The increase in temperature during the composting process was caused by the heat generated from the respiration and decomposition of sugar, starch and protein by the population of microorganisms. The increment in temperature is a good indicator that there is microbial activity in the compost pile, as a higher temperature denotes greater microbial activity . The temperature pattern showed that there is a rapid progress from the initial mesophilic phase to the thermophilic phase for both these treatments, which points to a high proportion of readily degradable substances, e.g. vegetables and fruits (contained in a green waste material). According to Haug , the composting temperature has to be above 55°C for three consecutive days to kill the pathogen. Although both the compost pile treatments met the requirement, pile C1 has the longest period of time above 55°C, that is, for 6 consecutive days, while C2 only remained for three consecutive days. Even though no microbial test was done on the sample, it can be assumed that the compost treatment with EM (C1) has high microbial activity compared to C2 based on the high temperature achieved.
The declining TOC value, which was 49% for C1 and 36% for C2, is similar to that reported by Benito et al.. According to Tumuhairwe et al., large TOC losses suggest pronounced microbial activity in the former. The t-test indicated that the EM application had a significant difference (p < 0.05) on the compost, which means that treatment C1 lost more carbon (C) than C2 during the composting process. Rice straw is a material that consists of high C content. Diaz et al. reported that during composting, C is a source of energy for microorganisms to build up cells. Almost all of the C is absorbed by the microorganisms and transformed to CO2 during the metabolism process of the cells. The left over C will be changed into membrane and protoplasm form. Throughout the composting process this organic matter is decomposed by microorganisms through which the organic carbon will be oxidized in aerobic condition to CO2 gas to the atmosphere and thus lower the C/N ratio.
The increase in the total nitrogen (N) contents at the end of the composting process differs from the values obtained by Tognetti et al. who found that total N decreases overtime. According to Viel et al. the increase in total N may be due to the dry mass net loss as the loss of organic C as CO2 during composting. In addition, the N values might also increase due to the nitrogen-fixing bacteria activity that commonly occurs at the end of composting . Although a decrease of N can occur due to leaching of NO−3-N and ammonia volatilization, in this experiment, both piles were covered with plastic to retain the moisture and avoid moisture from outside, which could lead to the result obtained. Moreover, in high technology composting, where they can control the leaching problem, the resultant compost achieves an increment in total N [23, 24]. The dependent pair t-test done on the treatments showed that the application of EM also had a significant (P < 0.05) positive effect on the net mineralization of N. The increase in the N value at the end of the composting period might occur due to the usage of N by microorganisms to build up cells, thus reducing the N, and some of the organisms will eventually die, which will be recycled as N and thus contribute to the increase . The increased amount of N at the end of composting is due to this stored source of N.
The phosphorus (P) content decreases in both treatments where the P value in the C1 compost dropped by 4.3% while C2 dropped by 19%. These results are in accordance with other previous reports [26–28]. According to Tumuhairwe et al., the loss of P during the composting process is possibly due to the leaching of P in the soluble organic solute. In contrast, there are other reports that obtain an increase in P value [29–32]. From these studies, there is a significant (P < 0.05) difference between the two treatments. Although both piles decrease in P value, the final output of C1 treatment has the potential to produce compost with a P value higher than the C2 treatment.
K is known as the element that is easily leached out . However, in this study the K value increased to 55% in the C1 compost compared to 17% in C2, which shows that the compost with EM are significantly (P < 0.05) higher than without EM. The use of rice straw as a medium that can absorb moisture and maintain its structural integrity and porosity, might avoid the loss of K in compost . Both treatments (C1 = 1.7; C2 = 1.4) pass the recommended value of K in compost, which is 1%. K plays an important role in plant growth where its function is to increase the elongation of the root, control ion balance, improve protein synthesis, encourage enzyme reaction, and improve the photosynthesis process and food development .
The accumulation of heavy metal (Zn and Fe) increases during composting, except for Cu whereas the values decrease proportional to the time. The increase of Zn and Fe are in line with previous research done by Paré et al.. According to both reports, during composting the accumulation of nutrient and heavy metals significantly increased, which indicates the maturity of the product. Pare et al. also concluded that the stability of biosolid compost can be correlated with the accumulation of heavy metals and nutrients, and, thus extractability and exchangeability of heavy metals.
In both treatments, the final Zn value increased but varied between the initial days until the end. A small increment was achieved by these two treatments, which are only 2% for C1 and 6% for C2. However, both composts are in the safe limit set by the European Standard  for heavy metals in compost, that is, in the range of 210–4000 mg kg−1. The decrease in Cu values in this study is similar to the result obtained by Md Sabiani . Both treatments show that the decline of Cu is proportional to the days of composting, however, for the C2 treatment, there is an increase for day 45 before it decreases again, which differs from pile C1where the Cu increases a bit at the end of the composting period. The Fe values increased in both treatments, and it was found that the Fe contained in C1 was much higher with a range of 560–2624 mg kg−1 during the composting process. This could occur due to the longer exposure to high temperature above 55°C for C1 compared to C2. The high temperature increases the loss in moisture content and C in C1 whereas in C2, the Fe accumulation is low with the range of 650–2379 mg kg−1. The low temperature achieved during the composting process compared to C1 contributed to the low accumulation of Fe. This happened due to the final resultant compost still being high in moisture content, which encourages the leaching process to occur, and, thus, reduces the Fe value. This theory correlates with the finding of Md Sabini  in which compost that loses its moisture content and C accumulate more Fe. The Fe also increases when there is a reduction of substrate pH in the early composting period, which increases the solubility of metals, including metallic microelements . The t-test conducted on the heavy metals showed that there is no significant difference between the treatment with EM and without EM on the value of heavy metals except for Fe. The compost applied with EM shows a significant (P < 0.05) difference in Fe accumulation.
Compost usually contains heavy metals based on their initial raw material. Generally, these heavy metals (micronutrients) are required by the plant for perfect growth. The heavy metals present in the compost, such as Zn, Cu and Fe, are absorbed by plants during the fertilizing process. In a small quantity, these trace elements are necessary for plant growth but in large quantities they can cause phytotoxicity . Kabata-Pendias and Pendias  also described in their research that Zn, Cu, Mn and Fe are useful as trace elements for crop growth. However, constant and intense application of organic compost containing heavy metals will lead to accumulation in the soil and an increase in toxic levels.