Introduction

In recent years, interest has grown in South Korea toward vermicomposting (a biological treatment method that uses manure as a feed source for earthworms) due to its advantages over conventional composting systems. Internationally, this eco-friendly approach to managing organic wastes, including livestock manure, has been practiced since the 1980s (Hand et al., 1988). Vermicomposting, involves stabilizing and reducing organic waste through the digestive activity of earthworms (Haimi and Huhta, 1987).

Unlike many other manure management techniques, vermicomposting minimizes secondary pollution, reduces odor and pest occurrence, and decreases the population of pathogenic microorganisms (Hwangbo and Jo, 2014; Jo and Hwangbo, 2014). Moreover, the earthworms produced during the process are valuable as a protein-rich feed resource (Son and Jo, 2003). The casts excreted by earthworms serve as a high-quality organic fertilizer (Song et al., 1993a) and can be applied to farmland, contributing to sustainable nutrient cycling in agriculture (Lee, 1995).

Earthworm feeding accelerates decomposition, and the enzymatic activity within the earthworm gut increases the humic acid content, a key component of humus (Mitchell et al., 1977). Vermicompost is typically rich in exchangeable cations, available phosphorus, and essential minerals such as calcium, magnesium, potassium, and phosphate, while also containing a high proportion of organic matter (OM) (Lavelle, 1988). Nitrogen content generally ranges from 1 - 2%, sufficient to supply plants with readily available mineral nitrogen without causing root damage (Lee, 1995; Lee and Lee, 1999).

In addition, vermicompost contains plant growth-promoting substances (Syers et al., 1979), exhibits a slightly alkaline pH, and possesses a granular structure that enhances soil porosity, aeration, and water retention, thereby improving soil physical properties (Tomati et al., 1987). As a result, it has considerable potential as both a soil conditioner and a plant growth stimulant (Oyege and Bhaskar, 2023). Beyond its fertilizer value, vermicompost contains carbon and amino acids that contribute to odor reduction during the composting process (Yoo and Lee, 2007). Because of these benefits, vermicompost is now used widely in both agriculture and landscaping. In addition, it has found use in areas such as odor control, removal of sulfur compounds, and as a medium for delivering microbial inoculants. This study was designed to investigate the effects of incorporating reed (Phragmites communis) into cow manure at different ratios on the chemical composition and heavy metal content of earthworm casts. With conventional composting methods facing environmental and efficiency limitations, vermicomposting offers an environmentally sustainable alternative for organic waste treatment. The integration of reed into livestock manure for vermicomposting not only reduces the risk of secondary pollution but also produces vermicast with significant potential for agricultural application.

Therefore, this study aimed to investigate how mixing reed with cow manure in different ratios affects the chemical properties and heavy metal content of the produced vermicast. It also explored how this vermicast could be used in agriculture as fertilizer and soil conditioner, and considered the possibility that such use might increase its commercial value and give more environmental benefits.

Materials and Methods

Experimental design

The vermicast analyzed in this study was obtained from a previous experiment titled “Effects of Reed Mixing Ratios in Cow Manure on the Biological Characteristics of Earthworms” (Lee and Hwangbo, 2025). In that experiment, the feed was prepared by mixing pure cow manure (collected from Hanwoo cows housed at the Gyeongkuk National University experimental farm and free of bedding or urine) with reed (P. communis) harvested as a forage resource. Five mixing ratios (v/v) were tested: 100% manure + 0% reed, 90% + 10%, 80% + 20%, 70% + 30%, and 60% + 40%.

For each treatment, 1,500 g of feed mixture was placed into a rearing container, and twenty-five adult Eisenia foetida of similar body weight were introduced. Each ratio was replicated five times, and the containers were arranged in a completely randomized design. The rearing period lasted 70 days. At the end of the trial, the solids from each container were dried at 60℃ for 48 h in a drying oven and then sieved through a 2 mm mesh to separate the vermicast fraction (< 2 mm), which was used for subsequent analyses.

Organic matter, pH, electrical conductivity, total carbon, total nitrogen, and C/N ratio of vermicast

Ash content was determined by combusting dried samples in a muffle furnace at 550℃ for 3 h. OM was calculated as 100-ash (%). pH and electrical conductivity (EC) were measured with a pH meter and EC meter, respectively. Total carbon (TC) was estimated according to the California Univ., Berkeley method using the formula (100−ash%) / 1.8 (University of California, Berkeley, 1953). Total nitrogen (TN) content was measured using the Kjeldahl method (AOAC International, 1995). The carbon-to-nitrogen (C/N) ratio was derived from the TC and TN values.

Heavy metal content of vermicast

For heavy metal analysis, the method of Nahm was followed (Nahm, 1992). Cadmium (Cd), chromium (Cr), copper (Cu), and lead (Pb) were determined by atomic absorption spectrophotometer (AAS; SpectrAA-200HT, Varian Inc. [now Agilent Technologies], USA), while arsenic (As) and mercury (Hg) were measured using inductively coupled plasma atomic emission spectrometer (ICP-AES; Liberty Series II, Varian Inc. [now Agilent Technologies], USA). Hg was not detected in any samples and thus omitted from the results table. The physico-chemical composition and heavy metal contents of the feed used in the preceding earthworm trial are presented in Tables 1 and 2.

Statistical analysis

All experimental data were analyzed using the SAS package program (SAS, 2023). Treatment means were compared by analysis of variance (one-way ANOVA) followed by Duncan’s multiple range test at a 5% significance level.

Results

Organic matter, pH, and electrical conductivity of vermicast

The OM content, pH, and EC of vermicast produced from different mixing ratios of cow manure and reed are presented in Table 3.

The OM content of the feed mixtures ranged from 61.1 to 67.1% (Table 1), while in the vermicast it was lower, ranging from 51.60 to 57.63%. In both the feed and vermicast, OM increased significantly (p < 0.05) with higher proportions of reed. The highest value was observed in the 40% reed treatment (57.63%), whereas the lowest values were recorded in the pure manure (51.60%) and 10% reed treatments (52.43%).

The pH of the feed ranged from 7.65 to 7.81 (Table 1), but in the vermicast it was lower, between 6.77 and 7.25. Increasing reed proportion significantly reduced the pH of the vermicast (p < 0.05), with the lowest value (6.77) recorded in the 40% reed treatment.

EC in the feed ranged from 0.603 to 0.945 mS·cm^-1 (Table 1), while in the vermicast it ranged from 0.576 to 0.830 mS·cm^-1, showing an overall reduction after digestion. Such reductions in EC are beneficial when vermicast is applied to soil, as they help mitigate the risk of salt accumulation. EC values declined significantly with increasing reed content (p < 0.05), following a pattern similar to pH.

Total carbon, total nitrogen, and C/N ratio of vermicast

The TC, TN, and C/N ratios of vermicast according to manure-reed mixing ratios are shown in Table 4.

The pure manure treatment recorded the lowest TC content (28.67%) (p < 0.05), whereas TC increased significantly with higher reed proportions, reaching the highest value (32.01%) in the 40% reed treatment (p < 0.05). TN content ranged from 1.40 to 1.55% in reed-mixed treatments and was 1.58% in the pure manure group, with the pure manure and 10% reed treatments showing the highest values (p < 0.05).

The C/N ratio in the feed ranged from 21.3 to 27.8 (Table 1) but was generally lower in the vermicast (18.11 - 22.84). The lowest ratio was observed in the pure manure treatment (18.11) (p < 0.05), while the highest ratio was found in the 40% reed treatment (22.84) (p < 0.05).

Heavy metal content of vermicast

The heavy metal concentrations of vermicast are presented in Table 5.

As content in the feed ranged from 16.4 to 26.5 ppm (Table 2) and was lower in the vermicast, ranging from 12.79 to 21.73 ppm. Increasing reed proportion significantly reduced As levels (p < 0.05), and treatments with 20% or more reed showed significantly lower concentrations than the pure manure group (p < 0.05). Hg was not detected in either the feed or vermicast (Table 2).

Cd content in the feed ranged from 1.4 to 1.7 ppm (Table 2), while in the vermicast it was 1.42 ppm for the pure manure group and 1.10 - 1.28 ppm in reed-mixed treatments. Cd levels declined significantly with increasing reed proportion (p < 0.05). Cr, Cu, and Pb contents also showed overall reductions in vermicast compared to feed, with significantly lower concentrations in treatments containing higher proportions of reed compared to pure manure (p < 0.05).

Discussion

After consuming and digesting their feed, earthworms leave behind what is known as vermicast. This by-product differs in both its physical form and chemical makeup compared to the material they first consumed. Vermicast is enriched with microorganisms and enzymes, enhancing its value as a fertilizer (Syers et al., 1979), and its granular structure contributes to improving soil physical properties (Tomati et al., 1987). Due to these characteristics, it has been recognized as a promising resource for use as both a soil conditioner and an organic fertilizer (Oyege and Bhaskar, 2023).

In this study, the OM content of the vermicast (51.60 - 57.63%; Table 3) exceeded the minimum requirement for by-product composts specified in the Korean Fertilizer Control Act (≥ 25%) (RDA, 2019). Compared with the feed, the OM level decreased by an average of 16.1%, a greater reduction than typically reported for conventional composting. This observation aligns with the findings that earthworms accelerate the mineralization of OM (Lee et al., 2005). Conversely, higher reed proportions resulted in increased OM content, likely due to the slower decomposition of reed, which is rich in lignin and cellulose.

The pH of the vermicast was within the neutral range (6.77 - 7.25; Table 3) but lower than that of the feed. This reduction is consistent with the report which attributed pH decreases to CO₂ release during earthworm respiration and organic acid production through microbial activity (Hartenstein and Hartenstein, 1981). EC (0.576 - 0.830 mS·cm^-1; Table 3) also declined compared to the feed, suggesting that earthworm activity reduces soluble salt concentrations (Song et al., 1993b).

TC content increased with higher reed proportions (Table 4). This trend supports previous reports that adding fibrous plant material to manure can maintain more stable forms of organic carbon during microbial and earthworm digestion (Edwards et al., 2010; Wang et al., 2022). The notably high OM content in the 40% reed treatment (Table 3) may indicate that an optimal fiber supply enhanced carbon retention.

TN content was highest in the pure manure and 10% reed treatments (Table 4), possibly because lower reed inclusion provided relatively nitrogen-rich feed and minimized nitrogen losses. In contrast, higher reed proportions may have diluted nitrogen due to increased carbon input. The C/N ratio (18.11 - 22.84; Table 4) met the compost standard requirement (C/N ≤ 50) (RDA, 2019) in all treatments and was generally lower than that of the feed, indicating accelerated mineralization of OM by earthworms.

Since vermicast is often applied back to the soil as a fertilizer or soil amendment, checking its heavy metal content is essential to ensure safety. In this experiment, the heavy metal levels in the feed materials met the allowable limits set by the Korean Fertilizer Control Act (As ≤ 50 ppm, Hg ≤ 2 ppm, Cd ≤ 5 ppm, Cr ≤ 300 ppm, Cu ≤ 300 ppm, Pb ≤ 150 ppm) (RDA, 2019). The actual values were As 16.4 - 26.5 ppm, Cd 1.4 - 1.7 ppm, Cr 8.6 - 13.1 ppm, Cu 12.6 - 21.2 ppm, Pb 22.2 - 35.6 ppm, and Hg was not detected. In the vermicast samples, As, Cd, Cr, Cu, and Pb were consistently lower than in the feed, with significant reductions observed as the proportion of reed increased. This pattern is probably explained by a combination of the dilution effect from reed and the binding of metals by increased OM. OM can create stable complexes with metals, limiting their uptake by plants and microorganisms (Negim and Sweed, 2020). The lack of Hg in all treatments most likely reflects its minimal presence in the original feedstock.

Overall, adding reed to cow manure during vermicomposting increased OM and carbon in the vermicast. At the same time, it reduced pH, EC, and heavy metal levels in the finished product. These changes help make vermicast both safer and more useful as a soil amendment and organic fertilizer. The results also suggest that reed and other fibrous plants can be good additions for improving manure composting. More studies carried out in different climates, with various rearing times and material combinations, would help develop stronger and more sustainable vermicomposting practices.

Conclusion

This study examined the changes in chemical composition and heavy metal content of vermicast produced by E. foetida when fed cow manure mixed with varying proportions of reed. Increasing the reed proportion significantly elevated the OM and TC contents of the vermicast, while pH, EC, and concentrations of As, Cd, Cr, Cu, and Pb were significantly reduced. TN content was highest in the pure manure and 10% reed treatments, and the C/N ratio in all treatments met the compost standard specified in the Korean Fertilizer Control Act. Hg was undetected in all samples, and the levels of other heavy metals were well below the permissible limits.

These results show that mixing reed with cow manure during vermicomposting can boost OM and carbon in vermicast, while reducing salts and heavy metals. This makes the product safer and more valuable as a soil amendment. Using roughly 40% reed may be a good balance, giving gains in OM and carbon along with clear drops in heavy metal content.

Therefore, the cow manure-reed vermicomposting approach offers an effective method for reducing environmental pollution from manure composting while producing safe, sustainable organic fertilizer for agricultural use. Further research under diverse climatic conditions, rearing durations, and combinations of plant-based materials is recommended to broaden its applicability in field settings.