Progress in biostimulation-based remediation of TPH-contaminated soils: a comprehensive review

Functional genes, biofilms, and microbial interactions in TPH degradation

The efficiency of biostimulation in petroleum hydrocarbon remediation is fundamentally driven by the metabolic pathways, gene expression, and cooperative interactions within soil microbial communities. Understanding these molecular and ecological processes provides a mechanistic foundation for optimizing biostimulant design and application.

One of the most critical aspects of microbial hydrocarbon degradation lies in the expression of functional genes encoding catabolic enzymes. Genes such as alkB (alkane monooxygenase), CYP153 (cytochrome P450 monooxygenase), nahA (naphthalene dioxygenase), and rdhA (reductive dehalogenase) are frequently associated with the degradation of aliphatic and aromatic hydrocarbons. Their transcription is often upregulated in the presence of petroleum hydrocarbons and is strongly influenced by nutrient availability, oxygen content, and biostimulant composition (Wang et al., 2021b; Ren et al., 2022). Quantitative PCR (qPCR) and metagenomic sequencing have revealed that nutrient-rich amendments (e.g., poultry manure, compost, or mineral fertilizers) not only increase microbial biomass but also significantly elevate the copy numbers of these functional genes.

In addition to gene-level regulation, biofilm formation plays a pivotal role in maintaining active microbial populations during bioremediation. Biofilms—dense microbial aggregates embedded in extracellular polymeric substances (EPS), enhance survival under environmental stress, improve substrate access, and promote intercellular signaling. Petroleum hydrocarbon degraders such as Pseudomonas putida and Mycobacterium spp. are known to form robust biofilms on soil particles and biostimulant surfaces like biochar and straw. These structured communities exhibit enhanced resistance to toxic compounds and maintain high local concentrations of degradative enzymes, thereby accelerating hydrocarbon breakdown (Saeed et al., 2021; Shi et al., 2022).

Furthermore, microbial cooperation and consortia dynamics are essential for complete mineralization of complex hydrocarbon mixtures. Diverse microbial taxa exhibit complementary catabolic activities; for instance, one group may oxidize long-chain alkanes, while another converts aromatic intermediates into simpler organic acids. The addition of biostimulants selectively favors the growth of key degraders such as Rhodococcus, Acinetobacter, Bacillus, and Sphingomonas and modulates community composition to optimize synergistic degradation. Studies have shown that mixed microbial communities stimulated by plant litter or aged refuse exhibit higher metabolic diversity and resilience compared to single-strain inocula (Zhang et al., 2021b; Chen et al., 2019b).

Elucidating these microbial mechanisms through the study of gene expression, biofilm behavior, and microbial community interactions enables a more predictive and targeted approach to biostimulation. It also supports the design of consortia-based strategies and the use of biostimulants that foster stable, highly active microbial networks for robust petroleum hydrocarbon degradation.

Agricultural residue

(1) The use of agricultural residue as a biostimulant serves not only microbial remediation efforts but also addresses the challenge of agricultural residue disposal. Zhang, Wu & Ren (2020) utilized swine factory wastewater as a nitrogen source for bioremediation and as a substitute for process water, and combined remediation with bran as well as exogenous strains of bacteria, which significantly boosted microbial colonization numbers, and the petroleum hydrocarbon degradation rate under the optimal medium conditions reached 68.27 ± 0.71%. There is a limited amount of research on using natural organic matter derived from plant waste to improve soils contaminated with petroleum. Utilizing it as a biostimulant can greatly enhance soil quality, supply microorganisms with the necessary N, P, and K nutrients vital for their growth and metabolic processes, encourage microbial proliferation, and play a crucial role in the bioremediation of soils contaminated with petroleum (Liu et al., 2020; Yao et al., 2024).

Huang et al. (2019) investigated the remediation of petroleum hydrocarbon-contaminated soils by paddy straw and sawdust, of which the degradation of TPH by paddy straw and mineral fertilizer was 45.2%, while there was no significant difference between sawdust and the control. There was a significant difference in the microbial community structure in different treatments, and the abundance of petroleum degrading bacteria in the soil increased with the addition of rice straw and sawdust. Zhang et al. (2021b) but the TPH degradation rate of the mixed treatment was higher than that of single-litter treatments, possibly due to synergistic effects in phosphorus supplementation, increased enzyme activity (invertase and dehydrogenase), and greater abundance of petroleum hydrocarbon-degrading bacteria in the mixed treatment. showed significant synergistic effects in supplementing effective phosphorus, improving invertase and dehydrogenase and the abundance of petroleum hydrocarbon degrading bacteria was different between the mixed plant litters and single apomictic treatment. Plant litter biostimulants provide microbial nutrients which support degrading bacteria growth while improving soil enzyme activity for petroleum hydrocarbon decomposition. The price for this substance remains inexpensive while its acquisition remains straightforward. The multiple advantages achievable through the mixture of plant wastes prove that numerous combining possibilities exist with promising results expected from these applications.

(2) Utilizing animal manure as a biostimulant represents a sustainable approach to remediating petroleum hydrocarbons. This method not only enhances the efficiency of bioremediation but also aligns with economic and sustainability goals (Adewoyin & Arimoro, 2023; Cunningham et al., 2020; Ruley & Tomor, 2024). Douglas et al. (2024) explored the use of cow and poultry manure to remediate soils contaminated with petroleum hydrocarbons. They discovered that poultry manure was generally more effective, achieving a TPH degradation rate of 36%, compared to 23% for cow manure. Additionally, they observed that increasing the amount of manure led to a rise in the number of heterotrophic microorganisms, which in turn suggested a greater degradation of petroleum. Robichaud et al. (2019) compared different biostimulants and found that horse manure had the highest petroleum hydrocarbons removal efficiency and was more effective in improving soil structure as well as fertility. Utilizing animal manure as a biostimulant proved to be more effective in enhancing the soil’s physicochemical characteristics. Compared with other organic fertilizers, manure can not only provide nutrients for microorganisms but also may contain petroleum hydrocarbon-degrading bacteria, which is more conducive to the remediation of petroleum hydrocarbons.

Domestic waste-based nutrient additives

(1) Food waste is a readily available and low-cost type of biostimulant. Xu et al. (2019) used food waste (mainly rice, noodles, vegetables, eggs and meat) from a school cafeteria to make a waste supernatant (FS) to be added to petroleum hydrocarbon contaminated soils. Incorporating FS offered a substantial supply of carbon and nitrogen to the IHD (indigenous hydrocarbons degradation bacteria), facilitating the proliferation of indigenous hydrocarbons degradation bacteria. As a result, over a period of 30 days, the TPH degradation rate reached 49.3%. It was observed that an increased C/N ratio accelerated the growth of IHD, suggesting that a higher C/N ratio may promote dominance of IHD. Liu et al. (2019) used meat bone meal (MBM), a by-product of meat processing, as a novel biostimulant, and found that its petroleum hydrocarbon degradation ability was comparable to that of traditional fertilizers (urea-treated), and MBM-treated soil avoided the usual negative effects of urea on soil pH, and its diesel degradation rate was 91.2% within 99 days under aeration treatment.

(2) Aged refuse serves as an effective soil enhancer, biostimulant, and microbial agent, capable of boosting soil fertility and supporting microbial life, while also containing numerous bacteria that degrade petroleum. Due to the high specific surface area and porosity, excellent physicochemical and hydraulic properties, aged refuse is also a good microbial carrier, which is conducive to the growth and aggregation of indigenous petroleum hydrocarbon-degrading bacteria. Liu et al. (2018) used sterilized and non-sterilized aged refuse to remediate petroleum hydrocarbon contaminated soils, it was found that the TPH removal rate of sterilized mineralized refuse was 74.64% after 98 days, compared to 89.83% of non-sterilized mineralized refuse, due to the synergistic effect of conditioner addition, biostimulation, and bioaugmentation leading to better bioremediation. Chen et al. (2019b) found that after 30 weeks of addition of mineralized refuse of TPH degradation was 63.7%, while the combination of aged refuse and nutrients reached 90.2%.

Composting

Compost made from organic waste serves a dual role by supplying essential nutrients and activating indigenous petroleum hydrocarbon-degrading bacteria, thereby enhancing microbial development (Barakwan, 2025), thus enhancing soil TPH removal (Sokač et al., 2022; Rahmeh et al., 2021; Wu et al., 2018). Zhang et al. (2011) used compost made from fresh waste (a mixture of leaves, twigs, and greens) as a biostimulant to be added to petroleum hydrocarbon contamination and found that after 60 d, the PAH concentration decreased by 50.5 ± 14.8%, and the significant rise in gram-positive bacteria in the soil demonstrates that compost supports the growth and development of microorganisms. Liu et al. (2021) explored the impact of inorganic salts, organic composts, and sawdust on soils contaminated with petroleum hydrocarbons. Their findings indicated that composting was the most successful method for enhancing the breakdown of TPH, achieving a degradation rate of 35.7% after 60 days. Compost can not only stimulate microbial communities, but also improve soil physicochemical properties. The advantages of using organic compost are similar to those of animal manure and aged refuse. This dual functionality, nutrient enrichment and microbial inoculation, makes compost an effective biostimulant for accelerating the biodegradation of petroleum hydrocarbons, thereby introducing them to petroleum hydrocarbon-contaminated soil, which enhances the degradation process of these hydrocarbons. It can also improve the physical and chemical properties of soil, which is conducive to the survival of microorganisms. However, this process is more complicated in the production steps.

Mineral fertilizers

Mineral fertilizers are used to provide elements such as N, P, and K to microorganisms by adding inorganic salts to petroleum hydrocarbon-contaminated soils, thus stimulating the growth of indigenous microbial-degrading bacteria and increasing the degradation rate of TPH. Common mineral fertilizers include NH₄NO₃ and KH₂PO₄, and the use of mineral fertilizers such as ammonium and phosphate is a good choice (Cai et al., 2020; Ugbune et al., 2025). Mineral fertilizers are generally most effective when used in combination with other remediation methods, such as microbial inoculation or organic amendments, especially when high TPH concentrations lead to nutrient limitations in later stages of remediation (Safdari et al., 2018; Ou et al., 2025). Yaman (2020) used NH₄Cl and KH₂PO₄ as biostimulants and found that their combination with inoculated microorganisms resulted in higher TPH removal than BS and BA alone, with a degradation rate of 74%.Varjani, Pandey & Upasani (2021) used NH₄NO₃ and Na₂HPO₄ as biostimulants, it was found that the combination of BS and BA gave the best treatment (93.14 ± 1.75% in 42 d). The use of mineral fertilizers and nitrogen and phosphorus as biostimulants is an effective restoration method. However, when the concentration of TPH to be remediated is high, there is a problem of nutrient deficiency at the later stages of remediation. Therefore, regular supplementation of nutrients in contaminated soil will be more effective for effective degradation of difficult to degrade petroleum hydrocarbons. Wu et al. (2020) compared the use of single application of nutrients and regular supplementation of nitrogen and phosphorus nutrients using a drip irrigation unit as nutrients and the results showed that the drip irrigation unit was more effective after 90 d with a TPH degradation rate of 53.5%. This combination strategy helps overcome the limitations of mineral fertilizers alone by ensuring sustained microbial activity and improved bioavailability of nutrients throughout the remediation process.

The use of mineral fertilizers as biostimulants can provide microorganisms with the required nutrients in a targeted manner, making it easier to control the proportion and amount of nutrients used, provide microorganisms with a suitable nutrient environment, and help microorganisms to use petroleum hydrocarbons as their carbon source, thus improving the efficiency of remediation. Mineral fertilizers can help solve the problem of a lack of nutrients for microorganisms, so they are often used in combination with other remediation methods. For example, in a combined chemical and microbial remediation method, the subsequent growth of microorganisms often results in insufficient N. At this time, through the addition of mineral fertilizers, N, P, and other elements can be targeted to control the microbial nutrient environment, stimulate subsequent microbial growth, and promote the degradation of petroleum hydrocarbons. However, mineral fertilizers are costlier and less convenient than most organic fertilizers. Table 3 summarizes Cases of organic fertilizer remediation.

Biochar remediation alone

To begin with, biochar possesses a significant ability to adsorb substances and exhibits enhanced stability, which can efficiently decrease the levels of heavy metals, petroleum hydrocarbons, and various other contaminants (Ali et al., 2019; Mansoor et al., 2021). Biochar primarily enhances the breakdown of petroleum hydrocarbons by stimulating biological activity. It creates an optimal environment for microorganisms, encouraging the proliferation of native microbial populations, which in turn boosts the rate of TPH degradation (Ren et al., 2022; Kong et al., 2018; Li et al., 2022b). Yu et al. (2019) remediated petroleum hydrocarbon contaminated soils by using biochar and mineral fertilizer, and the comparison of them found that their dominant bacterial flora were different, which proved that biochar could promote the certain microbial growth. Second, biochar can improve the water content of soil, which is favorable for the microbial degradation of petroleum hydrocarbons (Saeed et al., 2021; Lee et al., 2022b; Zhang et al., 2019). In addition, biochar materials have surface functional groups that can supplement specific nutrients to soil microorganisms, thereby promoting their growth (Dike et al., 2021; Zama et al., 2018). Saeed et al. (2021) found that the nitrogen content of biochar-treated soils increased by 23% and 16%, phosphorus content by 10–5%, and potassium content by 5–3%, respectively, under 10% and 15% oil contamination compared to the non-biochar treatment. The role of biochar in restoring petroleum hydrocarbon-contaminated soils is illustrated in Fig. 1, highlighting mechanisms such as sorption, microbial stimulation, and reduction of contaminant mobility. Biochar can act through two distinct mechanisms: sorption and nutrient amendment. Sorption refers to biochar’s ability to physically adsorb or chemically bind petroleum hydrocarbons onto its porous surface, reducing their mobility and toxicity. In contrast, nutrient amendment involves biochar supplying essential nutrients (such as nitrogen or phosphorus) to the soil, thereby stimulating microbial activity and enhancing biodegradation. These two mechanisms work independently but can also synergize during remediation.

Combined remediation of biochar and surfactants

Biochar is rarely used alone in remediation; it often functions more effectively when paired with other amendments such as surfactants, mineral fertilizers, or organic compost. Biochar serves as an ally for other biostimulants during the process of soil remediation from petroleum hydrocarbon contamination. Bioremediation uses surfactants as one of the most widespread biostimulant groups. Surfactants provide dual lipophilic and oleophilic properties and reduce surface tension to increase the availability of contaminants (Rong et al., 2021; Karlapudi et al., 2018). The application of surfactants generates several negative side effects that affects how effectively petroleum hydrocarbons break down (Sydow et al., 2018). Such negative impacts can be reduced when biochar works together with surfactants as part of a synergistic remediation strategy according to Wei et al. (2020b).

According to Zhen et al. (2021), biochar modification with rhamnolipid decreased biochar surface area and elevated oxygen content which led to improved petroleum hydrocarbon degradation capacity in soil with modified biochar sustaining 32.9% TPH reduction after 90 days while rhamnolipid and biochar co-treatment showed lower degradation performance. The incorporation of rhamnolipids with biochar enables better microbial interaction along with stronger ability to decompose petroleum hydrocarbons from soil.

Combined remediation of biochar and mineral fertilizers

Beyond surfactants, mineral fertilizers are another category that shows synergistic potential with biochar. Soils with organic contamination cannot be directly remediated by biochar due to insufficient biochar remediation ability and the soil’s unfavorable carbon-to-nitrogen ratio balance (Wu et al., 2017; Shamloo, Ebrahimi & Charati, 2025). According to Kong, Liu & Wu (2023), the application of biochar together with urea has proven effective for petroleum hydrocarbon polluted soil remediation. The combined method removed 78.6% of total petroleum hydrocarbons from the soil after 80 days of application. Soil pH and total nitrogen along with ammonium ions and microbial population received enhancement when biochar was combined with nitrogen fertilizer while this mixture also elevated dehydrogenase and catalase activity and alkB and CYP gene abundance relative to the control.

The trio of biochar, surfactants and mineral fertilizers can also be combined for remediation. Wei et al. (2020a) used a combination of biochar, rhamnolipid (RL) biosurfactant and nitrogen (N). The results showed that after 50 d, biochar + RL, biochar + N, and biochar + N + RL lowered the total soil pH by 32.3%, 73.2%, and 80.9%, respectively. It showed a synergistic effect and was more effective than individual applications, and the composite treatment significantly promoted the adsorption of aromatic compounds, while RL and N promoted the degradation of heavy and light aliphatic compounds.

Combined biochar and compost remediation

In addition to mineral fertilizers, compost can also be co-applied with biochar to enhance microbial activity and organic matter turnover. Biochar features a porous architecture, a significant specific surface area, and numerous surface functional groups, all of which can serve as a habitat and supply nutrients for microorganisms involved in composting. Incorporating biochar into the composting process shortens the composting duration, enhances the breakdown of organic materials and the formation of humus, curtails greenhouse gas emissions, boosts microbial populations, and adjusts the structure of the microbial community (Pang et al., 2023; Zhou et al., 2019). Moreover, the use of biochar has a notable impact on the moisture levels, available oxygen, temperature, pH, and carbon-to-nitrogen ratio during the composting of organic waste, thereby enhancing microbial proliferation (Nguyen et al., 2022).

According to Lv et al. (2022) petroleum hydrocarbon contaminated soil received treatment with rice husk biochar and aerobic composting which resulted in heavy oil degradation to 74.82% during 26 days. Results demonstrated that adding rice husk biochar impact microbial community development throughout composting thus accelerating heavy oil decomposition in the compost process. Biochar holds strong abilities to adsorb to surfaces and precipitate chemicals and possess high ash amounts yet compost (SMC) functions through organic matter content and microbial communities with enzyme activities. The characteristics of rice husk biochar allow better interaction with hydrocarbons and pollutants which decreases their bioavailability and NaCl ion availability. The combined toxic effects on microbial communities and nutrient deficiency become less severe because of this process which results in efficient oil contamination treatment for soil (Bao et al., 2020). The main mechanism through which TPH removal functions in biochar-assisted composting relies on biostimulation instead of adsorption (Lv et al., 2023b). Uyizeye, Thiet & Knorr (2019) reported that combined compost and biochar remediation reached 62% TPH degradation during 90 days which outpaced single biochar and compost remediation results. PAH remediation in the treatment remained ineffective due to the combination of biochar with compost thereby restricting bacterial activities and PAH-degrading functional genes within soil (Bao et al., 2020). Overall, biochar demonstrates the highest remediation potential when used in combination with surfactants, fertilizers, or compost. These integrated strategies enhance nutrient delivery, microbial activation, and hydrocarbon breakdown.

Owing to its unique structure and chemical composition, biochar can synergize with other remediation methods to improve the remediation efficiency. As mentioned earlier, it can synergize well with surfactants, mineral fertilizers, and organic compost. Therefore, in the future, the use of biochar should not be limited to individual remediation, but should focus more on the synergistic effect of other types of remediation agents and research on the remediation effect, Table 4 summarizes the instances of remediation using biochar-based methods.

Restoration of plants alone

In rhizoremediation and phytoremediation, the chemical toxicity of pollutants is not the only factor influencing plant growth and development; physical impacts like decreased oxygen levels and poor soil structure can also impact the effectiveness of phytoremediation (Hussain et al., 2019; Nwajiobi et al., 2025). Therefore, it is important to select suitable plants for the remediation of petroleum hydrocarbon-contaminated soils. For example, preference should be given to the use of native plants that are tolerant to petroleum hydrocarbons over exotic plants, as this will provide an economically viable and environmentally sustainable option for reclaiming petroleum hydrocarbon-contaminated sites at any given location (Correa-García et al., 2018; Hoang et al., 2021b; Singha & Pandey, 2021). Camacho-Montealegre, Rodrigues & Tótola (2019) tested 12 plants Sorghum arundinaceum Desv, Oryza longistaminata A., Hy- parrhenia rufa Nees, Abelmoschus ficulneus L. and other native plants for remediation of petroleum hydrocarbon contaminated soils and found that H. rufa (Gramineae) had the highest degradation of petroleum hydrocarbons of 74.4% after 120 d and had the highest potential for petroleum tolerance. The pristine environment plants themselves in some petroleum contaminated areas are sufficient to remediate petroleum hydrocarbon contamination without human intervention. Meištininkas et al. (2023) found that the pristine environment of an island had a range of 66% to 75% biodegradation of total petroleum hydrocarbons after 60 days of aerobic conditions, which proves that the rhizosphere microbiome in pristine soils have the ability to eliminate hydrocarbons through biodegradation. In rhizoremediation and phytoremediation, both chemical toxicity and physical constraints such as poor soil structure and low oxygen availability can hinder plant growth and remediation effectiveness (Hussain et al., 2019; Nwajiobi et al., 2025). Selecting appropriate plant species is crucial. Native, petroleum-tolerant species are generally preferred over exotic plants, as they offer cost-effective and environmentally sustainable solutions for remediating contaminated sites.

• (1)

  The nitrogen fixation capabilities of specific legumes make them optimum for TPH-contaminated soil remediation which produces better results compared to all other non-legume plants. The trials of Meištininkas et al. (2023) demonstrated that M. sativa and L. corniculatus together with M. albus displayed the highest petroleum oil removal performance and survival tolerance reaching 95% after 90 days of remediation. M. albus (MA) had the best restoration of soil nutrients (nitrogen and phosphorus) (Allamin et al., 2020). The best performance for remediation of soil nutrients was achieved by L. corniculatus (LC) combined with M. albus (MA). The combined ability of M. albus (MA) brought restoring soil nutrients to their best possible levels with phosphorus and nitrogen. Studies conducted by Farouq et al. (2022) showed Vigna unguiculata root exudates establish major soil alterations which boost both microbial community quantities and functionality. Xu et al. (2021) evaluated the remediation performance of five different plants Susanna, Seep weeds, and Sea-lavender for petroleum hydrocarbon treatment in contaminated soils. Results demonstrated Sesbania produces large amounts of biomass and exhibits superior petroleum hydrocarbon degradation capacity than other studied plants thus qualifying it as an excellent choice for coastal wetlands hydrocarbon-contaminated soil remediation.

• (2)

  Scientists have documented how Poaceae plants deliver persuasive pollution removal capabilities (Olson et al., 2007; Dong et al., 2019). The high plasticity together with environmental adaptability of Poaceae enables these plants to work effectively at cleaning petroleum hydrocarbon contaminated soils (Guarino et al., 2020). Tall fescue, a perennial poaceae, is widely used for rhizoremediation because of its tolerance to pollutants and its fibrous root system (Dong et al., 2019; Steliga & Kluk, 2020). Lee et al. (2021) demonstrated the remediation potential of tall fescue by finding that the TPH removal rate of tall fescue for 37 d was 30.2%, which was significantly higher than that of unplanted soil at 19.4%. Lee et al. (2022a) investigated tall fescue’s 317 d long-term remediation of petroleum hydrocarbon contaminated soil capacity, the results showed that the maximum remediation rate of 75.6% occurred in 61 d, and it was found that the structure of its rhizosphere microbiome changed with the season, and the degradation rate of TPH was positively proportional to that of the petroleum hydrocarbon degrading bacteria, such as Lysobacter. The Poaceae Cynodon spp. and Agropyron desertorum proved to be effective agents for phytoremediation (TPH degradation rates of 38.9% and 30.4% respectively) based on findings reported by Saraeian et al. (2018). Two varieties of the Poaceae family named Brachiaria decumbens and Megathyrsus maximus prove suitable for petroleum hydrocarbon-contaminated soil remediation while enhancing soil fertility levels (Uribe, Peñuela & Pino, 2022). Ma et al. (2023) investigated how different sweet sorghum varieties performed at remediation of petroleum hydrocarbons from petroleum-contaminated saline soils achieving maximum remediation rate at 69.3%. The remediation potential of additional plant families besides the described two still needs to be explored for petroleum hydrocarbon-contaminated soils; for example, plants in the family of Sanguisorba are also able to effectively remediate petroleum hydrocarbon-contaminated soils (Hoang et al., 2021a).

The selection of suitable phytoremediation should be based on local conditions, that is, it is best to utilize local plant species to remediate local contaminated sites, which is conducive to the adaptation of plants to the environment, thus improving remediation efficiency. The sustainability of phytoremediation is strong because of the mutually beneficial symbiotic relationship between plant rhizoremediation and petroleum hydrocarbon-degrading microorganisms. However, compared to other biostimulants, the short-term degradation rate of petroleum hydrocarbons may be lower; therefore, a longer restoration period is required. Currently, most plant species that are suitable for degrading petroleum hydrocarbons are from the Poaceae and legume families, and there is a need to explore plant species with high degradation efficiency in the future. The primary mechanisms of phytoremediation, including phytoextraction, phytodegradation, and phytostabilization, are summarized in Fig. 2 to demonstrate plant-based remediation functions.

Combined plant and organic fertilizer restoration

Phytoremediation alone is often limited and influenced by environmental conditions, so adding other biostimulants to phytoremediation can lead to better remediation of petroleum hydrocarbon contaminated soils (Li et al., 2023b). Rhizoremediation is a synergistic form of bioremediation and phytoremediation that improves the overall remediation performance and remediation rate through the synergistic action of plant roots and microorganisms. The overall performance and efficiency of rhizoremediation can be improved by selecting appropriate plant and microbial pairings and adding soil amendments, such as organic and inorganic amendments (Hussain et al., 2018).

Addition of organic fertilizers to rhizoremediation can provide microorganisms with nutrients and other elements, thus improving the remediation efficiency. Ruley et al. (2020) used organic fertilizer cow dung in combined remediation with plants, and the results demonstrated that the application of cow dung combined with an effective phytoremediation agent significantly increased the reduction of TPH as compared to natural attenuation or the use of manure or phytoremediation agent alone. Seo & Cho (2020), Wang et al. (2021a) added organic compost to phytoremediation, and the results showed that organic compost treatment resulted in a decrease in soil pH, an increase in the removal of TPHs, alkanes, and aromatics, an increase in the activities of soil dehydrogenase and PPOs enhanced, and a relative increase in PAHs and alkane-degrading bacteria. In addition, the organic fertilizer significantly increased aboveground length, root vigor, and aboveground and root dry weights of Vetiveria zizanioides. The targeted addition of mineral fertilizers (N, P) to plant rhizoremediation can also significantly improve the removal efficiency of TPH (Hoang et al., 2022).

Combined plant and exogenous bioremediation

Additional factors that determine the success of TPH remediation through rhizoremediation include how well plants tolerate TPH and the capability of the degrading microorganisms to occupy developing root systems and their ability to break down TPH compounds in the first place. The success of TPH remediation becomes more efficient when suitable plant-microbe partnerships are developed. Petroleum hydrocarbon-degrading bacteria isolated from petroleum-contaminated areas can be utilized for combined remediation with plants. Promsinga et al. (2021) utilized petroleum hydrocarbon-degrading bacteria screened from petroleum-contaminated soils with vetiver and the results showed that Vetiveria zizanioides and the bacteria M. luteus WN 01 or Kocuria sp. MU01 had a stronger remediation of petroleum hydrocarbon-contaminated soils and that after 45 d. The combination of vetiver and M. luteus WN01 showed the best TPH degradation after 45 d (TPH degradation rate of 50.25%). Eze et al. (2022) demonstrated that petroleum-degrading bacteria isolated from petroleum-contaminated sites can effectively increase the biomass of plants, and the combination of these with M.sativa resulted in a TPH degradation rate of 91% after 60 d. The incorporation of petroleum hydrocarbon-degrading bacteria leads to better TPH breakdown alongside lower toxicity against plant life and supports better plant growth development (Ali et al., 2023; Auti et al., 2019). Therefore, the combination of microorganisms isolated from inter-roots of plants and plants can also effectively improve the TPH degradation rate (Song et al., 2021; Afegbua & Batty, 2019). Lee, Lee & Cho (2023) found that Novosphingobium sp. CuT1, isolated from the inter-roots of tall fescue, can effectively promote plant growth and TPH degradation (Afegbua & Batty, 2019). Ani et al. (2021) also demonstrated that the combination of inoculated rhizoremediation microorganisms and plants was much more effective in TPH degradation than isolated microorganisms and plants alone.

Based on combined bioaugmentation and phytoremediation, the addition of mineral fertilizers (N, P, K, etc.) to stimulate microorganisms can achieve improved TPH degradation (Shahzad et al., 2020). Bhuyan, Kotoky & Pandey (2023) added NPK for bioaugmentation based on plant inoculation with rhizoremediation-isolated bacteria. The results demonstrated that when bacterial strains were used with NPK biostimulant, they could improve plant growth and attenuated plant and soil enzyme activities at petroleum contaminated sites, and bacterial aggregation-NPK biostimulation also altered the rhizoremediation microbiome and increased TPH degradation in crude oil contaminated soils. Ubogu, Odokuma & Akponah (2019) has also demonstrated that combined remediation of mineral fertilizers and exogenous petroleum hydrocarbon-degrading bacteria with plants is more effective than biostimulation or bioaugmentation alone. Anwar-ul-Haq et al. (2022) employed a combination of surfactants, exogenous microorganisms, and phytoremediation, thus proving that this combined application can help in the restoration of petroleum-contaminated soils for agricultural use.

Combined plant and biochar remediation

Plant root exudates are beneficial to biochar, and coapplication of biochar with root secretions can address the reduced bioavailability of hydrocarbons associated with biochar research (Ni et al., 2018; Dike et al., 2022). Biochar and inter-root soils were found to be enriched with some PAH-degrading bacteria, and the combination of these two promoted synergistic effects among PAH-degrading bacteria (Li et al., 2020). Therefore, the combined application of plants and biochar amendments is beneficial for the remediation of contaminated soil. Yousaf et al. (2022) showed that the TPH removal rate of the combined remediation of biochar and plants was higher than that of plants alone as well as biochar remediation, which proved that biochar promoted phytoremediation. In addition, the addition of biochar to phytoremediation not only promotes rhizoremediation to increase the TPH degradation rate but also promotes plant growth and development (Hussain et al., 2022). Combined phytoremediation strategies consistently outperform plant-only remediation by enhancing microbial activity, nutrient cycling, and contaminant bioavailability. Organic fertilizers such as compost and cow dung not only improve plant growth but also stimulate microbial degradation of TPHs. The addition of biochar enhances soil physicochemical properties and supports rhizobacterial colonization. Inoculation with petroleum-degrading microbes further accelerates biodegradation. Overall, the synergistic use of plants with biostimulants whether organic amendments, exogenous microbes, or biochar leads to faster, more complete remediation. Future efforts should focus on tailoring plant microbe amendment combinations based on site-specific conditions to optimize efficiency and sustainability.

Adopting the joint restoration of plants and other biostimulants can compensate for the shortcomings of plant restoration alone, improve restoration efficiency, and shorten the restoration cycle. Adding other biostimulants to phytoremediation can also improve the restoration environment for plants. For example, adding fertilizers can provide nutrients for plants, and adding biochar contributes to the enhancement of soil physicochemical properties., which is conducive to phytoremediation. The sustainability of phytoremediation is strong, but the efficiency of plants alone is not high. Adding other biostimulants to phytoremediation can improve the remediation efficiency, so the main trend in the future is to use phytoremediation in combination with other biostimulants.

Nanotechnology

The rapid evolution of biotechnological and environmental engineering tools has expanded the potential of biostimulation strategies for remediating petroleum hydrocarbon-contaminated soils. Emerging technologies such as nanotechnology, genetic engineering of microbial strains, and advanced monitoring systems are revolutionizing the way biostimulants are applied and monitored in the field.

Nanotechnology-based interventions have gained momentum due to their ability to enhance bioavailability and degradation kinetics of hydrophobic contaminants. Nanoparticles such as nano-iron oxides, nano-zeolites, and carbon-based nanomaterials (e.g., carbon nanotubes, graphene oxide) have demonstrated the capacity to improve nutrient delivery, adsorb hydrocarbons, and provide catalytic surfaces that stimulate microbial activity (Ren et al., 2022; Rong et al., 2021). These materials can be engineered for targeted release and controlled dispersion in subsurface environments, minimizing nutrient losses and enhancing contaminant-microbe interactions.

Genetically engineered microbes (GEMs)

Genetically engineered microorganisms (GEMs) offer another frontier for biostimulation enhancement. Through synthetic biology approaches, native strains can be modified to overexpress hydrocarbon-degrading enzymes or stress-tolerant traits, thereby improving their survival and activity in harsh contaminated environments. For example, engineered strains with elevated expression of alkB or CYP450 genes have been shown to significantly accelerate the breakdown of long-chain hydrocarbons. Furthermore, the development of microbial consortia with complementary metabolic capabilities allows for more comprehensive degradation of complex pollutant mixtures (Ren et al., 2022; Bhuyan, Kotoky & Pandey, 2023).

AI-based monitoring systems

Concurrently, biosensor-integrated and AI-enhanced monitoring systems are enabling real-time assessment of remediation progress and microbial activity. Biosensors based on microbial reporters or enzyme activity can detect specific hydrocarbon metabolites, allowing in situ tracking of degradation dynamics. Additionally, machine learning algorithms applied to omics data (e.g., metagenomics, proteomics) and environmental variables can predict system behavior, optimize nutrient ratios, and reduce reliance on trial-and-error strategies. These technologies support precision biostimulation, where interventions are continuously adjusted to match microbial needs and environmental responses (Kong, Liu & Wu, 2023; Anwar-ul-Haq et al., 2022).

Together, these emerging technologies offer substantial improvements in efficiency, adaptability, and environmental control. Integrating nanomaterials, GEMs, and smart monitoring tools into biostimulation strategies represents a paradigm shift in bioremediation, moving toward more predictive, responsive, and site-specific remediation systems.

Field-scale implementation challenges and solutions

At larger scales, the uniform distribution of biostimulants becomes technically complex. Subsurface heterogeneity can lead to uneven amendment delivery, nutrient leaching, or local microbial inhibition. To overcome this, precision delivery systems such as drip irrigation, deep soil injection, or controlled-release nutrient capsules have been developed to optimize amendment dispersion while reducing operational costs and nutrient loss (Wu et al., 2020). Additionally, in situ aeration techniques or passive oxygenation through biochar incorporation can help maintain oxygen levels crucial for aerobic hydrocarbon degradation.

Economic viability is another key consideration. Organic waste-based biostimulants like compost, manure, or food waste slurry are typically low-cost and locally available, making them suitable for large-scale remediation in resource-limited settings. In contrast, engineered solutions (e.g., nano-fertilizers or biosensors) may involve higher upfront costs but offer greater efficiency and monitoring precision. Thus, cost-benefit analysis should guide the selection of biostimulation strategies based on project scale, urgency, and regulatory constraints.

Risk management and regulatory alignment

Despite its sustainability, biostimulation can introduce environmental risks if poorly managed. Over-application of nutrients may lead to eutrophication of nearby water bodies or alter soil microbial equilibrium. To mitigate such risks, field applications should incorporate real-time monitoring of nutrient levels, microbial activity, and degradation kinetics using biosensors or periodic soil sampling. Risk assessments should also evaluate potential by-products of incomplete degradation (e.g., polycyclic aromatic hydrocarbons or intermediate metabolites).

On the policy front, regulatory guidance remains limited for biostimulation in many regions. There is a growing need for the development of standardized field protocols and approval pathways for biostimulant materials, particularly for those derived from waste streams or involving microbial inoculants. Establishing clear thresholds for TPH reduction, post-remediation soil quality, and ecological restoration benchmarks will support both public acceptance and environmental compliance.

Industry and stakeholder engagement

Industry professionals and environmental managers require practical guidance on the operationalization of biostimulation. This includes best practices for site assessment, biostimulant selection based on soil chemistry and contaminant profile, and timelines for expected remediation outcomes. Case studies demonstrating successful field-scale projects, particularly under regulatory oversight or public-private partnerships, could foster wider adoption. Stakeholder engagement—including local communities, environmental agencies, and waste management sectors—can further enhance the feasibility and accountability of field applications.

In summary, engineering-scale applications of biostimulation demand interdisciplinary coordination, technology transfer from lab to field, and policy frameworks that balance environmental efficacy with economic and regulatory realities. Addressing these challenges will be key to unlocking the full potential of biostimulation in restoring petroleum-contaminated soils at a national and global scale.