Organic waste recycling by vermicomposting amended with rock phosphate impacts the stability and maturity indices of vermicompost

ABSTRACT

Recycling organic waste can help the land be nourished, properly disposed of, and protected from the negative impacts of chemical fertilizers. Organic additions like vermicompost can help restore and preserve the quality of the soil, however, producing vermicompost of a high enough standard is difficult. This study was planned to prepare vermicompost by utilizing two different organic wastes viz. household waste and organic residue amended with rock phosphate and further evaluate their stability and maturity indices during vermicomposting for quality of produce. For this study, the organic wastes were collected and vermicompost was prepared by using earthworm (Eisenia fetida) and with or without enriching with rock phosphate. Results showed that pH, bulk density, and biodegradability index were decreased and water holding capacity and cation exchange capacity was increased with the gradual progress of composting starting from 30 to 120 days of sampling/composting (DAS). Initially (upto 30 DAS) water-soluble carbon and water-soluble carbohydrate increased with rock phosphate enrichment. The population of the earthworms and enzymatic activities (CO₂ evolution, dehydrogenase, and alkaline phosphatase) were also increased on enriching with rock phosphate and with the progression of the composting period. Rock phosphate addition (enrichment) also reflected the higher content of phosphorus (106% and 120% for household waste and organic residue, respectively) in the final product of vermicompost. The vermicompost prepared from household waste and enriched with rock phosphate showed greater maturity and stability indices. Overall, this can be concluded that the maturity and stability of vermicompost depend on the substrate used and improves on enriching with rock phosphate.

Implications: Our study concludes that the quality of vermicompost depends on different substrates, composting period, and enrichment with rock phosphate. The qualities of vermicompost were best found under household waste-based vermicompost enriched with rock phosphate. The efficiency of vermicomposting process using earthworms was found maximum for enriched and without enriched household-based vermicompost. The study also indicated that several stability and maturity indices are influenced by different parameters and hence cannot be determined by a single parameter. The addition of rock phosphate increased the cation exchange capacity, phosphorus content, and alkaline phosphatase. Nitrogen, zinc, manganese, dehydrogenase, and alkaline phosphatase were found higher under household waste-based vermicompost relative to organic residue-based vermicompost. All four substrates promoted earthworm growth and reproduction in vermicompost.

Introduction

India showed stagnated/declined crop yield due to the injudicious application of synthetic fertilizers (Juhi et al. 2022). Challenges to food security for the growing population can be addressed through improved crop productivity (Padbhushan et al. 2016), which can be achieved vide sustaining soil quality using organic sources either as integration or supplementation of chemical inputs (Padbhushan et al. 2021; Sharma, Padbhushan, and Kumar 2019). India generates a huge quantity of municipal and domestic waste (3 billion tonnes annually) whose quality and quantity depend upon several factors like population density, economic status, levels of urbanization, commercial activities, etc., in the foreseeable future; major growth is also anticipated (Sharma and Shah 2005). The disposal and management of these solid wastes are another big challenge. These solid wastes can be converted into a usable form for agriculture through the composting process for sustainable development (Padbhushan et al. 2016).

Vermicomposting is the technique of utilizing organic wastes such as organic residues, household wastes, and municipal wastes in the presence of earthworms to convert them into high-quality vermicompost that predominantly contains wormcast and decomposed organic debris (Devi and Prakash 2015). Vermicompost is a finely divided material that resembles peat and is abundant in micronutrients, and carbon-based matter, improving aeration, drainage, structure, porosity, and moisture retention in the soil and is helpful in the improvement of the soil properties. Studies suggested the addition of rock phosphate to the substrates of vermicomposting improved the nutrient composition and enzyme activity of the final product (vermicompost) (Adhami, Hosseini, and Owliaie 2014).

Stability and maturity are two important terms that define the decomposition rate of compost and its transformation from organics (Zmora-Nahuma et al. 2005) and are used in determining the quality of prepared compost. The firmness of the vermicompost is deeply associated with the action of microbes and diversity and can be assessed using various respirometry measures and/or by examining variations in the properties of the vermicompost. The maturity of the vermicompost has been evaluated using a variety of criteria and characteristics, albeit the majority of them apply to compost prepared by households. Physical traits including color, smell, temperature, moisture, moisture retention, porosity, and bulk density provide a rough notion of the stage of decomposition achieved, but they don’t provide much about the degree of maturation. The determination of compost stability has been reported using a wide range of methodologies (Wang et al. 2004). Chemical properties of vermicompost stability include pH, electrical conductivity (EC), cation exchange capacity (CEC), total organic carbon (TOC), forms of carbon (water-soluble carbon (WSC), water-soluble carbohydrate (WSCHO), and microbial biomass carbon), carbon to nitrogen (N) ratio, and ammonium ion to nitrate ion. The rate of microbial action in compost is determined by biological stability, which can be represented by a variety of metrics, including respiration, the dehydrogenase enzyme, phosphatase, and microbial biomass carbon (Viaene et al. 2017). Studies are more concentrated on the specific maturity index of the composts prepared under similar composting conditions, or of similar substrates. So far rare information is available to understand the impact of stability and maturity indices on vermicompost prepared by utilizing two different organic wastes viz. household waste and organic residue enriched with rock phosphate. Keeping these facts in view this study was focused on two main objectives: To develop the maturity and stability indices for organic residues and household wastes enriched with rock phosphate-based vermicompost and to assess the nutrient composition and enzymatic activities of the prepared vermicompost.

Materials and methods

Experimental site and materials required for experiment

The investigation was undertaken to measure the different types of vermicompost prepared from carbon-based residue and household waste with or without enriched rock phosphate at RPCAU, Pusa, Samastipur (Bihar). The experimental site is located at the coordinate’s 25° 58‘12“N latitude, 85° 41’ 24” E longitudes, and at 55 m above mean sea level.

The different organic materials used for vermicompost preparation were organic residues and household wastes. Organic residue namely, crop straw was collected from the research farm of RPCAU, Pusa, Samastipur. The household wastes were collected from the university residential blocks which were screened for plastic bags, polythene bags, and unwanted materials before utilizing for vermicompost preparation. Then, all these waste materials were dried in shade for about 2–3 days then it was processed for vermicompost preparation. To each of the composting masses, cattle dung was supplemented as usual inoculants to speed up disintegration. TOC of the substrates was 22.1%, 44.7%, and 47.3% for cow dung, organic residues, and household wastes, respectively. The detailed chemical characteristics of household wastes, organic residues, and cow dung have been shown in Table 1. Rock phosphate was used for enriching the vermicompost with P₂O₅ content was 18–20%.

Table 1. Chemical characteristics of substrate used for composting.

Chemical Characteristics	Household waste	Organic residue	Cow-dung
pH	7.35	7.19	8.55
EC (ds m^−1)	2.41	0.93	1.41
Total Organic Carbon (%)	47.3	44.7	22.1
Total nitrogen (%)	2.46	0.57	0.58
Total phosphorus (%)	0.21	0.13	0.26
Total potash (%)	2.39	1.07	0.78
Total Iron (mg kg^−1)	1191	998	1067
Total Copper (mg kg^−1)	16.0	12.0	24.9
Total Manganese (mg kg^−1)	99.6	51.9	349.5
Total Zinc (mg kg^−1)	141.2	51.7	92.9

Quality-enriched vermicompost production and treatment details

Mass production of enriched vermicompost using household wastes and organic residues of paddy and low-grade rock phosphate was done at the Vermicompost production unit of RPCAU, Pusa. The treatment combinations for vermicomposting were: 1). Household-waste (65%) +Cow-Dung (35%) +Earthworm, 2). Household-waste (65%) + Cow-Dung (35%) + Earthworm + Rock Phosphate (2.5% P₂O₅), 3). Organic-Residue (65%) + Cow-Dung (35%) + Earthworm, and 4). Organic-Residue (65%) +Cow-Dung (35%) +Earthworm+ Rock Phosphate (2.5% P₂O₅).

Vermicompost was prepared using household waste and organic residues mixed with cow dung separately in a 65:35 ratio and as per treatment, it was enriched with rock phosphate @ 2.5% P₂O₅ (w/w) by mechanical turnings used to ensure proper mixing of the composting matter. The total number of windrows was 4 with 5 replications and the size of individual windrow was 3.7 m × 0.8 m × 0.5 m dimension made under the shade. Suitable species of epigeic earthworms (Eisenia fetida) @ 2 kg per tonne of material at a suitable temperature (37 0C) and moisture (50–60%) were inoculated in prepared windrows. The earthworms were not disturbed during the entire period of composting. Throughout the composting process, water was sprinkled to keep the moisture level (about 50–60%) constant. At 30, 60, 90, and 120 days of composting or sampling (DAS) samples were drawn for analysis.

Sampling and analysis of prepared compost

A sample weighing 2000 g was collected from each windrow at different dates (30, 60, 90, and 120 DAS) to analyze pH change, moisture availability, BI, earthworm population, and enzyme activity. Further, the nutrient compositions of matured compost were estimated at 120 DAS.

To achieve the above, these samples were well mixed and separated into two parts. The initial portion was utilized to analyze total N and biochemical characteristics after being maintained in a refrigerator at 40°C. The remaining fraction was air-dried first in the shade, followed by an oven-dry at 65°C for 1 day, ground to pass through 2-mm sieves, completely combined, and then the total nutrients were analyzed. To represent the data on a dry-weight basis, sub-samples of fresh compost were also taken and their moisture level was calculated using the oven-dry method (65°C for 1 day).

The method outlined by Jackson (Jackson 1973) and explained in FCO (FCO Fertilizer Control Order 1985) was used to assess the pH and EC of the compost. The water holding capacity (WHC) of vermicompost was analyzed using Keen Rackzowski box methods explained by Piper (Piper 1966). Samples were tapped in a 250-ml cylinder and the bulk density (BD) was determined as given in FCO (FCO Fertilizer Control Order 1985).

Maturity and stability indices of compost

At 30, 60, 90, and 120 DAS, a wet sample was obtained from each window to examine the maturity and stability index characteristics. Using a conventional technique, the TOC, N, WSC, WSCHO, CEC, and BI of matured compost were examined. Nelson and Sommer’s dry combustion method (Nelson and Sommers 1982) with a muffle furnace heated to 550°C was used to assess TOC. N was estimated by the Micro-Kjeldahl method (Bremner and Mulvaney 1982). CEC was estimated by the neutral normal ammonium acetate saturation method (Black 1965). WSC content by McGill et al. (McGill et al. 1986) whereas WSCHO content was measured following the procedure outlined by Chesire and Mundie (Cheshire and Mundie 1966). Dehydrogenase and phosphatase activities by Klein et al (Klein, Loh, and Coudling 1971) and Tabatabai and Bremner (Tabatabai and Bremner 1969), respectively. The BI, which is a function of TOC and WSCHO over time was calculated using the following equation (1)(1) $\mathrm{B}\mathrm{i}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{b}\mathrm{l}\mathrm{e}\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}\left(\mathrm{B}\mathrm{I}\right)=3.166+0.039\mathrm{T}\mathrm{O}\mathrm{C}+02.832\mathrm{W}\mathrm{S}\mathrm{C}\mathrm{H}\mathrm{O}-0.011$(1) given by More et al. (Morel, Jacquin, and Gucket 1979).

(1) $\mathrm{B}\mathrm{i}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{b}\mathrm{l}\mathrm{e}\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}\left(\mathrm{B}\mathrm{I}\right)=3.166+0.039\mathrm{T}\mathrm{O}\mathrm{C}+02.832\mathrm{W}\mathrm{S}\mathrm{C}\mathrm{H}\mathrm{O}-0.011$(1)

Statistical analysis

Through the Duncan Multiple Range Test (DMRT), the variation in the data for various parameters was examined while considering the treatments represented by the treatments with the least significant difference at 5%. The data of the mean values were presented in the table/graph.

Results and discussion

Properties of vermicompost

The materials (organic residue and household waste) and enrichment technique have no impact on the WHC, BD, pH, and EC of prepared compost with time, however, the progression of the period resulted in greater values of WHC & EC value (Figures 1 and 2) and lesser BD & pH values (Figures 1 and 2). The average WHC was 20%, 22%, 25%, and 30% at 30, 60, 90, and 120 DAS, respectively. Similarly, values of BD at 60, 90, and 120 DAS were 1.03, 0.97, 0.86, and 0.76 Mg m−3, respectively. A lesser value of BD with time is an indication of compost maturity as well as improved porosity.

Figure 1. Effect of substrates and enrichment technique used in vermicompost preparation on water holding capacity and bulk density with the progression of period.

Figure 2. Effect of substrates and enrichment technique used in vermicompost preparation on pH and electrical conductivity with the progression of period.

The average pH was 8.46, 8.18, 7.86, and 7.74 at 30, 60, 9,0, and 120 DAS, respectively. On advancing substrates toward maturity, the decomposition of carbon-based matter took place resulting in the lowering of pH from 30 to 120 DAS. EC ranged from 0.78 to 0.87 dSm⁻¹. Highest EC (0.87 ds m⁻¹) was found at 90 DAS and the lowest EC (0.78 dS m⁻¹) was determined at 30 DAS. Cation exchange capacity was found significantly improved with the progression of the composting period up to 120 (Figure 3). The average CEC was 55, 67, 77, and 83 cmol (p+) kg⁻¹ for starting from 30 to 120 days of maturity at 30 days intervals, respectively. Highest CEC was observed in the vermicompost of household waste enriched with rock phosphate at maturity (120 DAS). The use of rock phosphate augmented the CEC of the vermicompost prepared from both substrates (organic residue and household waste).

Figure 3. Effect of substrates and enrichment technique used in vermicompost preparation on cation exchange capacity with the progression of period.

Nitrogen, zinc (Zn), and manganese (Mn) contents were found significantly affected due to varying substrates (Table 2). Household waste had higher N, Zn, and Mn contents compared to the organic residue. No variation in the content of phosphorus (P), potassium (K), iron (Fe), and copper (Cu) was observed between organic residue and household waste. Enriching with rock phosphate increased P content in both organic residues from 1.07% to 2.35% and household waste from 1.24% to 2.55% (Table 2).

Table 2. Nutrient composition of the vermicompost prepared from different materials.

	N	P	K	Zn	Fe	Cu	Mn
Prepared vermicompost	%			mg kg^−1
Organic residue + Cow-dung + Earthworm	1.86a	1.07a	1.28a	150a	1392a	24a	303a
Organic residue + Cow-dung + Earthworm + Rock phosphate	1.87a	2.35b	1.29a	159a	1466a	25a	320a
Household Waste + Cow-dung + Earthworm	2.14b	1.24a	1.33a	161b	1483a	25a	384b
Household Waste + Cow-dung + Earthworm + Rock phosphate	2.15b	2.55b	1.35a	167b	1509a	26a	407b

Note. Same letter denotes significant similar at P = 0.05 using DMRT.

Carbon fractions and C:N ratio

The substrates (organic residue and household waste) have an impact on the WSC and WSCHO of prepared compost with time; however, the progression of the period resulted in lower values of WSC and WSCHO (Figure 4). The average WSC was 2.52%, 2.00%, 1.31%, and 1.15% at 30, 60, 90, and 120 DAS, respectively, and the average WSCHO was 0.83%, 0.63%, 0.45%, and 0.36% at similar DAS as mentioned above. Enrichment techniques improved WSC up to 60 DAS and WSCHO up to 30 DAS for both substrates.

Figure 4. Effect of substrates and enrichment technique used in vermicompost preparation on water-soluble carbon and water-soluble carbohydrate with the progression of period.

The substrates and enrichment techniques have an impact on the total nitrogen (TN) of prepared compost with time; however, the progression of the period resulted in greater values of TN from 60 to 120 DAS (Figure 5). The average TN significantly increased from 60 DAS to 120 DAS. The average TN was 1.65%, 1.70%, 1.85%, and 2.01% for 30 DAS, 60 DAS, 90 DAS, and 120 DAS, respectively (Table 3). Total organic carbon was not found affected by varying substrates, applying rock phosphate (Figure 5). The average TOC decreased significantly up to 60 DAS and further decreased non-significantly. The ratio of carbon and nitrogen has shown no significant change due to varying substrates and enriching rock phosphate (Figure 5). The average C:N decreased significantly up to 90 DAS and further increased non-significantly. The average C:N was 21.17, 20.92, 20.38, and 20.45 for DAS from 30 to 120 at 30 days intervals, respectively.

Figure 5. Effect of substrates and enrichment technique used in vermicompost preparation on total nitrogen, total organic carbon, and C:N ratio with the progression of period.

Table 3. Maturity and stability indicators of vermicompost.

Prepared vermicompost	Units	Organic residue + cow dung + earthworm	Organic residue + cow dung + earthworm + rock phosphate	Household waste + cow dung + earthworm	Household waste + cow dung + earthworm + rock phosphate
Maturity indices
WSC	%	1.23b	1.25b	1.05a	1.08a
WSCHO		0.35a	0.37a	0.34a	0.36a
TN		1.86a	1.87a	2.14b	2.15b
TOC		33.70a	35.04a	34.12a	35.76a
C/N ratio		18.12a	16.46a	18.20a	16.75a
Stability indices
CO2 evolution	mg g^−124 hr ^−1	26.41a	26.63a	27.43a	28.36a
Dehydrogenase	μg TPF g^−1 soil h^−1	10.49a	10.97a	11.12a	11.66a
Alkaline Phosphatase	μg PNP g^−1 h^−1	557a	607b	658c	674c

Note. Same letter denotes significant similar at P = 0.05 using DMRT.

Biodegradability index and population of earthworm

The substrates and enrichment techniques have no impact on the BI of prepared compost with time, however, the progression of the period resulted in lower values of BI (Figure 6). The BI was decreased with increasing DAS. No statistically significant change was observed in the BI value from 30 to 60 DAS, further, it reduced suggestively at 90 DAS and was non-significant at 120 DAS. The average BI was 5.43, 5.13, 4.92, and 4.80 for DAS from 30 to 120 at 30 days intervals, respectively.

Figure 6. Effect of substrates and enrichment technique used in vermicompost preparation on biodegradability index, and population of earthworm with the progression of period.

Data shown in Figure 6 shows that the population of earthworms increased on variable substrates and applying rock phosphate (Figure 6). The population of earthworms decreased significantly with increasing DAS from 30 to 120. The average population of earthworms increased with increasing DAS from 30 to 120. The average population of earthworms was 630, 784, 851, and 986 for DAS from 30 to 120 at 30 days intervals, respectively.

Enzymatic activity

The substrates and enrichment techniques have an impact on the enzyme activities of prepared compost with time; however, the progression of the period resulted in greater values of enzyme activities (Figure 7). CO₂ evolution was found higher in household waste compared to the organic residue and increased on enriching with rock phosphate (Figure 7). The average CO₂ evolution increased significantly with increasing DAS from 30 to 120. The average CO₂ evolution was 16.6, 21.8, 25.4 and 27.2 mg g⁻¹24 hr ⁻¹ for DAS from 30 to 120 at 30 days intervals, respectively. Dehydrogenase was found to affect varying substrates and was not affected by enriching with rock phosphate (Figure 7). The average dehydrogenase was increased significantly with increasing DAS from 30 to 120. The average dehydrogenase was 6.8, 8.0, 9.2, and 11.1 μg TPF g⁻¹ soil h⁻¹for DAS from 30 to 120 at 30 days intervals, respectively. The alkaline phosphatase was found to affect varying substrates and was not affected by enriching with rock phosphate (Figure 7). The average alkaline phosphatase did not vary from 30 DAS to 60 DAS, and further significantly decreased at 90 DAS. The average alkaline phosphatase was 735, 700, 661, and 624 μg PNP g⁻¹ h⁻¹ for DAS from 30 to 120 at 30 days intervals, respectively.

Figure 7. Effect of substrates and enrichment technique used in vermicompost preparation on enzyme activity with progression of period.

Maturity and stability indicators

Data about maturity and stability indicators of prepared vermicompost are given in Table 3. At maturity, WSC was significantly higher in organic residue-based vermicompost compared to household waste-based vermicompost. No effect on WSC was recorded on enriching with rock phosphate. WSCHO content did not change with varying substrates and enriching with rock phosphate. TN was significantly higher in household waste-based vermicompost relative to organic residue-based vermicompost. No effect on TN was recorded on enriching rock phosphate. TOC content did not change with varying substrates and enriching with rock phosphate. Among stability indices, CO₂ evolution and dehydrogenase enzyme were not found affected by varying substrates and enriching with rock phosphate, however, alkaline phosphatase was found significantly higher in household waste-based vermicompost than organic residue-based vermicompost. Rock phosphate improved alkaline phosphatase in organic residue-based vermicompost than household waste-based vermicompost.

Discussion

The research demonstrated that altering the substrates and enriching the vermicompost with rock phosphate had an impact on its characteristics. This resulted in the disintegration of organic debris and the interactions between bacteria and earthworms.

Effect of different substrates, composting period, and enrichment with rock phosphate on the physical and chemical quality of vermicompost

Gradually increased value of WHC with vermicomposting period and maximum at maturity stage was the findings of the experiment; whereas reverse trends with regard to BD were observed. This is due to improved mature pore space and the overall carbon-based matter content of mature compost. However, changes in WHC and BD did not get affected due to varying substrates and enriching with rock phosphate. Sayed and Khate (Sayed and Khater 2015) also reported a similar effect on WHC and BD during the composting period. As the day neared its end, Schaub-Szabo and Leonard (Schaub-Szabo and Leonard 1999) reported that vermicompost made from four different substrates increased WHC; this may be due to the direct influence of organic particles (having high WHC) on substrates and/or the impact of other physical properties (BD, porosity, etc.) of prepared compost.

During composting, with time progression organic matter is broken down resulting in the production of organic acids resulting in reduced pH values of the mature vermicompost. Chaudhari et al. (Chaudhuri et al. 2000) vermicompost made from kitchen garbage utilizing Perionyx excavates had a lower pH when it was mature. Also, Nath et al. (Nath, Singh, and Singh 2009) reported similar findings about the pH of the vermicomposting period. Suthar (Suthar 2008) recorded a decrease in pH with cow dung vermicomposting using Eisenia fetida and Lampitomauritii. In our results, the use of rock phosphate did not statistically affect the pH of the vermicompost, however numerically pH decreased on enriching with rock phosphate. Some of the studies such as Li et al. (Li et al. 2012) reported a lowering of pH due to the application of rock phosphate in the vermicompost. This might be due to the formation of different acids during the processing of rock phosphate breakdown. Another potential explanation for the pH drop is the adsorption of ammonia and other cations by rock phosphate (Wong, Fung, and Selvam 2009). The pH of the vermicompost used in this study, which came from several substrates, ranged from 8.5 to 7.5. Ghinea and Leahu (Ghinea and Leahu 2020) also reported that in microbial degradation of organic materials, the range of pH is between 8.5 and 7.5.

Electrical conductivity reflects their salinity and appropriateness for plant growth (prefer EC range 2 to 3.5 dS m⁻¹); it has a significant effect on the quality of vermicompost as well. After the active phase, this parameter increased most likely as a result of the release of soluble salts such as ammonium, nitrates, and phosphate brought on by the breakdown of easily biodegradable organic substrates (Lazcano, Gomez-Brandon, and Dominguez 2008). Calcium, magnesium, K, and P are released as minerals when organic substrates decompose, and these minerals are released as cations in vermicompost (Guoxue et al. 2001). According to Amouei et al. (Amouei, Yousefi, and Khosrav 2017), earthworm activity, the breakdown of organic matter, and the mineralization of chemicals enhanced their solubility and mobility, which raised the EC of the substrate material during the vermicomposting process. Hernandez et al. (Hernandez-Rodriguez et al. 2012) reported the increase in EC with the period of vermicomposting, and our results were found to be consistent with their findings. Ansari and Rajpersaud (Ansari and Rajpersaud 2012), as well as Jadia and Fulekar (Jadia and Fulekar 2008), reported similar outcomes. CEC is a significant factor that is frequently utilized as a maturity index (Karak et al. 2013). During the various stages of decomposition, the humidification process generates different aromatic functional groups such phenolic-OH, alcoholic-OH, and aromatic-COOH and enhances the oxidation of the organic matter, which raises CEC. After the composting process (mature compost), CEC increases are phytotoxic-free and have a higher germination index (Ameen, Ahmad, and Raza 2016). The enrichment of organic elements with cation exchange characteristics in the composting mass is one reason why CEC the vermicompost increases during composting (Saharinen 1998). In our study, CEC was increased on maturity even varying substrates for vermicompost preparation and was found higher in the vermicompost prepared from household waste relative to organic residue-based compost. The use of rock phosphate also increased the CEC of the vermicompost and hence found highest under vermicompost prepared from household waste enriched with rock phosphate.

Both macronutrients and micronutrients, which are necessary for plant growth and soil health, are integral constituents of vermicompost. The vermicompost at the final harvest (120 DAS) from different windows was darker in color and granular structure. According to the nutrient content of the compost, the vermicompost made from household waste + cow dung + earthworm + rock phosphate had the highest efficiency of the vermicomposting process using Eisenia fetida (epigeic species). This was followed by household waste + cow dung + earthworm, organic residue + cow dung + Earthworm + rock phosphate, and organic residue + cow dung + earthworm. Nutrient content was increased at the maturity of vermicompost. Total N was obtained higher under household waste-based vermicompost relative to organic residue-based vermicompost. Our study also showed an increase in the TN at maturity (120 DAS) compared to intermediate composting periods (30 DAS, 60 DAS, and 90DAS). Decomposition of proteinous substance, as well as conversions of one form of N into another form, may be causes of improved N status of the compost at maturity. Enzymatic activity occurs during the digestion of organic matter as it moves through the earthworms’ guts, which leads to the breakdown of proteins and nitrogen-containing substances (Bhat, Singh, and Pal 2017). Total P content was increased significantly in enriched and without enriched vermicompost with the vermicomposting process. Additionally, substrates enhanced with rock phosphate had greater P contents. Wei et al. (Wei et al. 2016) likewise reported similar results. Nishanth and Biswas (Nishanth and Biswas 2008) also noted that the addition of rock phosphate to composted rice straw boosted its P content. Insoluble P can be transformed into soluble molecules by earthworms (Venkatesh and Eevera 2008). Vermicompost showed a rise in the total K as the day grew closer to maturity. This might be caused by an increase in K mineralization, which boosted microbial and enzymatic activity in the earthworm stomach (Parthasarathi and Ranganathan 2000). Mineralization of organic wastes reduces biomass volume which concentrates the elemental levels, and the inclusion of a bulking agent is the variable that results in an enhancement in micronutrients in the vermicompost. The process of vermicomposting tends to result in higher levels of micronutrients in the composting process (Sudha and Kapoor 2000). Our study also found that the end product of composting increased micronutrient content in all the vermicompost and higher in the enriched vermicompost relative to without enriched vermicompost. Micronutrients like Cu, Zn, Fe, and Mn were found to be improved in the enriched vermicompost as found by (Chaudhuri et al. 2000).

Effect of different substrates, composting period, and enrichment with rock phosphate on the carbon content of vermicompost

Our study showed that TOC in vermicompost decreased for all the substrates at maturity (120 DAS). This may be because some TOC was digested by the microbial biomass and some were lost as CO₂ to microbial respiration and mineralization of organic materials, which further increased TN. In addition to using carbon as a source of energy, bacteria also use organic matter to break down (Garg and Kaushik 2004; Pattnaik and Reddy 2010; Yatoo et al. 2022). Song et al. (Song et al. 2014) noted that the reduction in TOC in vermicomposting was caused by the organic matter’s mineralization, as earthworms use carbon as food and turn some of it into their biomass. Similar trends in the decline of TOC concentration after composting various organic wastes were also documented by (Goyal et al. 2005).

The C:N ratio can be used as a universal metric to evaluate the vermicompost’s quality and maturity because it directly reflects these factors. Because organic carbon is converted to CO₂, the C:N ratio rapidly decreases as vermicomposting develops. The maturation step takes longer to complete for substrates with greater C:N ratios. Carbon is lost during the vermicomposting process as a result of the partial mineralization of carbonaceous elements such as cellulose, hemicelluloses, and lignin, which partially disintegrate in a later stage but degrade swiftly in the initial stage of compost (Bernal, Albuquerque, and Moral 2009). According to observations made by Hait and Tare (Hait and Tare 2011) and Alidadi et al. (Alidadi et al. 2016), the drop in the C:N ratio was brought on by a greater loss of carbon through microbial respiration in the form of CO₂, as well as an increase in N and stability of waste by the activity of worms. According to Manna et al. (Manna et al. 2003), earthworm inoculations considerably raised the percentages of N content and decreased the percentages of the C:N ratio in all the feeding materials after 12 weeks, which caused the composts to mature more quickly. Because microorganisms have a high metabolic rate and can use readily, the C:N ratio declines.

Many researchers have utilized the WSC and WSCHO as a metric for evaluating compost maturity, which continuously declines during the composting process (Chanyasak and Kubota 1981; Manna et al. 1997). Except for the soluble fraction of fulvic acids, the WSC contains sugars, organic acids, amino acids, and phenols, making it the most readily biodegradable carbon component throughout the vermicomposting process (Garcia, Hernindez, and Costa 1991). All varieties of vermicompost were found to have decreasing WSC and WSCHO levels as they approached the compositing stage. This might be a result of substrates being passed through the intestines of epigeic earthworms, which raised their amount and finally decreased their amount in the final vermicompost. WSC decrease served as a measure of compost maturity (Chefetz et al. 1998; Garcia, Hernindez, and Costa 1991). Given the vast range of composting materials, Bernal et al. (Bernal et al. 1998). established WSC 1.7% as a measure of stable and mature compost, but this limit was not deemed appropriate in the current experiment. Eisenia fetida (epigeic species) injection appears to have dramatically reduced WSC and WSCHO levels in all composting litter breakdowns. The WSC and C:N ratio of all the composts dramatically reduced during the composting process (Manna et al. 2003). Inoculation with Eisenia fetida lowered WSCHO among the three epigeic earthworms substantially (P = 0.05) in all the substrates (Manna et al. 2014).

Effect of different substrates, composting period, and enrichment with rock phosphate on biological quality of vermicompost

The microbial activity as assessed by baseline respiration reaching an extreme worth of organic material during the dynamic phase may have been influenced by easily degradable components that stimulate the microbial community of the initial cattle dung. The stability of compost is typically evaluated using respiration indices (Goyal et al. 2005). These indices measure the level of biological activity in a substrate, which, under ideal circumstances, reflects the extent of substrate material decomposition. Since bacteria require oxygen for respiration and exhale CO₂ during aerobic decomposition, it is possible to calculate the rate of respiration using either CO₂ generation or oxygen consumption. Our study’s results are in line with those of Goyal et al. (Brinton 2007), who asserted that one of the most accurate measures of compost maturity and stability is CO₂ evolution.

Dehydrogenase is a crucial indicator of microbial action and the stability of carbon-based materials since it directly contributes to the process of microbial respiration (Nikaeen et al. 2015). The enhanced microbial activity during the vermicomposting process may be to blame for the increase in dehydrogenase activity levels (Asha et al. 2020).

Different organic phosphate compounds present in diverse materials, variations in earthworm and microbial activity, and the stage of vermicomposting could all contribute to the variable in alkaline phosphatase activity indicated by various substrates produced for vermicompost. The usage of rock phosphate amendment in the composting method, which may speed up the growth of microorganisms associated with phosphorus mineralization, increased the alkaline phosphatase activity in the vermicompost. With an increase in composting time up to 120 days, both in unenriched and enriched vermicompost, the alkaline phosphatase activity dropped. Our results were found to be consistent with other studies that demonstrated a similar trend throughout the composting of different bio-wastes, with a peak in phosphatase activity occurring after 3 weeks of composting followed by a sharp fall (Albrecht et al. 2010; Ros, Garcıa, and Hernandez 2006). Acid phosphatase and humic acid of the ultimate stabilized products rose as a result of the passage of carbon-based material via the earthworm guts, and their content was further boosted by inoculation of ligninolytic fungus or free-living N-fixing bacteria (Pramanik, Ghosh, and Banik 2009).

Effect of different substrates, composting period, and enrichment with rock phosphate on biodegradable index and earthworm population in vermicompost

The BI reduced as the number of days spent vermicomposting increased, reaching an average value of 4.80 after 120 days. During the first 90 days of decomposition, the values of BI suggested by Morel et al. (Sayed and Khater 2015). varied from 3.5 to 4.6 in injected earthworms and from 3.9 to 2.9 in inoculated controls. According to the experiment’s findings by Sahu et al. (Sahu et al. 2019), non-inoculated plant residues varied from 3.2 to 3.8, increasing immaturity, while inoculated plant residues had BI values between 2.6 and 3.0 at the stage of humification. As a result, it was determined that inoculation substrates matured because the BI rating in the humification phase reached 2.71. According to Garcia et al. (Garcia et al. 1992), if the BI was higher than 2.9, the compost was not yet mature.

Population dynamics of epigeic earthworm (Eisenia fetida) during vermicomposting were evaluated based on an increase in earthworm of the substrate. The use of appropriate organic substrates is important for earthworms to increase their growth and reproduction performance. The substantial rise in average worm biomass during vermicomposting suggests that feeding earthworms substrate is a possibility. Our research made it abundantly evident that the earthworms’ life cycle was significantly influenced by the substrate’s quality, temperature, and moisture content. The growth rates of epigeic earthworms were inversely correlated with temperature (Eisenia fetida). According to Manna et al. (Manna et al. 1997), the quality of the substrate can have an impact on the growth dynamics of P. excavatus. Manna et al. (Manna et al. 2003), Eisenia fetida outperformed P. excavatus and D. bolaui in overall populations, survival, and vermicompost quality. Similarly, Bhat et al. (Bhat, Singh, and Pal 2017), the press mud and cattle dung feed mix considerably increased average body weight, cocoon production, and hatchling formation as well as dramatically increased Eisenia fetida growth.

Conclusion

Vermicomposting is a low-cost technique for pollution control that aids in the recycling of organic wastes and the reduction of environmental pollutants. Our study concludes that the quality of vermicompost depends on different substrates, composting period, and enrichment with rock phosphate. The qualities of vermicompost were best found under household waste-based vermicompost enriched with rock phosphate. The efficiency of vermicomposting process using earthworms was found maximum for enriched and without enriched household-based vermicompost. The study also indicated that several stability and maturity indices are influenced by different parameters and hence cannot be determined by a single parameter. The addition of rock phosphate increased the CEC, P content, and alkaline phosphatase. Nitrogen, Zn, Mn, dehydrogenase activity, and alkaline phosphatase were found higher under household waste-based vermicompost relative to organic residue-based vermicompost. All four substrates promoted earthworm growth and reproduction in vermicompost. Therefore, it can be inferred from this that vermicompost matures and stabilizes differently depending on the substrate utilized and does better when enriched with rock phosphate.

Nomenclature

BD	=	Bulk density
BI	=	Biodegradability index
CEC	=	Cation exchange capacity
Cu	=	Copper
DAS	=	Days of sampling/composting
DMRT	=	Duncan Multiple Range Test
EC	=	Electrical conductivity
Fe	=	Iron
K	=	Potassium
Mn	=	Manganese
N	=	Nitrogen
P	=	Phosphorus
TN	=	total nitrogen
TOC	=	Total organic carbon
WHC	=	Water holding capacity
WSC	=	Water-soluble carbon
WSCHO	=	Water-soluble carbohydrate
Zn	=	Zinc

Acknowledgment

The authors are thankful to the Director of Research, and Head of the Soil Science and Vermicompost unit, RPCAU, Pusa, for providing research laboratory facilities for sample analysis.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article.

Additional information

Notes on contributors

Rajesh Kumar

Rajesh Kumar is an Assistant Professor-cum-Junior Scientist at Bihar Agricultural University, Sabour, India. He has more than 10 years of teaching and research experience specializing in Soil Physics. He has published several research articles/reviews in reputed journals.

Shankar Jha

Shankar Jha is a Soil Scientist at Dr. RPCAU, Pusa, India. He has 15 years of experience in research and specialized in Natural Resource Management. He has published various research articles/reviews/technical reports and authored books in different publications. He has also served as editor/co-editor for different journals and has received prestigious awards at different scientific forums.

Shiveshwar Pratap Singh

Shiveshwar Pratap Singh as Soil Scientist has more than 15 years of research experience in the field of soil fertility, nutrient management, salt-affected soils, waste recycling, and vermicomposting. He has published 35 peer-reviewed research articles in national and international journals and several books.

Mukesh Kumar

Mukesh Kumar is a Professor and Head of the Department of Soil Science at Dr. RPCAU, Pusa, India. He has a vast experience in research, teaching, and extension. He has guided several students in their thesis. He has published several research articles/reviews and authored books fin reputed publications. He has also served as editor/co-editor for different journals and has received prestigious awards at different scientific forums.

Ragini Kumari

Ragini Kumari is an Assistant Professor-cum-Junior Scientist at Bihar Agricultural University, Sabour, India. She has more than 10 years of teaching and research experience specializing in Soil fertility and Chemistry. She has published 20 research articles/reviews in reputed journals.

Rajeev Padbhushan

Rajeev Padbhushan is an Assistant Professor-cum-Junior Scientist at Bihar Agricultural University, Sabour, India. He has more than 10 years of teaching and research experience specializing in Soil Fertility and Chemistry. He has published more than 50 research articles/reviews in peer-reviewed journals. He has also acted as a reviewer for more than 20 International journals.