1 Introduction

Biochar derived from different feedstocks has been used as an efficient amendment for improving the physical, chemical, and biological properties of soil: maintaining and supplying soil nutrients (Atkinson et al. 2010); enhancing soil water retention (Razzaghi et al. 2020); increasing fertilizer utilization efficiency (Gong et al. 2018); improving soil microbial diversity and populations (Xiao et al. 2019; Pokharel et al. 2020); and enhancing soil productivity and agricultural sustainability (Atkinson et al. 2010). Furthermore, biochar amendment is favorable for providing mechanical operations, preventing plant pests and diseases, increasing carbon sequestration, and decreasing greenhouse gas emission (Bridgwater 2003; Lehmann et al. 2006; Lehmann and Joseph 2015; Meng et al. 2019). Although several studies have examined the effects of biochar application on soil organisms (especially, earthworms), to the best of our knowledge, there is still a lack of systematic review bringing together evidence to resolve the underlying reasons and factors.

Earthworms are important in regulating and maintaining many soil processes. Based on their specific ecological function and adaptability, earthworms are divided into three groups: epigeic species (living on soil surface and consuming surface litter), endogeic species (living in organic horizons and ingesting organic–mineral materials by horizontal burrowing), and anecic species (feeding on surface litter, but inhabiting mineral soil horizons) (Bottinelli et al. 2020). Earthworms are considered as excellent soil engineers with the ability to increase soil pH, improve nutrient availability, accelerate soil organic matter (SOM) decomposition and mineralization, facilitate nutrient cycling, develop soil macroporosity, and favor soil structural formation and maintenance (Lavelle et al. 2001; Curry and Schmidt 2007). Besides, incorporation of earthworms increases the community and abundance of soil microorganisms (e.g., bacteria, fungi, and algae), enhances soil enzyme activity, and inactivates harmful parasites (Pietikainen et al. 2000; Brown et al. 2004). The stimulated microbial activities can consequently enhance the production of plant growth regulators, indirectly increase the phyto-availability of water and nutrients in the rhizosphere, and maintain a well-fed plant nutritional status of crop plants (Brown et al. 1999; Scheu 2003). Moreover, earthworm casts can act as a carrier of soil nutrients and microorganisms to the rhizosphere area and promote crop yield and quality (Lavelle et al. 1997). Numerous studies have demonstrated that invasive earthworms can change the soil properties and nutrient supplementaiton (Van Groenigen et al. 2019; Ferlian et al. 2020), and further affect plant growth and ecology (Van Groenigen et al. 2014; Xiao et al. 2018). However, the effects of earthworm on soil characteristics in biochar-amended soil have not yet been comprehensively explored.

Earthworms are extremely susceptible to external environmental conditions, such as vegetation, food source, artificial disturbance (e.g., irrigation, fertilization, treading), climatic conditions (e.g., temperature and moisture), etc., and are subject to extensive barrier-free uptake of soil pollutants (Lanno et al. 2004). Therefore, changes in earthworm biomarkers, such as physical activity, growth rate, reproduction and mortality rates, enzyme activity, and cellular organelles and genetic damage, have been extensively studied as indicators for assessing soil quality (Yeardley et al. 1996; Tang et al. 2019). In addition, earthworm biomarkers have also been confirmed to be an effective indicator for evaluating pollutant bioavailability in contaminated soils (Logsdon 2008; Gomez-Eyles et al. 2011; Weyers and Spokas 2011). As such, we anticipated that the application of biochar would lead to alterations in earthworm biomarkers. Nevertheless, it’s worth noting that contradictory findings have also been frequently documented. For instance, the weight and survival rate of earthworms have been found to significantly increase (Sanchez-Hernandez et al. 2019), and higher soil microbial biomass and soil productivity have been observed in biochar-amended soil (Paz-Ferreiro et al. 2015). However, some contrasting results of increased body weight loss and avoidance rate of earthworms have also been established after biochar addition (Li et al. 2011; Tammeorg et al. 2014).

Furthermore, global climate changes and intensive farming have gradually deteriorated arable soil quality, leading to soil acidification, salinization, erosion, contamination, and SOM depletion (Kopittke et al. 2019). A substantial number of studies have found that biochar is a feasible and affordable amendment for the recovery of these degraded soils (Jien and Wang 2013; Feng et al. 2021; Cui et al. 2023; Qian et al. 2023). More recently, several researchers have proposed soil fauna (particularly earthworms) as a potential biological tool for the enhancement of biochar effects (Nahidan and Ghasemzadeh 2022; Yang et al. 2022). Therefore, reviews summarizing the works on co-application of earthworms and biochar for improving soil health and fertility could be crucial for research on biochar amendment.

Therefore, in this review, the available information and related studies on earthworm activity in biochar-amended soil are summarized to better understand the effects of biochar amendment on earthworm activity. The specific objectives of this review were to: (1) elucidate the causes responsible for earthworm activities in biochar-amended soil, (2) identify the influencing factors of biochar that contribute to earthworm performance, (3) assess the potential synergistic effects of biochar and earthworms on soil health and sustainability, and (4) provide suggestions for promoting the synergetic effects of biochar and earthworms co-application on improving soil productivity and health.

2 Knowledge mapping analysis of earthworm activities in biochar-amended soil using CiteSpace

For exploring earthworm responses to the biochar-amended soil, a search was conducted from Google Scholar's database of relevant publications, which 3018 articles in the latest 10 year (January, 2012 to date), with the query formulations of “TS = (SOIL*) AND TS = (BIOCHAR* OR BLACK CHARCOAL OR CARBON OR BLACK CARBON OR ACTIVATED CARBON) AND TS = (EARTHWORM* OR Aporrectodea* OR Amynthas* OR Eisenia* OR Eudrilus* OR Enchytraeus* OR Folsomaia* OR Lumbricus* OR Metaphire* OR Pontoscolex* OR Pheretima*)” in Boolean form. A semantic analysis of the 179 perfectly related articles, manually selected from the above literatures, made it possible to identify the ten most promising keywords in temporal order: black carbon, sorption, charcoal, polycyclic aromatic hydrocarbons (PAHs), water, pyrolysis temperature, toxicity, enzyme activity, sludge, microbial community (Table 1).

Table 1 Top 10 keywords with the strongest citation bursts on biochar and earthworm study between 2012 and 2023
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The co-citation network analysis, as depicted in Fig. 1, illuminated prolific authors and the extent of their collaborative efforts. Notably, the most productive authors, including Bastos AC, Jiang X, Shen F, and others, collectively contributed 4 articles. It's worth mentioning that while numerous authors have made significant contributions with their respective publications, there appears to be limited collaboration among them. Furthermore, the centrality of these authors within the network was relatively modest, indicating the potential for further high-quality and expansive collaborations in the future.

Fig. 1
figure 1

The network of author co-citation. The size of each node indicates the appearance of the authors in all 179 publications. Nodes with common subjects are assigned to a color-coded cluster

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Keyword co-occurrence analysis identified the key words fitting into more than two literatures and quantified their co-occurrences to reveal implicit knowledge of related content and feature items. Hence, the co-word network can be used to determine the relationship between subjects and reveal the hot research direction and research frontier of specific topics. Therefore, the following 10 clusters in numerical order were designated in the keyword co-occurrence analysis map (Fig. 2), i.e., soil fertility, contaminated soil, eisenia fetida (eisenia foetida), hydrothermal carbonization, vermiremediation, activated carbon, toxicity, soil, biochar amendment, and mycorrhizal fungi. Accordingly, the researches include the change of earthworm parameters in response to biochar addition, the influencing factors and underlying mechanisms involved, and co-application of earthworms and biochar for improving soil quality from the summarization of these papers.

Fig. 2
figure 2

Keywords co-occurrence analysis map. The size of each node indicates the appearance of the keyword in all 179 publications. Nodes with common attributes are assigned to a color-coded cluster

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3 Earthworm activity changes in response to biochar amendment

The effects of biochar amendment on earthworm activities can be estimated by the following earthworm biomarkers: individual, physiological and morphological, and biochemical and molecular indicators (Fig. 3). The selected changes in earthworm biomarkers after biochar incorporation are shown in Table 2.

Fig. 3
figure 3

Current and potential earthworm biomarkers for evaluating the effects of biochar addition on their activity in soil

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Table 2 Changes in earthworm parameters due to biochar incorporation (white, gray, and dark gray shading represent positive, alterable, and negative effects of biochar on earthworm parameters, respectively)
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3.1 Whole-individual-level biomarkers

Whole-individual-level earthworm indicators (e.g., weight loss, avoidance, mortality, and cocoon hatching) have been extensively used in earthworm toxicological investigations (OECD 1984; Renu et al. 2020). These parameters are mostly applied as a measure of earthworm activity, owing to the intuitive and efficient characteristics of earthworms’ response to additives (Yeardley et al. 1996; Schaefer 2004). For instance, the average body weight of E. fetida has been found to increase by 6% and 8%, but decrease by 7%, after incorporation of 1%, 3%, and 10% wheat straw biochar to soil, respectively (Zhang et al. 2019). Furthermore, addition of 10% and 20% apple wood sawdust derived biochar to artificial soil has been noted to result in a 40% increase in body weight loss of E. fetida after 28 days of contact, although no significant differences in its survival and reproduction were found among the treatments (Li et al. 2011).

Owing to their importance in evaluating the health status of earthworm populations, previous studies have estimated earthworm cocoon hatching and juvenile viability in laboratory settings. The production rate of E. fetida cocoon has been reported to increase by 213% in the fourth week and the average number of larvae has been found to increase by 11-fold in the sixth week after the amendment of mixtures of wood sawdust, sewage sludge, and sludge derived biochar to the cultural soil (Malinska et al. 2017). A conclusion that biochar-compost mixtures with low biochar application rates under temperate climate conditions have positive, or no adverse effects on earthworm communities, was made by Honvault et al. (2023). However, the cocoon production of E. fetida was noted to be inhibited by the application of a high dosage (15%) of biochar to cow dung–soil (Cao et al. 2021).

Whole-individual-level earthworm parameters are important indicators of soil health as well as potential early warning markers of contamination and stress in soil ecosystems. For example, the concentrations of diethylenetriaminepentaacetic acid extractable Cd, Pb, and Zn decreased and the weight loss of earthworms declined after incorporation of rice husk and sludge derived biochar to soil (Wang et al. 2019). Similarly, the survival rate of E. fetida increased by 8.0–17.5 times after addition of 0.7–11.1% wood waste derived biochar to polychlorinated biphenyl (PCB) contaminated soil, with the best result achieved following addition of 2.8% wood waste derived biochar (Denyes et al. 2012). However, the application of > 5% pine tree chips derived biochar is considered to increase the survival rate and decrease the biomass loss of E. fetida in glyphosate spiked OECD soil (Dlamini and Otomo 2022).

In general, epigeic earthworms mainly feed on plant litter or manure accumulated on soil surface, whereas anecic earthworms (which usually have permanent burrows in which they source soil surface litter and dung mixed with surface minerals) ingest large amounts of soil particles (Wang et al. 2014) and are thus strongly affected by incorporated biochar. For instance, the survival rates of Lumbricus terrestris (an anecic species) have been found to be about 20% and 70% in OECD artificial and Kettering loam soils amended with 20% wheat straw biochar, respectively, whereas those of E. fetida (an epigeic species) have been noted to be about 53% and 87% under the same treatments, respectively. A clearer evidence comes from the higher variation in weight loss (mean mass per earthworm, g) observed in L. terrestris (Elliston and Oliver 2020). However, the opposite trend might also be possible, with preferential epigeic and endogeic earthworm activities on soil surface layer leading to a higher variation in individual parameters after biochar incorporation, when compared with anecic earthworm activity. Despite the insufficient available evidence indicating that epigeic and endogeic earthworms are more sensitive to their environmental changes, a few reports support this possibility. For instance, the survival rate of Aporrectodea icterica (an endogeic species) after a 28-day exposure to biochar-free and 2% poultry-manure derived biochar amended silty loam soils has been found to decrease from 90% to 82%, respectively, whereas that of Aporrectodea longa (an anecic species) remained at 100%; similarly, the weight loss of A. icterica has been observed to increase from about 4.0% to 10.5%, respectively, while that in A. longa was less pronounced, presenting an increase from about 4.3% to 9.6%, respectively (Marchand et al. 2017).

3.2 Physiological and morphological biomarkers

Physiological and morphological studies include immunohistochemical examination, electron microscopy, and pathological sections, of the earthworm’s coelomic cavity, gastrointestinal tract, reproductive organs, etc. The biochars with high surface area, high porosity and, high content of oxygenated functional groups are deemed to have great potential for improvement of the internal environment of earthworm intestines. For instance, co-application of biochar and earthworms increases the relative abundance of dominant bacterial groups within the earthworms’ gut microbiota (Alsamhary 2023; Zhen et al. 2023), and the introduction of biochar significantly increased the copy number of bacteria by 21.59% (Gao et al. 2022).

Simultaneously, the added biochar can also ameliorate the detrimental impacts of soil pollutants on the physiological and morphological indicators of earthworms. Alsamhary (2023) found that rich microbial communities in the intestinal tract of E. fetida were recorded in wheat straw biochar combined treatments and their subsequent alleviation of Cd stress on earthworm. Soil fauna’s intestinal tract is susceptible to oxidative stress when exposed to microplastics (MPs) through ingestion of food and drink or inhalation (Sun et al. 2021), and the introduction of amended peanut shell biochar can reduce the migration ability of MPs within the soil matrix (Wang et al. 2022). Consequently, the biochar amendment is expected to mitigate the impact of MPs on the physiological and biochemical indicators of earthworms.

However, more cases have revealed that alterations in the soil environment can cause significant deterioration of the intestinal tract, intestinal mucosal epithelial cells and microvilli, ciliated cells, and mitochondria of earthworms (Tang et al. 2019; Jia et al. 2023). Boughattas et al. (2023) demonstrated that eucalyptus waste biochar addition increased the bioavailability of Cd to Eisenia andrei, leading to excessive Cd accumulation in the tissue and cell, and subsequent damage to the epidermal surface, intestine tract, and coelomocytes. Meanwhile, the biochar particles ingested by the earthworms may absorb nutrients and digestive enzymes released from the gastrointestinal lumen, and inhibit digestion process and absorption of nutrients (Sizmur et al. 2017; Zhu et al. 2017), which may also negatively affect the physiological and morphological responses of the earthworms.

The membrane stability of subcellular components of earthworms, especially lysosomes, has been applied as a potential biomarker for ecotoxicity monitoring of the contaminated soil (Shi et al. 2017, 2021). The degree of lysosome damage has been predominantly determined by staining of lysosome with neutral red dye, and the preservation time of neutral red in lysosome has been measured to assess the biological toxicity response of earthworms to soil pollution (Svendsen et al. 2004). The neutral red retention time has been reported to decrease with the increasing exposure time and cow dung biochar doses, indicating the disruption of lysosomal membrane stability in earthworm coelomocytes and toxicity of biochar to the earthworms (Shi et al. 2021; Boughattas et al. 2023).

3.3 Biochemical and molecular biomarkers

The responses of earthworms at biochemical and molecular levels can be elucidated by nucleic acid alterations, enzyme structure and function, and protein damage (Rola et al. 2012). Analysis of DNA damage in earthworms exposed to environmental changes has been predominantly performed using single cell gel electrophoresis. The broken DNA fragments produced from any injury form a fluorescence tail in gel electrophoresis, and the degree of DNA damage can be ascertained based on the tail length. The dosage of biochar determines the alleviation of the associated toxicity of pollutants to earthworms, as well as their DNA damage responses. For example, sewage sludge caused inhibition of esterase and phosphatase activities as well as induced DNA damage of Pheretima posthuma, which were ameliorated by corncob biochar through the comet assays (Khalid et al. 2022). The incorporation of 2% rice husk biochar was found to enhance the presence of dominant bacteria in earthworms, thereby alleviating the stress caused by Cd exposure and diminishing the extent of DNA damage in earthworms (Alsamhary 2023). However, multiple studies have reported that the application of high dosages of biochar can result in DNA damage to soil fauna. When 5% biochar was applied to the soil, the concentration of 8-hydroxydeoxyguanosine, a biomarker of oxidative DNA damage, in earthworms was significantly increased after 28 days of contact, when compared with that in the control, confirming that biochar stimulates oxidative DNA damage in earthworm tissues in a higher concentration of biochar (> 5%) (Kim et al. 2014). Furthermore, application of 10% biochar to pentachlorophenol and pyrene contaminated soil inhibited the growth of earthworms and caused DNA damage by comet assay; however, 1% and 5% biochar amendment had no significant influence on earthworm growth and DNA damage (Cui et al. 2009). More recently, transcriptomic analysis demonstrated that chloramphenicol acetyltransferase and glutathione S-transferase genes related to oxidative stress were upregulated in E. andrei exposed to eucalyptus wastes biochar amended organic farmland soil, leading to severe cellular and DNA damage (Boughattas et al. 2023).

Numerous studies have documented the enzyme activities in the organs and extracellular matrix of earthworms in biochar-amended arable and contaminated soils (Zhang et al. 2021; Dlamini and Otomo 2022). For instance, the gastrointestinal tract environment of L. terrestris was altered and the digestive enzyme activities increased after the soil in which the earthworms were incubated with spent coffee grounds biochar, and the effect of 5% biochar treatment was more pronounced than that of 1% biochar (Sanchez-Hernandez et al. 2019). In a previous study, Sanchez-Hernandez (2018) confirmed that Aporrectodea caliginosa and L. terrestris activated the extracellular enzymes (associated with biogeochemical and bioremediation pathways) in Anthrosol amended with spent coffee grounds and pine needles derived biochar.

During growth under significant environmental variations, antioxidant enzymes eliminate excess reactive oxygen species, maintaining normal metabolism. Thus, the degree of oxidative stress in earthworms can be evaluated by analyzing the antioxidant enzyme activities (Zhang et al. 2013). In general, the activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) increase at the beginning of oxidative stress to remove excess H2O2 accumulation in the cells (Liu et al. 2014), but decrease with further environmental deterioration. In contrast, the content of malondialdehyde (MDA), an in vivo index of lipid peroxidation, gradually increases with environmental deterioration (Zhang et al. 2014). Biochar incorporation to soil can usually alleviate oxidative stress damage in earthworms, and thus can alter the antioxidant enzyme activities in earthworms (Nyoka et al. 2021). The MDA content in E. fetida decreased with biochar application to Cd-polluted soil after 28 days of incubation, indicating that the amended biochar alleviated the oxidative damage caused by Cd pollution (Guo et al. 2020). Similar findings have also been reported in mesotrione-polluted soil, in which the CAT, SOD, and POD activities, and MDA content in earthworms increased after application of low concentration of wheat straw biochar (Zhang et al. 2019).

4 Influencing factors and underlying mechanisms of earthworm biomarkers responses in biochar-amended soil

4.1 Influencing factors

Many factors may influence earthworm biomarkers responses in biochar-amended soil, such as biochar properties and dosage, soil characteristics, earthworm species, as well as exposure duration (Lehmann et al. 2011; Ameloot et al. 2013).

4.1.1 Biochar properties and dosage

Biochar can be categorized according to its feedstock materials as wood (e.g., wood residues, sawdust), crop residue (e.g., wheat straw, rice husk, switch grass), manure (dairy or poultry waste), and sewage sludge derived biochar. In general, biochars derived from manure and sewage sludge have lower carbon content, surface area, and cation exchange capacity, when compared with lignocellulose biochar, but have higher pH, salinity, and ash, inorganic elements (N, P, K, Ca and Mg), volatile fatty acids, and ammonium contents (Rombola et al. 2015). For instance, poultry litter biochar amendment significantly increased the mortality of earthworms, probably because of the presence of toxic ammonia gas, in addition to high pH (10.3) and salinity; in contrast, no significant changes were observed in the earthworm biomarkers in soil amended with 10% pine biochar (Liesch et al. 2010). Furthermore, similar lignocellulose biochar treatments may produce diverse effects. For example, 10% rice husk biochar amendment promoted the growth of E. fetida, whereas wheat straw biochar amendment decreased the survival rate of E. fetida, especially with increasing contact time. Biochar byproducts encompass a variety of inorganic salts and organic substances, including carbonates, tar, PAHs, volatile organic compounds, and more. Wine tree cuttings biochar and low tar hardwood lump charcoal increased the accumulation of PAHs in earthworms (Malev et al. 2016), leading to an increase of MDA enzymes produced by the oxidative stress response (Godlewska et al. 2021). This further corroborated the internal or external abrasion of earthworms by wheat straw biochar particles, elevated soil pH, as well as increased PAHs concentrations in both the treated soils and earthworms residing in them (Elliston and Oliver 2020).

Besides the biomass feed properties, preparation protocol, pyrolysis temperature, and time also play important roles in determining the obtained biochar properties (Ippolito et al. 2020). In general, with the increasing pyrolysis temperature and time, the contents of ash, nutrients and carbonates, and the specific surface area, alkalinity, and porosity of biochar increase, whereas the surface functional groups, volatile matter, dissolved organic carbon, and cation exchange capacity of biochar decrease; these differences in biochar characteristics produce varying effects on earthworm biomarkers. For example, the mass loss of E. fetida was about 13% and 22% after contact for 28 days in soil incorporated with 300 and 550 °C perilla biochar at a dosage of 5%, respectively. The reason for greater weight loss of E. fetida following 550 °C perilla biochar treatment was due to the higher contents of available Zn, Cu, Cr, and Cd in the 550 °C perilla biochar treated soil (474.52, 131.60, 12.50, and 2.77 mg kg−1, respectively), which were about two-fold higher than those in the 300 °C perilla biochar treated soil (Kim et al. 2014). Moreover, earthworms thrived in the 450 °C biochar amended soil, which contained higher available P, when compared with 550 °C biochar amended soil (Chan et al. 2008).

Depending on its dosage, the added biochar had a double impact on earthworm metabolism. In general, low concentration of biochar is conducive to the growth and reproduction of earthworms, whereas high biochar dosage produces detrimental effects on earthworms. For example, the weight of earthworms significantly increased after 28 days of cultivation in 1% and 3% biochar-amended soils (Sanchez-Hernandez et al. 2019; Zhang et al. 2019). In contrast, the survival rate of E. fetida exhibited an increase, followed by a decrease after incorporation of 0.2%–11.1% wood waste biochar in PCB contaminated soil (Denyes et al. 2012, 2013). Addition of 10% biochar inhibited earthworm growth and induced DNA damage regardless of the soil moisture conditions (Zhang et al. 2019). Similarly, the earthworm survival rate decreased from 95% to 70%, and the live earthworms exhibited lumpy, irregular shape, and darker coloration when exposed to 20% wheat straw biochar (Elliston and Oliver 2020).

4.1.2 Soil characteristics

The same biochar application may produce varied effects on earthworm biomarkers in diverse soils, owing to the different physical, chemical, and biological properties of soil, especially soil pH and fertility status. For instance, L. terrestris ingested more biochar and wheat bran residue in acidic soil (pH 5.3) a mended with hardwood biochar; however, this outcome was not observed in neutral pH 7.4 soil (Elmer et al. 2015), mainly owing to greater improvement in the acidic soil pH. Similarly, after 48 h of contact, E. fetida exhibited obvious preference for acid ferrosol soil rather than calcareous soil when both the surface (0–10 cm) soils were amended with 10 t ha−1 sludge and waste wood chip derived charcoal (Van Zwieten et al. 2010). The interaction between biochar and earthworms displayed soil-type specificity. For instance, biochar and earthworms exhibited a more pronounced impact on clover growth in the Cambisol than the Andosol (Garbuz et al. 2020). The addition of biochar in low organic matter soils was more effective in mitigating the migration of MPs and alleviating their harm to earthworms when compared to high organic matter soils (Zhang et al. 2023).

4.1.3 Earthworm types

The responses of different types of earthworms to biochar amendment can vary, reflecting the diversities in the physical activities, feeding habits, burrowing, etc. of earthworms (Ferlian et al. 2020). For instance, the biota-soil accumulation factor, i.e., the pollutant concentration ratio in the earthworm and soil, of Metaphire guillelmi (an endogeic species) was about twice as high as that of E. fetida (an epigeic species), when both the types of earthworms were cultivated in 0.5% pine biochar amended soil polluted by 4.25 mg kg−1 atrazine (Wang et al. 2014). The reason for this observation may be attributed to the fact that endogeic earthworm species generally ingest higher amount of SOM and biochar, which get thoroughly mixed in their gut passage, resulting in higher biota-soil accumulation factor.

4.1.4 Exposure duration

Contact time is another important factor that affects earthworm biomarkers in biochar-treated soil. It has been reported that biochar addition caused negative effects on the earthworm population density and total biomass within a few days or weeks; however, these effects significantly reduced thereafter. Several earthworms were noted to escape from soil amended with 30 t ha−1 biochar derived from debarked spruce chips after 2 days of contact; nevertheless, the population density and individual biomass of earthworms in the biochar-amended soil were higher than those in the control treatment throughout the 14 day observation period (Tammeorg et al. 2014). Thus, the negative effects of biochar on earthworm biomarkers resulting from the marked pH changes in the biochar-amended soil, temporary release of toxic and harmful substances from biochar particles, and internal or external abrasion of earthworms by dry biochar particles can be progressively eliminated with cultivation time. The combination of nanoscale zero valent iron-enriched biochar and acid-washing biochar treatment notably reduced the concentration of sulfamethoxazole in earthworms during the 15 to 30 days of cultivation (Zhang et al. 2022).

4.2 Underlying reasons for changes in earthworm activities

Based on the above-mentioned discussion, the following direct impacts of biochar amendment on earthworm biomarkers were inferred (Fig. 4). Depending on the feedstock used, biochar can contain abundant nutrients and mineral elements, which serve as food source for earthworms as well as affect their growth and reproduction (Topoliantz and Ponge 2003). Earthworms ingest the microorganisms or microbial metabolites on the biochar surface, which can generate a favorable intestinal environment (Pietikainen et al. 2000; Lehmann et al. 2011; Jin et al. 2022; Alsamhary 2023).

Fig. 4
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Mechanism diagram summarizing the physiological changes of earthworms in response to biochar addition

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However, excessive toxic and harmful substances (e.g., ammonia, PAHs, metal (loid)s, etc.) generated during biochar production can pose potential threat to earthworms (Gomez-Eyles et al. 2013; Malev et al. 2016). Biochar particle invasion can cause physical damage to the internal and external tissues of earthworms (Schmidt et al. 1999; Wang et al. 2023), and excessive salinization and alkalization of biochar-amended soil may have potential negative impacts on earthworms (Van Zwieten et al. 2010; Domene et al. 2015). Besides, concentrated pollutants generated owing to their strong affinity to biochar are also unfavorable for earthworms (Jones et al. 2011; Shi et al. 2023); nevertheless, these pollutants may be removed after biochar purification, thus mitigating the negative effects on earthworms (Lin et al. 2012; Buss and Masek 2014).

Meanwhile, biochar incorporation can affect several soil properties, such as pH, available nutrients, and water-holding capacity (WHC), thus altering the earthworm living conditions. It has been well established that earthworms are sensitive to soil pH, and that neutral or weak alkaline conditions are generally favorable for earthworm metabolism. In contrast, biochar is usually characterized by high alkalinity and pH buffer capacity, causing an increase in soil pH following its application to soil. Improved earthworm biomarkers have been frequently documented in biochar-amended acidic soil. Moreover, biochar amendment can increase soil WHC owing to the high water retention capacity, hydraulic conductivity, and air-filled porosity of biochar (Liao and Thomas 2019). Most of the earthworm species live in dark, moist, and humic soil habitats. Hence, with an increase in soil moisture, but to a lesser degree of waterlog, the earthworm activities significantly increased.

Moreover, the biochar ingested by earthworms detoxifies the pollutants and minimizes their association with enzyme activities, and the pyrolyzed lignin and cellulose in biochar produce reactive oxygen species, which may protect the earthworms against oxidative stress (Liao et al. 2014).

5 Co-application of earthworms and biochar for improving soil health and fertility

The co-application of earthworms and biochar for improving soil health and fertility, and other ecological functions is summarized in Fig. 5. A mutual interaction exists between biochar and earthworms in cultivating soil, with the applied biochar affecting earthworm biomarkers and the earthworm activity influencing biochar efficiency. For instance, earthworms may transport biochar into the deeper soil layer via bound particles on epidermis mucus. Furthermore, the biochar ingested by the earthworms mixes with other particles in their intestines to form highly stable organic complexes through the digestive tract, thus homogenizing the cast with soil matrix and consequently enhancing carbon sequestration potential. These results are supported by the enrichment of charcoal fragments in the body of P. corethrurus in dark organic soil (Ponge et al. 2006), which partly explains charcoal enrichment in terra preta deep soil layers in the Amazon region (Glaser et al. 2000). All of these studies suggest that the persistence and efficiency of amended biochar increases in the presence of earthworms. Few studies have indicated that amended biochar is less efficient when co-applied with earthworms, because ingestion of biochar particles by the earthworms reduces the mobility of biochar in the soil environment (Ameloot et al. 2013; Domene 2016; Wang et al. 2023).

Fig. 5
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Synergetic effects of biochar and earthworm co-application on soil health and fertility

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Earthworm activities can improve microbial growth conditions around the biochar particles, owing to the improved production of extracellular enzymes, increased adsorption capacity on the biochar surface, and homogenized biochar in the soil. Numerous studies have demonstrated the ability of earthworms to promote soil enzyme activities after biochar application (Garcia et al. 1997; Sanchez-Hernandez et al. 2019; Garbuz et al. 2020; Noronha et al. 2022), and an enriched soil microbial population can enhance the absorption of nutrients (e.g., N, P, and K), which is conducive to promoting soil productivity and crop yield (Vishwakarma et al. 2016).

As described earlier, both live earthworms and biochar incorporation can enhance SOM mineralization, inhibit pathogens growth, stimulate microbial metabolism, increase WHC and aeration efficiency, and thus promote plant growth. Co-application of biochar and earthworms has cumulative and synergistic effects on soil health and productivity. The combined application of biochar and earthworms to the soil has been reported to enrich the microorganisms in the earthworms and activate N processing enzymes, resulting in an increase in inorganic N supply and rice yield (Noguera et al. 2010). After incorporation of biochar derived from sludge, deinking sludge, miscanthus, and pine to a tropical soil with earthworms, the number of fruits per plant in soil with P. corethrurus and biochar was higher than that in soil without earthworms, and the fruit yield in soil amended with sludge biochar and earthworms was nearly 50% higher than that in soil incorporated only with biochar (Paz-Ferreiro et al. 2014).

Co-application of earthworms and biochar not only improved the health and fertility of the amended soil (Subler et al. 1997; Sheehan et al. 2006; Barot et al. 2007; Noronha et al. 2022; Alsamhary 2023), but also increased C/N fixation and reduced CO2 and NOx emissions (Liu et al. 2018; Gao et al. 2022). Incorporation of endogeic earthworms, ammonium sulfate, and 4% biochar derived from peanut hull and miscanthus into two soils containing different organic matter indicated that biochar application reduced NO2 emission of earthworms in both high and low organic matter containing soils; however, the effect was more profound in low organic matter containing soil (e.g., co-application of biochar and earthworms reduced NO2 and CO2 emissions by 42% and 43%, respectively), when compared with the control treatment (Augustenborg et al. 2012).

Soil amendments comprising earthworms and biochar could play positive roles in correcting polluted soils. A previous study reported that the concentrations of As and Cu in soil leachate decreased following incorporation of biochar and earthworms, which can be attributed to the decline in dissolved organic carbon concentration and reduction in pore water seepage through the chamber owing to the presence of earthworms (Beesley and Dickinson 2011). Another study indicated that the CaCl2-extractable Pb and atrazine concentrations significantly decreased after 15 days in soil subjected to co-application of E. fetida and dairy manure biochar, when compared with the control treatment without amendments, which further confirms that the addition of biochar and earthworms decreased Pb and atrazine availabilities (Cao et al. 2011). More recently, Xiao et al. (2021) and Alsamhary (2023) have presented results on the increase in Cd-remediation efficiency after co-application of biochar, earthworm, and specific microorganisms.

Accordingly, the following suggestions are proposed to promote the synergetic effects of biochar and earthworms in improving soil productivity and health through their co-application: (1) optimization of the biochar application dosage (< 5%) to diminish its ecotoxicity (Denyes et al. 2013; Kim et al. 2014; Dlamini and Otomo 2022); (2) utilization of biochar with appropriate alkalinity, salinity, and nutrients, depending on the amended soil properties (Liesch et al. 2010; Rombola et al. 2015) to ensure the transformation of the amended soil into a neutral pH, well-hydrated, and fertile environment; (3) selection of biochar with low content of ammonia, PAHs, metal (loid)s, and other toxic substances, or purification/modification of biochar containing excess of these components (Lin et al. 2022); (4) moistening of soil in advance or in a timely manner to avoid temporary internal or external abrasion of earthworms resulting from over-dried biochar particles (Li et al. 2011); and (5) selection of different earthworm species according to the soil and biochar characteristics as well as soil amendment purposes (Augustenborg et al. 2012).

6 Conclusions and perspective

In this review, we summarized individual, physiological and morphological, and biochemical and molecular indicators of earthworms in biochar-amended soil. Our findings indicated that the activity of the aforementioned earthworms can be influenced by various factors, including the properties and dosage of the biochar used, soil properties, the specific earthworm species involved, and the duration of exposure. This variation can be attributed to both direct effects of the added biochar, such as the provision of ample nutrients and essential minerals, as well as potential exposure to excess toxins and hazardous substances, and indirect effects, which encompass alterations in soil properties and the detoxification of pollutants. Co-application of biochar and earthworms has the potential to enhance soil health and productivity in following ways, including pH correction, enhanced enzyme activity, augmentation of microbial species and communities, decreased CO2 and NOx emissions, detoxification of heavy metals, organic pollutants, and emerging contaminants. As a result, this combined approach can effectively promote overall soil health and productivity.

Nevertheless, more case studies are required to improve our understanding of earthworm activity in biochar-amended soil, especially for a prolonged contact period in field trials. Moreover, besides the most commonly studied E. fetida, the responses and parameters of other earthworm types and species to biochar amendment should also be investigated. Furthermore, research on the detoxification effects of biochar and earthworms co-application on multiple contaminated soil limited. Although the present fragmented observations have yielded substantial insights into the process of soil fauna activity, further analysis of omic datasets is not only recommended, but is also significantly crucial for the validation of methodology prior to practical application for refining earthworm health exposure assessment.