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Relationship between Compost Stability and Extractable Organic Carbon. L. Wu and L. Q. Ma*. ABSTRACT et al., 1991; Jimenez and Garcia, 1992). Adani et al.
Relationship between Compost Stability and Extractable Organic Carbon L. Wu and L. Q. Ma* ABSTRACT

et al., 1991; Jimenez and Garcia, 1992). Adani et al. (1995) speculated that this discrepancy is due to the fact that NaOH-extractable organic carbon (OC) contains a considerable biodegradable nonhumus fraction, especially during the initial stages of composting. Chefetz et al. (1998a) developed the concept of a core-HA fraction, which was obtained by removing the nonhumic substance from the HA fraction via successive extractions with an organic solvent followed by sulfuric acid treatments, and finally by an alkaline solution. The process is believed to be inert with respect to the coreHA fraction. Their research found that the OC content of the core HA relative to total dry mass increased while the HA decreased steadily with composting. Adani et al. (1995) found that the difference in OC between HA and core-HA fractions decreased during composting and proposed to use the ratio of core HA to HA as a new compost stability index (organic matter evolution index, OMEI), expressed as a number between zero and one. Though the relation of core HA to HA sheds light onto OC evolution during the composting process, it cannot be used as a practical indicator for compost stability, due to the extensive laboratory work needed to calculate this index. In addition to the OMEI, the ratio of HA to FA, another humification-related parameter, has been used for assessing organic matter stability. The ratio of HA to FA invariably increased during composting, especially when the fraction of NaOH-soluble OC at pH ⬍ 2 is treated as FA (Garcia et al., 1991). This is because the FA fraction decreased more than the HA fraction during composting. However, due to the variability among composts of different material sources, the suggested critical value of this index covers a large range for different composts, even within the same composting system and time (Bernal et al., 1998). As a result, it is not feasible to use the ratio of HA to FA as the sole indicator of compost stability. The primary objective of this study was to examine the relationship between water- and NaOH-extractable OC, and compost stability estimated by the CO2 evolution rate. The extracted OC was further separated into FA and HA fractions to examine the relationship of these fractions to compost stability. The ultimate goal of this research is to develop a simple and reliable way to estimate the stability of composts from different source materials.

Establishing a simple yet reliable compost stability test is essential for a better compost quality control and utilization efficiency. The objective of this study was to examine the relationship between extractable organic carbon (OC) and compost stability based on 18 compost samples from five composting facilities. The compost samples were extracted sequentially with water for 2 h [water(2h)] and 0.1 M NaOH for 2 and 24 h [NaOH(2h) and NaOH(24h), respectively]. The extractable OC was further separated into fulvic acid (FA) and humic acid (HA) fractions by adjusting the pH to ⬍2. The mass specific absorbance (MSA) of OC in the six fractions was measured. Compost stability was estimated with a CO2 evolution method. The extractable OC concentration was influenced by the total volatile solids and decreased with curing time for compost with a high level of extractable OC. The OC levels in each fraction were significantly correlated (p ⬍ 0.05) to each other except for the water(2h)–extractable HA. In addition, all the FA and HA fractions except for water(2h)–extractable HA were highly (P ⬍ 0.01) and linearly correlated to CO2 evolution, but multiple regression showed that NaOH(24h)–extractable OC was insignificant for CO2 evolution. The relatively high slope of NaOH(2h)–extractable FA versus CO2 evolution suggests that this fraction may contribute the most to compost CO2 evolution. The water(2h)– and/or NaOH(2h)– extractable FA tests are recommended for measuring compost stability because of their high correlation with CO2 evolution. This estimation can be obtained through a simple photometric method covering a wide range of carbon concentrations up to 4000 mg L⫺1.

H

umification is widely considered an important process during the composting of organic materials where humic substances form and nonhumic substance decompose (Bernal et al., 1998; Hsu and Lo, 1999; Leita and Denobili, 1991; Miikki et al., 1997; Sanchez-Monedero et al., 1999; Schnitzer et al., 1993). As composting progresses, the percentage of humic substances is expected to increase relative to the total dry mass or the total organic matter. As a result, humification-related parameters have been examined to represent compost stability and maturity (Adani et al., 1997; Garcia et al., 1991; Jimenez and Garcia, 1992; Veeken et al., 2000). The method commonly used to extract humic substances from composts is similar to that used to extract organic matter from soils, where humic substances are extracted with a dilute sodium hydroxide solution, sodium pyrophosphate solution, or a mixture of the two. The fraction that precipitates at pH ⬍ 2 is referred to as humic acid (HA), whereas the fraction remaining soluble at pH ⬍ 2 is defined as fulvic acid (FA). Contrary to what is expected, concentrations of HA either decreased during composting or in some cases the differences were not significant (Bernal et al., 1996; Garcia

MATERIALS AND METHODS Compost Sample Collection and Preparation

Department of Soil and Water Science, Univ. of Florida, Gainesville, FL 32611-0290. Approved for publication as Florida Agricultural Experiment Station Journal Series no. R-08752. Received 8 June 2001. *Corresponding author ([email protected]).

Biosolids compost samples were collected from full-scale composting facilities (sample sets identified as Meadow, RegAbbreviations: FA, fulvic acid; HA, humic acid; MSA, mass specific absorbance; OC, organic carbon.

Published in J. Environ. Qual. 31:1323–1328 (2002).

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J. ENVIRON. QUAL., VOL. 31, JULY–AUGUST 2002

Table 1. Selected physical-chemical properties of tested biosolids compost samples. Compost method

Other wastes and amendment

Force-aerated windrow

food waste, yard waste, animal manure, and wood chips

Force-aerated windrow

food waste, yard waste, animal manure, and wood chips

In vessel

ground yard waste

In vessel

sawdust

Windrow

yard waste

Windrow

yard waste

Sample Register A-1 Register A-2 Register A-3 Register B-1 Register B-2 Register B-3 Register B-4 Winslow-1 Winslow-2 Winslow-3 Sunset-1 Sunset-3 Sunset-5 Meadow-1 Meadow-2 Puerto Rico-1 Puerto Rico-2 Puerto Rico-3

Curing d 0 7 30 0 9 21 25 0 7 30 0 14 35 30 90 0 7 14

WHC†

Volatile solids g

1148 1116 991 1376 1415 1169 1127 1279 1075 1312 1468 1423 1428 669 473 716 699 662

681 737 668 852 884 681 646 562 614 598 914 916 902 221 164 358 324 297

Total N

TOC‡

C to N ratio

28.4 25.8 26.1 43.0 41.0 36.0 37.0 16.8 18.0 18.2 13.6 14.1 13.8 11.3 6.9 13.6 12.2 12.6

398 413 392 481 505 387 365 319 342 346 517 522 511 124 90 203 183 168

14 16 15 15 16 14 13 19 19 19 38 37 37 11 13 15 15 13

kg⫺1

† Water holding capacity. ‡ Total organic carbon.

ister A, Register B, Winslow, Sunset, and Puerto Rico) in Florida and Puerto Rico. The Register A and B samples were collected from the same composting facility at different seasons. The composition and ratios of biosolids to other feedstock materials varied greatly from one facility to another but were consistent within each facility. Two to four samples were collected at each facility from different stages of the curing process as defined by the facility (Table 1). Collected samples were placed in polyethylene bags, packed with ice in a cooler and shipped to the lab on the same day. Upon arrival, the samples were sieved through a 9.5-mm screen to remove large particles and air-dried. The Puerto Rico samples were collected and freeze-dried before shipping to our lab. Compost mass is expressed on a dry weight basis in this paper. The chemical and physical properties of the compost samples are listed in Table 1. For details on measurement of these parameters, please see Wu et al. (2000).

which is defined as the absorbance per unit mass of DOC (L mg⫺1 m⫺1 ).

Carbon Dioxide Evolution Compost stability was measured based on CO2 evolution rate, a measure of the microbial respiration of the compost samples using a modified procedure of Iannotti et al. (1994). Approximately 10 g dry weight of compost sample at 60% (w/w) moisture content was sealed in a 0.5-L vessel along with a beaker containing a known volume of 0.5 M NaOH solution. The samples were incubated for 7 to 8 d at room temperature (24 ⫾ 2⬚C). During the incubation, the released CO2 was captured by the NaOH solution, which was then analyzed titrimetrically at regular intervals. Since there is a large variation in the evolution of CO2 during incubation, the peak CO2 evolution rate was used to represent compost stability.

Sequential Extraction and Analysis of Organic Carbon

Statistical Analyses

A three-step extraction procedure was employed to separate the OC from the composts into different fractions. Samples were first extracted with water for 2 h [water(2h)], then with a 0.1 M NaOH solution for 2 h [NaOH(2h)], and finally 0.1 M NaOH solution for 24 h [NaOH(24h)], with the solid to solution ratio of 1:10. The NaOH extraction was conducted by shaking the suspensions in a horizontal shaker at room temperature under N2. At the end of 2 and 24 h of shaking, the suspension was centrifuged at 10 000 rpm for 10 min and filtered through a 0.45-␮m membrane filter. An aliquot of the filtrate was adjusted to pH ⬍ 2 with 6 M HCl and then centrifuged at 10 000 rpm for 10 min and filtered through a 0.45-␮m membrane filter. The precipitated pellet collected at the bottom of centrifuge tube is called humic acid (HA), while the filtrate containing fulvic acid and other nonhumic substances is referred to as fulvic acid (FA) (Swift, 1996). Organic carbon concentrations in the aqueous solutions were determined with a Shimadzu (Kyoto, Japan) TOC-5050A carbon analyzer. The OC concentration of the HA fraction was calculated as the difference between total OC and FA. The pellet containing HA was re-dissolved with 0.1 M NaOH. The absorbance of FA and HA solutions at 420 nm was measured with a Shimadzu UV160U UV-visible spectrophotometer with samples diluted to approximately 500 mg C L⫺1 for FA and 100 mg C L⫺1 for HA solutions. The readings were then used to calculate the mass specific absorbance (MSA),

The SAS (SAS Institute, 1996) procedure GLM with LSMEAN and PDIFF options was used to compute the p value of statistical differences between samples. Since compost stability and maturity degrees differed greatly from one facility to another, samples from each facility were treated as nested by facility. Correlation coefficients between parameters were calculated with the CORR procedure. The stepwise multiple correlation procedure was used to derive the multiple regression between CO2 evolution rate and other parameters.

RESULTS AND DISCUSSION Compost samples used in this study varied greatly in their physical and chemical properties (Table 1), which directly affect compost stability measurement. In general, the Sunset samples had the highest water holding capacity and C to N ratio as a result of their high volatile solids and OC, whereas the Register samples had relatively high total nitrogen and OC. On the other hand, the Meadow and Puerto Rico samples had generally low values for all the parameters determined in this study. These differences among compost samples were mainly due to the differences in their source materials (Table 1).

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Table 2. Peak CO2 evolution rate and organic carbon content of sequentially extracted organic carbon. Mass of compost is expressed on a dry weight basis. Water Compost

Curing

CO2

HA†

FA‡

0.1 M NaOH for 24 h Sum

HA/FA

HA

FA

0 7 30 0 9 21 25 0 7 30 0 14 30 60 90 0 7 14

HA/FA

HA

FA

Sum

HA/FA

Total

g kg⫺1

d Register A Register A Register A Register B Register B Register B Register B Winslow Winslow Winslow Sunset Sunset Sunset Meadow Meadow Puerto Rico Puerto Rico Puerto Rico

Sum

0.1 M NaOH for 24 h

768 487 185 686 606 80 17 65 90 62 60 53 45 46 23 54 58 40

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

29 14 7 15 50 9 5 2 2 3 2 1 4 6 2 2 5 1

3.6 6.0 4.5 3.7 5.9 9.3 9.2 2.5 2.4 1.4 3.4 2.9 2.2 0.4 0.1 0.7 0.6 0.8

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

1.1 1.4 0.1 1.9 0.2 0.2 0.5 0.1 0.2 0.1 0.0 0.0 0.1 0.0 0.0 0.5 0.0 0.0

25.2 20.0 14.2 31.1 35.5 17.4 9.4 4.9 5.0 2.7 8.8 8.9 7.1 1.2 0.9 6.3 3.7 5.0

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

1.0 1.8 0.1 0.0 1.0 0.6 0.0 0.1 0.1 0.0 0.3 0.2 0.1 0.0 0.1 0.5 0.0 0.1

28.8 26.0 18.7 34.7 41.5 26.7 18.6 7.4 7.3 4.1 12.2 11.8 9.3 1.6 1.0 6.9 4.3 5.9

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.1 0.4 0.1 2.0 1.2 0.4 0.4 0.1 0.2 0.1 0.3 0.2 0.0 0.0 0.1 0.0 0.0 0.0

0.14 0.30 0.31 0.12 0.17 0.54 0.99 0.51 0.48 0.51 0.38 0.33 0.32 0.34 0.17 0.11 0.15 0.16

23.1 19.0 19.8 16.6 16.5 16.5 23.1 11.5 7.9 13.7 10.7 6.1 10.1 8.0 6.2 4.9 3.9 3.4

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.6 0.2 1.6 0.4 0.3 0.4 0.4 0.2 2.1 1.1 1.8 1.1 0.5 0.5 0.7 0.3 0.3 0.1

17.8 14.4 11.7 19.3 17.3 9.6 6.6 10.6 10.7 6.6 7.9 8.2 7.3 6.1 5.9 8.4 7.3 8.2

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.1 0.5 0.8 0.1 0.3 0.2 0.1 0.5 0.1 0.7 0.1 1.1 0.2 0.8 0.6 0.2 0.0 0.2

40.9 33.4 31.4 35.9 33.8 26.0 29.8 22.0 18.6 20.3 18.7 14.3 17.4 14.2 12.1 13.3 11.2 11.6

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.5 0.7 0.7 0.3 0.0 0.5 0.5 0.3 2.0 0.3 1.8 0.0 0.3 0.3 0.1 0.4 0.3 0.1

1.30 1.32 1.70 0.86 0.95 1.72 3.48 1.09 0.74 2.11 1.35 0.75 1.39 1.32 1.07 0.58 0.54 0.41

32.6 26.5 23.3 31.3 34.2 26.5 27.3 24.5 21.5 2.41 10.5 10.7 11.2 20.8 21.3 6.8 6.2 6.4

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

1.1 1.3 0.0 0.3 6.8 1.5 1.1 1.7 0.1 0.2 1.5 2.6 0.3 1.3 0.4 0.8 0.7 0.6

15.1 15.2 11.7 16.1 14.0 9.2 6.9 13.0 11.7 6.0 7.5 7.6 7.5 12.8 9.7 5.9 6.1 5.8

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.6 0.8 0.3 1.1 1.3 0.4 0.2 0.9 .01 0.3 0.2 0.1 0.5 1.7 1.9 0.4 0.7 0.0

47.8 41.7 35.0 47.4 48.2 35.7 34.2 37.4 33.2 30.1 18.0 18.3 18.7 33.6 31.0 12.7 12.4 12.2

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.5 2.1 0.3 0.9 5.5 1.9 1.3 0.8 0.0 0.1 1.7 2.7 0.7 3.0 1.6 1.3 0.0 0.6

2.16 1.74 2.00 1.95 2.47 2.87 3.94 1.90 1.84 4.01 1.41 1.42 1.50 1.63 2.23 1.14 1.03 1.11

117.4 101.1 85.1 118.0 123.6 88.5 82.6 66.8 59.1 54.5 48.8 44.4 45.4 49.4 44.2 32.9 27.9 29.7

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.1 3.2 1.1 0.8 6.7 0.9 1.4 1.1 1.8 0.2 0.5 2.5 0.1 3.3 1.7 1.7 0.3 0.4

† HA, humic acid. Organic carbon soluble at pH ⬍ 2. ‡ FA, fulvic acid. Organic carbon insoluble at pH ⬍ 2.

Carbon Dioxide Evolution Rate Generally speaking, CO2 evolution rates decreased with curing (Table 2). The decrease in CO2 evolution rates was significant (p ⬍ 0.01) in the Register samples, which suggested that these samples were not stable. In comparison, decreases in the remaining samples were minor, indicating that they were relatively stable compost samples (Table 2). Hue and Liu (1995) used the mean plus two standard deviations of the CO2 evolution rate from 14 commercial composts as the threshold level for a stable compost, which is 120 mg CO2 kg⫺1 h⫺1 based on the average of the last 2 d of a 3-d incubation test. Based on this standard, all compost samples in the current study were stable except for the five samples from the Register facility having CO2 evolution rates ⬎ 120 mg CO2 kg⫺1 h⫺1 (Table 2). This was consistent with the conclusions drawn from the changes in CO2 evolution rate with curing. The average CO2 evolution rate for the remaining samples was 55 mg CO2 kg⫺1 h⫺1 with a standard deviation of 21 mg CO2 kg⫺1 h⫺1. A value of 98 mg CO2 kg⫺1 h⫺1 was obtained as the threshold level based on this current study following the method of Hue and Liu (1995), which was close to 120 mg CO2 kg⫺1 h⫺1. This suggests that an index based on CO2 evolution rate may be a viable parameter to use in screening for unstable composts with diverse source materials.

Water- and Sodium Hydroxide–Extractable Organic Carbon The total extractable OC content (sum of all three extractions) ranged from 27.9 to 123.6 g kg⫺1 dry mass, with the Register samples having the greatest concentration among all the compost samples (Table 2). The total extractable OC correlated highly (r ⫽ 0.881, p ⬍ 0.01) with the total volatile solids with the exception of the Sunset samples (Table 1). The Sunset samples had a

lower extractable OC than other samples, probably due to the high amount of sawdust added to the compost. As expected, the distribution of extractable OC (sum of HA and FA) across the three extractions increased in the order water(2h), NaOH(2h), and NaOH(24h), which accounted for 2 to 34, 27 to 40, and 39 to 70% of the total extractable OC, respectively. This direct correlation with extraction intensity can be attributed to the proportional increase in HA (Table 2), raising the ratio of HA to FA. It is generally believed that FA is more soluble, and thus more easily extractable than HA. It is reasonable to assume that the water-extractable OC would be more degradable than the NaOH-extractable OC, which was confirmed by our data. With curing, the water-extractable OC decreased much more significantly than the NaOH-extractable OC (Table 2). In the Register A and B samples, for example, reduction in total water-extractable OC was 39 and 46% compared with 26 and 28% for the NaOH-extractable OC, respectively. This is partially because of higher ratios of HA to FA in the NaOH-extractable OC than in the waterextractable OC. However, no consistent trend with curing was observed between the NaOH(2h)– and NaOH(24h)– extractable OC [i.e., the NaOH(2h)–extractable OC was not necessarily more degradable than the NaOH(24h)– extractable OC] (Table 2). As discussed earlier, total extractable OC in each extraction decreased with curing (Table 2). The reduction was more pronounced in samples with high extractable OC, such as the Register samples. The decrease of water-extractable OC with composting has been reported previously (Bernal et al., 1998; Chefetz et al., 1998b; Hue and Liu, 1995). Hue and Liu (1995) suggested using 10 g of water-extractable OC per kg dry matter as the threshold value for stable compost. Use of this value in the current study yields results similar to those obtained based on CO2 evolution rate (i.e., all compost samples, except for Register, were stable) (Table 2).

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Composting is believed to be a humification process, thus, concentration of humic substances is expected to increase with composting. Due to the differences in HA and FA, they behave differently during composting. Using a single NaOH extraction, inconsistent changes in HA have been reported (Bernal et al., 1998; Calace et al., 1999; Pascual et al., 1997; Sanchez-Monedero et al., 1999). On the other hand, significant and consistent reduction in FA concentrations was observed in almost all studies. Our results are consistent with these previous studies. The ratios of HA to FA generally increased with curing primarily due to the consistent and substantial reduction of FA during curing. However, such changes were inconsistent, which agreed well with data published by Bernal et al. (1998). This was most likely because our samples were collected from different curing ages, which span a relatively short period of time (⬍30 d). Thus, changes in the ratio of HA to FA during this period were probably insignificant. The ratios of HA to FA also varied greatly with composting source materials, and seemed to be less correlated with other parameters that measure compost stability. Similar to the changes in extractable OC, the ratios of HA to FA increased in the following order: water(2h) (0.11 to 0.99), NaOH(2h) (0.41 to 3.48), and NaOH(24h) (1.03 to 4.01), or with increased extraction intensity. It is apparent that these three fractions contain OC of different degrees of polymerization and aromaticity, which can be evaluated based on mass specific absorbance (MSA). The MSA has previously been used to characterize the degree of humification of a watersoluble humic substance (Battin, 1998). The greater the MSA, the higher the degree of polymerization and aromaticity. Thus, MSA of HA is expected to be greater than that of FA, and MSA of the NaOH-extractable HA and FA greater than that of the water-extractable HA and FA (Table 3). In general, MSA of HA and FA increased with curing (with some exceptions), indicating an occurrence of humification during composting (Table 3). We examined nine fractions of OC, including HA, FA, and HA ⫹ FA fractions of the three extractions.

Carbon concentrations in different OC fractions were positively correlated with each other, having correlation coefficients ranging from 0.56 to 0.98 with a few exceptions (data not shown). The correlation coefficients between the NaOH-extractable HA and NaOH-extractable FA [0.562 and 0.76 for NaOH(2h) and NaOH(24h)] were greater than those between the water-extractable HA and water-extractable FA (r ⫽ 0.556). This fact, along with the fairly high concentration of FA in the NaOH-extractable fraction, seems to support the theory that NaOH-extractable FA might not be a truly independent part of the humic substance, rather it is a partial product of the hydrolysis of HA during the NaOH extraction (Orlov, 1999). Longer reaction time and more vigorous shaking during the extraction procedure would result in more FA compared with the actual amount of FA present.

Relation of Extractable Organic Carbon to Carbon Dioxide Evolution Rate Concentration of OC in each of the nine fractions was highly (p ⬍ 0.01) and linearly correlated to CO2 evolution rate (r ⫽ 0.59 to 0.89), except for the water(2h)– extractable HA (r ⫽ 0.27) (data not shown). The NaOH(2h)–extractable FA had the highest correlation coefficient (r ⫽ 0.95, p ⬍ 0.001) followed by the water(2h)–extractable FA fraction (r ⫽ 0.89, p ⬍ 0.001) (Fig. 1). The combination of the water(2h)–extractable FA and NaOH(2h)–extractable FA was also highly correlated with CO2 evolution rate, with r ⫽ 0.93 (p ⬍ 0.001). Thus, FA concentrations of NaOH(2h) or water can be used to indicate compost stability. In addition, FA can be easily measured by a simple photometric method, since the MSA of FA is within a relatively narrow range of 0.05 to 0.18 L mg⫺1 m⫺1 (Table 3). In fact, FA concentration was highly correlated to its absorption with r ⫽ 0.95 (p ⬍ 0.001) for the water(2h)–extractable FA and r ⫽ 0.87 (p ⬍ 0.001) for NaOH(2h)–extractable FA fraction (Fig. 2). Concentrations of OC in these compost extracts covered a wide range up to 4 g L⫺1 C, which makes it possible to determine OC concentration in a simple fashion.

Table 3. Mass specific absorbance of water- and NaOH-extractable fulvic acid (FA) and humic acid (HA). Water Compost Register A Register A Register A Register B Register B Register B Register B Winslow Winslow Winslow Sunset Sunset Sunset Meadow Meadow Puerto Rico Puerto Rico Puerto Rico

Curing d 0 7 30 0 9 21 25 0 7 30 0 14 30 30 90 0 7 14

FA 0.06 0.09 0.10 0.08 0.07 0.10 0.13 0.09 0.10 0.15 0.10 0.10 0.12 0.07 0.10 0.05 0.06 0.05

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.1 M NaOH for 24 h HA

0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00

0.33 0.44 0.68 1.06 0.64 0.49 0.46 0.23 0.31 0.94 0.71 0.80 1.14 0.40 1.26 0.84 0.45 0.41

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.07 0.05 0.07 0.56 0.00 0.02 0.04 0.02 0.03 0.05 0.01 0.02 0.07 0.00 0.26 0.06 0.00 0.04

FA 0.09 0.11 0.12 0.10 0.11 0.14 0.16 0.12 0.12 0.18 0.10 0.11 0.12 0.11 0.16 0.14 0.15 0.16

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.1 M NaOH for 24 h

HA L mg⫺1 m⫺1 0.00 0.90 ⫾ 0.05 0.00 1.18 ⫾ 0.02 0.01 1.39 ⫾ 0.08 0.00 0.93 ⫾ 0.00 0.00 0.88 ⫾ 0.00 0.00 1.41 ⫾ 0.02 0.01 1.30 ⫾ 0.04 0.02 0.93 ⫾ 0.05 0.00 1.30 ⫾ 0.39 0.02 1.58 ⫾ 0.10 0.00 0.62 ⫾ 0.09 0.00 1.04 ⫾ 0.22 0.01 0.74 ⫾ 0.03 0.01 0.38 ⫾ 0.03 0.01 0.83 ⫾ 0.08 0.00 0.72 ⫾ 0.03 0.00 0.90 ⫾ 0.08 0.01 1.36 ⫾ 0.07

FA 0.09 0.10 0.11 0.09 0.10 0.13 0.11 0.07 0.09 0.13 0.09 0.08 0.09 0.08 0.10 0.13 0.13 0.17

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

HA 0.00 0.01 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.02 0.00 0.01 0.01

0.85 1.13 1.24 0.96 1.00 1.46 1.34 0.89 1.00 1.38 0.61 0.59 0.66 0.45 0.69 0.72 0.88 1.01

⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾

0.01 0.02 0.05 0.00 0.20 0.11 0.04 0.01 0.06 0.02 0.07 0.12 0.04 0.01 0.04 0.01 0.09 0.07

WU & MA: COMPOST STABILITY AND EXTRACTABLE ORGANIC C RELATIONSHIP

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Fig. 1. Relationship between compost CO2 evolution and organic carbon concentration. Water(2h)–extractable fulvic acid and NaOH(2h)–extractable fulvic acid represent samples sequentially extracted for 2 h with water and NaOH, respectively.

To further understand the contribution of each fraction of OC to CO2 evolution rate, a stepwise multiple regression procedure was used. The regression equation (p ⬍ 0.01) is as follows: CO2 ⫽ ⫺0.252 ⫹ 0.013FAwater2h ⫺ 0.037HAwater2h ⫹ 0.025FANaOH2h ⫹ 0.013HANaOH2h

[1]

The regression model includes water(2h) and NaOH(2h) fractions, which account for 95% (p ⬍ 0.01) of the variability, and indicates that the NaOH(24h)–extractable OC does not significantly contribute to CO2 evolution. It may also indicate that the NaOH(24h)–extractable OC is either a stable humic substance or a lignin-type material that is resistant to decomposition, although the percentage of total extractable OC is the highest for this fraction.

It is interesting to note that the water(2h)–extractable HA had a negative effect on CO2 evolution (Eq. [1]), while both NaOH(2h) fractions contributed positively to CO2 evolution. Based on fractionation and NMR data, Chefetz et al. (1998b) concluded that one fraction of water-extractable HA was related to real HA and this fraction represents an intermediate state in the humification process. The negative correlation between CO2 evolution rate and the OC content of water(2h)–extractable HA leads one to speculate that the water(2h)–extractable HA fraction was not an easily available carbon source for microbial growth. Rather, it may ultimately polymerize and precipitate into water-insoluble HA as the compost further matures. The ratio of HA to FA is not significantly related to

Fig. 2. Relationship between organic carbon concentration of fulvic acid and absorbance at 420 nm. Water(2h)–extractable fulvic acid and NaOH(2h)–extractable fulvic acid represent samples sequentially extracted for 2 h with water and NaOH, respectively.

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CO2 evolution rate if all of the samples are considered together, although it was significantly (p ⬍ 0.01) correlated to CO2 evolution rate for Register samples (data not shown). A similar observation was made by Jimenez and Garcia (1992) and Bernal et al. (1998). They attributed this inconsistency to the dependency of this parameter on the differences in compost source materials.

CONCLUSION Extractable OC decreased with curing, which was influenced by the composition of compost source materials. Except for water(2h)–extractable HA, the OC contents in the FA and HA fractions of each fraction correlated to each other and the CO2 evolution rate, but multiple regression data showed that the NaOH(24h) fraction is insignificant in contributing to the total CO2 evolution. The water(2h)–extractable FA and/or NaOH(2h) tests are recommended as indicators for compost stability due to their high correlation with compost stability estimated by CO2 evolution. The FA concentrations can be estimated by a simple photometric method covering a wide range of C concentrations. REFERENCES Adani, F., P.L. Genevini, F. Gasperi, and G. Zorzi. 1997. Organic matter evolution index (OMEI) as a measure of composting efficiency. Compost Sci. Util. 5:53–62. Adani, F., P.L. Genevini, and F. Tambone. 1995. A new index of organic matter stability. Compost Sci. Util. 3:25–37. Battin, T.J. 1998. dissolved organic matter and its optical properties in a blackwater tributary of the upper Orinoco river, Venezuela. Org. Geochem. 28:561–569. Bernal, M.P., A.F. Navarro, A. Roig, J. Cegarra, and D. Garcia. 1996. Carbon and nitrogen transformation during composting of sweet sorghum bagasse. Biol. Fertil. Soils. 22:141–148. Bernal, M.P., C. Paredes, M.A. Sanchez-Monedero, and J. Cegarra. 1998. Maturity and stability parameters of composts prepared with a wide range of organic wastes. Bioresour. Technol. 63:91–99. Calace, N., M. Capolei, M. Lucchese, and B.M. Petronio. 1999. The structural composition of humic compounds as indicator of organic carbon sources. Talanta 49:277–284. Chefetz, B., F. Adani, P. Genevini, F. Tambone, Y. Hadar, and Y.

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