Abstract
Vermicomposting is a process by which the organic waste is converted into manure with the help of earthworms. Growth rate, onset of maturity (clitellum development), rate of reproduction (cocoon production) and population buildup of earthworm during vermicomposting have been depend upon the conditions like temperature, moisture and physico-chemical properties of the feed mixtures. Eisenia fetida was superior to other epigeic species and tolerate wide range of temperature, moisture and pH. Endogeic species produced lesser cocoon than epigeic species and cocoon production decreased at low temperature. Maintenance of temperature and moisture content is the critical step for vermicomposting. Growth and maturation of earthworms was best at 20–25°C temperature with 80–85% moisture content. Increase temperature upto 30°C accelerated growth rate of earthworms and lessened the time to sexual maturity. Earthworms can survive in the soil contaminated with heavy metals by accumulating heavy metals in their tissues.
Keywords
- earthworm
- growth rate
- physico-chemical properties
- heavy metals
- vermicomposting
1. Introduction
Over the past few years disposal and management of organic solid wastes has become more problematic and rigorous due to rapidly increasing population, intensive agriculture and industrialization. Aristotle, the Greek philosopher, documented very early the importance of earthworm in the ecosystem, and called them “intestines of the earth”. After that, Darwin [1] highlighted the role of earthworm in breakdown of animal matter as well as dead plant. Transformation of organic waste through vermicomposting is of multifold interest as along with checking pollution of the environment, it results in production of a rich, more stable and homogenous product as compared to composting [2, 3]. Vermicompost acts as a buffer, has a significantly lower volatile solid content and high N, P, K content in the plant available form [4, 5].
Out of the 3200 species of earthworm reported from the world over, 509 species belonging to 67 genera are known to occur in the Indian subcontinent and about 374 species have been reported from different habitats of India alone [6, 7]. Certain epigeic earthworms, which have natural ability to colonize organic waste; show tolerance to wide range of environmental factors; short life cycle; high reproductive rates; high rates of feedstock consumption, digest and assimilate organic matter shows good potential for vermicomposting [8]. Few surface feeders earthworms species which contain all these characteristics and widely used in vermicomposting of a wide variety of organic wastes are
2. Suitable species of earthworm used for vermicomposting
Epigeics species are useful for biosolid waste management as these worms can hasten the composting process to a significant extent and produce better quality of vermicomposts, compared with those prepared through traditional methods [11].
Earthworm species
3. Effect of temperature and moisture on earthworm
Reinecke and Kried [23] and Reinecke and Venter [24] concluded that
Edwards et al. [28] studied the life cycle of
Dominguez et al. [17] made an observation on the biology and population dynamics of
Bhattacharjee and Chaudhuri [9] studied the reproductive biology of seven Indian species of earthworm, viz.
Elvira et al. [22] studied the growth rate and reproduction of the epigeic species
Soobhany et al. [19] resulted that moisture content within the feed mixture decreased with an increase in temperature by employing
4. Growth rate of earthworm
The earthworms grow best in easily metabolizable organic matter and non-assimilated carbohydrates; these also favor their reproduction [31]. There was a positive correlation between Growth and reproduction with volatilable solid content of the waste [29]. Earthworm growth slows down when C: N ratio and temperature is high [32, 33]. The biomass gain by
Reinecke and Viljoen [44] observed that cocoon production by
5. Changes in physico-chemical quality of the feed wastes during vermicomposting
The physio-chemical composition of the vermicompost is known to be influenced by the different kind of feed given to the animal, bedding material used and the way the waste is collected, stored and handled before utilization [56]. A detailed review of various changes in physico-chemical parameters of feed material during vermicomposting is given in the following section.
5.1. pH and electrical conductivity (EC)
The differences in the pH of vermicompost are directly dependent on the type of raw materials used for vermicomposting. Different substrates used for vermicomposting resulted in different types of intermediates products which shows a different behavior in pH shift. The neutral pH throughout the vermicomposting is ideal for the growth of earthworm [57]. The occurrence of acidic environment may be attributed to the bioconversion of organic acids or higher mineralization of the nitrogen and Phosphorus into nitrites /nitrates and orthophosphate, respectively [42, 58, 59, 60]. The pH of cow dung and sheep manure vermicompost came out to be 8.48 and 8.6 [60], cattle manure had a pH of 6.0–6.7 [61, 62], pig manure had a pH of 5.3–5.7 [63, 64] and the one derived from sewage sludge had a pH of 7.2 [65]. The lower pH of the final vermicomposts might be due to production of CO2 and organic acids by microbes during the process of bioconversion of different substrates in the feed given to earthworms [66, 67]. The decline in pH might be due to reduction in quantity of different types of volatile solids and to the growth of earthworm’s biomass. The larger the increase in biomass growth, there was greater the reduction in volatile solids and hence the more shift toward the acidic condition [68, 69]. A decrease in pH might be an key factor in nitrogen retention as this element is lost as volatile ammonia at higher pH value. The lower pH was due to production of fulvic acid and humic acid during decomposition [70].
The change of mesophilic to thermophilic condition changes pH from acidic to alkaline due to conversion of organic –N- to NH4+ [71, 72, 73, 74]. Rynk et al. [75] suggested that the excess of organic nitrogen not required by microbes was released as ammonia which got dissolved in water and increased the pH of the vermicompost. Datar et al. [76], Singh et al. [77], Goswami et al. [78], Huang et al. [79] and Lleo et al. [80] also reported an increase in pH during vermicomposting of solid waste, beverage biosludge, tea factory coal ash, fruit & vegetable waste and home waste respectively. They asserted that an increase in pH during composting and vermicomposting process was due to progressive utilization of organic acids and an increase in mineral constituents of the waste. On the other hand Song et al. [67] and Ravindran et al. [81] observed decrease in pH during vermicomposting of fermented tannery waste and animal manure spiked with mushroom residue respectively. They attributed that production of CO2, organic acids and joint action of earthworms and microbes lead to low pH of the vermicompost.
Electric conductivity (EC) is a good indicator of the suitability and safety of vermicompost [82]. The reports regarding electrical conductivity during vermicomposting process are contradictory, some workers reported decrease in electrical conductivity [77, 83, 84, 85] and others an increase in electrical conductivity [67, 69, 86, 87]. The decrease in pH might be due to decrease in ions after forming a complex, whereas the increase in pH might be due to the degradation of organic matter to release various types of cations of different mineral salts in available forms such as phosphate, ammonium and potassium [88, 89] or may be due to loss of organic matter [16].
5.2. Nitrogen
Earthworms may influence microbial N transformation such as mineralization, nitrification and denitrification through their interaction with soil biota and increase concentration of ammonia in the fresh vermicasts [90]. Nitrogen generally declines during aerobic composting due to use of nitrogen by the rapidly multiplying heterotrophic bacteria but it increases during vermicomposting [69, 77, 91]. Chaudhuri et al. [66] reported the decrease in potassium and nitrogen content during the vermicomposting of kitchen waste with the help of
Whalen et al. [106] found that microbial biomass was responsible for maximum of nitrogen released from decomposing earthworm tissue. Whalen et al. [107] observed that juvenile of
5.3. Organic carbon and C:N ratio
The C:N ratio is one of the most common indicator used for estimating compost maturity [111]. A decline in C: N ratio <20 indicates an advanced degree of organic matter stabilization and reflects a satisfactory degree of maturity of organic wastes but a C:N ratio ≤15 is preferred for agronomic application [10, 112]. According to Song et al. [67], C:N ration <12 indicated that vermicompost had the preferable properties for field application. Speratti and Whalen [113] observed that mean N2O and CO2 fluxes during the study period tended to be greater from enclosures with added earthworms than the control (no earthworms added), but were non-significantly different due to the low survival rate of introduced earthworms. Better control of earthworm populations in the field is required to fully assess the impact of earthworms on CO2 and N2O fluxes from temperate agro-ecosystems. Similar results was also reported by Tognetti et al. [114] and observed that the rate of CO2 production from vermicompost was much higher as compare to traditional compost. Cabrera et al. [115] reported faster decline in C:N ratio during vermicomposting as compared to compost without earthworm.. However, Atiyeh et al. [116] reported that the C:N ratio of the manure with or without earthworms decreased progressively.
The loss of organic carbon may be mainly due to high CO2 emission via strengthened carbon mineralization due to respiratory activity of earthworms and microorganism [117] which cause faster reduction in carbon and lowering of C:N ratio during vermicomposting. The total organic carbon reduction ranged from 10 to 45% during vermicomposting of organic waste [118] while Singh et al. [82] observed increased in organic carbon content from soil to vermicast. The C:N ratio of vermicompost reduced to 12–17:1 from 21–69:1 [11, 16, 55, 119, 120]. Saha et al. [121] and Pramanik [122] observed that decrease in C:N ratio attributed to an increase in earthworm abundance which leads to rapid decrease in organic carbon due to enhancement in organic matter oxidation. Aira and Dominguez [123] reported that rise in microbial biomass during vermicomposting increase carbon losses. Briones et al. [124] suggested that calciferous organs of worms provided a mechanism of CO2 regulation and both environmental and metabolic CO2 could be fixed by this organ.
5.4. Phosphorus
Phosphorus is an important nutrient for growth of plants and is used for protein formation, metabolism, photosynthesis, seed germination and flower and fruit formation [125]. However, phosphorus in soil is in mineral form which was readily available for plants but the potential activity of earthworm and phosphate solublising microorganisms increases phosphorus availability for plants [120, 126].
Gomez et al. [85]; Pramanik [122]; Lim et al. [127]; Singh et al. [128]; Hanc and Chadimova [129] asserted that the rise in total Phosphorus during vermicomposting was probably due to mineralization and mobilization of phosphorus as a result of bacterial and fecal phosphatase activity of earthworms. When organic matter passed through the earthworm gut, some amount of phosphorus is converted into more available form due to enzyme phosphatase and further release of might be attributed to the phosphorus solublizing microorganisms present in the cast [20]. In 1999, Patron et al. [130] noted that earthworm activity accelerated transformation of organic Phosphorus to plant available phosphorus form. Lim et al. [127] and Bayon and Binet [131] observed an increase of phosphorus by 25% and 2.4–49.5% by employing E. fetida and
According to Kaviraj and Sharma [16], organic matter decomposition by microbes resulted into acid production which is the major mechanism for solubilization of insoluble phosphorus and potassium. Therefore, presence of a huge number of gut microbes in earthworm might play an important role in increasing phosphorus content in the vermicompost. Mba [133] and Wan and Wong [134] studied the effects of
5.5. Potassium
There are contradictory reports regarding the total potassium content in vermicomposts obtained from different substrates due to the differences in the chemical nature of the initial raw materials [135]. Song et al., [67], Gomez et al. [85], Benitez et al. [110], Lim et al. [127] and Bhat et al. [136] have reported higher potassium concentrations during vermicomposting process. Increase in potassium content in vermicompost suggested that earthworms have symbiotic gut microflora with secreted mucus and water to degrade the ingested substrate which cause release of easily assailable metabolites [89]. Garg et al. [83], Singh et al. [128] and Elvira et al. [137] reported that total potassium concentration decrease in vermicompost. This decrease in concentration of potassium may attributed to the variation in chemical composition of initial feed mixture or due to leaching of potassium because of low water holding capacity of the vermicompost [128].
Guerra-Rodriguez et al. [72], Delgado et al. [138] and Suthar [139] revealed that mineralization process significantly enhanced the concentration of exchangeable potassium during vermicomposting. Suthar [20] and Nahrul Hayawin et al. [96] also reported higher potassium content in the vermicompost produced from distillery sludge, oil farm waste and food industrial sludge respectively. Lim et al. [127] observed 15–121.4% increase in potassium content by using
5.6. Bioaccumulation of heavy metals and its effect on earthworms
The increasing exploitation of natural resources by human beings during the past few centuries has adversely affected the global balance of heavy metals causing a gradual increase in the concentration of metals in the soil ecosystem [140]. In order to maintain the environmentally sound soil quality, investigators are seeking methods to reduce the mobility of heavy metals from wastes to soil ecosystem. Metal mobility and availability can be reduced by raising the soil pH [141]. Phyto-remediation is known as the most viable and environment friendly technology. But, a limited number of plants have been found to have phyto-accumulation ability and a very less number can be used for field phyto-remediation because of low biomass production. Therefore, earthworms appears to be a valuable substitute for control of metals in contaminated soils [142]. According to Hopkin [143], the earthworms have capacity to control metals, particularly trace metals, such as Cu and Zn, in their bodies. Earthworms can also be used as bioindicators for assessing the level of soil contamination with agricultural runoff, heavy metals, acid rain, pesticides etc. [144].
The capability of earthworms to mitigate the heavy metal toxicity and to increase the nutrient profile of organic wastes might be useful in sustainable land restoration practices [20]. Heavy metals have the capability to bind with ligands of the tissues and thus leads to their bioaccumulation in the food chain [145]. A positive correlation between metal concentrations in the earthworms and those in the soils were observed with differences in bioaccumulation factors for different metals, this could be due to a variable metabolic requirement of earthworms for metals [146]. The effects of sub lethal concentrations of lead nitrate on reproduction and growth of
Earthworms are have the capability to inhabit and survive in sites contaminated with metals [148] and have the ability to accumulate heavy metals in the cells of yellow tissue [149]. Earthworm populations may develop mechanism by which they can tolerate or resist the effect of metal induced stress. Such tolerance or resistance acquired by earthworms either through a variation in their genetic structure or reversible changes in an earthworm’s physiology. Toxicity tests done by various authors have shown that heavy metal pollution negatively affects life-history of earthworms such as growth, reproduction and survival [150]. Beyer et al. [151] studied the bioaccumulation of methyl-mercury in the
Maenpaa et al. [155] showed that the treatment of high Phosphorus significantly reduced lead, zinc and cadmium bioavailability to the earthworm which was due to formation of metal–phosphate complex in the soils. This amendment reduce ecological risk to soil-inhabiting invertebrates exposed to heavy metal contaminated soils. Malley et al. [156] reported that earthworm act as an indicator for heavy metals toxicity that are present in the materials and are bioconverted, giving an indication of potential environmental hazard. The capacity of earthworm to uptake and redistribute heavy metals in their body lead to a balance between uptake and excretion which helps them to survive in metal contaminated soil. Kızılkaya [157] observed that the earthworm
Udovic and Lestan [160] reported that bioavailability of Pb and Zn before and after soil leaching with EDTA with two earthworm species,
Song et al. [67] conducted a pilot scale trial to investigate the response of heavy metals and nutrients changes to composting animal manure spiked with mushroom residues with and without earthworms. They resulted that composting without earthworm have high concentration of heavy metals, that is, As, Pb, Cu, Zn, while that in vermicompost concentration of heavy metals decreased significantly relative to the compost. The decrease of metals concentrations in the vermicompost occurred for at least two reasons. First, vermicompost processed by earthworms had high level of humic acid which posed a stronger sorption effect on formation of stable metal humus complex especially for Cu and Zn [167]. Second, bioaccumulation of heavy metals by earthworms tissues with the help of epithelial layer and body fluids [168]. Singh et al. [82] and Kharrazi et al. [120] also observed decrease in concentration of heavy metals in the final vermicompost material. Soobhany et al [19] concluded that the reduction in toxic heavy metals by inoculating earthworm in the organic waste might be helpful in gaining clean environment.
6. Conclusion
Growth rate of earthworm, clitellum development, cocoon production and population buildup of earthworm were depend upon the physico-chemical composition of the feeding materials, types of feed mixture and environmental conditions like temperature, moisture and pH determine the sexual maturation in earthworms. Out of the various species of earthworms,
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