been recognized as an important pillar for recycling of biomass, contributing to the carbon capture and bioenergy industries, and remediation of contaminated soil, sediments and water. This paper aims to critically review the application potential of biochar with a special focus on the synergistic and antagonistic effects on contaminant-degrading microorganisms in single and mixed-contaminated systems. Owing to the high specific surface area, porous structure, and compatible surface chemistry, biochar can support the proliferation and activity of contaminant-degrading microorganisms. A combination of biochar and microorganisms to remove a variety of contaminants has gained popularity in recent years alongside traditional chemical and physical remediation technologies. The microbial compatibility of biochar can be improved by optimizing the surface parameters so that toxic pollutant release is minimized, biofilm formation is encouraged, and microbial populations are enhanced. Biocompatible biochar thus shows potential in the bioremediation of organic contaminants by harboring microbial populations, releasing contaminant-degrading enzymes, and protecting beneficial microorganisms from immediate toxicity of surrounding contaminants. This review recommends that biochar-microorganism co-deployment holds a great potential for the removal of contaminants thereby reducing the risk of organic contaminants to human and environmental health.

The characteristics of alkaline phosphatase activity and phoD gene community in heavy-metal contaminated soil remediated by biochar and compost

This research was carried out to determine the influence of biochar and compost addition on the characteristics of potential alkaline phosphatase (ALP) activity and phoD gene community in heavy metal polluted soils. The ALP activity, the abundance and structure of phoD gene were systematically determined. Results showed that biochar and compost significantly changed soil properties, and promoted the microbial transformation of phosphorus. Compost addition significantly increased the ALP activity. Biochar and compost addition markedly increased the phoD gene abundance. The addition of biochar increased the proportion of Actinobacteria, Euryarchaeota, and Proteobacteria. By contrast, Betaproteobacteria, Deltaproteobacteria, and Gammaproteobacteria were the dominant taxa in soils with compost addition. Electrical conductivity critically controlled the expression of phoD and changed the structure of phoD-coding microbial communities in heavy-metal polluted soils that remediated by biochar and compost.

Acid-Modified Biochar Impacts on Soil Properties and Biochemical Characteristics of Crops Grown in Saline-Sodic Soils

Soil salinity and sodicity is a potential soil risk and a major reason for reduced soil productivity in many areas of the world. This study was conducted to investigate the effect of different biochar raw materials and the effects of acid-modified biochar on alleviating abiotic stresses from saline-sodic soil and its effect on biochemical properties of maize and wheat productivity. A field experiment was conducted as a randomized complete block design during the seasons of 2019/2020, with five treatments and three replicates: untreated soil (CK), rice straw biochar (RSB), cotton stalk biochar (CSB), rice straw-modified biochar (RSMB), and cotton stalk-modified biochar (CSMB). FTIR and X-ray diffraction patterns indicated that acid modification of biochar has potential effects for improving its properties via porous functions, surface functional groups and mineral compositions. The CSMB treatment enhanced the soil’s physical and chemical properties and porosity via EC, ESP, CEC, SOC and BD by 28.79%, 20.95%, 11.49%, 9.09%, 11.51% and 12.68% in the upper 0–20 cm, respectively, compared to the initial properties after the second season. Soil-available N, P and K increased with modified biochar treatments compared to original biochar types. Data showed increases in grain/straw yield with CSMB amendments by 34.15% and 29.82% for maize and 25.11% and 15.03% for wheat plants, respectively, compared to the control. Total N, P and K contents in both maize and wheat plants increased significantly with biochar application. CSMB recorded the highest accumulations of proline contents and SOD, POD and CAT antioxidant enzyme activity. These results suggest that the acid-modified biochar can be considered an eco-friendly, cheaper and effective choice in alleviating abiotic stresses from saline-sodic soil and positively effects maize and wheat productivity.

Silicon fertilizers, humic acid and their impact on physicochemical properties, availability and distribution of heavy metals in soil and soil aggregates

It has been confirmed that silicon (Si) fertilizer and humic acid (HA) could effectively decrease the heavy metals in soil. Nonetheless, the impact of these additives on soil aggregate characteristics was ignored. Therefore, the effects of Si fertilizer, HA, and their combinations on the physicochemical characteristics, availability of heavy metals (Cu, Cd, Pb, Zn), and fraction changes in soils and soil aggregates were investigated in this research. The results showed that Si fertilizer and HA significantly modified soil properties such as soil pH, electrical conductivity total organic carbon, water-soluble organic carbon, and nitrate‑nitrogen. HA and Si-HA (SHA) supplementation significantly decreased the availability of Cu, Cd, Pd, and Zn. Besides, there was no significant difference in physicochemical properties between soil and soil aggregates. The availability of Cu, Cd, Pd, and Zn in soil aggregates could be significantly inhibited by the addition of HA and SHA, and the content in microaggregates was greater than that in macroaggregates. After the addition of the three additives, the main fractions of heavy metals in different particle sizes were changed and eventually transformed to the residue state. These results indicated that Si fertilizer, HA, and SHA were influential in physicochemical properties and metal availability in soil aggregates. Therefore, it is of great scientific significance to study the impact of heavy metal pollution on the ecological environment in different aggregates, which will provide reference data for future sustainable management of heavy-metal polluted soils.

Remediation of cadmium contaminated soil by composite spent mushroom substrate organic amendment under high nitrogen level

Cadmium (Cd) contamination in soil poses a serious threat to ecological environment and crop quality, especially under high nitrogen level. Here, the efficiency of composite organic amendment (spent mushroom substrate and its biochar) on remediation of Cd contaminated soil under high nitrogen level has been studied through a 42 days' soil incubation experiment. The results showed: (i) the application of composite organic amendment minimized the repercussions of high nitrogen and significantly reduced the exchangeable Cd proportion by 28.3%-29.5%, especially for Ca(NO₃)₂ treatment; (ii) the application of composite organic amendment improved the physicochemical properties of soil, such as pH, CEC and organic matter content increased by 0.63-0.99 unit, 39.69%-45.00% and 7.77%-11.47%, and EC decreased by 16.21%-44.47% compared with non-amendment Cd-contaminated soil, respectively; (iii) the application of composite organic amendment significantly increased the soil enzyme activities and microbial biomass, among which urease activity was increased most by 12.06-16.42 mg·g^-1·d^-1, and the copy number of AOA was decreased by 30.6%- 92.0%, and the copy number of AOB was increased most by about 45 times. In brief, the composite organic amendment can alleviate the adverse effects of Cd and nitrogen on the soil, but its long-term efficacy needs to be verified in further field study.

Biochar and nitrogen fertilizer co-application changed SOC content and fraction composition in Huang-Huai-Hai plain, China

Biochar can significantly enhance soil organic carbon (SOC) and crop yield, and it is therefore the preferred material for soil improvement in medium-low yield fields. In this study, a field experiment was designed to explore the impacts of biochar application on SOC content and fraction composition. Results indicated that incorporation of biochar into soil increased the SOC content by 26.9%-65.3% in the surface layer (0-10 cm) and 30.3%-63.0% in the subsurface layer (10-20 cm) of soil, while water-soluble organic carbon (WSOC) of the two layers was increased by 2.2-40.0% and 2.3-39.8%, respectively. Microbial biomass carbon decreased under conventional nitrogen treatments and increased with biochar addition under increased nitrogen application. The C:N value increased with biochar application, while the water-soluble C:N value of soil applied with 30 t ha^-1 biochar was lower than that of soil applied with 15 t ha^-1 biochar, both in the two tested soil layers. Wheat yield is evidently correlated with SOC, with the correlation coefficients of 0.919 and 0.952 in the surface and subsurface soil layers (P < 0.01), respectively. Particularly, increasing fulvic and humic acid-like compounds of WSOC promoted the bioavailability of nutrient elements, thereby increasing the crop yields. Therefore, biochar application is an effective means to fertilize middle-low yield soils through increasing SOC sequestration and nutrient reserves, or adjusting soil C:N value to a proper range, thereby reducing nutrient loss and increasing wheat yield.

Impact of Biochar Amendment on Soil Properties and Organic Matter Composition in Trace Element-Contaminated Soil

The application of biochar as an organic amendment in polluted soils can facilitate their recovery by reducing the availability of contaminants. In the present work, the effect of biochar application to acid soils contaminated by heavy metal spillage is studied to assess its effect on the quantity and composition of soil organic matter (SOM), with special attention given to soil humic acids (HAs). This effect is poorly known and of great importance, as HA is one of the most active components of SOM. The field experiment was carried out in 12 field plots of fluvisols, with moderate and high contamination by trace elements (called MAS and AS, respectively), that are located in the Guadiamar Green Corridor (SW Spain), which were amended with 8 Mg·ha⁻¹ of olive pit biochar (OB) and rice husk biochar (RB). The results indicate that 22 months after biochar application, a noticeable increase in soil water holding capacity, total organic carbon content, and soil pH were observed. The amounts of oxidisable carbon (C) and extracted HAs in the soils were not altered due to biochar addition. Thermogravimetric analyses of HAs showed an increase in the abundance of the most thermostable OM fraction of the MAS (375–650 °C), whereas the HAs of AS were enriched in the intermediate fraction (200–375 °C). Spectroscopic and chromatographic analyses indicate that the addition of biochar did not alter the composition of the organic fraction of HAs, while Cu, Fe, and as were considerably accumulated at HAs.

Biochar Addition Inhibits Nitrification by Shifting Community Structure of Ammonia-Oxidizing Microorganisms in Salt-Affected Irrigation-Silting Soil

Biochar has been widely recognized as an effective and eco-friendly ameliorant for saline soils, but information about the mechanism of how biochar influences nitrification in salt-affected agroecosystem remains fragmented. An incubation experiment was performed on the salt-affected soil collected from a three-consecutive-year experiment at biochar application gradients of 7.5 t⋅ha−1, 15 t⋅ha−1 and 30⋅t ha−1 and under nitrogen (N) fertilization. Responses of the nitrification rate (NR), numbers of ammonia monooxygenase (amoA) gene copies, and community structures of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to biochar application were investigated. The results indicated that, under N fertilization, the NR and numbers of amoA-AOB and amoA-AOA gene copies negatively responded to biochar addition. Biochar application increased the community diversity of AOB but decreased that of AOA. Biochar addition and N fertilization shifted the AOB community from Nitrosospira-dominated to Nitrosospira and Nitrosomonas-dominated, and altered the AOA community from Nitrososphaera-dominated to Nitrososphaera and Nitrosopumilus-dominated. The relative abundance of Nitrosospira, Nitrosomonas and Nitrosopumilus decreased, and that of Nitrosovibrio and Nitrososphaera increased with biochar application rate. Soil SOC, pH and NO3−-N explained 87.1% of the variation in the AOB community, and 78.1% of the variation in the AOA community was explanatory by soil pH and SOC. The SOC and NO3−-N influenced NR through Nitrosovibrio, Nitrosomonas, Norank_c_environmental_samples_p_Crenarchaeota and amoA-AOB and amoA-AOA gene abundance. Therefore, biochar addition inhibited nitrification in salt-affected irrigation-silting soil by shifting the community structures of AOB and AOA and reducing the relative abundance of dominant functional ammonia-oxidizers, such as Nitrosospira, Nitrosomonas and Nitrosopumilus.

Accelerated humification and alteration of microbial communities by distillers' grains addition during rice straw composting

This research explored the influence of distillers' grains amendment on the humification performance and microbial communities during rice straw composting. The composition of dissolved organic matter and maturity index were analyzed by the fluorescence excitation emission matrix spectroscopy and parallel factor analysis. High-throughput sequencing and redundancy analysis were employed for revealing the structure dynamics for microbial community and their shaping factors, respectively. Results indicated that addition of distillers' grains effectively increased the microbial activity, which was beneficial to the organic matter degradation and nitrogen conservation. Microbial community structures were significantly changed with different amendment strategies. Nitrate, water soluble carbon, organic matter, ammonium were the key parameters influencing the variation of bacterial community in different treatments. Water soluble carbon significantly affected the dominant fungal community dynamics. These results showed that addition of distillers' grains effectively improved the nutritional status and changed the microbial communities during rice straw composting.

Characteristics and Influencing Factors of Microbial Community in Heavy Metal Contaminated Soil under Silicon Fertilizer and Biochar Remediation

Silicon fertilizer and biochar have been widely used to remediate soil contaminated by heavy metals. The effects and mechanism of silicon fertilizer and biochar addition on the heavy metal availability, soil biological properties, and microbial community characteristics need further study in soils contaminated by heavy metals. Therefore, this research determined how silicon fertilizer, biochar, and their combined using affected microbial communities related with nitrogen and phosphorus cycling. The abundance and composition of the microbial community were evaluated by quantitative PCR and phospholipid fatty acid analysis, respectively. Results showed that silicon fertilizer and biochar addition significantly changed soil properties, including pH, total organic carbon, ammonium, nitrate. The Cd and Zn speciation were significantly reduced by silicon fertilizer, biochar, and their integrated application. Microbial community abundance and structure were also significantly changed. Principal component analysis shows that the difference in soil microbial community structure is the most obvious under the combined addition of biochar, silicon fertilizer and biochar. In addition, the results of fluorescence quantitative PCR showed that with biological addition, the number of soil bacteria was significantly reduced. This study reveals the influence of silicon fertilizer and biochar on bacterial and fungal communities in heavy metal soils and the effect of soil heavy metal availability.

Heavy metals passivation driven by the interaction of organic fractions and functional bacteria during biochar/montmorillonite-amended composting

The aim of this study was to identify critical driving factors and pathways of mitigating heavy metals (HM) bioavailability during biochar/montmorillonite-amended composting: emphasize on the interaction effect between organic constituents and functional bacteria. Organic components, such as humus (HS), humic (HA) and fulvic acid (FA) and dissolved organic carbon (DOC), exhibited indivisible links with Cu and Zn speciation, which confirmed their vital roles on deactivating Cu and Zn. Network analysis indicated that biochar/montmorillonite obviously increased the diversity of Cu resistant/actor and Zn actor bacteria, which aided in HM passivation. Although multiple pathways were involved in regulating Cu/Zn passivation, the interaction of bacteria and organic constituents was the most critical driving factor. Given that, promoting potential HM resistant/actor bacteria utilizing and transforming low-humification organic fractions coupling with elevating high-humification constituents were the optimal pathway. This study is helpful to practical application of biochar/montmorillonite to inactivate HM for industrial composting.

Effects of landfill refuse on the reductive dechlorination of pentachlorophenol and speciation transformation of heavy metals

Landfill refuse is a mixture of inorganic minerals and organic matter that is capable of undergoing complexation and redox reactions due to its active functional groups. Organic matter often combines with minerals in landfill refuse and it remains unclear whether this combination involves electron transfer. Therefore, the effects of landfill refuse composition on reductive dechlorination and speciation transformation of heavy metals were investigated in this study. Results show that landfill refuse comprises protein- and humic-like substances, aliphatic structures, and a large number of hydroxyl, carboxyl, quinoid and other active functional group. The electron donating capacity (0.09-0.26 μmol/g(C)) of landfill refuse was found to be higher than its electron accepting capacity (0.03-0.23 μmol/g(C)), indicating that electron donating groups (hydroxyl) were the main redox-active moieties, facilitating the reductive dechlorination of pentachlorophenol (PCP) by microorganism. Fe₂O₃, FeO and SiO₂ were the main inorganic minerals affecting PCP dechlorination. The speciation distribution of heavy metals in landfill refuse was determined by the BCR sequential extraction method. Results showed that Zn and Ni have high potential migration capacity, poor stability and the highest bioavailability, while Cr, Cu and Pb are relatively stable and have weak migration potential. The oxygen- and nitrogen-containing functional groups, aliphatic structures and aromatic carbon in landfill refuse can promote the transformation of Ni and Cr from an unstable to stable state. Protein-like substances exhibit a strong Cu binding ability, allowing Cu to combine with organic matter more easily than other assessed heavy metals. Both Fe₂O₃ and FeO affected the stability of Cu. FeO promoted the stabilization of Zn, whereas Fe₂O₃ and SiO₂ promoted Cu instability. These results could provide some references for the treatment of organic chlorides and the stabilization of heavy metals in landfill refuse in China.