Biol Fertil Soils (2009) 45:305–313 DOI 10.1007/s00374-008-0335-x

ORIGINAL PAPER

Physicochemical, including spectroscopic, and biological analyses during composting of green tea waste and rice bran Mohammad Ashik Iqbal Khan & Kihachi Ueno & Sakae Horimoto & Fuminori Komai & Kinji Tanaka & Yoshitaka Ono

Received: 13 May 2008 / Revised: 24 September 2008 / Accepted: 26 September 2008 / Published online: 15 October 2008 # Springer-Verlag 2008

Abstract The aims of this study were to monitor the changes in physicochemical, including spectroscopic, and biological characteristics during composting of green tea waste–rice bran compost (GRC) and to define parameters suitable for evaluating the stability of GRC. Compost pile temperature reflected the initiation and stabilization of the composting process. The pH, electrical conductivity, NO3−N content, and carbon-to-nitrogen ratio were measured as chemical properties of the compost. The color (CIELAB variables), humification index (the absorption ratio Q4/6 = A472 /A664 of 0.5 M NaOH extracts), absorption at 665 nm of acetone extracts, and Fourier-transform infrared (FT-IR) spectra were measured to evaluate the organic matter transformation; germination of komatsuna or tomato seeds was measured to assess the potential phytotoxicity of composting materials during composting. No single parameter was capable of giving substantial information on the composting process, the nutrient balance, phytotoxicity, and organic matter decomposition. The FT-IR spectra at 3,300, 2,930, 2,852, and 1,065 cm−1 provided information on the molecular transformation of GRC during composting and they decreased over the composting. Most of the assayed parameters showed no further change after about 90 days of composting suggesting that GRC can be used for agricultural purposes after this period. M. A. I. Khan : K. Ueno (*) : S. Horimoto : F. Komai : K. Tanaka : Y. Ono Faculty of Agriculture, Saga University, Kuboizumi, Saga 849-0903, Japan e-mail: [email protected] M. A. I. Khan : K. Ueno : F. Komai : K. Tanaka : Y. Ono The United Graduate School of Agricultural Sciences, Kagoshima University, Kagoshima, Japan

Keywords Green tea waste . Rice bran . Composting . Stability indices

Introduction The disposal of large quantities of agro-based industrial waste causes energy, economic, and environmental problems. However, since these wastes have a high content of organic matter and mineral elements, they can potentially be used to restore soil fertility. Composting is useful for waste recycling and produces a chemically stable material that can be used as a source of nutrients and for improving soil structure (Castaldi et al. 2005). During composting, most of the biodegradable organic compounds are broken down and a portion of the remaining organic material is converted into humic-like substances, with production of a chemically stabilized composted materials. The agricultural application of partially decomposed or unstable compost causes nitrogen immobilization and decreases the oxygen concentration around root systems due to the rapid activation of microbes. In addition, chemically unstable compost is phytotoxic due to the production of ammonia, ethylene oxide, and organic acids (Mathur et al. 1993; Tam and Tiquia 1994). Therefore, evaluation of compost stability prior to its use is essential for the recycling of organic waste in agricultural soils. Tea is one of the world’s most popular beverages, with more than three million tons of tea leaves produced annually. Twenty percent of this production is green tea (ITC 2006). After extraction of the green tea from the processed tea leaves, the remainder of the leaves is discarded as waste, and most of the waste is burned or dumped into landfills. Burning and damping of these waste management practices have a serious negative impact on

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the economy and the environment (Kondo et al. 2004). Rice is the staple food of half of the world’s population and rice bran accounts for 5–8% of rough rice. Rice bran contains oil, protein, vitamins, and essential minerals and helps to maintain a high temperature in a compost pile when it is mixed with other materials for composting (Khan et al. 2007a). Moreover, rice bran compost is potentially useful as weed control in organic farming systems (Khan et al. 2007b). Although several indices and methods have been proposed for evaluating compost stability, to date, there is no single method that can be used for organic-residuederived compost. The discrepancy between methods is related to the measure of widely different chemical characteristics of organic waste (Benito et al. 2003; Mondini et al. 2003). Here, the dynamics of green tea waste and rice bran composting were analyzed by different physicochemical parameters such as temperature, pH, electrical conductivity (EC), nitrate nitrogen (NO3−-N) concentration, the carbon-to-nitrogen (C/N) ratio, spectroscopic characteristics (CIELAB color space, UV-Vis, and Fourier-transform infrared (FT-IR) spectroscopy), and a biological parameter (phytotoxicity). These parameters have been reported as good indicators of the stability of compost from different sources (Benito et al. 2003). Particularly, FT-IR spectroscopy was utilized to provide a modern molecular-based analysis of the composting process. The effect of green tea waste–rice bran compost (GRC) on soil fertility and productivity has been previously established (Khan et al. 2007a). However, little information is known about the physicochemical, spectroscopic, or biological characteristics of GRC during composting. The green tea waste–rice bran in a 30:70 ratio (v/v, on a dry basis) was found to produce the best compost quality among those tested, including the ability to enhance spinach growth and the potential for weed control (Khan et al. 2007a, c). Therefore, in this study, GRC with a 30:70 ratio was used to evaluate the composting process.

Materials and methods Composting of green tea waste and rice bran Green tea waste and rice bran were collected from JA Beverage, Saga Co. Ltd., Japan and Field Science Center, Saga University, Japan, respectively. The compost was prepared by mixing green tea waste and rice bran in a 30:70 ratio (v/v, on a dry basis) in a 45-L plastic bucket (height 49.5 cm and diameter 42.7 cm, Belc, Risu Corporation, Japan). Compost was prepared in triplicate (three buckets maintained separately) and allowed to decompose for 246 days (starting from April 2006) in a glasshouse of the Field Science Center, Saga University, Japan (Khan et al.

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2007a). The moisture content was adjusted to 65–70% at the start of composting and moisture was adjusted to around 50% in later stages of the composting when the temperature had stabilized. The composting buckets were turned upside down once a week. Every 2 weeks, ten subsamples were taken from each bucket covering the profile (from top to bottom) and pooled together to give a composite sample which was air-dried, ground to ≤850-μm particle size, and kept frozen (−30°C) until analyzed. Physicochemical analyses The temperature of the compost was measured hourly at a depth of 15 cm from the surface of the compost using a Thermo Recorder (RT-12, ESPEC MIC Crop, Japan). The pH and EC of the samples in distilled water (1:10 w/v on a dry weight basis) were measured by a pH meter (SARTORIUS, Professional meter PP-20) and an EC meter (EC Testr 11+, Oakton instruments), respectively. The concentration of NO3−-N, organic C, and total N were determined using the cadmium reduction method (Hach Company 1995a), the Tyurin (1931) method, and a Kjeldahl method (Hach Company 1995b), respectively. Spectroscopic analyses The organic matter transformation during composting was evaluated using CIELAB color space, Ultraviolet-Visible (UV-Vis) spectroscopy, and FT-IR spectroscopy. Changes in compost color over composting were measured directly from the bottom of a glass Petri dish filled with ground oven-dried sample using a Minolta Color Reader, CR-13 (0° viewing angle and CIELAB color space). The color reader was initially calibrated with a clean empty Petri dish placed on a white tile. We used the following parameters L*, a*, b*, and ΔE*ab to evaluate changes in color. The symbol L* is known as “lightness” and its value extends from 0 (black) to 100 (white). The symbols a* and b* are indicators of color ranging from red through green, yellow, and blue. A high a* value means that the sample is more red and less green in color, and a high b* value means that the sample is more yellow and less blue in color. The color change of the compost over time (ΔE*ab) was calculated by subtracting the color values of the initial material from the color values of the compost on a specific day of composting and inserting these values into the following formula: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ΔE
ab ¼ ðΔL
Þ2 þðΔa
Þ2 þðΔb
Þ2 (CIE 1986). To determine the humification index (HI), the material (1 g) was placed in a 250-mL polyethylene flask and it was

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The phytotoxicity of the composting materials was evaluated by the germination of komatsuna (Brassica rapa var. pervidis) and tomato (Lycopersicon esculentum) seeds. One hundred grams of dried compost sample (dried overnight with forced air at 60°C) and 1 L of distilled water were mixed and shaken for 12 h at high speed (250 rpm) at 4±1°C; then, the mixture was filtered through two layers of filter paper (Advantec No. 5A). Twenty-five seeds of either komatsuna or tomato were imbibed and incubated for 48 or 72 h, respectively, at 25°C under completely dark conditions on a double sheet of filter paper (Advantec No. 2). The paper was moistened with 5 mL of either compost water extract or distilled water (control) in a covered 9-cm glass Petri dish. The Petri dishes were sealed with parafilm (Parafilm “M” Laboratory Film American National Can TM). The results were expressed as the percentage of seed germination with compost water extract considering the number with distilled water equal to 100%. The experimental design was a completely randomized design and the treatment was repeated four times. Statistical analyses All the results reported in this paper were expressed as means of three replicates. The relationships among the selected parameters of GRC stability evaluation were

Results and discussion The compost pile temperature during composting The temperature of the composting material reached the thermophilic phase (>45°C) within 5 days of composting reflecting the initiation of the composting process (Fig. 1). From days 9 to 65, the daily average temperature was maintained, with some fluctuation, at the optimal values 50–60°C for effective composting (Wong et al. 2001). The highest hourly temperature (59.4°C) was attained within 40–50 days of composting. The temperature increased immediately after turning over the composting materials at the initial stage of composting. However, at the later stages of composting, the temperature did not increase any further even after addition of water and turning over the compost. The temperature of the composting materials reached a plateau on day 90, indicating that the compost had become stabilized. Temperature has been widely used as one of the most important parameters for evaluating compost stability since compost pile temperature is related to microbial activity and to the rate of decomposition during composting (Tiquia and Tam 2002). Moreover, a high temperature (50–60°C) is important for the destruction of weed seeds and for killing pathogens of the compost. The use of the compost pile temperature for evaluating the composting process is limited by its dependency on factors such as the type of material that is composted, the volume and moisture content of materials, the composting procedure, the season, and other variables (Ko et al. 2008). 60

o

Biological analysis

analyzed by Pearson’s correlation coefficient using the SPSS statistical software (SPSS for Windows, version 12.0.1; SPSS Inc, Chicago, IL, USA).

Temperature ( C)

extracted with 50 mL 0.5 M NaOH by shaking for 2 h; then, the flask was left overnight. The next day, the suspension was centrifuged at 600×g for 25 min and the absorbance (A) of the supernatant was measured at 472 nm (A472) and at 664 nm (A664) so as to calculate the absorbance ratio Q4/6 =A472 /A664 often taken as the humification index (Zbytniewski and Buszewski 2005). As an index of decomposition, we measured the chlorophyll-type compounds (mainly chlorophyll a, chlorophyll b, pheophytin, chlorophyllide, and pheophorbide) of the composting materials over time (Hoyt 1966; Rajbanshi and Inubushi 1998) by extracting 100 mg of an oven-dried and finely ground compost sample with 20 mL 90% acetone for 24 h (<5°C in the dark); then, the mixture was centrifuged at 600×g for 25 min before determining the absorbance of the extractants at 665 nm (A665). The FT-IR spectra of materials at four stages of the composting process (0, 56, 140, and 246 days) were obtained at wavenumber ranging from 4,000 to 400 cm−1 in an FT-IR Spectrum 2000, Perkin Elmer Ltd.; the compost (2.4 mg) was oven-dried, finely ground, and mixed with 100 mg of KBr; then, the mixture was compressed into pellets before FT-IR measurement.

307

50

Compost Ambient

40 30 20 10 0

0 4/26

30 60 90 120 5/26 6/25 7/25 8/24 Composting time (days)

150 9/23

Fig. 1 Changes in the daily average temperature of compost over the first 146 days of composting compared to the ambient temperature. Arrows indicate the addition of water during mixing of compost

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The pH of the compost dropped from 6.3 to 5.9 during the first 28 days of composting and thereafter gradually increased (Fig. 2a). The initial decrease in pH was probably due to the production of organic acids in the initial stage of composting by the rapid stimulation of microbial activity and to the decrease in the NH4+ content (Meunchang et al. 2005). The EC value increased from 3.8 to 8.4 in the first 42 days of composting and tended to stabilize thereafter (Fig. 2b). Such an increase may be due to the increase in salt concentration and to the mineralization of organic matter (Michel and Reddy 1998). However, due to the lack of a distinct pattern of change in pH over time and early stabilization of EC, these parameters were not considered suitable for monitoring the composting process or for evaluating the nutrient balance or chemical stability of the composting materials. The NO3−-N concentration increased from 0.59 to 1.7 mg kg−1 over the composting process (Fig. 2c). The rate of increase was very slow in the first 84 days of composting but increased sharply thereafter. The increase in NO3−-N concentration after day 84 is indicative of the beginning of the active nitrification. It has been reported that ammonification occurs during the initial composting process and that nitrification exceeds the ammonification when the ammonification rates slow down (Tognetti et al. 2007). In the same study, it was also found that the highest nitrification rates occurred after 80 days of composting when temperature values became more favorable for nitrifying bacteria (Tognetti et al. 2007). The decrease of NH4+ concentrations and the appearance of NO3− in the composting materials indicate that the compost can be used (Finstein and Miller 1985). The C/N ratio decreased sharply

-1

EC (mS cm )

pH

6.5 6.0 0

42

84 126 26

240

(c)

2.0 1.5 1.0

C/N

-1

NO3-N (mg kg )

5.5

0.5 0.0

0

42

84 126

240

12 10 8 6 4 2 16 14 12 10 8 6

(b)

Color changes during composting Determination of color change is currently widely used to evaluate changes in soil properties (Sánchez-Marañ n et al. 1997). The International Commission on Illumination has laid down guidelines with respect to the measurement of color (CIE 1986) and recommends the use of CIELAB as a uniform space (CIE 1995). However, the use of CIELAB color space for compost characterization is a new approach in compost science. The principal advantage of using CIELAB color space is the uniformity in its associate chromaticity diagrams. The changes in CIELAB variables (L*, a*, b*, and ΔE*ab) calculated over the composting time are illustrated in Fig. 3. The lightness L* value decreased from 50.7 to 44.1 over composting, whereas the a* values increased from 0.8 to 5.8 and the b* values decreased from 25.5 to 10.5. In general, composting materials gradually turn black due to the formation of humic substances. Therefore, as expected, the value of L* gradually decreased over time. The small increase in L* at a later stage of composting (126 days) may be due to the evolution of white color actinomycetes. It has been reported that the yellow color (b* value) decreases gradually and turns more blue with increasing composting time (Brinton 60

0

42

84 126

240

(d)

CIELAB unit

(a)

7.0

from 14.6 to 8.3 in the first 84 days of composting and tended to stabilize thereafter (Fig. 2d). The C/N ratio is widely used as an indicator of compost stabilization and is expected to stabilize over time (Meunchang et al. 2005). The C/N ratio of pig manure compost decreased rapidly from an initial value of 21 in the raw material to a value of 10 after only 18 days of composting. Then, the ratio decreased less sharply to a value of 7.4 after 49 days and stabilized at a value of approximately 7 for the rest of the process (Hsu and Lo 1999). However, there is no general agreement as to which value the C/N ratio indicates stabilization of organic matter and compost. In this study, the C/N ratio of the stabilized compost was around 8.0.

42

84 126 12

240

a*

∆ E*ab

60

45

45

30

30

15

15

0 0

L* b*

Color difference (∆ E*ab)

Chemical properties of the compost

0

42 84 126 240 Composting time (days)

0

Composting time (days) Fig. 2 Changes in pH (a), EC (b), NO3−-N (c), and the C/N ratio (d) of compost over time. Vertical bars indicate standard deviation (±)

Fig. 3 Time course of the color (CIELAB variables) during composting of GRC. Error bars (standard deviation) are not shown since they are smaller than the symbols

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and Droffner 1994). The changes in compost color are not only due to simple changes in physical properties like moisture but also due to changes in the content of carbon dioxide or volatile organic acids in the compost sample (Brinton and Droffner 1994). In this study, change in the measured CIELAB variables of the GRC reached a plateau after 84 days of composting, indicating that composting was stabilized after day 84. This study suggests that there is great potential for the use of CIELAB color space for characterizing the transformation of organic matter as well as the evaluation of compost stability during composting. To our knowledge, this is the first report for the characterization of compost organic matter and for the evaluation of compost stability using compost color measured by CIELAB color space. Further research is therefore necessary before CIELAB color space can be used for the characterization of compost organic matter as well as for the evaluation of compost stability.

ratio was not considered a suitable parameter for GRC stability evaluation. Absorbance at 665 nm of acetone extracts of composting materials decreased with the composting time from 0.145 to 0.087 (Fig. 5). Decomposition of chlorophyll-type compounds, as determined by the assay of light absorption of acetone extracts of compost, is an index used to evaluate compost stability (Inubushi et al. 1982). During composting of the allelopathic plant leaves Eupatorium adenophorum and Lantana camara, the absorbance of acetone extracts decreased by 0.432 and 0.481 units, respectively, by the end of composting (Rajbanshi and Inubushi 1998). In GRC, we have observed a decrease by 0.058 units. Due to the small difference in light absorption between the initial and the final (246 days old) composting period observed here, this index was not considered suitable for evaluation of the complex composting process. FT-IR spectroscopy

UV-Vis spectroscopy of compost The absorbance ratio Q4/6 (A472/A664) of 0.5-M NaOH extracts of compost has been suggested to reflect the degree of stability of organic matter in bulk compost (Zbytniewski and Buszewski 2005), the absorbance of extracts at 460– 480 nm being an indicator of the status of the organic material present at the beginning of humification and the absorbance at 600–670 nm indicative of strongly humified material. The absorbance ratio Q4/6 is therefore termed the humification index (Zbytniewski and Buszewski 2005). In this study, Q4/6 decreased sharply over the first 42 days of composting and roughly stabilized thereafter being lower than 5 as it has been reported for the humified materials (Gieguzyńska et al. 1998; Fig. 4). A low Q4/6 ratio indicates a high degree of humification. But, due to early stabilization of HI with respect to other reliable parameters, the Q4/6

FT-IR spectroscopy is based on the interaction of infrared light with matter and detects chemical functional groups (Smith 1999). FT-IR has been used to determine changes at the molecular level during the biological treatment of organic waste (Meissl et al. 2007) and has also been used to assess compost stability (Smidt et al. 2005; Smidt and Meissl 2007). These investigations have shown that FT-IR spectra provide a considerable amount of information about the chemical status of complex samples during composting. The major changes in absorbance over the time of composting of GRC are illustrated in Fig. 6. The width of a broad band at approximately 3,300 cm−1 (Fig. 6a) probably depends on the alcohol and phenol association (Castaldi et al. 2005) and the width of this band gradually decreased at the end of composting. This confirms a previous finding showing a marked reduction with composting of the broad

0.20 Absorption (665 nm)

Humification index (Q4/Q6)

15

10

5

0

0

42 84 126 Composting time (days)

240

Fig. 4 Time course of humification during composting of GRC. Error bars (standard deviation) are not shown since they are smaller than the symbols

0.15 0.10 0.05 0.00

0

42 84 126 Composting time (days)

240

Fig. 5 Time course of light absorbance (λ=665 nm) of acetone extracts of compost during composting. Vertical bars indicate standard deviation (±)

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Fig. 6 Partial Fourier-transform infrared spectra of compost at different stages of the composting process: range 3,600–3,000 cm−1 (a), 3,000–2,800 cm−1 (b), 2,400–2,300 cm−1 (c), and 1,200–950 cm−1 (d). The samples were prepared after their mixing with KBr

(a)

246 day

(b) 246 day

0.0059

0.0198

140 day

56 day 0.0579

0.0477

56 day

0.0427

3300 0.0652

140 day

0.0165 Absorbance

Absorbance

0.0270

0.0033 0.0029

2930

0.0585

0 day

2852

0.0532 0 day

3600

3400

3200

3000

3000

(c) 0.0484 0.0360

0.0222

246 day

140 day

0.0486

Absorbance

Absorbance

0.0701 0.0471

56 day 0.0881 0.0687

0.0780

140 day

56 day

1065 0 day

2361

0.0814 0.0649

2400

2800

(d)

246 day

0.0878 0 day

2900

2340

2350

band at the 3,400–3,300 cm−1 with disappearance of OH groups (Castaldi et al. 2005). During the GRC process, two distinct peaks at 2,930 and 2,852 cm−1, due to aliphatic C–H bonds, were noticeably reduced (Fig. 6b). It has been reported that during composting the aliphatic methylene

2300

1200 Wavenumbers (cm-1)

1125

1050

975

bands (2,925 and 2,850 cm−1) decrease with time due to the breakdown of the aliphatic skeleton of macromolecules (Smidt et al. 2005). Furthermore, measurement of methylene bands has been suggested to be a better indicator of decomposition than measurement of the amount of organic

Seed germination (% of control)

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311

100

indicator peaks. At the end of the process, the heterogeneous raw materials were transformed into a product with a uniform chemical structure as suggested by a decrease in the intensity of the majority of the peaks.

80 60

Phytotoxicity of compost

40

komatsuna tomato to

20

The seed germination test is a widely accepted protocol for evaluating the compost phytotoxicity as well as the compost stability (Tiquia et al. 1996; Zucconi et al. 1981). The changing pattern of the germination of komatsuna and tomato (test species) seeds in a water extract of GRC (0.1 g mL−1) over composting is shown in Fig. 7. From the beginning of the composting up to 14 days, the germination rate of seeds of both tests decreased compared to the rate of the distilled water control. The initial reduction in germination may be attributed to the release of a high concentration of ammonia and to the presence of low-molecular-weight organic acids (Wong 1985). The percentage of seeds that germinated at the later stages of composting increased significantly and were above 80% of the distilled water control value at 42 and 84 days of composting for komatsuna and tomato seeds, respectively. These results suggest that phytotoxicity of GRC is selective and that the tomato is more sensitive than the komatsuna to the phytotoxic effects of GRC. A previous report also found that water extracts of the composts from activated sludge and coffee residue had different phytotoxic effect on seed germination of specific legumes (Nagaoka et al. 1996). The extent of phytotoxicity caused by insufficient composting can also be taken as an indicator of the compost chemical instability (Wu and Ma 2001). However, due to the selective toxicity of different composting materials towards

0 0

42 84 126 Composting time (days)

240

Fig. 7 Time course of the germination of komatsuna and tomato seeds in water extracts of compost (0.1 g mL−1). Vertical bars indicate standard deviation (±). Seed germination of komatsuna and tomato in a distilled water control was always 100±0%

matter (Smidt and Meissl 2007). Measurement of changes in band intensity at 2,925–2,930 cm−1 has also been shown to be useful for evaluating the composting processes (Grube et al. 2006). In this study, the intensity of the doublet peak at 2,361 and 2,340 cm−1 initially increased (up to the 56th day of composting) and then gradually decreased (Fig. 6c). The high initial value may be due to the initial respiration rate of compost microflora (Nakasaki et al. 1985), leading to an increase in CO2 evolution. The broad peak at the 1,065 cm−1 zone decreased gradually with composting (Fig. 6d). The absorption band at 1,100–1,000 cm−1 has been assigned to carbohydrates and has been reported to be useful for evaluating the composting process (Grube et al. 2006). The analyses by FT-IR suggested that transformation of the materials during composting proceeded gradually with time as indicated by a change in the intensity of the

Table 1 Correlation coefficient among the used parameters Parameters

Pearson’s correlation coefficient (r) Temperature

Temperature – NO3−-N C/N ratio CIELAB variables L* b* ΔE*ab Phytotoxicity Komatsuna Tomato

NO3−-N

−0.715a –

NS nonsignificant relationship Significant relationship at the 0.01 level b Significant relationship at the 0.05 level a

C/N ratio

NS −0.747a –

CIELAB variables

Phytotoxicity

L*

b*

ΔE*ab

Komatsuna

Tomato

NS −0.636b 0.823a

NS −0.611b 0.978a

NS 0.619b −0.973a

−0.706a 0.760a −0.914a

−0.950a 0.741a −0.583b

0.828a

−0.879a −0.988a –

−0.764a −0.893a 0.849a

−0.558b −0.526b NS

–

–

–

0.841a –

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seeds from different species, it will be necessary to select species that are sensitive to the specific composting materials before this test can be used for the evaluation of compost stability (Bernal et al. 1998).

different types of organic wastes. Among the FT-IR spectra, those at 3,300, 2,930, 2,852, and 1,065 cm−1 decreased over the composting time and these spectra can be also used to assess GRC stability with other parameters.

Relationships among the parameters

Acknowledgments We are grateful to the Ministry of Education, Culture, Sports, Science, and Technology, Japan for providing a scholarship to Mohammad Ashik Iqbal Khan and thankful to the JA Beverage, Saga Co. Ltd. for providing green tea waste. We are also grateful to Prof. Dr. Kazuo Morita, Faculty of Agriculture, Kagoshima University and Dr. Takashi Someya, Laboratory of Soil Microbiology, Faculty of Agriculture, Saga University, Japan for their valuable suggestions in the writing of this manuscript.

To find out the single parameter that can evaluate the stability of GRC, we calculated the correlation coefficients (r), which are represented in Table 1. The compost pile temperature showed significant (p≤0.01) and negative correlation with NO3−-N concentration and compost phytotoxicity but there was no significant correlation with C/N ratio and CIELAB variables, although CIELAB variables can provide the information on organic matter characterization during composting. The NO3−-N concentration showed significant correlation with all measured parameters of GRC stability and C/N ratio showed a significant correlation with CIELAB variables and phytotoxicity. Among the CIELAB variables, L* value showed the most significant relationship with NO3−-N concentration and tomato seed germination and b* value showed the strongest correlation with C/N ratio and germination of komatsuna seeds. The germination of komatsuna seeds showed comparatively stronger correlation with other parameters than test (based on germination) of tomato seeds. Although many reports have already been published on the most promising indicator of the degree of compost stability as evaluated by regression analysis (Iglesias Jiménez and Pérez García 1992; Bernal et al. 1998), it is impossible to find a single parameter which can assess a complete process such as the formation of a stable compost. In this experiment, most of the measured parameters (temperature, C/N ratio, and CIELAB variables) of GRC stability were stabilized around day 90 of composting because these parameters were directly and/or indirectly related with organic matter decomposition during composting. In addition, the NO3−-N concentration and phytotoxicity test also indicated the stabilization of compost at after 90 days of composting.

Conclusion Compost pile temperature, NO3−-N concentration, C/N ratio, and L* or b* value of CIELAB are useful for assessing organic matter characterization and germination of komatsuna seeds can be measured for reliable information on GRC stability. However, a single parameter cannot be used to evaluate a complete process such as the formation of a stable compost. Although CIELAB color space was sensitive in characterizing the organic matter decomposition, further research is necessary by using

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