The harmful effects of microplastics (MPs) pollution in the soil ecosystem have drawn global attention in recent years. This paper critically reviews the effects of MPs on soil microbial diversity and functions in relation to nutrients and carbon cycling. Reports suggested that both plastisphere (MP-microbe consortium) and MP-contaminated soils had distinct and lower microbial diversity than that of non-contaminated soils. Alteration in soil physicochemical properties and microbial interactions within the plastisphere facilitated the enrichment of plastic-degrading microorganisms, including those involved in carbon (C) and nutrient cycling. MPs conferred a significant increase in the relative abundance of soil nitrogen (N)-fixing and phosphorus (P)-solubilizing bacteria, while decreased the abundance of soil nitrifiers and ammonia oxidisers. Depending on soil types, MPs increased bioavailable N and P contents and nitrous oxide emission in some instances. Furthermore, MPs regulated soil microbial functional activities owing to the combined toxicity of organic and inorganic contaminants derived from MPs and contaminants frequently encountered in the soil environment. However, a thorough understanding of the interactions among soil microorganisms, MPs and other contaminants still needs to develop. Since currently available reports are mostly based on short-term laboratory experiments, field investigations are needed to assess the long-term impact of MPs (at environmentally relevant concentration) on soil microorganisms and their functions under different soil types and agro-climatic conditions.
Enhancing crop yield to accommodate the ever-increasing world population has become critical, and diminishing arable land has pressured current agricultural practices. Intensive farming methods have been using more pesticides and insecticides (biocides), culminating in soil deposition, negatively impacting the microbiome. Hence, a deeper understanding of the interaction and impact of pesticides and insecticides on microbial communities is required for the scientific community. This review highlights the recent findings concerning the possible impacts of biocides on various soil microorganisms and their diversity. This review's bibliometric analysis emphasised the recent developments' statistics based on the Scopus document search. Pesticides and insecticides are reported to degrade microbes' structure, cellular processes, and distinct biochemical reactions at cellular and biochemical levels. Several biocides disrupt the relationship between plants and their microbial symbionts, hindering beneficial biological activities that are widely discussed. Most microbial target sites of or receptors are biomolecules, and biocides bind with the receptor through a ligand-based mechanism. The biomarker action mechanism in response to biocides relies on activating the receptor site by specific biochemical interactions. The production of electrophilic or nucleophilic species, free radicals, and redox-reactive agents are the significant factors of biocide's metabolic reaction. Most studies considered for the review reported the negative impact of biocides on the soil microbial community; hence, technological development is required regarding eco-friendly pesticide and insecticide, which has less or no impact on the soil microbial community.
Treatment of petroleum-contaminated soil to a less toxic medium via physical and chemical treatment is too costly and requires posttreatment. This review focuses on the employment of phytoremediation and mycoremediation technologies in cleaning hydrocarbon-contaminated soil which is currently rare. It is considered environmentally beneficial and possibly cost-effective as it implements the synergistic interaction between plants and biosurfactant producing mycorrhiza to degrade hydrocarbon contaminants. This review also covers possible sources of hydrocarbon pollution in water and soil, toxicity effects, and current technologies for hydrocarbon removal and degradation. In addition to these problems, this review also discusses the challenges and opportunities of transforming the resultant treated sludge and treating plants into potential by-products for a higher quality of life for future generations.
In understanding stress response mechanisms in fungi, cold stress has received less attention than heat stress. However, cold stress has shown its importance in various research fields. The following study examined the cold stress response of six Pseudogymnoascus spp. isolated from various biogeographical regions through a proteomic approach. In total, 2541 proteins were identified with high confidence. Gene Ontology enrichment analysis showed diversity in the cold stress response pathways for all six Pseudogymnoascus spp. isolates, with metabolic and translation-related processes being prominent in most isolates. 25.6% of the proteins with an increase in relative abundance were increased by more than 3.0-fold. There was no link between the geographical origin of the isolates and the cold stress response of Pseudogymnoascus spp. However, one Antarctic isolate, sp3, showed a distinctive cold stress response profile involving increased flavin/riboflavin biosynthesis and methane metabolism. This Antarctic isolate (sp3) was also the only one that showed decreased phospholipid metabolism in cold stress conditions. This work will improve our understanding of the mechanisms of cold stress response and adaptation in psychrotolerant soil microfungi, with specific attention to the fungal genus Pseudogymnoascus.
Bioaugmentation helps to obtain a microbiome capable of remediating polycyclic aromatic hydrocarbons (PAHs). In this study, acclimation of microorganisms to soil supplemented with phenanthrene (PHE) led to enrichment with PAH-degraders, including those in Actinobacteriota and in the genera Streptomyces, Rhodococcus, Nocardioides, Sphingomonas, and Mycobacterium. Aqueous (28 °C, pH 6.5) and soil cultures inoculated with PHE-acclimated soil showed a high PHE (ca. 50 mg L-1) degradation efficiency. The PHE degradation kinetics in aqueous and soil incubations fitted to the Gompertz equation and the first-order kinetic equation, respectively. Indigenous microorganisms adapted to PHE in their environment, and this increased their capacity to degrade PHE. The effect of co-contaminants and pathway intermediates on PHE degradation showed that the degradation of PHE improved in the presence of diesel while being hindered by lubricant oil, catechol, salicylic and phthalic acid. Our findings provide theoretical and practical support for bioremediationof PAHs in the environment.
A novel kinetic model has been developed to explain the degradation of total petroleum hydrocarbons. Microbiome engineered biochar amendment may result in a synergistic impact on degradation of total petroleum hydrocarbons (TPHs). Therefore, the present study analyzed the potential of hydrocarbon-degrading bacteria A designated as Aeromonas hydrophila YL17 and B as Shewanella putrefaciens Pdp11 morphological characterized as rod shaped, anaerobic and gram-negative immobilized on biochar, and the degradation efficiency was measured by gravimetric analysis and gas chromatography-mass spectrometry (GC-MS). Whole genome sequencing of both strains revealed the existence of genes responsible for hydrocarbon degradation. In 60 days remediation setup, the treatment consisting of immobilization of both strains on biochar proved more efficient with less half-life and better biodegradation potentials compared to biochar without strains for decreasing the content of TPHs and n-alkanes (C12-C18). Enzymatic content and microbiological respiration showed that biochar acted as a soil fertilizer and carbon reservoir and enhanced microbial activities. The removal efficiency of hydrocarbons was found to be a maximum of 67% in soil samples treated with biochar immobilized with both strains (A + B), followed by biochar immobilized with strain B 34%, biochar immobilized with strain A 29% and with biochar 24%, respectively. A 39%, 36%, and 41% increase was observed in fluorescein diacetate (FDA) hydrolysis, polyphenol oxidase and dehydrogenase activities in immobilized biochar with both strains as compared to control and individual treatment of biochar and strains. An increase of 35% was observed in the respiration rate with the immobilization of both strains on biochar. While a maximum colony forming unit (CFU/g) was found 9.25 with immobilization of both strains on biochar at 40 days of remediation. The degradation efficiency was due to synergistic effect of both biochar and bacteria based amendment on the soil enzymatic activity and microbial respiration.
Widespread and inadequate use of Monocrotophos has led to several environmental issues. Biodegradation is an ecofriendly method used for detoxification of toxic monocrotophos. In the present study, Msd2 bacterial strain was isolated from the cotton plant growing in contaminated sites of Sahiwal, Pakistan. Msd2 is capable of utilizing the monocrotophos (MCP) organophosphate pesticide as its sole carbon source for growth. Msd2 was identified as Brucella intermedia on the basis of morphology, biochemical characterization and 16S rRNA sequencing. B. intermedia showed tolerance of MCP up to 100 ppm. The presence of opd candidate gene for pesticide degradation, gives credence to B. intermedia as an effective bacterium to degrade MCP. Screening of the B. intermedia strain Msd2 for plant growth promoting activities revealed its ability to produce ammonia, exopolysaccharides, catalase, amylase and ACC-deaminase, and phosphorus, zinc and potassium solubilization. The optimization of the growth parameters (temperatures, shaking rpm, and pH level) of the MCP-degrading isolate was carried out in minimal salt broth supplemented with MCP. The optimal pH, temperature, and rpm for Msd2 growth were observed as pH 6, 35 °C, and 120 rpm, respectively. Based on optimization results, batch degradation experiment was performed. Biodegradation of MCP by B. intermedia was monitored using HPLC and recorded 78% degradation of MCP at 100 ppm concentration within 7 days of incubation. Degradation of MCP by Msd2 followed the first order reaction kinetics. Plant growth promoting and multi-stress tolerance ability of Msd2 was confirmed by molecular analysis. It is concluded that Brucella intermedia strain Msd2 could be beneficial as potential biological agent for an effective bioremediation for polluted environments.
Ecosystem functions directly depend upon biophysical as well as biogeochemical reactions occurring at the soil-microbe-plant interface. Environment is considered as a major driver of any ecosystem and for the distributions of living organisms. Any changes in climate may potentially alter the composition of communities i.e., plants, soil microbes and the interactions between them. Since the impacts of global climate change are not short-term, it is indispensable to appraise its effects on different life forms including soil-microbe-plant interactions. This article highlights the crucial role that microbial communities play in interacting with plants under environmental disturbances, especially thermal and water stress. We reviewed that in response to the environmental changes, actions and reactions of plants and microbes vary markedly within an ecosystem. Changes in environment and climate like warming, CO2 elevation, and moisture deficiency impact plant and microbial performance, their diversity and ultimately community structure. Plant and soil feedbacks also affect interacting species and modify community composition. The interactive relationship between plants and soil microbes is critically important for structuring terrestrial ecosystems. The anticipated climate change is aggravating the living conditions for soil microbes and plants. The environmental insecurity and complications are not short-term and limited to any particular type of organism. We have appraised effects of climate change on the soil inhabiting microbes and plants in a broader prospect. This article highlights the unique qualities of tripartite interaction between plant-soil-microbe under climate change.
This study aims to investigate the positive effects of the combined use of Enterobacter cloacae and biochar on improving nitrogen (N) utilization. The greenhouse pots experimental results showed the synergy of biochar and E. cloacae increased soil total N content and plant N uptake by 33.54% and 15.1%, respectively. Soil nitrogenase (NIT) activity increased by 253.02%. Ammonia monooxygenase (AMO) and nitrate reductase (NR) activity associated with nitrification and denitrification decreased by 10.94% and 29.09%, respectively. The relative abundance of N fixing microorganisms like Burkholderia and Bradyrhizobium significantly increased. Sphingomonas and Ottowia, two bacteria involved in the nitrification and denitrification processes, were found to be in lower numbers. The E. cloacae's ability to fix N2 and promote the growth of plants allow the retention of N in soil and make more N available for plant development. Biochar served as a reservoir of N for plants by adsorbing N from the soil and providing a shelter for E. cloacae. Thus, biochar and E. cloacae form a synergy for the management of agricultural N and the mitigation of negative impacts of pollution caused by excessive use of N fertilizer.
Vast amounts of plastic waste are causing serious environmental issues and urge to develop of new remediation methods. The aim of the study is to determine the role of inorganic (nitric acid), organic (starch addition), and biological (Pseudomonas aeruginosa) soil amendments on the degradation of Polyethylene (PE) and phytotoxic assessment for the growth of lettuce plant. The PE-degrading bacteria were isolated from the plastic-contaminated soil. The strain was identified as Pseudomonas aeruginosa (OP007126) and showed the highest degradation percentage for PE. PE was pre-treated with nitric acid as well as starch and incubated in the soil, whereas P. aeruginosa was also inoculated in PE-contaminated soils. Different combinations were also tested. FTIR analysis and weight reduction showed that though nitric acid was efficient in degradation, the combined application of starch and bacteria also showed effective degradation of PE. Phytotoxicity was assessed using morphological, physiological, and biochemical parameters of plant. Untreated PE significantly affected plants' physiology, resulting in a 45% reduction in leaf chlorophyll and a 40% reduction in relative water content. It also had adverse effects on the biochemical parameters of lettuce. Bacterial inoculation and starch treatment mitigated the harmful impact of stress and improved plants' growth as well as physiological and biochemical parameters; however, the nitric treatment proved phytotoxic. The observed results revealed that bacteria and starch could be effectively used for the degradation of pre-treated PE.
Soil ecosystems are home to a diverse range of microorganisms, but they are only partially understood because no single-cell sequencing or whole-community sequencing provides a complete picture of these complex communities. Using one of such metagenomics approaches, we succeeded in monitoring the microbial diversity and stress-response gene in the soil samples. This study aims to test whether known differences in taxonomic diversity and composition are reflected in functional gene profiles by implementing whole gene sequencing (WGS) metagenomic analysis of geographically dispersed soils from two distinct pristine forests. The study was commenced by sequencing three rainforest soil samples and three peat swamp soil samples. Soil richness effects were assessed by exploring the changes in specific functional gene abundances to elucidate physiological constraints acting on different soil systems and identify variance in functional pathways relevant to soil biogeochemical cycling. Proteobacteria shows abundances of microbial diversity for 52.15% in Royal Belum Reserved Forest and 48.28% in Raja Musa; 177 out of 1,391,841 and 449 out of 3,586,577 protein coding represent acidic stress-response genes for Royal Belum and Raja Musa, respectively. Raja Musa indicates pH 2.5, which is extremely acidic. The analysis of the taxonomic community showed that Royal Belum soils are dominated by bacteria (98% in Sungai Kooi (SK), 98% in Sungai Papan (SP), and 98% in Sungai Ruok (SR), Archaea (0.9% in SK, 0.9% in SP, and 1% in SR), and the remaining were classed under Eukaryota and viruses. Likewise, the soils of Raja Muda Musa are also dominated by bacteria (95% in Raja Musa 1 (RM1), 98% in Raja Musa 2 (RM2), and 96% in Raja Musa 3 (RM3)), followed by Archaea (4% in RM1, 1% in RM2, and 3% in RM3), and the remaining were classed under Eukaryota and viruses. This study revealed that RBFR (Royal Belum Foresr Reserve) and RMFR (Raja Musa Forest Reserve) metagenomes contained abundant stress-related genes assigned to various stress-response pathways, many of which did not show any difference among samples from both sites. Our findings indicate that the structure and functional potential of the microbial community will be altered by future environmental potential as the first glimpse of both the taxonomic and functional composition of soil microbial communities.
Microbial-based fertilizer has been widely used as a healthier and better alternative to agrochemical products. However, the effects of biofertilizers on the rhizospheric microbiota has rarely been investigated. Thus, the aim of this study was to investigate the effects of symbiotic fungus Trichoderma asperellum SL2-based inoculant on the soil bacterial population through next generation sequencing using a metabarcoding approach. The treatment plots were treated with T. asperellum SL2 spore suspension, while the control plots were treated with sterilized distilled water. The results showed similar bacterial microbiome profiles in the soil of control and T. asperellum SL2-treated plots. In conclusion, the application of the T. asperellum SL2 inoculant had not exerted a negative impact towards the bacterial population as similar observation was reflected in control plots. Nonetheless, future research should be conducted to investigate the effects of repeated application of T. asperellum SL2 over a longer period on the rice microbiota communities.
Oil palm (OP) plantations are gradually replacing tropical rainforest in Malaysia, one of the largest palm oil producers globally. Conversion of lands to OP plantations has been associated with compositional shifts of the microbial community, with consequences on the greenhouse gas (GHG) emissions. While the impact of the change in land use has recently been investigated for microorganisms involved in N2O emission, the response of the aerobic methanotrophs to OP agriculture remains to be determined. Here, we monitored the bacterial community composition, focusing on the aerobic methanotrophs, in OP agricultural soils since 2012, 2006, and 1993, as well as in a tropical rainforest, in 2019 and 2020. High-affinity methane uptake was confirmed, showing significantly lower rates in the OP plantations than in the tropical rainforest, but values increased with continuous OP agriculture. The bacterial, including the methanotrophic community composition, was modified with ongoing OP agriculture. The methanotrophic community composition was predominantly composed of unclassified methanotrophs, with the canonical (Methylocystis) and putative methanotrophs thought to catalyze high-affinity methane oxidation present at higher relative abundance in the oldest OP plantation. Results suggest that the methanotrophic community was relatively more stable within each site, exhibiting less temporal variations than the total bacterial community. Uncharacteristically, a 16S rRNA gene-based co-occurrence network analysis revealed a more complex and connected community in the OP agricultural soil, which may influence the resilience of the bacterial community to disturbances. Overall, we provide a first insight into the ecology and role of the aerobic methanotrophs as a methane sink in OP agricultural soils.
This study was initiated following the serendipitous discovery of a unialgal culture of a Stichococcus-like green alga (Chlorophyta) newly isolated from soil collected on Signy Island (maritime Antarctica) in growth medium supplemented with 100 µg/mL cycloheximide (CHX, a widely used antibiotic active against most eukaryotes). In order to test the generality of CHX resistance in taxa originally identified as members of Stichococcus (the detailed taxonomic relationships within this group of algae have been updated since our study took place), six strains were studied: two strains isolated from recent substrate collections from Signy Island (maritime Antarctica) ("Antarctica" 1 and "Antarctica" 2), one isolated from this island about 50 years ago ("Antarctica" 3) and single Arctic ("Arctic"), temperate ("Temperate") and tropical ("Tropical") strains. The sensitivity of each strain towards CHX was compared by determining the minimum inhibitory concentration (MIC), and growth rate and lag time when exposed to different CHX concentrations. All strains except "Temperate" were highly resistant to CHX (MIC > 1000 µg/mL), while "Temperate" was resistant to 62.5 µg/mL (a concentration still considerably greater than any previously reported for algae). All highly resistant strains showed no significant differences in growth rate between control and treatment (1000 µg/mL CHX) conditions. Morphological examination suggested that four strains were consistent with the description of the species Stichococcus bacillaris while the remaining two conformed to S. mirabilis. However, based on sequence analyses and the recently available phylogeny, only one strain, "Temperate", was confirmed to be S. bacillaris, while "Tropical" represents the newly erected genus Tetratostichococcus, "Antarctica 1" Tritostichococcus, and "Antarctica 2", "Antarctica 3" and "Arctic" Deuterostichococcus. Both phylogenetic and CHX sensitivity analyses suggest that CHX resistance is potentially widespread within this group of algae.
Fusarium wilt (FW) caused by Fusarium oxysporum f. sp. cubense Tropical Race 4 (TR4) is a soil-borne disease that infects bananas, causing severe economic losses worldwide. To reveal the relationship between bacterial populations and FW, the bacterial communities of healthy and TR4-infected rhizosphere and bulk soils were compared using 16S rRNA gene sequencing. Soil physicochemical properties associated with FW were also analyzed. We found the community structure of bacteria in the healthy and TR4 infected rhizosphere was significantly different compared to bulk soil within the same farm. The rhizosphere soils of infected plants exhibited higher richness and diversity than healthy plant with significant abundance of Proteobacteria. In the healthy rhizosphere soil, beneficial bacteria such as Burkholderia and Streptomyces spp. were more abundant. Compared to the infected rhizosphere soil, healthy rhizosphere soil was associated with RNA metabolism and transporters pathways and a high level of magnesium and cation exchange capacity. Overall, we reported changes in the key taxa of rhizospheric bacterial communities and soil physicochemical properties of healthy and FW-infected plants, suggesting their potential role as indicators for plant health.
Rice, the main staple food for about half of the world's population, has had the growth of its production stagnate in the last two decades. One of the ways to further improve rice production is to enhance the associations between rice plants and the microbiome that exists around, on, and inside the plant. This article reviews recent developments in understanding how microorganisms exert positive influences on plant growth, production, and health, focusing particularly on rice. A variety of microbial species and taxa reside in the rhizosphere and the phyllosphere of plants and also have multiple roles as symbiotic endophytes while living within plant tissues and even cells. They alter the morphology of host plants, enhance their growth, health, and yield, and reduce their vulnerability to biotic and abiotic stresses. The findings of both agronomic and molecular analysis show ways in which microorganisms regulate the growth, physiological traits, and molecular signaling within rice plants. However, many significant scientific questions remain to be resolved. Advancements in high-throughput multi-omics technologies can be used to elucidate mechanisms involved in microbial-rice plant associations. Prospectively, the use of microbial inoculants and associated approaches offers some new, cost-effective, and more eco-friendly practices for increasing rice production.
Oil palm trunks (OPT) are logged for replantation and the fiber residues are disposed of into the palm plantation area. The fiber residues are expected to increase soil fertility through recycling of carbon and minerals via fiber decomposition. This study investigated the effects of OPT fiber disposal and other lignocellulosic biomass on plant growth and microbial diversity in the soil environment. Four treatment plots were tested: (A) soil+OPT fiber (1:20), (B) soil+sugarcane bagasse (1:20), (C) soil+cellulose powder (1:20), and (D) unamended soil as a negative control. Low plant height, decreased chlorophyll content, and low biomass was observed in corn grown on soil mixed with OPT fiber, cellulose, and sugarcane bagasse, when compared with those of the control. The plants grown with OPT fiber were deficient in total nitrogen and magnesium when compared with those without fiber amendment, which suggested that nitrogen and minerals in soil might be taken up by changing microflora because of the OPT fibers presence. To confirm differences in the soil microflora, metagenomics analysis was performed on untreated soil and soil from each lignocellulose treatment. The microflora of soils mixed with OPT fiber, cellulose and sugarcane bagasse revealed substantial increases in bacteria such as families Cytophagaceae and Oscillospiraceae, and two major fungal genera, Trichoderma and Trichocladium, that are involved in lignocellulose degradation. OPT fiber resulted in a drastic increase in the ratios and amounts of Trichocladium in the soil when compared with those of cellulose and sugarcane bagasse. These results indicate that unregulated disposal of OPT fiber into plantation areas could result in nutrient loss from soil by increasing the abundance of microorganisms involved in lignocellulose decomposition.
Soil potassium (K) supplement depends intensively on the application of chemical fertilizers, which have substantial harmful environmental effects. However, some bacteria can act as inoculants by converting unavailable and insoluble K forms into plant-accessible forms. Such bacteria are an eco-friendly approach for enhancing plant K absorption and consequently reducing utilization of chemical fertilization. Therefore, the present research was undertaken to isolate, screen, and characterize the K solubilizing bacteria (KSB) from the rhizosphere soils of northern India. Overall, 110 strains were isolated, but only 13 isolates showed significant K solubilizing ability by forming a halo zone on solid media. They were further screened for K solubilizing activity at 0 °C, 1 °C, 3 °C, 5 °C, 7 °C, 15 °C, and 20 °C for 5, 10, and 20 days. All the bacterial isolates showed mineral K solubilization activity at these different temperatures. However, the content of K solubilization increased with the upsurge in temperature and period of incubation. The isolate KSB (Grz) showed the highest K solubilization index of 462.28% after 48 h of incubation at 20 °C. The maximum of 23.38 µg K/mL broth was solubilized by the isolate KSB (Grz) at 20 °C after 20 days of incubation. Based on morphological, biochemical, and molecular characterization (through the 16S rDNA approach), the isolate KSB (Grz) was identified as Mesorhizobium sp. The majority of the strains produced HCN and ammonia. The maximum indole acetic acid (IAA) (31.54 µM/mL) and cellulase (390 µM/mL) were produced by the isolate KSB (Grz). In contrast, the highest protease (525.12 µM/mL) and chitinase (5.20 µM/mL) activities were shown by standard strain Bacillus mucilaginosus and KSB (Gmr) isolate, respectively.
Rhizospheric organisms have a unique manner of existence since many factors can influence the shape of the microbiome. As we all know, harnessing the interaction between soil microbes and plants is critical for sustainable agriculture and ecosystems. We can achieve sustainable agricultural practice by incorporating plant-microbiome interaction as a positive technology. The contribution of this interaction has piqued the interest of experts, who plan to do more research using beneficial microorganism in order to accomplish this vision. Plants engage in a wide range of interrelationship with soil microorganism, spanning the entire spectrum of ecological potential which can be mutualistic, commensal, neutral, exploitative, or competitive. Mutualistic microorganism found in plant-associated microbial communities assist their host in a number of ways. Many studies have demonstrated that the soil microbiome may provide significant advantages to the host plant. However, various soil conditions (pH, temperature, oxygen, physics-chemistry and moisture), soil environments (drought, submergence, metal toxicity and salinity), plant types/genotype, and agricultural practices may result in distinct microbial composition and characteristics, as well as its mechanism to promote plant development and defence against all these stressors. In this paper, we provide an in-depth overview of how the above factors are able to affect the soil microbial structure and communities and change above and below ground interactions. Future prospects will also be discussed.
The term "Total petroleum hydrocarbons" (TPH) is used to describe a complex mixture of petroleum-based hydrocarbons primarily derived from crude oil. Those compounds are considered as persistent organic pollutants in the terrestrial environment. A wide array of organic amendments is increasingly used for the remediation of TPH-contaminated soils. Organic amendments not only supply a source of carbon and nutrients but also add exogenous beneficial microorganisms to enhance the TPH degradation rate, thereby improving the soil health. Two fundamental approaches can be contemplated within the context of remediation of TPH-contaminated soils using organic amendments: (i) enhanced TPH sorption to the exogenous organic matter (immobilization) as it reduces the bioavailability of the contaminants, and (ii) increasing the solubility of the contaminants by supplying desorbing agents (mobilization) for enhancing the subsequent biodegradation. Net immobilization and mobilization of TPH have both been observed following the application of organic amendments to contaminated soils. This review examines the mechanisms for the enhanced remediation of TPH-contaminated soils by organic amendments and discusses the influencing factors in relation to sequestration, bioavailability, and subsequent biodegradation of TPH in soils. The uncertainty of mechanisms for various organic amendments in TPH remediation processes remains a critical area of future research.