Visual observation in terms of texture, color, odor, and microbial evolution during the whole period of composting revealed that physical characteristics of all compost piles were similar during the first week of composting. Insignificant physical changes were noticeable except that an appearance of white fungi was observed from day 1 of composting. During the second and third week of composting, composting materials were of smaller sizes with brownish color given that white fungi could still be visually observed. From the fourth week of composting onwards, qualities of coir pith compost prepared conformed well to those of the organic compost standard specified by the Department of Agriculture of Thailand (2005) in that compost became loose, easy to pulverize, and of blackish brown color with humus-like fragrance.
Composting process performance
Temperature evolution directly reflects microbial activities during composting Golueke (1991) which may be considered as a good indicator of the bio-oxidative phase. In this study, the bio-oxidative phase could also be classified into two phases: the thermophilic (70°C to 40°C) and mesophilic (40°C to 25°C) phases Kuo et al. (2004) which are typically reported in the literature as lasting at 11 and 23 days, 9 and 25 days, and 11 and 23 days, respectively, for treatments 1, 2, and 3. It is evident from Figure 1 that the temperature profiles for the three treatments were of similar pattern. Further, a rapid increase in temperature was noticeable since day 1 post-composting, reaching 56°C to 58°C, the thermophilic temperatures at eight, fourth, and fifth days of composting, respectively, and maintained for 9 to 11 days. Results found are in line with that recommended by the USEPA in that temperature of the compost pile should be higher than 40°C and maintained for at least 5 days in order to ensure elimination of pathogenic microorganims USEPA (1993). After the thermophilic stage, temperatures of all treatments rapidly declined until day 14 post-composting and then gradually decreased to that of the ambient temperature. Subsequently, from day 21 onwards, temperature remained at ambient until the end of composting process indicating maturation phase providing that composting materials whose color were more intense became loose in character (Diaz and Savage 2007).
An increase in temperature observed in all treatments on day 8 may possibly be due to a result of delayed microbial growth stemming from pile turning and moisture content adjustment on day 7. Generally, pile turning could reactivate the composting process due to an increase in oxygen availability to microorganisms present during composting Cayuela et al. (2006). From day 8 onwards, temperature of treatment 2 declined more rapidly than those of treatments 1 and 3. Results further suggested that, for 25:1 C/N formulation, high microbial activities occurred leading subsequently to rapid degradation of organic matters of the compost. The results are in line with that of Zhu 2007 who investigated the effect of low initial C/N ratio on composting of wheat straw with pig manure and reported that formulation containing 25:1 C/N ratio exhibited a more rapid decrease in temperature than pile formulated with the 20:1 C/N ratio.
As is obvious from Figure 2, the initial C/N ratios affected pH changes during the thermophilic phase insignificantly during the first 2 weeks of composting process (p > 0.05). For all three compost preparation, pH increased gradually from 4.8 to 5.4 on day 1 to 7.3 to 8.1 on day 7 post-composting. After 2 weeks of composting, the pH values of all composts prepared gradually declined from 8.0 to 8.3 to 6.9 to 7.3. Additionally, the pH values of all treatment piles remained rather constant (p > 0.05) since day 35 post-composting. At the end of the process (day 120), the pH values of all treatments were not significantly different (p > 0.05) which abided by the Thai standard of organic compost Department of Agriculture of Thailand (2005).
An increase in pH of all compost piles at the commencement of composting, especially during the thermophilic phase, was due likely to the metabolic degradation of organic acids Satisha and Devarajan (2007) and the ammonification process taking place during organic matter degradation Mahimairaja et al. (1994). After the 14th day, the pH values of all piles were significantly declined which is likely to be a consequence of phenolic compound production and organic acid production during biodegradation of lignocellulose in the compost piles Satisha and Devarajan (2007).
The C/N ratio is usually employed to indicate the maturity degree of compost Bernal et al. (1998). A decrease in C/N ratio implies an increase in the degree of humification of organic matter. As can be observed from Figure 3, the C/N ratio of the three piles decreased substantially until 21 days of post-composting and became comparatively stable. The compost piles containing 30:1 and 25:1 C/N ratio yielded a more rapid decrease in C/N ratio than that observed with the 20 C/N ratio formulation. Such evolution coincided well with the significant reduction in organic matter (Figures 4 and 5) indicating a rapid biodegradation rate.
Organic matter degradation
The evolution of organic matter during composting of coir pith with different cow manure concentrations is shown in Figure 4. For treatments 1, 2, and 3, organic matter declined from 75.8%, 84.5%, and 80.1% at day 1 to 68.1%, 64.8%, and 63.7%, respectively, at the end of the process (day 120). The decrease in organic matter indicated the biodegradation within the compost piles.
n general, OM is oxidized to CO2 and H2O including microbial biomass during the aerobic composting process. Therefore, the rate of organic matter loss reflects the overall composting rate Diaz and Savage (2007). As is shown in Figure 5, OM degradation of the three compost piles during the initial phase of composting was more rapid than that of the latter composting stage, from day 14 onwards. The OM degradation for an entire composting process, expressed in terms of OM loss, fitted well to the first-order kinetic (p < 0.05) of the form Paredes et al. (2000):
where A, k, and t are maximum OM degradation (%), degradation rate constant (day−1), and composting time (days), respectively.
The high reaction rate constant (k), ranging from 0.08 day−1 (C/N ratio of 20) to 0.31 day−1 (C/N ratio of 25) indicated the rapid biodegradation of the composting materials, coir pith and cow manure. Formulation containing C/N ratio of 25 (pile 2) appeared to degrade the most rapid, as demonstrated by the highest values of both k (0.31 day−1) and the product A × k (18.4% day−1) indicating an extent of material degraded per time (Table 2). For treatment 2, 57.5% of the initial OM was mineralized during the first 14 days of composting given further that by the end of composting 66.1% OM loss was attained (Figure 5). When initial C/N ratio was either higher or lower than 25, namely, 20 and 30 in this study, the total OM losses during composting process were much lower; in other words, approximately 31.8% to 56.4% of the initial OM was mineralized by the end of the composting process.
Total nitrogen and nitrogen losses
Table 3 shows total nitrogen (TN) changes observed for treatments 1, 2, and 3 during composting. The TN of all treatments tended to increase from 1.27%, 1.79%, and 1.90% on the first day of composting to 1.80%, 2.35%, and 2.00%, respectively, at the end of process (120 days). The increase of total nitrogen during composting process might be caused by the weight loss of the compost piles during composting process Dias et al. (2010).
In general, the TN loss was controlled by the initial C/N ratio Goyal et al. (2005; Dias et al. (2010) as well as the OM degradation Barrington et al. (2002). For all three compost treatments, TN losses conformed satisfactorily to the first-order degradation kinetic Paredes et al. (2000) with coefficients of determination (R2) of greater than 90% (Table 2) except for nitrogen loss of the treatment containing initial C/N ratio of 30. TN losses during composting process were approximately −7.3%, 41.9%, and 43.2%, respectively, for piles 1, 2, and 3. Although the TN losses of treatments 2 and 3 were apparently higher than that of treatment 1, TN content in the final compost of treatment 2 was significantly higher than that of treatment 1 (p < 0.05).
The lowest nitrogen loss was observed for treatment 1 whose C/N ratio was 30, due to the high initial C/N ratio of composting mixtures which favored the microbial growth and, at the same time, enhanced immobilization of inorganic nitrogen Liang et al. (2006), therefore, reducing TN losses in the form of N-NH3 Dias et al. (2010). In contrast, high nitrogen losses were evident for both treatments 2 and 3, approximately 41.9% to 43.2% of the initial N (Figure 5), suggesting higher loss of nitrogen as ammonia under the low initial C/N ratio Tiquia and Tam (2000). High organic matter degradation generally leads to high TN losses Dias et al. (2010) due possibly to high degree of mineralization through high ammonia volatilization and, in turn, regulates the concentrations of NH4+ available in the compost pile. The degrees of TN loss found for both treatments 2 and 3 were similar to those obtained by Dewes (Martin and 1992) who reported that 37% to 54% of the initial TN was lost during the co-composting of rice straw and cattle manure under the low initial C/N ratios ranging from 12.9 to 14.4. Further, Barrington et al. (2002) also reported that approximately 85% of initial TN available was degraded, whereas 70% of C was lost during immobilization process under low initial C/N ratio composting of approximately 20.
Compost maturity and quality
Several indices have been employed to assess the state of composting including color, odor, temperature, and C/N ratio Zmora-Nahum et al. (2005). According to the physico-chemical changes as well as compost qualities (Figures 1,3,5), compost prepared with initial C/N ratio of 25 reached maturity faster, approximately 1 week, than those prepared with initial C/N ratios of 20 and 30. By the end of composting, qualities of compost (Table 4) conformed well to that specified by the Thai standard for organic compost Department of Agriculture of Thailand (2005). Furthermore, the efficacy, in terms of maturity, of composting process using the natural aeration under low C/N ratio for co-composting of coir pith in this study was comparable to those reported by Ghosh et al. (2007) who composted coir pith fortified with edible oyster mushroom and urea in a multilayer heap fashion given that perforated PVC pipes were employed for the purpose of aeration. The authors found that the multilayered heap composting of coir pith led to compost maturity within 21 days. Results found in this study indicated that forced aeration during composting of coir pith was not necessary if coir pith was fortified with cow manure, rice bran, and coconut juice and operated under low C/N ratio; however, maturity was reached at 35 days post-composting.