3.2. Physio-Chemical Characteristics
The mean variation of physio-chemical parameters over time for the four runs is presented in
Table 2. Differences in physio-chemical properties were significant (
p < 0.05) between weeks of measurement but not significant (
p > 0.05) between runs. There was also no significant (
p > 0.05) interaction between runs and weeks of measurement. The initial characteristics of cattle manure added to the vermicomposting units are presented at week 0 while those of the harvested vermicompost are presented at week 12. From
Table 2, it can be observed that pH, TOC, VS, and C/N ratio significantly decreased (
p < 0.05) while TS, ash content, TKN, TP, and TK significantly increased (
p < 0.05) over time.
The overall decrease in pH was about 20% which is more than the 15% and 14% reduction reported by Yadav et al. [
37] and Bhat et al. [
48], respectively, during vermicomposting of cow dung. A fall in pH during vermicomposting may be attributed to mineralization of phosphorus and nitrogen compounds and production of fulvic and humic acids [
47].
The overall increase in TS content was about 41%. This is in agreement with Lalander et al. [
35] who reported a 50% increase in TS content during vermicomposting of cattle manure and food waste. An increasing TS content (reduction of moisture) is indicative of the progress in composting [
49], which may be attributed to a high consumption rate of cattle manure by earthworms, thereby making it friable [
50].
There was a 20% total decrease in VS content from week 0 to week 12. This decrease is lower than the 58% reported by Lalander et al. [
35] during vermicomposting of cow manure and the 33% reported by Varma et al. [
51] during vermicomposting of water hyacinth and cow manure. A reduction in VS is attributed to the bioconversion of waste material by earthworms, and it indicates the degree of maturity of the product [
47]. This thus shows that the harvested vermicompost for this study was matured.
The overall decrease in TOC was about 41%. This was higher than the 36% and 21% reduction reported by Yadav et al. [
37] and Xie et al. [
52], respectively, during vermicomposting of cow dung. The decrease in the amount of carbon may be attributed to its role as an energy source for both earthworms and microorganisms [
53].
TKN in vermicompost was about 48% more than that of initial cattle manure (
Table 2). This was comparable to the 49% increase reported by Bhat et al. [
48] but lower than the 237% reported by Yadav et al. [
37] during vermicomposting of cattle dung. Increase in total nitrogen content of vermicompost may be attributed to earthworm activity, such as the addition of mucus, growth-stimulating hormones, nitrogenous excretory substances, and microbe mediated transformations [
48]. The further increase in nitrogen content in the last weeks of the experiment (
Table 2) could be attributed to decomposing earthworms as proposed by Atiyeh et al. [
54]. Furthermore, an increase in nitrogen content over time may also be attributed to a decreasing pH since nitrogen is easily lost as volatile ammonia at high pH values [
55].
This study established an overall C/N ratio reduction of about 60%. This is lower than the 81% but greater than the 35% and 25% reduction reported by Yadav et al. [
37], Taeporamaysamai and Ratanatamskul [
56], and Xie et al. [
52], respectively during vermicomposting of cow dung. A fall in C/N is associated with a loss of carbon as well as an increase in TKN. C/N ratio is an indicator for the maturity of compost/vermicompost [
57] and usually values <12 signify compost maturity and stabilization [
58]. Hence, it is indicated from the C/N ratio of 10 obtained in week 12 that the vermicompost at the end of the experiments reached full maturity (
Table 2).
TP registered an overall increase of about 120%. This is more than the 83% and 99% increase reported by Bhat et al. [
48] and Yadav and Garg [
12], respectively, during vermicomposting of cow dung. The increase in TP may be attributed to the presence of earthworm gut phosphatase, and phosphorous solubilizing microorganisms in the worm casts that enhance the release of phosphorus in various forms [
59]. Ghosh et al. [
60] reported the presence of phytase enzymes in vermicompost that also enhances mineralization of phosphorus as time progresses. In addition, mineralization and mobilization of organic matter by the combined effect of microorganisms and fecal phosphate activity of earthworms probably increases TP content in the final vermicompost [
12]. These factors could thus have caused an overall increase in TP.
The overall increase in TK was about 47% which is greater than the 31% and 34% increase reported by Yadav and Garg [
12] and Lalander et al. [
35], respectively. The increase in TK could be attributed to the solubilizing of insoluble potassium as a result of acid production by microorganisms [
61]. TK content within vermicompost is also directly related to phosphatase enzymes present in the digestive systems of earthworms and bacteria [
62]. In addition, the gut of an earthworm has a big population of microflora which could enhance the release of potassium in vermicompost [
63]. These factors could thus have contributed to an overall increase in TK over time.
3.3. Gaseous Emissions
The change in the mean composition of gaseous emissions over the course of the experiment is shown in
Table 3. A two-way ANOVA indicated that there was no significant difference (
p > 0.05) in the composition between the different runs but a significant difference (
p < 0.05) in the different weeks of measurement. CO
2 increased by 1.1% from week 4 to week 8 and then decreased by 0.8% from week 8 to week 12. CH
4 was highest in week 4, decreased slightly by 0.06% in week 8, and further decreased by 0.06% in week 12. The concentration of N
2O was greatest in week 4, decreased in week 8 by 2.25 ppm, and was not detected in week 12. The same trend was observed for the emission factors (
Figure 3). The cumulative emission factors were 102 g CO
2 kg
−1 DM, 7.6 g CH
4 kg
−1 DM, and 40 mg N
2O kg
−1 DM.
Ammonia (NH
3) emissions were not detected during the measurement period for the four runs. This could be attributed to relatively low temperatures and pH throughout the experiment. According to Velasco-Velasco et al. [
26], ammonia volatilization is promoted by relatively high temperatures and pH, and this usually occurs in the first week of vermicomposting. High temperatures (>45 °C) and pH (>9) promote NH
3 volatilization due to an inhibition of nitrification [
22]. However, vermicomposting is a mesophilic process (<30 °C) implying that NH
3 volatilization is less likely to occur [
64]. The absence of NH
3 emissions in this study could also be associated with the high earthworm density in the vermicomposting units. In their experiment measuring gaseous emissions during vermifiltration of pig slurry, Robin et al. [
65] reported an absence of NH
3 emissions in their units, and this was attributed to the presence of a big earthworm population.
Emissions of methane (CH
4) decreased over time. The high CH
4 content in week 4 may be attributed to low oxygen concentrations resulting from the relatively high moisture content of the cattle manure substrate in the vermicomposting units compared to the other weeks with measurements (
Table 3). Low oxygen content and high moisture content cause anaerobic conditions in vermicomposting units, hence stimulate the development of methanogenic bacteria that produce CH
4 [
66,
67]. A reduction in the CH
4 concentration in week 8 and further in week 12 may be attributed to an increase in the earthworm population. The burrowing activity of earthworms increases aeration, which inhibits those anaerobic conditions in which CH
4 producing methanogens would thrive [
23]. Earthworms also fragment large substrate particles and hinder the development of anaerobic conditions that promote methane production [
20]. The cumulative CH
4 emissions measured in this study (7.6 g CH
4 kg
−1 DM) were greater than the 0.011 g CH
4 kg
−1 DM reported by Komakech et al. [
28] during vermicomposting of cow manure and food waste. This could be the result of higher uncertainty since measurements were performed only on a single day by Komakech et al. [
28].
Emissions of nitrous oxide (N
2O) decreased over time. This may be attributed to an increasing density of earthworms within the vermicomposting units. The burrowing action of these earthworms reduces anaerobic denitrification processes within the earthworm guts, hence reducing N
2O emissions [
27]. The decrease in emissions of N
2O by earthworms may also be as a result of the earthworms gut content being lower in available nitrogen than the fresh organic matter that was added [
65]. In addition, Wang et al. [
23] reported that the addition of earthworms in treatment units of duck manure reportedly decreased cumulative N
2O emissions by 7% when compared to units with no earthworms within a comparative period of 45 days. The cumulative N
2O emissions of this study were higher than the 7.5 mg kg
−1 DM [
22] but lower than 180 mg kg
−1 DM [
23] measured during vermicomposting of vegetable waste and duck manure, respectively. This could have been caused by the lower nitrogen content of cattle manure (1.4%) used in this study compared to that of duck manure (2.3%) as reported in Wang et al. [
23].
The low concentrations of CO
2 at the start of the experiment may be attributed to the then low rate of biological degradation of organic matter [
68]. After the initial decrease in CO
2 concentration, it increased. This was probably caused by earthworms facilitating the enzymatic and microbial degradation of organic matter resulting in the transformation of wastes into organic humic forms with a significant release of CO
2 [
67]. The decrease in CO
2 emissions in week 12 of this study may be attributed to stabilization of the compost as reported by Nada et al. [
25]. The cumulative CO
2 emissions obtained in this study (102 g CO
2 kg
−1 DM) were less than the 264 g CO
2 kg
−1 DM reported by Wang et al. [
23] during vermicomposting of duck manure. This might have been caused by the addition of red straw in the duck manure vermicomposting system which caused longer durations of higher temperatures, in turn, promoting duck manure decomposition [
23] and high CO
2 emissions.
From the cumulative emission factors (
Figure 3) and using the environmental factors of 1, 28, and 265 for CO
2, CH
4, and N
2O, respectively [
43], the study established a total GWP of 324 kg CO
2 eq t
−1 waste. From active composting of feedlot cattle manure, Hao et al. [
69] reported a GWP of 401.4 kg CO
2 eq t
−1 waste. During composting of straw bedded and wood-chip bedded cattle manure, Hao et al. [
70] reported GWP of 368.4 kg CO
2 eq t
−1 waste and 349.2 kg CO
2 eq t
−1 waste, respectively. On the other hand, GWP from dairy cattle manure composting, stockpiling, and anaerobic digestion (slurry) was 605.08 kg CO
2 eq t
−1 waste, 741.4 kg CO
2 eq t
−1 waste, and 605 kg CO
2 eq t
−1 waste, respectively, after three months of evaluation [
71]. The GWP from beef cattle manure management using the same management methods for the same evaluation period were 388.2 kg CO
2 eq t
−1 waste, 433.81 kg CO
2 eq t
−1 waste, and 372.18 kg CO
2 eq t
−1 waste, respectively [
71]. It can thus be concluded that vermicomposting is a better cattle manure management technology in terms of mitigating greenhouse gas emissions.
3.4. Material and Substance Flows
The MFA was based on the average mass flows (
Table 1) and average nutrient composition of cattle manure (week 0) and vermicompost (week 12) for the four runs as indicated in
Table 2. The nutrient composition of earthworms was derived from literature, as shown in
Table 4.
The MFA showed results with low uncertainties (< ± 15%) for all the flows except for N emissions into the atmosphere with an uncertainty of 25.6%. A total wet weight (ww) of 384 kg of materials (cattle manure, water, and earthworm biomass) entered into the vermicomposting unit of which 46% was transformed into vermicompost, 2% into earthworm biomass, and 52% was lost to the atmosphere in the form of various gases. Through data reconciliation and error propagation performed by the software STAN based on all the inputs of materials and substances as well as their outputs, the predicted mean output of vermicompost was about 178 kg as opposed to a mean of 168 kg of the four runs (6% increase). This may be attributed to precision errors by the weighing scales used in the field. On a dry weight basis, 75.33 kg of total solids entered into the vermicomposting system of which 72% was transformed into vermicompost, 28% lost to the atmosphere, and 0.2% was transformed to earthworm biomass. About 89% of weight was lost to the atmosphere in the form of water vapor. The flows of nutrients within the vermicomposting systems are shown in
Table 5. Inputs are shown as total kilograms/year and transferred in percentage to the outputs.
The calculated N loss of 18.18% to atmosphere is less than the 22.2% reported by Nigussie et al. [
22] during vermicomposting of cattle manure and vegetable waste. However, the calculated C loss of 68.49% is greater than the 45.5% reported by Nigussie et al. [
22]. Of the C losses, 30% was in the form of CO
2, and 2.2% was lost as CH
4. Of the N losses, only 0.28% was lost as N
2O. However, it is assumed that most of the lost nitrogen to the atmosphere is in the form of N
2 with no impact on the climate [
30,
72]. The C and N losses that are not accounted for may be attributed to the difficult collection of homogeneous and representative samples for gaseous emissions over a long period [
73]. There is also the possibility that gaseous emissions were emitted before the start of the monitoring campaign and in-between the sampling. Furthermore, Whittaker et al. [
74] recently reported that the static chamber technique may not completely account for spatial and temporal gaseous variations which may lead to an underestimation of emission rates. Possibly C and N were also lost via volatile compounds [
72].
The nitrogen loss uncertainty (±25.6%) is less than that reported by Jensen et al. [
34] of 126% uncertainty for N emission to the atmosphere during combined dry anaerobic digestion and post-composting of household wastes. This high uncertainty was associated with few and uncertain measurements of N emissions. Hence, they suggested that frequent emission measurements onsite would reduce these uncertainties. When compared to other manure management methods (
Table 6), vermicomposting proved to enable high retention of nutrients. Higher losses of carbon to the atmosphere in this study (68.52%) are most likely due to easily degradable initial C content in cattle manure.