Introduction

Increased proliferation of green landscape areas, institutions, hotels and restaurants generate lawn (LW) and kitchen wastes (KW) which has a major mismanagement issue. Institution messes generate huge amount of biodegradable organic wastes. The most prominent part of these biodegradable wastes includes lawn wastes and uncooked vegetable wastes emitted from hostel’s messes, canteens and lawns of the institutional premises. Yard waste comprises pruned branches and leaves from trees and shrubs, detritus waste, and lawn waste like grass clippings and trimmings from mowed lawns from gardens, green areas and parks. It has been estimated that 1.3 billion metric tons of food is wasted annually that is approximately equal to one-third of the total food production [1]. On the other hand, yard waste generation is also rising continuously due to proliferation of green urbanized areas which comprises the greatest contributor of municipal solid waste [2,3,4]. Management of lawn waste, which is most prominently produced yard waste is costly due to their collection and transportation to the treatment facilitating establishment. Consequently, KW and LW are openly dumped, burned or thrown to the landfill along with the toxic, non-biodegradable and hazardous waste materials. These conventional methods not only enhance the load on sanitary landfill but also act as a source of noxious greenhouse gases. Moreover, direct dumping of nutritious-rich biodegradable organic waste (NBOW) is also not recommended, because it renders the soil unproductive by degradation of soil physical structure and reduction of macronutrient uptake capacity [5].

There are clear indications that intensive farming practices have depleted the soil nutrients status and farmers try to compensate it by chemical fertilizers. Extensive use of chemical fertilizers contaminate agricultural ecosystem and thereby affect public health [6]. Additionally, soil quality is also negatively affected which further reduces the microbial content that is essential for the sustaining soil health [7]. These negative effects of chemical fertilizers could be reduced through the use of bio-compost prepared from NBOW. Hence, to resolve aforementioned environmental issues, waste materials should be properly collected, processed and reused for sustainability. Lawn waste makes a significant proportion of the total yard waste stream and by diverting it to composting or vermicomposting; we can save a substantial landfill space. If lawn waste enters landfills, it degrades under anaerobic condition to produce methane, a gas with 25 times more greenhouse potential as compared to carbon dioxide. Volume-wise also lawn waste occupies extensive area in landfills. As per Environment Protection Agency (EPA) 2014 [2], the yard waste contributes to more than 13.5% of municipal solid waste.

In the past few years, some work has been done on vermicomposting and composting of vegetable wastes and yard wastes separately [8,9,10]. However, due to relatively high ligno-cellulosic content, yard waste is not found to be easily decomposed. Use of lawn waste, in combination with nutrient rich vegetable waste, and some dairy waste as bulking material, is likely to facilitate the bioconversion process. In Asian countries especially India, buffalo dung is abundantly available. However, this dairy waste has been seldom used in vermicomposting studies, while use of cow dung has been very common. The present study was, therefore, conducted to analyse the effectively of bioconversion of lawn waste (LW) into compost and vermicompost, on being mixed with other nutrient rich biowastes i.e. kitchen waste (KW) and buffalo dung (BD). Different ratios of the three types of the wastes in the substrate mixture were used to find out their suitability for earthworm growth and quality of the final bio manure. The study involved analysing the growth and fecundity of the earthworms, nutrient dynamics, heavy metal concentrations and enzyme activities such as dehydrogenase, urease and phosphatase during vermicomposting as affected by different combinations of lawn waste, kitchen waste and buffalo dung in comparison to simple composting. The major objective of the present study was to analyse how effectively the common biodegradable wastes such as lawn waste and kitchen waste generated regularly by the urban institutions could be converted to good quality value-added vermicompost. Eisenia fetida species of earthworm was employed for vermicomposting and quality of the bio-compost was assessed by studying germination index of tomato seeds.

Materials and methods

Kitchen waste, lawn waste, buffalo dung, earthworms and chemicals

Lawn waste (LW) was collected from K.I.E.T Group of institutions, Ghaziabad, India and it included the trimmings of lawn grass after cutting and mowing. Kitchen waste (KW) collected from the canteen of the institution comprised mainly of potato peels, vegetable peals and discarded parts of carrot, cauliflower and spinach. For the vermicomposting process, healthy unclitellated Eisenia fetida species was used which were collected from the stock vermiculturing unit (K.I.E.T Group of institutions) maintained in the laboratory using buffalo dung as a survival media. The chemicals used in the study were of analytical grade.

Pre-composting process

The substrates liable to vermicompost were pre-composted in a bin for three weeks before introducing the earthworms. The initial characteristics of different substrates are shown in Table 1. Various studies have recommended this process, because some toxic gases and heat is generated during initial decomposition (thermophilic) process, which may be lethal to earthworm species during vermicomposting process [11,12,13]. Thus, pre-composting of various organic substrates is a primary and crucial step prior to vermicomposting for the effective functioning of earthworm species.

Table 1 Initial physico-chemical characteristics of lawn waste (LW), Kitchen waste (KW), and Buffalo dung (BD) at 0 day
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Experimental design and vermibin preparation

The experiment was laid out in a completely randomized design (CRD) with different combination of pre-composted KW and LW with buffalo dung (BD) to investigate their bio-stabilization with and without earthworm species E. fetida. Experiments were conducted in plastic vermibins (diameter 40 cm, depth 12 cm). Each ratio treatment was triplicated. At the bottom of each bin, multiple holes were provided for leachate discharge. The composition of KW, LW and BD in different bins was as follows:

Bin No. 1 (C1): BD 40% + LW 20% + KW40% (2:1:2)

Bin No. 2 (C2): BD 60% + LW 10% + KW30% (6:1:3)

Bin No. 3 (C3): BD 30% + LW 10% + KW 60% (3:1:6)

Bin No 4 (C4): BD 50% + LW 0% + KW50% (1:0:1)

Bin No. 5 (V1): BD 40% + LW 20% + KW40% + E. fetida (2:1:2)

Bin No. 6 (V2): BD 60% + LW 10% + KW30% + E. fetida (6:1:3).

Bin No. 7 (V3): BD 30% + LW 10% + KW 60% + E. fetida (3:1:6)

Bin No. 8 (V4): BD 50% + LW 0% + KW50% + E. fetida (1:0:1)

The study was based on the hypothesis that the lawn waste (LW), which by itself is not readily decomposed due to high structural carbohydrates can be more successfully mineralized to compost or vermicompost if used along with additives like nutrient-rich kitchen waste (KW) and bulking agent like common dairy waste, buffalo dung (BD). The three types of solid biowastes were mixed in different ratios to find out the most suitable composition of substrate to get the best quality product. Some earlier studies showed use of 40–60% cow dung as suitable bulking agent for vermicomposting of municipal solid wastes and industrial sludge [14,15,16]. Hence, in the present study, the bulking agent BD was taken in the range of 40–60%, while remaining 60–40% was proportioned between LW and KW, keeping greater proportion of the soluble nutrient rich KW, while LW, which is more bulky and has less soluble nutrients, was kept in lower proportion (0–20%) in different treatments.

Different ratios of substrate were mixed for the respective vermicompost bin and compost bin using with and without earthworm species E. fetida. Non-clitellated and juvenile earthworms were taken from stock cultures, weighed and released in each vermibin selected for the experiment. 20 earthworms were introduced per kg of feedstocks in each vermibin. Comparison of vermicomposting to composting was essentially required to find to what extent difference came in the quality of manure produced with and without earthworms. For comparative analysis, identical set of composting bins with same substrate composition without earthworm were also taken. The moisture content was maintained up to 60% by sprinkling water time to time. Vermibins were covered with jute bags to protect from moisture loss and predators such as birds and prevent evaporation loss. Jute bags also provided congenial dark environment inside the vermibins to the photosensitive earthworms. To maintain aerobic environment, all the feedstocks were turned manually thrice a week. All the bins were kept under shade to prevent from sunlight and rain. The components of lawn waste includes pruned lawn grass waste. The vegetable waste constitutes major portion of potato waste, vegetable peals and waste such as carrot, cauliflower, spinach, tomato. The study was performed for 90 days and samples were taken from the feedstocks at 0, 30, 60 and 90 days. Immediately after collection, earthworms and their cocoons were carefully counted, sorted and placed back into vermicompost feedstock. Moisture was removed from the compost and vermicompost samples and homogenized by grinding into powder form through a steel grinder mixer and sieved through 2 mm mesh sieves for various physicochemical analyses.

Physicochemical analysis

Various physicochemical parameters of the compost and vermicompost samples were determined at 0th, 30th, 60th, and 90th day. 200 g of sample that was free of earthworms, hatchlings and cocoons was taken out, dried and passed through 0.2 mm sieve. The pH and electrical conductivity (EC) were determined using a digital pH meter (digital pH System, Labman, India) and EC meter (Decible, India), respectively, in the homogenized feedstock sample (1:10 w/v with distilled water). Total organic carbon (TOC) was assessed by a dry combustion method at 550 °C for 2 h [17]. Total nitrogen content was measured using Kjeldahl method after digestion of sample [18]. The C/N ratio is an important index for maturity of organic wastes during composting and vermicomposting process. C/N ratio was calculated by dividing the TOC content to the total nitrogen level. Potassium, sodium, and phosphorous levels were assessed using flame photometry [19].Concentration of heavy metals Cu, Zn, Pb, Cr, Co, Cd, and As (acid digest in a mixture of conc. HNO3, conc. H2SO4 and conc. HClO4; 5:1:1, v/v ratio) were assessed using Inductively coupled plasma mass spectrometry (ICP-MS, NEXION) at the beginning and end day of the study. Calcium content was determined by atomic absorption spectroscopy (AAS) as described by [20].

Worm growth biomass

Earthworms were taken out from the feedstock mixtures and cleaned with water, to remove the substrate. Weight of earthworms was estimated at respective time interval (0th, 30th, 60th and 90th day). In the feedstock mixture, cocoon and clitellum growth rate were also calculated.

Enzymatic analysis

Substrate enzymatic analysis was done to determine ureases, phosphatases and dehydrogenase activities with the help of method described previously [21,22,23].

Seed germination index

Germination index (GI) is widely used to assess vermicompost maturity. The effect of compost and vermicompost on seed germination index of tomato (Solanum lycopersicum) seeds was described by Zucconi and his co-workers. The compost or vermicompost produced at 90th day were mixed with distilled water in a ratio of 1:1 and poured into a filter paper-lined petri dish. The seeds are kept in the petri dish and subsequently incubated at 25 °C in a BOD incubator for five days. GI was calculated by determining the number and root length of germinated seeds.

GI = (Seed Germination × Root length of the treatment) × 100/(seed germination × root length of the control).

Statistical analysis

The results were represented as mean ± standard deviation (SD). The data were analyzed through two-way analysis of variance (ANOVA) with the help of GraphPad prism software. P < 0.05 was considered as statistically significant.

Results and discussion

Growth and reproduction of (E. fetida) in the vermibins

Growth of the earthworms was influenced by the ratio of KW, BD and LW in the substrate as reflected by biomass gain (mg/earthworm), mean growth rate and reproduction rate of the earthworm (cocoon/worm/d) at different stages of composting. Table 2 shows that maximum and minimum biomass gain/earthworm was in the V2 (790.56 mg) and V4 (575.81 mg), respectively. Moreover, maximum and minimum growth rate/worm/d was found in V2 (8.78) and V3 (6.40), respectively. While deviation of clitellum began within 5–6 weeks, cocoon production continued from 5 to 12 weeks. V2 showed high number of cocoons and its reproduction rate was also the highest (0.13) out of all the treatments. The maximum reproduction rate was observed in V2 vermibin (0.130) (Table 3). Thus, V2 vermibin showed the most suitable ratio to support growth and reproduction of earthworms. The worms grew constantly throughout the study period of 90 days and the differences in biomass become more pronounced in 60 and 90 days. There was a mean net gain of 575.8–790.6 mg by the earthworms in different vermibins with maximum growth rate in V4. The readily available nutrients present in BD, KW and LW enhanced the feeding activity, thereby augments the biomass of earthworms.

Table 2 Growth rate of earthworms at 0, 30, 60 and 90 days during the process of vermicomposting of KW and YW
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Table 3 Reproduction parameter of earthworms during the process of vermicomposting process
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pH and electrical conductivity (EC)

During the process of composting pH value was found decrease in all the bins during 90 days of study period (Fig. 1a). At day 0, pH of all samples was alkaline pH 8.00. However, it was gradually declined in all compost and vermicompost samples throughout the study period. Maximum decline in pH was observed in vermibin V2 where pH fell down to 6.54. The observed pH reduction may possibly be due to release of acidic metabolites such as malonic, fumaric and succinic acids, and carbon dioxide during breakdown of complex organic compounds [24]. Lowering down of pH was more prominent in vermicompost samples as compared to compost samples attributable to earthworm’s activities. pH of vermicompost samples differed significantly attributed to (P < 0.05) from their respective compost bins at 30, 60 and 90 days. Reports on trends of pH change during vermicomposting are not constant, while pH declined during vermicomposting with water hyacinth and grass waste [25, 26]. There was rise in pH during vermicomposting of sewage sludge [27]. Optimum pH required for proper functioning of earthworms is around 7.0 [28]. pH also acts a regulatory factor in the availability of nutrients in the soil. The micronutrients and macronutrients are likely to be less available at very high or low pH soil, respectively [25]. Therefore, an ideal pH of compost and vermicompost required to promote absorption available nutrients are near neutral.

Fig. 1
figure 1

Physico-chemical characteristics of compost and vermicompost during the study: (a) pH, (b) Electrical conductivity (EC), (c) total organic carbon (TOC), (d) total nitrogen (TN), (e) and carbon–nitrogen (C/N) ratio. Data are represented as Mean ± SD. *P < 0.05 compared with C1; #P < 0.05 compared with C2; $P < 0.05 compared with C3; @P < 0.05 compared with C4

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A gradual rise in EC was observed during 90 days of vermicomposting as well as simple composting (Fig. 1b). The increase in EC was in the range of 10.7–53.3% for compost samples and 147.5–212.9% for vermicompost samples, respectively. Electrical conductivity of vermicompost samples were significantly higher (P < 0.05) as compared to compost samples at 60 and 90 days. The significant increase in EC in vermicompost samples may be attributed to enhancement of decomposition rate by earthworms, solubilizing the nutrients release. This causes release of ammonium and free forms of minerals such as P, K, Ca, and Mg seem responsible for rise in EC that may lead to rise in EC during vermicomposting process as reported by different vermicomposting studies on green waste [29, 30]

Total organic carbon (TOC), Total nitrogen (TN) and CN ratio

Vermicomposting and composting of kitchen waste and lawn waste with buffalo dung led to decline in TOC% during 90th day. The TOC content in the order: V1 < V2 < V3 < V4 < C1 < C4 < C3 < C2 (Fig. 1c). The reduction in TOC was more in vermicompost as compared to compost. The initial TOC content of waste substrate was found in the range of 36.12–42.69% for vermicompost while it was 35.65–40.22% for compost. The TOC content of vermicompost was found to decline in the range of 21.89–29.0% for vermicompost and 31.86–33.76% for compost. at the end on 90th day. As observed in Fig. 1, 2, the percentage of organic carbon is lower in the vermicompost than in the parent raw material indicating that earthworms accelerate the microbial degradation of organic substrates as well as assimilation of polysaccharides and carbohydrates from the feed mixtures by earthworms [31]. Decline in TOC is reported during vermicomposting of green lawn wastes [13]. Decline in TOC content is an indicator of stability and maturity of compost. Organic carbon gets mineralized during vermicomposting leading to loss of TOC in the form of CO2, while some carbon gets accumulated in the biomass of earthworms resulting in reduction of TOC. During vermicomposting of tea factory coal ash, metal detoxification and accumulation potential was found with low TOC which indicated compost stability, maturity and humus richness in the final compost or vermicompost [32]. Total nitrogen increased on the other hand in all the bins. The augmentation of nitrogen in all vermibins was high in comparison to compost bins. TN content showed the following trend: V1 (1.89) > V2 (1.75) > V3 (1.29) > V4 (1.24) > C1 (1.26) > C2 (1.06) > C4 (1.03) > C3 (0.88) at 90th day of the study (Fig. 1d). V1, V2 and V3 groups showed significantly (P < 0.05) higher TN content in comparison to their respective (60, and 90 days) compost samples. Mucus, nitrogenous excreta and decaying tissues of earthworms can increase the TN content in the vermicompost samples. Enhanced degradation of organic matter also contributed to the TN content in the vermicomposting process [33]. In vermicomposting the association of microbial biota and earthworms enhances the production of enzymes which directly increases the nitrogen content in the vermicompost.

Fig. 2
figure 2

Nutrients value in compost and vermicompost during the study: (a) Potassium, (b) Calcium, (c) Sodium, and (d) Phosphorous levels. Data are represented as Mean ± SD. *P < 0.05 compared with C1; #P < 0.05 compared with C2; $P < 0.05 compared with C3; @P < 0.05 compared with C4

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C:N indicates humification rate of organic matter and compost maturity. At day 0, CN ratio showed the following trend: C4 (49.84) > V4 (43.99) > C3 (46.07) > V2 (39.91) > V3 (39.38) > C2 (36.73) > C1 (28.26) > V1 (27.61). At 90th day, the C:N ratio was found in the following manner: C3 (37.87) > C4 (33.19) > C2 (31.80) > C1 (25.71) > V4 (23.63) > V3 (22.43) > V2 (16.16) > V1 (11.65) (Fig. 1e). The present results clearly shown that decrease in CN ratio was more pronounced in vermicomposting in comparison to composting and the differences were statistically significant (P < 0.05) in V2 and V3 as compared to respective compost samples at 60th and 90th day. Whereas, V1 group showed significant (P < 0.05) reduction in CN ratio as compared to its respective compost sample at 90th day.

The carbon and nitrogen are crucial for microbial biodiversity as carbon act as an energy source and nitrogen structural synthesis elements. Nitrogen is a basic component for the synthesis of cellular constituents like amino acid, protein, enzymes, nucleic acid and coenzymes crucial for the growth and metabolism of cells [34]. CN ratio, being one of the most common indicator of organic waste stabilization, decreased below 20 indicates compost maturity and is most suitable for the agronomical purposes [35, 36]. The decrease in CN ratio is dependent upon the change in TOC and TKN. In the present study, the decline in CN ratio in the vermicompost may be attributed to simultaneous increase in TKN and decline in TOC%. At Both V1 and V2 samples had a C:N ratio below 20. The rapid decomposition of the organic matter, microbial respiration, excretion of nitrogenous waste contributed to reduction in C:N ratio [37].

Total potassium (K), calcium (Ca), sodium (Na) and phosphorous (P)

Macronutrient concentration showed marked increase during the composting and vermicomposting process throughout the study. The percentage rise in TK was observed in the following order of V2 > V4 > V1 > V3 > C3 > C1 > C4 > C2 (Fig. 2a). In different vermibins, increment in TK varied from (30–35%) being V2 (35.20%). The TK content was significantly (P < 0.05) more in all the vermicompost samples as compared to respective compost samples. These macronutrients play a crucial role in the plant growth. It has been demonstrated that vermicomposting process significantly increased the macronutrient content [27, 38]. Similar trend of TK variation during vermicomposting of rice straw and grass was observed in a previous study [36]. Intestinal microbes of earthworms enhanced the mineralization process that leads solubilization of insoluble potassium during vermicomposting. Moreover, loss of net dry mass during vermicomposting also acts as a major factor for TK increment [27]

Total calcium was also found to increase significantly (P < 0.05) with time in all the vermibins as compared to compost bins (Fig. 2b). At the end of the experiment, maximum TCa level was observed in V2 (0.99), while V3 (0.77) vermibin showed minimum concentration. The percentage increase in TCa content was in order of V2 (82.11%) > V1 (80.83%) > V3 (77.26%) > V4 (60.98%). Compost bin showed slight augmentation in TCa level. The maximum concentration of TCa was found in C2 (0.76) and minimum in C3 (0.57) in feed mixture without vermicomposting. The augmentation in TCa due to vermicomposting has also been reported previously in vermicompost of press mud, bagasse, and vegetable waste [16, 30]. Carbonic anhydrase secreted from the calciferous glands of earthworms causes production of CaCO3 from CO2, thereby increases TCa level [39].

Figure 2C shows that total sodium concentration in vermicompost was more than that in compost samples. The percentage increase in TNa was found in the following order: V2 (48.48%) > V1 (31.59%) > V3 (27.17%) > V4 (19.34%) > C2 (16.25%) > C3 (15.10%) > C1 (8.8%) > C4 (6.47%). TNa content varied from 0.63 to 0.68, the maximum being in concentration of V2, respectively. The augmentation of TNa has been reported in vermicompost earlier from sewage sludge and lingo-cellulosic waste [27, 40]. However, Singh and Suthar in their study on vermicomposting of herbal pharmaceutical industry solid wastes reported contrary result that earthworms may consume the available fraction of salts that may decline the sodium concentration from the final substrate [41].

Phosphorus, an important nutrient plays a determinant factor in the compost quality. In the present study, total available phosphorous (TAP) gradually increased in all the feed mixtures with earthworms (Fig. 2d). The percentage increase in TP% in all bins was found in the following order: V2 (142.54%) > V1 (140.80%) > V4 (123.66%) > V3 (100.91%) > C1 (72.54%) > C4 (59.14%) > C3 (56.54%) > C2 (48.20%). The highest available phosphorous content was again observed in V2 (0.44) vermibin. V1 and V2 vermicompost samples showed significant (P < 0.05) rise in TAP level as compared to their respective compost mixtures. Previous studies that performed vermicomposting of rice straw, sewage sludge and vegetable-market waste [42, 43]. Previous studies indicated that phosphate-solubilizing bacteria in earthworm’s cast increase the mineralization process and mobilize the phosphate in the feedstock which in turn resulted in to augmentation of TAP [38].

Heavy metals (Cu, Zn, Pb, Cr, Co, As and Cd)

Heavy metal (Cu, Zn, Pb, Cr, Co, As and Cd) concentration in compost and vermicompost at 90th day showed that the concentrations significantly declined in the final vermicompost as compared to initial feedstock (Table 4). The heavy metals are ubiquitous in nature and their toxicity is mainly influenced by concentration present in the substrate, mobility in free form, the rate of uptake mechanisms as well as bioaccumulation rate [44]. Thus, determination of heavy metal concentration in the vermicompost or compost is necessary before its agricultural use.

Table 4 Heavy metal concentration in different ratios of compost and vermicompost samples at initial and final day of the study.
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In the present study, maximum decline in Cu concentration was recorded in V2 (40.68%) followed by V4 (30.04%), V1 (29.94%) and V3 (28.52%) after 90 days of vermicomposting. On the other hand, % decline in Cu in compost samples were found in the following order C4 > C3 > C1 > C2. After 90 days, the Cu concentration in V1, V2, V3 and V4 was found significantly (P < 0.05) less as compared to that at 0th day. Cu is an essential co-factor for the functioning of various metalloproteins and enzymes [45]. However, beyond a critical level Cu has been reported to inhibit plants growth via inhibition of vital enzymes that play a critical role in plant cell membrane integrity and photosynthesis [46]. Therefore, optimum concentration of Cu is required by plants for their growth. Similar decrease in Cu concentrations during vermicomposting of biomass like water hyacinth have been reported in earlier studies too [38, 47].

Zn concentration also decreased significantly with maximum decline in V2 (44.02%) and minimum in V4 (23.18%) feed mixtures among vermicomposting samples. Feed mixtures without earthworms also showed some decline in Zn content after 90 days. The percentage decline in Zn was observed in the following order: V2 > V3 > V1 > V4 > C4 > C3 > C2 > C1. Vermicompost feedstocks (V1, V2 and V3) showed significantly (P < 0.05) lower Zn concentration on 90th day as compared to the 0th day of the study. Cu and Zn has been reported to inhibit absorption of each other [48]. Zn also plays a critical role in enzymatic activities (carbonic anhydrase and hydrogenase), cytochrome synthesis and cell membrane integrity in the plants. Previous reports on vermicomposting of sewage sludge also showed significantly reduce Zn levels [49].

A similar decrease in Co concentration was also seen in vermicompost after 90 days. Among vermicompost mixtures, Co concentration showed maximum decline in V4 (44.71%) and minimum in V4 (20.25%) vermibins. Among compost samples, the decline was less than vermicomposting and maximum and minimum decline in Co concentration were in C3 (18.18%) and C2 (8.33%), respectively. Overall, the percentage decline in Co concentration were in the following order: V2 > V3 > V1 > V4 > C3 > C4 > C1 > C2. Cobalt is an important component of plant enzymes that are involved in nitrogen fixation. However, its high concentration negatively affects the chlorophyll content and nutrient balance in plants [50]. Thus, the optimum Co content is required for plant growth.

Pb concentration in compost and vermicompost feedstocks showed maximum decline of Pb in V2 (36.33%) and minimum in V4 (25%) feed mixtures after 90 days of vermicomposting. The percentage decline in Pb with vermicomposting was in the order of V2 > V3 > V1 > V4. Vermicompost samples V1, V2, and V3 (P < 0.05) showed significant reduction as compared to their concentration at 0 day. Heavy metals such as, Cd, Ni, Cd, and Pb are not essential for the plant growth. They can inhibit metal-sensitive enzymes which eventually cause plant cell death. Pb has been found to inhibit seed germination, plant growth, and chlorophyll production via inhibition of essential enzymes and electron transport chain [51]. The reduction in Pb level due to vermicomposting has been also observed by a previous study [52].

In this study, the As concentration decreased during vermicomposting with maximum decline in V2 (48.53%) and minimum in V3 (35.61%) feed mixtures. Feed mixtures without earthworms also showed some decline in the As content. Overall, the percentage decline in As concentration was found in the following order: V2 > V1 > V4 > V3 > C4 > C2 > C3 > C1. At 90d, V1, V2 and V3 vermibins showed significant (P < 0.05) reduction in As concentration as compared to that at 0 day. As is easily absorbed and translocated in plants. It easily forms complexes with plant thiol containing enzymes and compounds due to their higher affinity with sulfhydryl group, thereby inhibiting vegetative growth [53]. Vermicomposting is a useful and effective technology to reduce the heavy metal concentration. A previous study also showed reduction of as content in waste sludge treated with earthworms [54].

Cd concentration also decreased by vermicomposting with maximum decline in V2 (45.05%). V2 feedstock showed significantly (P < 0.05) lower Cd concentration after 90 days of vermicomposting process in comparison to 0d. Feed mixtures without earthworms also displayed some reduction in Cd content. The range of decline observed in Cd concentration after 90 days was found in the order of V2 > V4 > V1 > V3 > C3 > C2 > C4 > C1. Cd, one of the common heavy metals inhibit uptake of essential nutrients by plants. Photosynthesis inhibition has been observed in the presence of toxic concentration of Cd [55]. Thus, it is important that to determine the Cd concentration in the final compost and vermicompost should not be high.

The percentage decline in total Cr content was found in the order of V2 > V1 > V4 > V3 > C2 > C4 > C1 > C3 after 90-day period. The Cr concentration was significantly (P < 0.05) reduced in V1, V2 and V4 vermibins after 90 days of vermicomposting in comparison to that of 0d. The maximum decline in Cr concentration after vermicomposting was also found in V2 (53.11%) feed mixtures. Composting process also showed some decline in concentration Cr concentration (8.68–7.13%). Cr has been reported to inhibit seed germination and plant growth mainly via nutrient imbalance, photosynthesis impairment and activation of oxidative stress [56]. Some previous studies also reported decline in Cr during vermicomposting of various substrates such as sewage sludge and water hyacinth [57, 58].

In an earlier study, reduction in heavy metal in the vermicompost produced from sewage sludge and additives such as fly ash, soil, straw and saw dust was observed [59]. During vermicomposting, organic wastes are mineralized and humified by various enzymes to form simple and short chain of organic acids like humic acid, which bind the metals forming stable metal complexes [49, 60]. Decline in heavy metals concentration in the final vermicompost may also be due to the bioaccumulation of heavy metals in the tissues of earthworms during ingestion, digestion of raw substrate or through skin absorption. The digestive system and its associated microflora contain several enzymes that retain the heavy metals. Moreover, cutaneous tissue of earthworms has been reported to easily absorb heavy metals through dermal route [61, 62].

Enzyme activities (Urease, Dehydrogenase and Phosphatase) in vermicompost/compost mixtures

The enzymatic activities i.e. urease, phosphatase and dehydrogenase determined at different time points of the study are shown in (Fig. 3a–c). In vermicomposting (V1, V2, V3 and V4), urease activity significantly (P < 0.05) increased from day-0 to day-30 as compared to respective compost mixture ratios. In composting, urease activity was found slightly increased at day-30. No significant difference was observed between urease activity at day-0 and day-30 in composting. V2 group showed highest level of urease activity (889.15 mg N/g/h) among vermicompost samples at day-30 (Fig. 3a). In the present study, the decline of urease activity after 30 days signifies the faster degradation and initiation of maturity of vermicompost. In vermicomposting samples, urease activity subsequently decreased at day 60 and day 90. Urease enzyme transforms the complex organic nitrogenous matter into simpler nitrogenous forms [63]. The initial augmentation in urease activity attributed to high level of NH4+ in the feedstocks. Moreover, earthworms assist in the colonization of microorganisms such as bacteria and fungi that stimulate urease activity in the feedstock mixture [64]. Such an initial rise in urease activity has been also reported in some previous studies [65]. However, towards maturation of the vermicompost urease activity declines as reported in the previous study [64].

Fig. 3
figure 3

Enzymatic activity analysis in compost and vermicompost during the study: (a) Urease, (b) Phosphatase, and (c) Dehydrogenase. Data are represented as Mean ± SD. *P < 0.05 compared with C1; #P < 0.05 compared with C2; $P < 0.05 compared with C3; @P < 0.05 compared with C4

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Phosphatase activity in vermicompost feedstocks (V1, V2, V3 and V4) were significantly higher (P < 0.05) in comparison to corresponding compost feedstocks on day 30 (Fig. 3b). The prominent activity of phosphatase in vermibins reflects the presence of high microbial activity and amount of organic phosphate matter [66]. The trend of phosphatase activity among different feedstock at day 30 was as follows: V2 > V1 > V3 > V4 > C3 > C2 > C1 > C4. Subsequently, phosphatase activity exhibited a declining trend in vermicompost samples till the completion of the study. Phosphatase enzyme transforms organic P to inorganic form of P which leads to easily availability of P by plants. The potential of microbes to perform mineralization process is reflected in phosphatase activity [67, 68]. Thus, phosphatase enzyme can be considered as an enzyme of agronomic importance. It has been suggested that decline in phosphatase activity reflects the end of decomposition process in the vermicomposting [69]. Similar trends in phosphatase activity was reported for vermicomposting of olive oil mill waste waters and filter cake [70, 71].

Earthworms were found to exert strong impact on the dehydrogenase activity. During initial 30 days, dehydrogenase activity increased more than two times in all the vermibins except V4. The rise in dehydrogenase activity reflects higher availability of complex molecules that increases microbial activity. The trend for dehydrogenase activity in all the feedstocks at day 30 was as follows: V1 > V2 > V3 > V4 > C1 > C2 > C3 > C4. The dehydrogenase activity was found significantly (P < 0.05) higher in V1, V2, and V3 samples as compared to the respective compost ratios at day 30 (Fig. 3c). Thereafter, sharp reduction in dehydrogenase activity was observed in V1, V2 and V3 samples. The decline observed in dehydrogenase activity after 30 days may be attributed to unavailability of substrate in the feedstock. A previous study reported that at the termination of maturity process decline in dehydrogenase activity is associated with maturity of composting process [72]. However, compost samples and V4 vermibin showed rise in the dehydrogenase activity up to 60 days followed by slight decline at day 90. Dehydrogenase activity in V1, V2 and V3 vermibins at day 90 was lower as compared to day 0. Presence of earthworms thus seems to accelerate the microbial process influencing dehydrogenases activity. Dehydrogenase activity assessment is widely used to determine the metabolic status of microbial in the feedstock mixture [73]. Previous study found similar results that indicates increase in the dehydrogenase activity in the initial 20 days followed by decreased activity during vermicompost of sewage sludge and cattle dung [74].

Seed germination index (Solanum lycopersicum) in vermicompost/compost

The seed germination index (GI) parameter is considered as suitable method for the determination of agronomical value of any substance [75]. The effect of mature composts and vermicomposts on the germination index of tomato seeds (Solanum lycopersicum) is shown in Fig. 4. GI of tomato seeds was significantly (P < 0.05) higher in vermicomposts (V2 > V1 > V4 > V3) as compared to composts. GI value was found maximum in V2 group (500) which is about three times that of C2 group (133.33). Earlier, a study suggested that food waste-derived vermicompost is microbial rich and possesses various enzymes, humic substances and hormones that impart positive effect on plant growth [76]. In another study on vermicompost prepared from industrial waste like fly ash and press mud, there was an enhancement in the seed germination index of Solanum lycosporium was found [77].Various characteristics showed that V2 vermicompost is the most appropriate one in terms of productivity of vermicompost which is further substantiated by germination of tomato seeds.

Fig. 4
figure 4

Germination index of tomato seeds in compost and vermicompost of buffalo dung, lawn waste and kitchen waste. Data are represented as Mean ± SD. One-way ANOVA was applied followed by Tukey’s test on the data. *P < 0.05 compared with C1; #P < 0.05 compared with C2; $P < 0.05 compared with C3; @P < 0.05 compared with C4

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Conclusion

Overall, our findings demonstrated that KW and LW can be employed as a reliable vermicompost for agronomical purposes. It was also concluded that buffalo dung can be a good substitute to other cattle dung as a bedding material. V2 vermibin ratio favors the growth and reproduction of earthworms. Consequently, optimum physicochemical properties (such as pH, EC, TOC, TN and C:N ratio), and high nutrient status (P, K, Na and Ca) with low heavy metal content were found in V2 vermibin. Thus, vermicomposting is an effective eco-friendly technology as compared to sole composting that can be utilized for the nutrient retrieval from the LW and KW.

Conflict of interest

The authors have none to disclose.