Introduction

Recently, there has been an increase in the demand for food crops in the wake of an ever-increasing population pressure associated with a global count of approximately 7.6 billion people (UN DESA 2017). The human population, mainly in developing countries, is increasing at an exponential rate (Etongo et al. 2018). Additionally, population growth is expected to result in a doubling of the estimated global cereal demand by 2050 (Singh and Singh 2017). Agriculture ecosystems are one of the basic requirements for human existence. With an increase in population and food constraints, agriculture is the only mode to satisfy our food needs. However, the intensification of agriculture systems has proven to have a notable environmental impact; on the one hand, food demand increases with the increase in the global population, and on the other hand, the area under cultivation is declining (Etongo et al. 2018). Unscientific and imprudent use of chemical fertilizers has resulted in the deterioration of the soil ecosystems by nutritional disparity, rhizosphere micro-ecological environment disturbance, soil acidification and increased heavy metal ions activity in the soil (Li et al. 2018; Lin et al. 2019). Thus, based on previously performed studies, it can be inferred that there is an urgent need for sustainable agriculture.

Environmental deterioration is the main problem for the living beings which occurred due to the implementation of the green revolution (Singh and Singh 2017). Soil fertility and the groundwater level have been declining rapidly because of the intensive farming practices (Gubbels and Brescia 2017). The intensive modern practices are expensive for crop production and eventually contribute to environmental risk such as land use/land cover change with a decline in soil aeration. It is desirable to shift toward cheap, eco-friendly practices to improve soil health and increase food quality (improved nutrient value). Many green technologies such as biofertilizers, green manuring, bacterization, algal biofertilizers and vermicompost promote sustainable agriculture. Such practices also involve the minimization of waste generated from agricultural crops. Nutrient availability in domestic and agricultural wastes after composting evidently improves plant growth when applied to soil (Wahi et al. 2019). The generation of agricultural residue in agricultural countries like India poses a great difficulty in utilizing the residues in a sustainable manner. About 910 Mt of agricultural residues per annum with a plethora of 321 Mt has been generated from different waste streams such as cereals, oilseeds, pulses, cotton stalk, coconut (coir), sugarcane, horticulture and other agricultural activities (Dhar et al. 2017). These residues unless otherwise utilized results in environmental hazards and disposal problems. Among the various other options to recycle agricultural and domestic wastes, it has been determined that earthworms can potentially convert these undesired by-products to vermicompost that can serve as fertilizer to improve soil quality. Vermiculture technology has been determined to be one of the best solutions for organic waste management. Recent studies on vermiculture applications have been shown to improve agricultural yield and increase soil restoration (Sinha and Herat 2012), by assisting plant growth regulator formation such as humates and fulvates (Edwards et al. 2011). Moreover, vermicompost provides plant growth hormones such as gibberellins, auxins and cytokinins (Ravindran et al. 2015). However, such studies are still uncommon that describe vermicompost enrichment by biochar and microorganisms, and night soil vermicomposting. This review focuses on the latest ecological applications of earthworms and provides an overview of the current findings in regard to soil health. To achieve this objective, the present review has attempted to (1) analyze the patterns of global agriculture, (2) highlight the chronological development of vermicomposting and (3) discuss emerging research issues, for instance, detailed applications of vermitechnology for soil health improvement that result in the framing of a novel approach for sustainable agriculture development.

Global agricultural patterns: environmental alteration

Land clearing and intensive use of existing lands for crop production can assist in addressing increasing food demands. However, the consequences and trade-offs associated with this option of agricultural extension affect soil health adversely. Agriculture expansion has many demerits, such as land clearing, habitat fragmentation and biodiversity loss. It has been assessed that approximately 75 billion tonnes of verdant soil is lost worldwide due to agrarian practices (Pimentel and Burgess 2013). Industrialization, urbanization and soil erosion are some of the major reasons for environmental alteration which convert arable land into non-farm land (Pimentel and Burgess 2013). Agrochemicals used in agro-systems also deteriorate soil health (Pimentel and Burgess 2013). Besides these circumstances, still today, about 820 million of the earth’s poorest people persist malnourished because regional agro-systems are not able to deliver sufficient nourishing food, and financial aspects prevent the impartial supply of accessible food (Searchinger et al. 2019). Based on present patterns, both livestock and crop production will require to surge at significantly swifter rates than they have surged above the previous 50 years to completely fulfill expected food demand (Searchinger et al. 2019). Moreover, the quality of food production is also a significant challenge that must be addressed, because millions of people are suffering from acute pesticide poisoning and thousands of people die annually as a result of the use of chemical fertilizers in developing countries (Sinha et al. 2013). Asia is ranked first in terms of global agrochemicals revenue in 2008 at 43.1% (Quaik and Ibrahim 2013). In India, agriculture has been under practice for more than 4000–5000 years and the native farming of the country is primarily organic farming (Singh and Singh 2017). The development of agro-ecosystem resulted in agricultural advancements in the mid-1980s, and the green revolution era led to a reduction in soil quality in the main irrigated agricultural regions due to the prominent use of fertilizers (Sinha and Herat 2012) (Fig. 1a).

Fig. 1
figure 1

Adopted from Lemtiri et al. (2014)

a Illustration of the anthropogenic activities on agriculture and its sustainable options; b process of vermicompost production and its consequences.

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Earthworms: the eco-biological engineers

An eco-biological engineer is an organism which improves the ecological and biological condition of the soil. As such, earthworm changes the properties of soil contaminated with pesticides or heavy metals and overcome human-triggered heavy metal and pesticide pollution (Šrut et al. 2019; Sanchez-Hernandez 2019). They assist in the reclamation of degraded land to revive soil health and serve as soil indicators. Over the years, new knowledge has been generated by the researchers who worked in various areas, viz. ecology (soil-rich earthworm casts, as mounds of soil on the pasture cover, can reduce photosynthesis, muddy the surface and lead to thinning, weed invasion and surface softening) (Boyle et al. 2019) and wastewater treatment (earthworms-biochemical and transcriptomic response of Eisenia andrei exposed to soils irrigated with treated wastewater) (Mkhinini et al. 2019). All these aspects need to be further explored including the substrates, pH, moisture and the conditions required for earthworm survival. Similarly, few studies have been performed on earthworm identification, e.g., the giant earthworm biome and its life cycle study in Brazil (Drumond et al. 2015). Earthworms can be used as bioindicators for the soil, in land reclamation, in organic waste recycling, in wastewater treatment, in remediation and as a biofertilizer. Earthworm’s end products (vermicomposts) also serve as a soil conditioner via nutrient and microbial enrichment and by virtue of biological control of pathogens through the volatile organic components produced by bacteria and actinomycetes in vermicompost/vermicast (Mu et al. 2017; Balachandar et al. 2018). Based on these properties, earthworms can be considered as an important organism in agriculture and as an eco-biological engineer in comparison with other green technologies. Vermicompost and humic fertilizer are reported to be suitable soil amendments for wheat crop which alleviate saline soils by controlling bacterial community and soil aggregates (Liu et al. 2019). Moreover, it has been shown to increase maize productivity (Oo et al. 2013). Sahariah et al. (2015) also evaluated the increase in microbial respiration rate, in addition to an enzyme that assists in metal remediation including the conversion of waste into valuable products via earthworm activity.

Earthworms are classified into 23 families consisting of nearly 700 genera and 7000 species. In India, Julka (1993) has reported 67 genera and 509 species of earthworms. In terrestrial ecosystems, the dominant soil macrofauna is usually earthworms belonging to Oligochaeta of the Phylum Annelida (Jusselme et al. 2015). They are found seasonally in all nadir of the soil, and their size varies from 0.028 yards up to 2 yards (Pechenik 2009). They improve the physicochemical and biological soil characteristics, in addition to its structure (Lim et al. 2015). This often results in increased microbial communities (Groffman et al. 2015). Earthworms also increased the delivery of organic carbon and nitrogen into soil aggregates, thereby improving soil organic matter stabilization and accretion in agrarian systems (Grdiša et al. 2013), and the C/N ratio regulates crop productivity (Awe 2015). All these actions depend on their ecological category. Ecologically, earthworms dwelling in terrestrial soils are divided into three categories: anecic, endogeic and epigeic (Bouché 1977). Anecic species burrow into the profound mineral soil sheets and tow organic matter into their tunnels from the surface of the soil. Endogeic species create temporary warrens in the top film of the soil, chiefly devour mineral soil substance and are identified as eco-biological engineers. Epigeic species survive on the surface of soil, do not form enduring tunnels and chiefly consume litter and humus, decomposed organic material, without mixing organic and inorganic matter (Demetrio et al. 2019). Earthworms are present in both temperate and tropical soil. Sensitivity comparisons of tropical and temperate species have shown contradictory results because of different soil and climate conditions; different earthworm species have shown different results (de Silva et al. 2009). Based on these results, one can conclude that some factors that are independent of the natural ecosystems are responsible for earthworm population. Intensively cultivated lands have generally exhibited a lower earthworm population (such as power-driven disturbance). It has been observed that the population of earthworms is approximately 8–10 times greater in fallow areas than in cultivated areas, which evidently shows that the population declines with soil degradation (Karthikeyan et al. 2015). Consequently, higher mortality rates are predominantly recorded in epigeic and anecic species due to the increase in cultivation activities (Briones et al. 2011).

Vermicomposting: basic process

Vermicompost and vermicast formation processes and the merits of their application are described diagrammatically (Fig. 1b). In this process, the overall digestion and excretion via the earthworm’s gut are discussed. Mainly, enzymes present in the earthworm gut play a major role (Ravindran et al. 2016). In the foregut, the calciferous glands present in earthworms increase the pH which helps to produce an upsurge of bacterial, fungal, algal and trophozoite population (Lemtiri et al. 2014), and increase the absorption of organic matter. Assimilation of C and N through the midgut released CO2 and N2O via the hindgut. In the midgut, the secreted antibacterial and antifungal substances help to reduce plant pathogens. The gut of the earthworm acts as a bioreactor with the presence of intestinal mucous to increase microbial activity. The activity of microorganisms is increased as a result of the nitrogenous compounds present in the mucus in the gut (Ravindran et al. 2016). Microbial activity also degrades organic matter which primarily includes cellulose, hemicelluloses, lignin and proteins. The enzymes, viz. lipase, cellulase, amylase and chitinase, help to split complex molecules into a simpler form. The feeding strategies of earthworm expand the surface area for proper microbial activity. Microbes create a humidified environment that converts oxidized unstable organic matter into stable forms.

Vermicompost as biofertilizers

The benefits of vermicompost on crop yield are as follows: (1) It increases the soil temperature and water retention because of the hydrophilic groups that are present and increases plant growth, leading to a better yield; (2) it increases soil fertility and productivity; and (3) it stabilizes soil particle aggregation. de Godoi Periera et al. (2014) have discussed the role of vermicompost in agrarian output (Table 1). Vermicompost increases the radical scavenging (antioxidant) property in tea plants.

Table 1 Application of vermicompost for sustainable agricultural benefits
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Vermicompost has various advantages such as the restoration of nutrients and long-term soil stabilization. Plants showed a higher growth rate because vermicompost contains plant growth hormones and humic acid (Song et al. 2015). Apart from increasing plant growth, humic acid increases root hair proliferation and mineral nutrient release and is involved in oxidative phosphorylation, cellular respiration, photosynthesis, protein synthesis and several enzymatic reactions. Hussain et al. (2018) observed that vermicompost of Salvinia molesta improved the germination success rate and the morphological growth and biochemical content of plant species. Truong et al. (2018) reported that vermicompost improved the productivity and quality of tomato plants. Rekha et al. (2018) observed that vermicompost improved plant nutrition, growth, photosynthesis and chlorophyll content of leaves. Benazoukk et al. (2018) demonstrated that vermicompost works as a protective agent and improves the salt-stress resistance in tomato crops. Vermicompost extracts with arbuscular mycorrhizal fungi increased the shoot and root nutrient uptake rate in plants (Khan et al. 2014). Vermiwash (liquid fertilizer) with AMF (arbuscular mycorrhizal fungi) simultaneously affected the C/N/P stoichiometry in plant shoots and plant growth. Encouraging substances in vermicompost improved the growth of crops with mycorrhizal colonization (Khan et al. 2014). Various species such as Streptomyces present in vermicompost act as an antifungal agent against many pathogenic fungal species (Gopalakrishnan et al. 2011). The antibacterial and antifungal activity of vermicast isolated actinomycetes is due to the production of a variety of antimicrobial metabolites (Balachandar et al. 2018, 2019).

Vermicompost plays a crucial role in influencing the nutrient cycling and increasing resistance to fungal diseases and crop pathogen in plants via microbial activities (Gomez-Brandon et al. 2015). The excellent composition of vermicompost enhances the leaf area and dry matter of plants and the fruit yield also increases (Singh et al. 2010a, b). Vermiwash produced by vermicompost can increase the total phenolic content and enhance the antioxidant properties of tea plants (Bagchi et al. 2015). Vermicompost also reduces the total and monomeric phenolic compounds which have inhibitory effects on plants (Masciandaro et al. 2010). Organic treatments have better SOD (superoxide dismutase) and catalase activity to reduce the action of superoxides and hydrogen peroxides. The mixing of vermicompost with commercial growth medium has also shown a beneficial impact on plant growth (Wonga et al. 2015). The quantitative and qualitative properties of crops are enhanced by vermicompost, such as capsaicinoid accumulation and the increase in the pod yield in Chiltepin peppers (Capsicum annum L.) (Diacono and Montemurro 2015). Moreover, it also affects soil fertility attributes such as soil organic carbon stock. Vermicompost has been shown to increase straw yield, biological yield with improvement in growth parameters such as total sugar, total suspended solids (TSS), fruit quality, seed germination, root and shoot length, total root nodules and leghemoglobin content (Khan et al. 2015). It has been associated with an increase in shoot biomass, the number of flowers, vegetable weight and the germination of plants. A progressive increase in vermicompost levels has been shown to produce significantly elevated protein content in the seeds of plants (Rajya Laxmi et al. 2015). The succulence and ascorbic acid contents of fruits are also improved via vermicompost application (Sheikh et al. 2015). The number of soil microbes such as Gluconacetobacter diazotrophicus, Pseudomonas putida, Azotobacter chroococcum, Azotobacter vinelandii, Bacillus stearothermophilus, Bacillus megaterium, Bacillus subtilis and Brevibacillus borstelensis is increased by the addition of vermicompost which further increases the total dry weight of root and shoot length and also improves the rate of seedling emergence. Gluconacetobacter species has an associative symbiosis which explains the N fixation process in the plant root zone. Pseudomonas putida induces plant growth and a plant defense mechanism. Vermicompost teas also exhibit a reduction of soil-borne diseases when used as soil drenches (Fernández-Gomez et al. 2012). The aged vermicompost tea (vermicompost maturity) composition also has an influential effect on plants. Mature composts are probably released with higher levels of solvable mineral nutrients and a lower level of phytotoxic organic acids, as well as a reduced amount of heavy metals than unformed composts (Gomez-Brandon et al. 2015). This technology results in increased profits for farmers and industries.

Bioenrichment of vermicomposts

Currently, various microbial inoculations are performed to enhance the properties of vermicompost. When vermicompost is inoculated with nitrogen-fixing Azospirillum lipoferum and Azotobacter chroococcum strains, the effect of the phosphate-solubilizing Pseudomonas striata on nitrogen (N) and phosphorus (P) matters was evaluated. The N2-fixing bacteria inoculation into vermicompost augmented N2 content. The addition of rock phosphate and P. striata notably improved the available phosphorus in the vermicompost. Vermicompost was prepared by leaves of pearl millet (Pennisetum glaucum), chopped stalk and cattle dung with a moisture content of 75% by decomposing under anaerobic conditions for 3 weeks. These substrates were mixed in the ratio of 1:3 by volume (dung: plant residues) and kept for enrichment at 32 °C for 9 weeks. Nitrogen, bacterial population and available phosphorus contents (g/100 g) were measured at 0, 15, 30, 45, 60 and 75 days. The addition of rock phosphate inoculated with P. striata led to more availability of P, most likely due to the production of organic acids by the bacteria which solubilized the rock phosphate. The incubation period duration affects the inoculated bacterial strains, and they quickly flourished which fixed N2 and solubilized the phosphate (Kumar and Singh 2001). Similarly, Alikhani et al. (2017) observed that inoculation of Pseudomonas and Azotobacter in vermicompost can potentially be used as a means of increasing quality. Moreover, the rapid increase in temperature reduces the effectiveness of the process. The incubation period is an important criterion in vermicompost enrichment. In addition, vermicompost inoculated by Thiobacillus produced explicit effects with respect to the conversion of rock phosphate into available P, as demonstrated by Burkholderia and Herbaspirillum (Rajiv et al. 2013). Pseudomonas improved the availability of P by releasing organic acids and increasing humic acids (Alikhani et al. 2017). From the above information, it is quite evident that microbial enrichment in vermicompost decreases inorganic fertilizer usage, improves diversity, increases soil biological activity, sustains the physical properties of soil and recovers environmental health. The survival rate of the biofertilizers, A. chroococcum, B. megaterium and Rhizobium leguminosarum, was found to be higher in vermicast carrier material than in lignite, indicating that the earthworm casts intact the viability of biofertilizers (Rajasekar and Karmegam 2010). Vermicompost (prepared by Ipomoea carnea, Eichhornia crassipes, rice straw mixed with biomass) augmented with A. chroococcum leads to an improvement in the leaf chlorophyll content, grain yield, plant growth and activity of nitrate reductase in rice crop, as a result of augmentation with Azotobacter brasilense. Plant nutrients, organic C, available N, P, K and CEC in post-harvest soil were also considerably enhanced by the application of enriched vermicomposts, and the best effect was found using Azotobacter-enriched vermicompost when incubated for eight weeks (Mahanta et al. 2012). Furthermore, vermicompost with plant growth-promoting bacteria and humate was apparent in both sapling development and growth (Olivares et al. 2015). Vermicompost augmented with B. subtilis IIHR BS-2 effectively manages soft rot disease and root-knot nematode complex in carrot (Rao et al. 2017). A recent study by Karmegam et al. (2019) revealed that the addition of leguminous green manure plants, Gliricidia sepium and Tephrosia purpurea along with cow dung while vermicomposting paper industrial sludge resulted in the production of nutrient-rich and less phytotoxic vermicompost.

Biochar is a carbon-rich solid material that can be obtained by a broad variety of plant and animal biomass through a thermal process in the absence of oxygen (Malinska et al. 2017). Biochar has several properties, viz. holding water and nutrients, contaminant binding ability and improving microorganism population. Various important roles of biochar are as a soil conditioner and an enricher (Ngo et al. 2013), an adsorbent for inorganic and organic pollutants (Ahmad et al. 2014) or an additional material for composting (Czekała et al. 2016). Similarly, biochar can be used as a capable amendment in vermicomposting of different organic wastes, involving sewage sludge resulting in higher growth rates and hence allowing the efficient transformation of sewage sludge into vermicompost (Malinska et al. 2016). Vermicomposting also improved the decomposition of organic waste and hastened the nitrogen mineralization process, while the rise in the C/N ratio might decrease GHG emission in sewage sludge vermicomposting (Lv et al. 2018).

Biochar addition to vermicomposting augmented earthworm growth, biomass, cocoon and juvenile numbers of Eisenia fetida also enhanced the enzyme activities such as dehydrogenase, urease, cellulase and alkaline phosphatase as compared to control (without biochar). Lignin degradation increased (13.9%) by biochar addition (Gong et al. 2018). It also enhanced vermicompost quality in cation exchange capacity, dissolved organic carbon, degradation, humification, nitrogen transformation, toxicity to germinating seeds and heavy metal concentrations (Gong et al. 2018). Cadmium and zinc bioavailability decreased to E. fetida by adding biochar (sewage sludge derived) in composting of municipal sewage sludge and wood chip mixtures (Malinska et al. 2017). Biochar addition also protected the organic matters from chemical oxidation and altered their vulnerability to biological decomposition, which signifies that biochar possibly will enhance the sequestration of carbon potential of vermicompost, manure and compost when amended in the mixture (Ngo et al. 2013). Similarly, Sarma et al. (2018) conducted an experiment with wheat green gram crops in an inceptisol for evaluating the biochar and vermicompost effects on soil organic carbon (SOC), its elements and carbon mineralization when utilized with inorganic fertilizers. Biochar application into a wheat green gram crop rotation fertility program with inorganic fertilizers would be better which can not only sustain crop yield but also sequester higher SOC in comparison with vermicompost (Sarma et al. 2018). The remediation impacts of vermicompost and biochar have been evaluated in cadmium-polluted soil under the acid rain threat, individually and associative conditions (Wang et al. 2018a) (Fig. 2a). The findings revealed that acid rain accumulation has an inhibiting effect on soil remediation process. The soil conditioners (biochar and vermicompost) indicated that they restore soil fertility and lower the acid rain stress (Wang et al. 2018b).

Fig. 2
figure 2

a Vermicompost and biochar as nutrients to enhance soil fertility and check heavy metals in cadmium-contaminated soil under acid rain stress (Adopted from Wang et al. 2018a, b); b accumulation of pollutants (POPs, herbicides, pesticides, PAHs, etc.) in soil and their effect on earthworm DNA

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Vermicomposting of night soil/human excreta

In the current scenario, low-income countries and rural areas throughout the world often have rudimentary onsite sanitation systems including improper disposal of fecal sludge. Animal wastes are already in use as fertilizers, but the use of human waste is limited, despite its high nutrient value. Ngone et al. (2018) found that plant nutrients, for instance, N, P and K, were increased by eight, two and two times, respectively, using fecal sludge in the mound burning method. Farmers prefer organic inputs on the agricultural field; however, supplies are limited and subsidized chemical fertilizers are inexpensive compared to organic fertilizers. The apparent profits of human excreta compost have decreased mainly due to the insanitary situations of conventional toilets, the initiation of modern toilets, a smaller workforce for this type of work activity, the modern society mindset, educational progress, social opinion and easy availability of agrochemical fertilizer. Additionally, the use of old-style dry toilets which readily enabled the conversion of human excreta into compost has declined with time in countries like India (Oinam, 2008). Therefore, the management of urban solid wastes including human excreta reuse in agriculture should necessarily incorporate decontamination, stabilization and maturation aspects to diminish the possible spread of disease and to achieve a more stable and matured product for application to the soil (WHO 2006).

Human excreta are considered to be one of the best fertilizers available because they tend to provide a quick response when they are applied as a surface layer. It should be applied in less quantity because it burns crops when used in huge quantities. Mashauri and Senzia (2002) demonstrated that night soil compost in Tanzania provides a better yield when no other fertilizer except for human excreta is used. However, the farmers still had doubts regarding its hygienic utilization. The windrow and vermicomposting methods are most effective in maintaining the pathogens below a threshold level. Hence, the elimination of pathogens from composts is considered to be a critical aspect. According to Mengistu et al. (2017), the vermicomposting method is recommended for organic waste composting including human excreta. The whole deactivation of complete coliforms was observed in a study on human feces processing, which highlighted the vermicomposting ability for the conversion of source-separated human feces (Yadav et al. 2010). Hence, this approach may eventually solve the problems associated with the improper disposal of human excreta/night soil.

Vermicompost maintain biological properties of soil

Earthworms or vermicast has localized spots for increased microbial activities, nutrient cycling and incorporation of SOM (soil organic matter) for transformation into rich humus (Blouin et al. 2013). Increased microbial activities and secondary metabolites formed by microbes secrete beneficial enzymes, plant growth hormones and humic acids. Subsequently, the enzymatic activity leads to a reduction in water demand by 30–40% with a large surface area to make the soil porous (Sinha and Herat 2012). Vermicompost enhanced the dehydrogenase activity which is used to calculate the microbial community respiration activity (Khan et al. 2015). Additionally, it also enhances the nitrogenase activity in the rhizosphere, which is responsible for N2 fixation in legumes. Moreover, the presence of the enzyme phosphatase in vermicompost increases phosphorus availability to plants (Padmavathiamma et al. 2008). It, further, increases beta-glucosidase and acid phosphatase (hydrolytic enzymes) activities that play key roles in C and P cycling (Lim et al. 2015). Vermicompost is also known to suppress plant diseases by inhibiting the parasitic fungi growth, for instance, Pythium, Rhizoctonia and Verticillum, and also inhibit nematode attacks (Singh et al. 2008). Vermicompost was shown to reduce aphid and jassid attacks in a groundnut field (Rao 2002). In addition, stimulated microbial activities transformed nitrogen into mucoprotein that prevents the leaching of nitrogen in the soil and also reduces the C/N ratio (Ansari and Ismail 2012). Vermicompost has a positive correlation between biomass and nutrient mineralization with a slower release in soil (Roy et al. 2010). Nevertheless, the available data pertaining to the growth of plants using vermicompost under a variety of climatic conditions with different soil types for different crops are sporadic; these parameters are potent indicators of soil quality.

Vermicompost alters soil physicochemical properties

The physical properties of soil, for example, resistance to corrosion, aeration, drainage, infiltration and porosity, are improved due to the addition of vermicompost (Arancon et al. 2008). The water holding capacity is improved due to the presence of organic matter and mucus formation in the gut of the earthworm that enhanced soil aggregation (Kale and Karmegam 2010). Bulk density, porosity, cation exchange capacity and oxidation potential also increase with an increase in microbial activity in soils that utilized vermicompost (Manivannan et al. 2009). Salt concentration is maintained in the soils with vermicompost in the form of leachate (Oo et al. 2013). The aggregation of soil mostly depends upon the level of carbohydrate present in the vermicast, which maintains the soil structure that is required for proper root growth and nutrient uptake in plants (Lim et al. 2015). The use of vermicompost improved available microelements such as Ca, Mg, Na, N, P, K, Zn, Fe, Cu and Mn to plants (Mondal et al. 2015). The ion concentration and raw materials used for vermicompost affect the electrical conductivity (Gutiérrez-Miceli et al. 2007). A reduced electrical conductivity appears because of the existence of stabilized raw material in vermicompost (Lim et al. 2011). The number of exchangeable Na+ and Ca+ ions increases with vermicompost use and subsequently lowers the electrical conductivity of the soil (Oo et al. 2013). Vermicompost application considerably differs according to plant species and their genotype (Lazcano et al. 2011).

Previous studies have revealed conflicting results as the soil pH changes for different soil types with vermicompost application (Valdez-Pérez et al. 2011). It has been reported that NH4+ and NO2− concentration was low, but NO3− was present in high concentration in vermicompost-incorporated soil due to microbial actions (Gutiérrez-Miceli et al. 2007). NH4+ was converted into NO3− by aerobic nitrification process because of augmented microbial activity, resulting in the slow release of nutrients that can be leached, added on to the negative charges of the substances or consumed by plants (Atiyeh et al. 2001). Vermicompost reduced nutrient loss and decreased the risk of NO3− leaching in soil (Masciandaro et al. 2010). In addition, vermicompost, with a large population of autotrophic nitrifiers, will cause nitrification and provides stable NH4+ and NO3− for plants. Based on these observations, it can be concluded that the vermicompost process results in different outcomes depending on the type of soil, climate condition, crop type and substrate used for vermicomposting.

Contribution in land reclamation

Earthworms have the ability to eliminate hydrocarbons and several toxic chemicals from the soils known as vermi-remediation. Several species such as Dendrobaena rubida, Dendrobaena veneta, Eiseniella tetraedra, E. fetida, Aporrectodea tuberculata, Lumbricus rubellus, Lumbricus terrestris and Allolobophora chlorotica are known to remove hydrocarbons and toxic compounds, pesticides, lipophilics and organic compounds like PAH (polycyclic aromatic hydrocarbon) from the soil (Sinha and Herat 2012). Primarily, PAHs exist in the soil of several industrially polluted sites. Contreras-Ramos et al. (2006) reported that earthworms detoxify the soil; in their research work, main PAHs were separated in only 11 weeks using 50 worms per kg of soil during winter season in Brisbane. Vermicompost plays a role in the deterioration of PAHs via its polarity contribution and thermodynamic tendencies. The tendency for thermodynamic reduction results in the organic chemicals being transferred to vermicompost, which has a high concentration of electric charge. As a result, vermicompost can reduce diuron (pesticide) accessibility (Fernandez-Bayo et al. 2008) due to the polarity of this chemical. Eradication of PAH is also performed using vermicompost, viz. anthracene, phenanthrene and benzo(a)pyrene, from the soil (Alvarez-Bernal et al. 2006). Shi et al. (2019) observed that earthworm casts might also act as biomarkers for the initial indication of soil pollution. This report presents the information on earthworm ecology in contaminated soil and also highlights that the earthworm’s cast can serve as an important marker for soil contamination assessment. The phenanthrene sensitivity varies based on the different properties of the casts (Shi et al. 2019). Moreover, the removal of PAH using vermicompost has been shown to exhibit a lower impact in comparison with the direct earthworm application such as biosolids. Therefore, in advanced studies, the utilization of biosolids with earthworms to enhance efficacy should be investigated.

Yang et al. (2014) reported that living earthworms or vermicasts can remove nonylphenol (NP) produced from surfactants used for domestic products and act as an endocrine disruptor. Earthworms enhanced the bioremediation of NP by promoting microbial action and growth via defecation of readily degradable carbon (Yang et al. 2014). The results suggested that the direct use of these organisms was not appropriate because of toxicity. Therefore, vermicasts are a better option for the bioremediation of NP. In particular, they can bioaccumulate toxic compounds in order to make the soil less polluted (Romero-Freire et al. 2015). In addition, L. terrestris, L. rubellus and D. rubida can bioaccumulate cadmium (Cd) and lead (Pb) in their tissues (Sinha and Herat 2012). A. vinelandii can fix nitrogen and also mobilize cadmium, mercury and lead in the soil (Mary et al. 2015). Earthworms increase metal mobility, but more studies are required to explore this aspect in further detail (Kale and Karmegam 2010). The major concern of protecting tainted soils by shielding topsoil can also be achieved by amending organic soil additives such as vermicompost and biofertilizers. Additives have enhanced nutrient supply capacity for soils by reducing the duration of humification action (Sinha and Herat 2012). This provides a stable environment for soil and litter-dwelling invertebrates.

Vermicompost production using olive cake with other organic sources decreased the herbicide amount after blending with calcareous soil (Delgado-Moreno and Penã 2009). The amount of herbicides, viz. cyanazine, terbuthylazine, simazine and prometryn, was similar to that of the control in combination with soil amendments at an initial stage, which was then decomposed via microbes using vermicompost, and there was no effect on the thermodynamic properties (de Godoi Pereira et al. 2014). Vermicompost was also used to decompose chemicals. For instance, pentachlorophenol (PCP) which is used for agricultural purposes was decomposed through PCP decomposition due to the presence of humic substances and a PCP humus complex formed which was degraded via pH neutralization of the soil (Lin et al. 2016). Microbes in vermicompost result in a decline in the amount of heavy metals (Lv et al. 2016) and herbicides in the soils. Moreover, it decreases the Cd and Cu ion concentration in sultry soils (Jordão et al. 2011).

Persistent organic pollutants (POPs) are steady organic pollutants that affect the ecosystems adversely due to higher persistence, toxicity and biomagnification along the food chains. Espinosa-Reyes et al. (2019) reported that DNA damage occurred in earthworms when exposed to POPs in the Coatzacoalcos River low basin, Mexico. Earthworms are bioindicators for assessing the effect of pollutants. POPs affect earthworm at the genetic level and cause damage to their DNA which can be assessed by comet assay. In the POP analysis, the maximum concentration was determined in individual samples collected from industrial sites, rural and urban areas (Espinosa-Reyes et al. 2019) (Fig. 2b).

Contribution to waste recycling

In the era of urbanization and the increasing consumerism of the human population, the rate of municipal solid waste generation has rapidly accelerated in developing countries. The quantity of waste produced in urban areas of the globe is approximately 1.3 billion tonnes annually, which is predicted to be 2.200 billion tonnes in 2025 (Hoornweg and Bhada-Tata 2012). The waste generation rate of an urbanized area was 219 kg/capita/year in 2010, which is estimated to increase to approximately 343 kg annually per capita by 2025 (Hoornweg and Bhada-Tata 2012).

India is a developing country with 31.16% of its population living in urban areas (Sudhir and Gururaja 2012). Municipal solid waste management is one of the main concerns in India. The municipal waste generation is approximately 109,598 tonnes per day and is projected to increase to approximately 376,639 tonnes/day in 2025 (Hoornweg and Bhada-Tada 2012). Approximately 90% of the municipality’s full budget is expended on waste collection in India. However, the collection efficiency is only approximately 70–72% (CPCB 2012). These data reveal the extent of pollution caused by man to the environment. Man has been unsuccessful in resolving the problem completely, and as such, there is a disharmony between man and nature. Appropriate processing, segregation and organic waste recycling can reduce environmental pollution to some extent.

Vermicomposting incurs a monetary cost and is manageable from an environmental perspective. Recycling of organic waste, similar to food industry waste, can also be performed using earthworms. Vermicomposting of bakery industry sludge mixed with cow dung can be converted into biofertilizer (Yadav and Garg 2019). Lalander et al. (2015) reported that vermicomposting is a feasible method for manure management that cuts waste by 45.9% through a conversion rate of 3.5% of waste to biomass. This process was used to reduce food waste and cow manure using Eudrilus eugeniae in Kampala, Uganda, within 172 days. Pattnaik and Reddy (2010) observed that around 15 tonnes of waste was produced in the vegetable market of Puducherry (Pondicherry). The pilot-scale project was implemented amid two European towns and Puducherry in connection with a French Agency for the Environment (ADEME) of Italy. This project was performed to address the problem of treating the organic portion of MSW. Vermicomposting is used in landscaping and has monetary value as a biofertilizer. Gurav and Pathade (2011) also investigated the benefits of vermicomposting using temple solid waste in Maharashtra. They inferred that vermicompost is an outstanding and green method for temple waste management. Similarly, the study by Nalgonda (Andhra Pradesh) showed that vermicomposting performs a dynamic role in city environments and municipalities (Venkatesham and Reddy 2009).

The results of various studies have shown that animal solid waste from tannery industries can be converted into nutrient-enriched products by using composting/vermicomposting processes (Ravindran et al. 2015). Vermicomposting of different organic/industrial wastes requires pre-treatment prior to the process (Table 2). The results of the application of this procedure to paper mill sludge combined with cattle manure applying E. andrei in a 6-month pilot study has been stated by Elvira et al. (1998). Nogales et al. (1999) investigated the possibility of dairy industry biosolids vermicomposting by the blending of bulking agents, for example, cereal straw or wood shavings, using E. andrei. Banu et al. (2001) informed on the paper mill sludge modification by two exotic epigeic and an indigenous anecic earthworm species. Garg and Kaushik (2005) performed various experiments to convert textile mill sludge combined with poultry droppings and cow dung into vermicompost with E. fetida. Suthar (2006) stated the guar gum industrial waste utilization in vermicomposting based on three dissimilar cow dung combinations and sawdust using Perionyx excavatus. Sen and Chandra (2007) considered the organic matter transformation and sugar industry waste humification in vermicomposting. Ravindran et al. (2015) observed solid waste vermicomposting from tannery industries using the epigeic earthworm E. fetida and E. eugeniae. Khwairakpam and Bhargava (2009) effectively utilized one local (Perionyx excavatus) and two exotic (E. fetida and E. eugeniae) earthworm species in combinations (polycultures) and individual (monocultures) to recycle pressmud. Karmegam and Daniel (2009a, b) explored the possibility of using Perionyx ceylanensis in vermicomposting of different organic substrates. Followed by, Prakash and Karmegam (2010) reported the possibility of utilizing P. ceylanensis in converting sugar industrial waste (pressmud) for vermicompost production. Singh et al. (2010a, b) reported the use of vermicomposting of biosludge from the beverage industry with or without cattle manure. John Paul et al. (2011) reported that the municipal solid waste in combination with cow dung promoted earthworm and microbial activity apart from the increase in nutrients. Yadav and Garg (2011) demonstrated that different industrial wastes can be recycled using earthworms in a compiled study. The availability of nutrients and transformation and some heavy metals were evaluated in the integrated composting–vermicomposting of both wastes activated sewage sludge and primary sewage sludge utilizing aged vermicompost as a native bulking material and applying E. fetida as the earthworm species (Hait and Tare 2012).

Table 2 Vermicomposting of different industrial and other organic wastes using different earthworm species
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In contemporary studies, the vermicomposting process has been shown to remove about 90% of the heavy metals from spent mushroom compost and sewage sludge combination (Azizi et al. 2013). Vermi-remediation of dyeing sludge present in a textile mill has also been demonstrated with the assistance of the exotic earthworm species E. fetida (Bhat et al. 2018). The influence of metal-rich tea factory coal ash on composting, reproduction and the metal deposition capability of E. fetida and Lampito mauritii has been observed by Goswami et al. (2014). Suleiman et al. (2017) observed that E. fetida, E. andrei and D. veneta accumulated various heavy metals; among those species, Eisenia fetida has shown highest ability to accumulate the heavy metals. Inoculation of local species for vermicomposting is a viable option and is recommended to the farming community for recycling of sugar industry wastes (Shah et al. 2015). Furthermore, domestic waste, kitchen waste, sewage sludge, digestate, diluted spent wash and crude spent wash require bulking agents, for instance, cardboard, newsprint, discarded paper, leaf litter and cow dung to improve aeration and better waste structure and absorb excess liquids (Sonia et al. 2016; Sharma and Garg 2018). Amendments make the waste more digestible for earthworm consumption (Busato et al. 2016; Huang et al. 2016). Sludge blended with swine manure and cow dung has a lower pH, C/N ratio, total organic carbon (TOC), NH4+–N and an elevated total available phosphorus having optimum maturity and steadiness (Xie et al. 2016). Several substrates can be modified using bulking agents to deliver the required nutrient content, for example, N (Hanc and Chadimova 2014). Further toxic wastes, for instance, water hyacinth and tannery sludge, used to extract heavy metals are poisonous to earthworms because of their high heavy metal concentration. Hence, pre-treatment with bulking agents is needed (Cunha et al. 2016; Ravindran et al. 2016). Palm oil mill effluent (POME) is often blended with other feedstocks to decrease their moisture content (Lim et al. 2014). The heavy metals in the substrates like thermal power plant fly ash could be eliminated primarily by the intestinal microbes and chloragocytes of the earthworms (Bhat et al. 2016). Spectroscopic, thermogravimetric and structural characterization analyses to compare municipal solid waste composts and vermicompost stability and maturity have been performed by Soobhany et al. (2017). The capability of inoculating cow dung–paper waste mixture (fly ash) plus a specialized microbial concoction termed as an effective microorganism in vermicomposting using E. fetida earthworms has been assessed by Mupambwa et al. (2016). In the reviews by Lee et al. (2018), it was reported that coffee industrial waste and paper mill wastewater sludge can be recycled to yield a value-added product using earthworms. The plentily available seaweed biomass in the coastal regions can also be utilized as a substrate for vermicomposting, indicating the wide array of this environment-friendly technology in organic matter utilization and nutrient recovery (Ananthavalli et al. 2019).

Contribution to wastewater treatment

In the present scenario of rapid industrialization, urbanization, population increase, and various other anthropogenic activities have resulted in serious depletion of natural resources (Singh and Singh 2017). Humans are facing a serious shortage of clean water for their domestic purposes, agricultural irrigation and other works. The major reason for this is poor wastewater management (Singh et al. 2017). The sewage treatment plant is a conventional method to manage wastewater, but it generates high amount of sludge after the process. The maintenance and operation cost also fail to achieve the target of increasing norms of disposal and discharge in these last years (Zhao et al. 2010). Hence, biological methods are helpful with higher efficiency to treat wastewater in comparison with other methods (Singh et al. 2017). The aerobic biological systems have been observed as better options than anaerobic systems in terms of pH, temperature and organic loading rate adoption (Singh et al. 2017). Hence, the vermifiltration method is an efficient method in India due to its cost-effective and ecological features. In fact, vermifilter has been largely used in municipal and domestic sewage, but nowadays this method has been used for industrial wastewater treatment (Sinha et al. 2008).

One of the studies has shown that vermifiltration technology attempted to reduce downstream contamination by the effluent (Manyuchi et al. 2018). Wastewater treatment using vermifiltration technology and the end products, for instance, clean effluent and vermicompost, a biofertilizer that is released from the procedure, are shown in detail (Fig. 3a) (Gupta 2016). The optimization of different factors such as hydraulic retention time, hydraulic loading rate, recirculation ratio, organic loading rate, earthworm abundance and reactor type on organic matter is needed for vermifiltration technology (Lourenco and Nunes 2017). Another hybrid method is macrophyte (Canna indica)-assisted vermifilter (macrophyte filter with earthworms) used to treat dairy wastewater. This method is one of the most economical processes for wastewater treatment in developing countries (Samal et al. 2018). Vermifiltration applied with the natural ingredients, viz. mud balls, glass balls, wood coal and river bed material, increased biological oxygen demand (BOD), chemical oxygen demand (COD) and total suspended solid removal. River bed material decreased the indicator organisms, viz. fecal streptococci, total coliform, Escherichia coli and fecal coliform. At the end of the process, the vermicompost obtained was rich in nitrate and phosphate that can be used in sewage farming (Kumar et al. 2015). Vermifiltration method is also used to remove heavy metals, and Jordão et al. (2009) reported Zn(II) adsorption from both kaolin industry wastewater and synthetic solution by vermicompost. But, this adsorption depends on various factors, viz. mass of vermicompost/volume of the Zn(II) solution ratio, contact time, particle size of the vermicompost, solution pH and temperature.

Fig. 3
figure 3

a Vermifiltration technology application in wastewater treatment (Adopted from Gupta 2016); b demerits of earthworms and vermicompost

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Earthworm and vermicompost: shortcomings

Various factors are involved that render earthworms and their products as being unsuitable for crop management (Fig. 3b). Vermicompost releases nutrients gradually in the soil during crop development compared to the chemical fertilizers which have cracking effect resulting in immediate release of nutrients in the soil. As such, farmers do not readily adopt vermiculture technology in their crop fields. Vermicompost production is a problematic task to maintain on a large scale, and the challenges related to the methodology are discussed. It is a difficult process because of the unavailability of incessant organic waste and water supply (Singh and Singh 2017). The cost of organic source transportation is high for this methodology. Maintaining the appropriate temperature, pH and moisture condition makes this a complicated process (Munroe 2007). The negative impact of earthworms on the environment, e.g., GHG emissions from the soil that results in global warming, has been reported (Singh and Singh 2019). Globally, GHG emission is currently at an alarming level. Earthworms can increase CO2 and N2O emissions from soils by 33% and 42%, respectively (Lubbers et al. 2013). Even though many reports are in support of improved germination, crop growth and yield, some reports claim reduction in germination ability of plants by the application of vermicompost. This might be due to a high concentration that can reduce aeration and porosity in the agro-systems (Atiyeh et al. 2001). Similarly, an increase in salt concentration, along with elevated levels of phytotoxic substances and heavy metals in vermicompost, can have detrimental effects on plants (Atiyeh et al. 2001). Unformed vermicompost inhibits seed germination, plant and growth root destruction (Singh and Singh 2017). Establishing a marketplace for vermicompost business is difficult, and the process requires the utilization of spaces other than residential areas because of the associated odor problem.

Moreover, there is an alarming situation of natural and high levels of metallic species present in vermicompost damaging soil which can lead to contamination of plants and various food chains. These elements can destroy the soil structure, viz. sodium in high concentration hastens microbial death and erosion, as well as lethal metals, for example, calcium and lead. Taking into account of the intrinsic worth of earthworms and vermicompost, the demerits recognized are very meagerly supported; hence, most of the points in focus seem to be least significant. Based on these considerations, further research is needed to effectively address the challenges that arise regarding unformed vermicompost application failure and its composition.

Conclusions

Though earthworm and vermicompost are used for various purposes, for instance, soil improvement, waste recycling and wastewater treatment, the vermicompost composition has limitations depending on the time required for maturity and nature of feedstock materials; hence, further research concerning the substrate composition is necessary. An interesting fact of pathogen reduction during vermicomposting and on the application in the crop field requires further insights to get more benefit from vermitechnology. Also, the following aspects related to earthworms and vermicompost should be investigated: (1) the composition of vermicompost whether or not it contains toxic substrates; (2) earthworm activities from soil that generate GHG emission; and (3) and for the recovery of soil health, some other biofertilizer could be mixed as an amendment.