Rice cultivation is a cornerstone of global food security, accounting for almost a fifth of global cereal consumption and ranking second only to wheat (FAOSTAT, 2021). In Africa, rice is a staple food of strategic importance to both rural and urban populations and its significance is growing alongside demographic expansion and evolving dietary preferences (Seck et al., 2013). In Côte d’Ivoire, annual rice consumption is estimated at 2.65 million tonnes, equivalent to around 80 kg per person per year, while national production amounts to approximately 1.30 million tonnes cultivated across over 325,000 ha (SNDR-2.0, 2022).
       
The expansion of rice production results in the generation of large quantities of agricultural residues, primarily rice straw and husk. These residues are predominantly lignocellulosic materials characterized by high levels of cellulose, hemicellulose and lignin, which greatly reduces their natural biodegradability (Chen et al., 2008). In Côte d’Ivoire, rice residues are commonly burned on site or discarded indiscriminately, contributing significantly to air and water pollution through the release of greenhouse gases, particulate matter (PM_2.5) and other toxic compounds (Lorn et al., 2022). Such emissions pose serious risks to human health, degrade ecosystems and exacerbate climate change (Romasanta et al., 2017). Furthermore, these practices result in the loss of valuable organic resources that could be recycled to improve soil fertility.
       
Composting is one of the most widely adopted organic waste management strategies in developing countries due to its low cost and technical simplicity (Folefack, 2008). While rice straw and husk can be successfully composted (Gayathri and Thiyageshwari, 2020; Dhatt et al., 2023; Yumnam et al., 2023), conventional composting is often limited by long processing times, significant nutrient losses (particularly of nitrogen and carbon) and the potential accumulation of heavy metals in the final product (Begum et al., 2022). These limitations raise concerns about the quality of the compost and its environmental safety when applied to agricultural soils.
       
Vermicomposting has emerged as an efficient, environmentally friendly way of stabilising and adding value to organic waste. This bio-oxidative process, driven by earthworms and associated microorganisms, speeds up the decomposition of organic matter, increases the availability of nutrients and stabilises the residues while producing a high-quality organic fertilizer (Yuvaraj et al., 2018). The African nightcrawler (Eudrilus eugeniae) has been shown to be highly efficient at vermicomposting various lignocellulosic waste products (Coulibaly et al., 2019, Domínguez et al., 2001). However, there is limited information on its performance in vermicomposting rice straw and husk under the agroecological conditions of Côte d’Ivoire. As substrate physicochemical characteristics are key determinants of earthworm growth, reproduction and overall vermicomposting efficiency, substrate composition is expected to strongly influence biostabilisation outcomes. The aim of this study was to assess the effect of substrate composition on the efficiency of Eudrilus eugeniae and on biostabilisation processes during the vermicomposting of rice straw and husk. The specific objectives were to: (i) characterize the physicochemical properties of rice straw- and husk-based substrates, (ii) evaluate the growth and reproductive performance of E. eugeniae under different substrate compositions and (iii) assess the degree of organic matter stabilisation and nutrient enrichment of the resulting vermicomposts.
       
This work aims to contribute to the development of sustainable strategies for rice residue management and soil fertility improvement in Côte d’Ivoire and similar tropical agroecosystems.

Study site
 
The study was conducted at the experimental station of Nangui ABROGOUA University, Abidjan, Côte d’Ivoire, from June to September 2025. Abidjan is located between latitudes 5°172  N and 5°312  N and longitudes 3°452  W and 4°222  W. The experiments were carried out in a room maintained at a temperature of 30.62±1.41°C and a relative humidity of 87.7±1.51%.
 
Biological materials
 
Rice straw was collected from the “LALOKA” rice-growing area of Sakassou, one of the largest rice-producing areas in Côte d’Ivoire.  As for the rice hull, it was obtained from the rice husking factory of the company “Agricultural and Management Company Food and Commerce (AMC-FC)” located in Bongouanou. Cow and chicken excrement were also collected to be used in the experiment as they are widely produced in the country. In this experiment, rice straw and husk are considered as principal residues. Cow and chicken excrements are secondary waste and are used as catalysts. These latter wastes are known as rich substrates and favorable substrates for the growth of E. eudrilus (Coulibaly et al., 2014). The different wastes were air dried before their use. Healthy adults of E. eugeniae, with a mean weigh of 0.85±0.2 grams each, were collected from stock cultures maintained by authors in the laboratory using mowed grass as culture medium.
 
Experimental design
 
The containers used in the experiment were plastic with a volume = 3 l, diameter = 50 cm, depth = 15 cm. Three groups were formed in the experiment. The first one was constituted of the two main wastes. Three hundred grams of each dried principal residue was taken as culturing material in circular plastic containers. The second group was formed of a half mixture of each principal wastes (150 g rice traw +150 rice husk). The third group was formed of each principal and secondary waste. Containers were filled firstly with 100 g of each main waste and completed with 100 g of one secondary waste. Secondly, the containers received 100 g of principal waste and 100 g of secondary waste. The fourth lot was constituted of 75 g of each type of waste. In total, eight treatments were set up for the study (Rice Husk (RH), Rice Straw (RSt), Rice Husk + Rice Straw (RH+RSt), Rice Straw + Cow Dung (RSt+CD), Rice Straw + Chicken Waste (RSt+CW), Rice Husk + Cow Dung (RH+CD), Rice Husk + Chicken Waste (RH+CW) and Rice Straw + Rice Husk + Cow Dung + Chicken Waste (RSt+RH+CD+CW). Fifteen repetitions were retained per test. The content of the containers was watered with distilled water and the moisture was adjusted to 70-80 per cent. The mixtures were turned over manually daily for two weeks to eliminate volatile gases which may be potentially toxic to earthworms. After the precomposting period, 5 individuals of E. eugeniae were put in the containers for mineralization. The containers were covered with their pierced cover for 90 days. After three months, homogenized samples of each feed material were taken. The cocoons, earthworms and hatchlings were removed manually from each sample. The samples were air dried in the shade and the pH, total organic carbon (TOC), total N, total K and total available P were analyzed in Biological Systems Engineering Laboratory at Washington State University (USA). Distilled water suspension of each waste was used to measure the pH after mechanical agitation for 30 min and filtration through a Whatman No. 42 filter paper. Total nitrogen, total carbon and hydrogen were determined using a TRUSPEC-CHN® (LECO, US) elemental analyzer (Coulibaly et al., 2020). Inductively coupled plasma mass spectroscopy (ICP-MS) analysis was used to analyze for the contents of total K, Mg, Ca and Na in the samples. This analysis was conducted based on the method described by Pecha et al. (2015) in an ICP-MS (Agilent 7500cx) instrument. A 100-mg, of dried sample in question, was mixed with 3 mL concentrated nitric acid (HNO3) (69-70 per cent) and 2 ml 30 per cent H2O2 and placed in a microwave digester (SPD, CEM corporation) at 300°C and 250 psi for 5 min. The filtrate obtained via the Whatman No. 42 filter paper was used for the ICP-MS analysis.
 
Statistical tests
 
Data were analyzed by factorial analysis of variance (ANOVA) using the SAS general linear model (GLM) procedure (SAS, 1999). Least Significant Difference (LSD) multiple range-tests were used to determine significant differences between chemical components of the different substrates before and after vermicomposting and the number of earthworms and cocoons in the media at the end of the experiment. Significant differences between means were determined by P-values < 0.05 (α = 0.05).

Initial physicochemical properties and their implications
 
The initial characteristics of the eight substrates are presented in Table 1. Moisture content ranged from 4.62 per cent (rice husk, RH) to 7.86 per cent (rice straw and cow dung mixture, RSt+CD). Ash content varied significantly (p<0.05), with the highest value (23.49±4.08 per cent) in RSt+CD and the lowest (17.11±3.15 per cent) in RH+chicken waste (CW). The pH ranged from 6.79 (RH) to 8.48, with all other substrates being alkaline. The highest Total Organic Carbon (TOC, 47.19±10.07 per cent) was recorded in RSt+CD, while the highest Total Nitrogen (TN, 1.47±0.06 per cent) was found in RSt. The lowest TN was in RH. The highest concentrations of Total Potassium (TK, 1.59±0.07 per cent), Total Phosphorus (TP, 0.43±0.03 per cent) and Magnesium (Mg, 0.79±0.06 per cent) were all measured in the RSt+CW mixture. This significant variation in initial properties can be attributed to the distinct biochemical compositions of the substrates. This finding is consistent with those of other studies on agro-waste (Asmadi et al., 2023). Despite this variability, all materials had suitable nutrient profiles for vermicomposting.


 
Properties of the substrates after vermicomposting
 
Following the 90-day vermicomposting period, there were significant differences among the substrates in all measured physicochemical parameters (p<0.001; Table 2). The moisture content of all vermicomposted substrates increased compared to their raw state, reaching a maximum of 6.84±1.82% in RSt+CW. This increase, particularly in treatments amended with animal waste, can be attributed to the burrowing activity of E. eugeniae and the water-holding capacity of their organically rich casts (Li et al., 2020). Earthworm activity enhances porosity and introduces hydrophilic organic colloids, thereby improving moisture retention (Ding et al., 2013; Hauser et al., 2012).


       
Final pH values ranged from 6.83 to 8.17. The modulation of pH towards this neutral to slightly alkaline range is a well-documented outcome of vermicomposting, reflecting the optimal conditions for earthworm and microbial metabolism (Rayela et al., 2025).
       
A consistent pattern emerged across all treatments: TOC and TK contents decreased, while TN, TP and Mg contents increased relative to initial values. The significant reduction in TOC indicates intense microbial respiration and mineralisation, processes that are notably accelerated by earthworm mucus and gut-associated enzymes, particularly in nutrient-rich substrates such as animal manure mixtures (Garg et al., 2006). The concurrent increase in TN is due to the concentration effect resulting from carbon loss, nitrogen mineralisation and the direct addition of nitrogenous compounds via earthworm excretions (Sofo et al., 2023). The decline in TK is likely due to leaching (Orozco et al., 1996), while the increase in TP is attributed to the mineralisation and mobilisation of phosphorus via microbial and earthworm gut phosphatase activity (Le Bayon and Binet, 2006).
 
Evolution of the C/N and C/P ratios
 
The carbon-to-nitrogen (C/N) and carbon-to-phosphorus (C/P) ratios decreased significantly (p<0.001) in all substrates after vermicomposting (Fig 1 and Fig 2). The most pronounced reduction in the C/N ratio occurred in the amended substrates, dropping from 51.85 to 15.05 in the RSt+CD mixture and from 40.53 to 13.51 in the four-component mixture (RSt+RH+CD+CW). The C/P ratio also decreased substantially across treatments, with percentage reductions ranging from 33.1 per cent (RSt+CW) to 94.91 per cent (RH+CD). This drastic reduction is a key indicator of successful bioconversion and compost maturity. Substrates amended with animal manure achieved C/N ratios below 20, meeting the established benchmark for stabilised organic amendments (Senesi, 1989). This decline highlights the efficient conversion of carbon by microbes and earthworms and the subsequent enrichment of nutrients (Atiyeh et al., 2000; Garg et al., 2006; Lakshmi et al., 2014).




 
Earthworm performance in relation to substrate
 
The growth and fecundity of Eudrilus eugeniae were highly dependent on substrate composition (Table 3). The highest final earthworm population (354±62.08), biomass and cocoon production (127.66±18.14) were achieved in the RSt+RH+CD+CW mixture. In stark contrast, pure RH supported the lowest final population, biomass and fecundity (23±16.23).


       
The superior performance of the four-component mixture demonstrates the critical importance of balanced substrate chemistry. This mixture likely provided an optimal, less recalcitrant resource with readily available nutrients and a more favourable microclimate. Earthworms preferentially consume organics with lower lignin and cellulose content (Manna et al., 2003; Sarangi and Lama 2013); conditions that are best met in co-amended substrates. The poor earthworm development in pure RH emphasises the limitations of highly lignocellulosic, nutrient-poor substrates for vermiculture and highlights the necessity for supplementation to achieve efficient waste valorisation.

This study demonstrates that vermicomposting using Eudrilus eugeniae is an efficient and viable biotechnology for the conversion of two abundant and problematic agro-industrial residues in Côte d’Ivoire: rice straw and husk. The key factor in the success of the process was the composition of the substrate. Although rice residues alone supported earthworm activity, adding nutrient-rich animal manure (cow dung and chicken waste) significantly increased the efficiency of bioremediation. This was evidenced by accelerated organic matter mineralisation, a pronounced reduction in the C/N and C/P ratios to levels indicating advanced stability and maturity (C/N <20) and a concurrent increase in plant-available nutrients, such as nitrogen and phosphorus. Furthermore, substrate quality directly modulated the biological performance of E. eugeniae. Mixed substrates, particularly those containing all four waste types, created a more favourable physicochemical environment, resulting in superior earthworm growth, biomass production and fecundity compared to monoculture residues. These findings highlight that the initial characteristics of the feedstock are critical for optimising vermicomposting systems, rather than being merely incidental.
       
From a practical standpoint, this research provides a scalable, sustainable waste management model. The recommended practice of co-composting rice residues with locally sourced animal waste transforms an environmental liability into a valuable organic fertiliser. This closed-loop strategy promotes circular agricultural economics, reducing reliance on open burning and the associated pollution and health risks while returning essential nutrients to the soil. Future research should focus on field-scale validation of the produced vermicompost on crop productivity and soil health parameters to fully quantify its agronomic and environmental benefits.

The present study was supported by Nangui ABROGOUA University, Côte d’Ivoire, through the provision of institutional support and research facilities necessary for conducting the study.
 
Disclaimers
 
The views and conclusions expressed in this article are solely those of the authors and do not necessarily represent the views of their affiliated institutions. The authors are responsible for the accuracy and completeness of the information provided, but do not accept any liability for any direct or indirect losses resulting from the use of this content.