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Published online 5 July 2005
Published in J Environ Qual 34:1392-1403 (2005)
DOI: 10.2134/jeq2004.0257
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Waste Management

Physicochemical Characteristics of Animal and Municipal Wastes Decomposed in Arid Soils

Farouk Faresa, Akram Albalkhia, Jerzy Decb,*, Mary Ann Brunsc and Jean-Marc Bollagb

a Department of Soil Sciences and Land Reclamation, Damascus University, Syria
b Laboratory of Soil Biochemistry, Penn State Institutes of the Environment, 107 Research Building C, The Pennsylvania State University, University Park, PA 16802
c Department of Crop and Soil Sciences, 116 Agricultural Sciences and Industries Building, The Pennsylvania State University, University Park, PA 16802

* Corresponding author (jdec{at}psu.edu)

Received for publication July 6, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The application of anaerobically processed animal manure to maintain adequate levels of organic matter in arid soils is becoming a common practice. The purpose of this study was to characterize two farm manure products as compared with municipal waste compost (MWC). The anaerobic processing to obtain a biogas manure (BM) product was much faster (25 d) than the aerobic composting of farmyard manure (FYM) (90 d). Drying by three different methods (solar-drying, vacuum-drying at 45°C, and freeze-drying) did not affect the quality of BM. Based on the chemical characteristics, FYM and BM products were comparable, and, containing less ash (30%) and heavy metals (50 mg Pb kg–1), seemed superior to MWC that contained 65% ash and 108 mg Pb kg–1. On the other hand, MWC had higher C content (69.9%), lower acidity (15.04 mol kg–1), and higher exothermic peaks (300–460°C) than BM and FYM (50% C, 20 mol kg–1, and 275–450°C, respectively), thus showing a greater extent of humification. Also, when the organic materials were incubated with arid soils and monitored for mean residence times (MRT), MWC was slightly more resistant to decomposition (MRT 175–180 d) than BM or FYM (MRT 161–166 d). The observed differences, however, were too small to dismiss any of the products as a valuable material for land applications to improve soil quality. In view of the results obtained, MWC may be considered an adequate substitute for BM or FYM, whenever the latter are in short supply.

Abbreviations: BM, biogas manure • DOP, dried organic product • DTA, differential thermal analysis • EC, electrical conductivity • FYM, farmyard manure • MRT, mean residence time • MWC, municipal waste compost • TEOM, total extractable organic matter • TGA, thermogravimetric analysis


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SOIL ORGANIC MATTER has a major impact on biogeochemical cycles, and is essential for maintaining soil quality and sustaining agricultural production (Doran et al., 1994; Allmaras et al., 2000). Conventional farming practices have been reported to result in decreasing humus content in soil (Beyer and Blume, 1990; Buyanovski and Wagner, 1998; Bruce et al., 1999), and soil dehumification constitutes a risk for the bioenergetic regime of soil ecosystems (Kovda, 1990). The rate at which organic matter is decomposed depends on its chemical composition and environmental conditions. Many factors such as temperature, moisture, pH, available nutrients, and C to N ratio may interact to control the size and turnover rate of soil organic matter (McDaniel and Munn, 1985; Burke et al., 1989; Hassink, 1994; Alvarez and Lavado, 1998).

Arid soils are generally poor in organic matter due to rapid oxidation, shortage of organic amendments, grazing or burning of crop residues after harvest, unsound decision making in terms of crop rotation, and destructive tillage (Nahal and Fares, 1995; Six et al., 2000). As a result of these practices, arid soils seldom contain more than 1 to 2% organic matter (USDA, 1979).

The beneficial effects of biosolids of both agricultural and urban origin on the content of organic matter in soil are well documented (Pierzynski, 1994). Many scientific papers discussed the climate-controlled organic matter storage in soil (C sequestration) (Lal et al., 1998; Jenzen et al., 1998; Grant et al., 2001; Izaurralde et al., 2001), especially in arid and hyper-arid areas. Little has been done, however, to examine how physicochemical characteristics of aerobically and anaerobically processed biosolids may influence their turnover in arid soils.

Subjecting cattle (Bos taurus) manure to composting or anaerobic processing to generate biogas are useful methods of obtaining a stabilized product that can be stored or spread with little odor emission or fly breeding (Eghball, 2000). Other advantages of composting or anaerobic treatment include considerable attenuation of pathogens and weed seeds, and the reduction of volume and weight of animal manure (Rynk et al., 1992; Eghball and Power, 1994; Eghball, 2000).

Composting has some disadvantages, such as loss of nutrients and organic C during aerobic processing. Eghball et al. (1997) found a 20 to 40% reduction in total N and a 46 to 62% decrease in total organic C during composting of beef cattle feedlot manure. There were also significant losses of K and N in runoff from composting manure windrows during rainfall (Eghball, 2000). In contrast, the residual organic matter, after producing biogas, seems to be associated with very slight loss in nutrients (N, P, and K) (Eghball, 2000).

The objectives of this study were to assess the quality of two manure products (composted farmyard manure and anaerobically processed manure) compared with compost from the Damascus (Syria) city composting plant. The farmyard manure was composted in windrows under aerobic conditions, and the biogas manure was processed anaerobically in a biogas generator. The comparisons were based on (i) determining the chemical composition and biochemical characteristics of the three organic materials and (ii) monitoring biodegradation rates of these products in two arid soils under controlled aerobic conditions.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Organic Materials
Farmyard manure (FYM) and biogas manure (BM) products were obtained by processing fresh dairy feces (without bedding) of Bos taurus, sub sp. Primigenius (Holestien Friesian) under aerobic or anaerobic conditions, respectively. Aerobic processing or composting (to obtain dry FYM product) took 90 d during the spring seasons of 1998 and 1999, and was completed twice using about 500 L of fresh feces placed in a pit with a capacity of 1 m3 excavated near the dairy cattle farm at Kharabo farm of the Faculty of Agriculture (Damascus University, Syria). The feces were turned over twice a week throughout the whole composting period to maintain aerobicity.

For the BM, about 980 L of a mixture of feces and water (1:1) was fed into a biogas reactor with a capacity of 1 m3 (Fig. 1) built near the pit. The anaerobic processing was performed in the spring seasons of the years 1998 and 1999. To initiate the operation, 20 L of an anaerobically fermented slurry containing approximately 8 to 10% of the solid phase material was collected from an active biogas generator (located at the same Faculty of Agriculture farm) and added as an anaerobic microbial inoculum to the total volume (about 980 L) of the initial inactive slurry mixture. After 25 d of the initial biogas reactor operation, a 40-L portion of the active slurry (4%) was removed daily and replaced with fresh 1:1 feces to water slurry. The temperature of the mixture was measured daily and ranged from 16 to 35°C. The pH, which was measured once a week, ranged from 6.1 to 7.8. Following each replacement, the slurry was mixed briefly by manual agitation of the biogas-floating collector (Fig. 1).



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  		Fig. 1. A general scheme of the biogas generator.

 
The anaerobic organic matter suspension collected through a side pipe was dried by three different methods: (i) solar (air) drying in windrows at the temperatures ranging between 16 to 35°C, (ii) rotary evaporation under vacuum at 45°C, and (iii) freeze-drying at –53°C in a Heto Drywinner 3 lyophilizer (Heto, Allerød, Denmark).

The municipal waste compost (MWC) was prepared by 60-d aerobic processing and air-drying at a local municipal waste facility.

Before characterization, dry FYM, BM, and MWC were finely milled (<250 µm). The total extractable organic matter (TEOM) was obtained by a standard procedure (Swift, 1996) that involved shaking each dried organic product (DOP) overnight in a 1:10 (w/v) solution of 0.1 M NaOH at 25°C, under N2 gas. The extract was separated by centrifugation at 5000 x g, and passed through an exchange resin (Dowex-50 H+; Dow, Indianapolis, IN); the pH of the extract ranged between 2.9 and 3.2. The resulting TEOM extracts from each of the products were divided in two; the first half was refrigerated at 4°C under N2, and the second was freeze-dried and refrigerated under vacuum.

Physicochemical Characterization of Test Materials
The gravimetric water content was determined after a 24-h drying of manure products at 100°C. Heating at 650°C for 24 h was used to determine the ash contents. Analyses for carbon, nitrogen, and hydrogen were performed using a CHN elemental analyzer (Vario Analyzer; Elemental Analysensysteme, Hanau, Germany). Oxygen content was determined by subtraction (assuming that C, H, N, and O contents constituted 100%). The pH was measured potentiometrically in 1:10 DOP to water and in 1:100 TEOM to water. The electrical conductivity (EC) of both DOP and TEOM was determined in extracts of DOP (1:10, DOP to water) and TEOM (1:100, TEOM to water), respectively. Total acidity and carboxylic and carbonyl group concentrations were determined by conventional titration methods (Schnitzer, 1982). Phenolic hydroxyl concentrations were obtained by subtraction. The UV/visible spectra of the TEOM were recorded over the wavelength range 200 to 750 nm, using a UV/VIS spectrophotometer (S750; Secomam, Domont, France). The absorptions at 465 nm and 665 nm were used for calculation of E4 to E6 ratios (Chen et al., 1977; Olk et al., 1999). Atomic ratios were calculated so that each element was represented by the quotient of its concentration and atomic weight.

A TGA/DTA instrument (TG-ATD 1600; Labsys, Caluire, France) for both thermogravimetric analysis (TGA) and differential thermal analysis (DTA) was used for determining the thermal properties of the different preparations of organic materials. The TGA scanning was done at a constant heating program ranging from 25 to 600°C at a temperature rate of 2°C min–1 under N2 atmosphere. The DTA thermograms were obtained at a temperature program of 5°C min–1 under a static air atmosphere. The Pt/Pt-Rh thermocouples were directly immersed in organic materials (15 mg) and a reference standard ({alpha}–Al2O3). The trace elements (Fe, Mn, Cu, and Zn) and heavy metals (As, Cd, Cr, and Pb) contents in DOP and TEOM were determined using an atomic absorption spectrophotometer (932 AA; GBC, Melbourne, Australia) following the digestion with 30% hydrogen peroxide (H2O2) at 90°C, addition of 1 M HCl, further heating at 90°C to achieve complete oxidation, and filtration.

Soil Analyses
The selected soils were Soil A, a brown calcareous clay loam soil (Aridisol Xerochrepts) of Damascus (Abou-Jarach), and Soil B, a fine montmorillonitic clay soil (Aridisol Calciorthids) of Daraa (Mahaja), which are common arid soils. Soil A had a high calcium carbonate content (47%) and relatively high content of organic matter (2.4%). Soil B had a high montmorillonitic clay content (45.5%) and a low organic matter content (0.67%). The soils were sampled from the 0- to 20-cm layer by multiple coring. Ten cores (about 550 g, wet weight) were collected for each soil. The cores were placed without thorough mixing in a plastic bag and stored at 4°C at their original field moisture until used for analysis or incubation with manure products.

Soil particle size analysis was determined by the hydrometer method (Gee and Bauder, 1986). Soil pH was measured in deionized water, and in 1 M KCl, using 1:2.5 soil to liquid ratios. The organic C content was estimated by rapid oxidation with a hot mixture of K2Cr2O7 and H2SO4 (Nelson and Sommers, 1996). Total and organic nitrogen were measured by the Kjeldahl method (Bremner and Mulvaney, 1982) on a Kjeltic System 1015 (Tecator, Höganäs, Sweden). Calcium carbonate was determined using a homemade calcimeter (Damascus University). The cation exchange capacity was determined by sodium acetate extraction according to Polemio and Rhoades (1977). Total phosphorus and potassium were determined according to Saunders and Williams (1955), and available P and exchangeable K were measured using the procedures of Olsen and Sommers (1982) and Richards (1954), respectively.

Incubation Experiments and Statistical Analysis
The degradability of BM, FYM, and MWC was determined by placing 100 g of Soil A or B (oven dry-weight, 105°C) in 250-mL flasks (16 flasks for each soil) and thoroughly mixing (in four replicates) with the dry organic products (2% of soil weight), so that the weight contents of DOP added were equivalent to 2 g (oven dry-weight, 105°C). Four soil samples (for each soil) without DOP amendments served as controls. Moisture content was adjusted to 80% of field capacity, which was 44% for Damascus soil (A) and 46% for Daraa soil (B). The soil–DOP mixtures (except for controls) were inoculated with 1 mL of fresh water extract from the refrigerated soils, and incubated in triplicates at 28 ± 0.1°C for 95 d. The incubation was performed under aerobic conditions. Soil moisture was maintained at the initial level by adding water (once a week) to compensate for the decrease of total weight (flask plus incubated soil) due to evaporation. The CO2 evolved during the incubation was trapped in 1 M KOH solution (placed in 20-mL vials suspended inside the flasks) and analyzed periodically by titration with 0.1 M HCl (Dec et al., 1990); 13 titrations were made for each sample over an incubation period of 95 d.

There was a total of 32 samples (16 for each of the two tested soils) distributed randomly throughout the incubation chamber. The experiments were replicated four times, which resulted in small coefficients of variation. Significance was set at P < 0.05 and at P < 0.01 for organic C content, total N, functional groups, ash content, trace elements, heavy metals, and the daily and cumulative CO2 evolved from the incubated organic matter–soil systems during 95 d. Least significant difference (LSD) for the tested soils was calculated as a measure of their intrinsic biodegradability potential at P < 0.05 and P < 0.01. Statistical analyses were performed with the STATISTICA for Windows 5.1 program (StatSoft, 2004).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Chemical Composition and Biochemical Characteristics of Dried Organic Materials
The effect of manure processing on the quality of the final organic products was assessed based on the changes in chemical and biochemical characteristics of the processed materials. This study mainly addressed the effect of (i) different manure processing conditions (aerobic composting vs. anaerobic biogas generation) and (ii) different drying techniques and operation procedures. To a limited extent, the effect of the applied analytical procedures was also considered.

Elemental Composition and Atomic Ratios
In general, although the differences in chemical composition of aerobically (FYM) and anaerobically (BM) processed manures were statistically significant, these two manure products can be considered similar to each other (Table 1). However, they both differed markedly, especially in terms of carbon and oxygen contents, from the MWC. The elemental composition was calculated on an ash-free basis and, due to the elevated ash content (65.41%), the organic C content of the aerobically processed MWC was higher (69.93%) than the organic C contents of FYM and BM, which ranged between 48.49 and 51.42%. In contrast, oxygen and hydrogen contents of MWC were significantly lower than those of the other organic materials (Table 1).


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  		Table 1. Elemental composition and atomic ratios of the dried organic products (DOP).{dagger}

 
The air-dried FYM showed a slightly lower organic C content than BM dried by three different techniques (Table 1). Inversely, the O and N concentrations in the former tended to be higher than those in the latter (Table 1), but statistically the O and N values did not differ. The vacuum applied during rotary evaporation (45°C) or freeze-drying (–53°C) caused negligible losses in N contents through volatilization, as indicated by minimal differences between C to N ratios for rotary-evaporated, freeze-dried, and air-dried BM (16.76, 16.46, and 16.15, respectively) (Table 1). This is consistent with previous investigations (Mahimairaja et al., 1990; Atallah et al., 1995), showing that freeze-drying caused limited loss of N when compared with other drying methods. In general, the C to N ratios for BM products and FYM were significantly smaller than the C to N ratio for MWC (Table 1).

The H to C ratio reflects the percent saturation of the C atom. According to Thurman (1985), an H to C ratio between 1:1 and 2:1 indicates that the carbon atoms are aliphatic, whereas an H to C ratio between 1:1 and 1:2 indicates aromatic structure. The H to C ratios for BM and FYM products (1.06–1.22) (Table 1) correspond well with those shown by a variety of humic substances (Steelink, 1985; Filip et al., 1999). The range of H to C ratios for BM and FYM (1.06–1.22) implied a largely aliphatic character, relatively low aromaticity, and relatively high biodegradability of the products. On the other hand, MWC showed a relatively low H to C ratio (0.67), which indicated an essentially aromatic character and reduced aliphatic component. Consequently, MWC may be less biodegradable than the other tested materials.

The O to C ratio is considered an indicator of the nature of humic substances (Steelink, 1985). In general, humic acid O to C ratios cluster around 0.5, while those for fulvic acids cluster around 0.7 (Stevenson, 1994). Many humic substances present in aquatic media and sediments show O to C ratios between 0.5 and 0.75 (Steelink, 1985). At similar oxygen contents, the aliphatic character of manure products implies low aromaticity, which is indicative of low reactivity and bioavailability (Gauthier et al., 1987; Christensen et al., 1998). The aerobically and anaerobically processed manures showed O to C ratios ranging between 0.58 and 0.66, suggesting high contents of carboxyl, phenolic, or carbohydrate moieties (Table 1). The municipal compost showed a relatively low O to C ratio (0.24), accompanied with low acidity and carboxylic moiety content: 15.0 and 1.0 mol kg–1, respectively, as compared with about 20.0 and 2.5 mol kg–1, respectively, for manure products (Table 2). In some cases high O to C ratios, such as those observed for the biogas manure, may be due to high content of carbohydrates (Malcolm, 1990; Christensen et al., 1998).


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  		Table 2. Chemical characteristics of dried organic products (DOP).{dagger}

 
The respective N to C atomic ratios were mainly dependent on the quality of the initial manure material and, to a lesser degree, on the duration of manure processing. Because C concentrations were quite similar in both biogas and farm manure products, the respective N to C ratios did not differ significantly (0.060–0.067) (Table 1). However, N to C ratio for the municipal compost (0.048) was significantly lower. The slightly higher N to C ratio in FYM (0.067) compared with BM (0.061) seemed to be the result of long composting under aerobic conditions (up to 90 d). For comparison, municipal waste was composted for 60 d showing a reduced N to C ratio (0.048). This probably reflects the low content of the initial N and the very high ash content (65.41%) (Table 1).

Functional Groups, pH, and Electrical Conductivity
Physicochemical characteristics of the organic materials (pH, electrical conductivity, acidity, ion concentrations) are presented in Table 2. The pH of the processed manures and municipal compost, ranging from 8.1 to 8.4, showed little difference with regard to processing procedure. Electrical conductivity (EC) is a particularly useful parameter for the characterization of organic products, as it represents the overall ionic strength of the measured samples. The BM products dried by evaporation and freeze-drying showed lower EC values than FYM and MWC (Table 2). Air-drying after anaerobic processing also resulted in an increased EC value (3.50 dS m–1). This seems to confirm the dependence of ion concentration on the drying technique and processing time (90 and 60 d). Similar results for dried manure materials were reported by Atallah et al. (1995).

Manure products showed small differences in the concentration of acidic functional groups (Table 2). The total acidity for aerobically and anaerobically processed manure was in a range of 19.91 to 21.21 mol kg–1. The municipal compost had a significantly lower total acidity value. Low total acidity values appear to be indicative of increasing maturity of organic products originating from animal manure or municipal composts (Senesi and Loffredo, 1999). In this study, the processing times were 30, 60, and 90 d for biogas manure, farmyard manure, and municipal compost, respectively. Time, then, appeared to have had some impact on the reactivity of tested materials as represented by their total acidity values.

The patterns of carboxylic, phenolic, and carbonyl group contents in manure products corresponded with the pattern of total acidity values in that the functional group contents in manure products were significantly higher than those in the municipal compost (Table 2). During humification, organic materials show increasing carbon content and decreasing aliphaticity, acidity, H to C, O to C, and N to C atomic ratios (Stevenson, 1994; Olk et al., 1999, 2000). As compared with manure products, the municipal waste compost had higher carbon content, lower aliphaticity and acidity, and smaller H to C, O to C, and N to C atomic ratios, thus showing a greater extent of humification.

UV/Visible Spectra
Following the extraction of manure and compost products with 1 M NaOH and Dowex-50 H+ resin purification, the pH of the final TEOM extract was about 3. The UV/visible spectra of TEOM are shown in Fig. 2 . Unlike soil humic substances, whose UV/visible spectra are generally featureless, showing no well-defined maxima or minima, all TEOM extracts showed prominent maxima ({lambda}max) at 350 nm with absorption ranging between 0.39 and 0.72. The {lambda}max absorptions for air-dried farmyard manure and compost (0.47 and 0.40, respectively) were lower than those for air-dried, rotary-evaporated, and freeze-dried biogas manures (0.57, 0.64, 0.72, respectively). The varying absorption values probably reflected differences in dissociation–protonation equilibria for carboxyl and phenolic hydroxyl and, consequently, in the exposure of the chromophores to specific wavelengths (Bloom and Leenheer, 1989). According to previous studies (MacCarthy and O'Cinneide, 1974; Tsutsuki and Kuwatsuka, 1979), the maximum at 350 to 360 nm may be caused by the ionized trihydroxybenzene components of the extractable humic material. With increasing wavelength, the absorption decreased and showed slight shoulders between 400 and 450 nm for farmyard manure and compost extracts.



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  		Fig. 2. UV/visible spectra of the total extractable organic matter (TEOM) from dried organic products (DOP). Each point represents a mean of three replications.

 
The ratio of absorptions at 465 and 665 nm, referred to as the E4 to E6 ratio, is believed to be inversely proportional to the degree of condensation, and has been used as an indicator of the degree of humification (Chen et al., 1977; Stevenson, 1994; Olk et al., 1999). As shown in Table 3, E4 to E6 ratios for manure extracts (6.20–7.50) were higher than the E4 to E6 ratio for the municipal compost extract (4.10). Previous studies (Kononova, 1966; Claus et al., 1999) suggested that low E4 to E6 ratios were indicative of enhanced condensation of aromatic constituents, which in turn indicated enhanced humification. The pattern of E4 to E6 ratios (Table 3) was consistent with that of H to C, O to C, and N to C ratios (Table 1) that were lower also in municipal compost than in manure products.


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  		Table 3. The E4 to E6 ratios (i.e., absorption at 465 nm/absorption at 665 nm) and {Delta} log K values ({Delta} log K = log K465 – log K665) of total extractable organic matter (TEOM) extracts.{dagger}

 
In the visible range (400–750 nm), TEOM absorption spectra approached linearity (Fig. 2), and their respective slopes ({Delta} log K) calculated from the equation {Delta} log K = log K465 – log K665 (Yoshida et al., 1978) are presented in Table 3 along with the E4 to E6 ratios. Previous studies (Kumada, 1965, 1987; Yoshida et al., 1978; Tan, 1998) indicated that, using the {Delta} log K, three types of humic substances could be distinguished: (i) humic acids with {Delta} log K values below 0.6 tended to occur in volcanic ash soils (Type A), (ii) those with {Delta} log K ranging from 0.6 to 0.8 were likely to form in forest soils and paddy soils (Type B), and (iii) those with {Delta} log K ranging from 0.8 to 1.1 were typically present in peat, decomposed grass, and animal manures (Type C). Our manure products with their {Delta} log K ranging from 0.83 to 1.04 (Table 3) belong to Type C (i.e., humic substances present in peat and manure) (Tan, 1998). As was the case for atomic ratios (Table 1), functional group contents, and E4 to E6 ratios, the municipal compost had a somewhat lower {Delta} log K value (0.7) than those shown by manure products (0.83–1.04). Despite the different natures, origins, and humification pathways of municipal compost and soil humic acids, the municipal compost product appeared to resemble Type B soils in terms of recalcitrance.

Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) of Dried Organic Materials
The results of TGA are presented in Fig. 3 . Up to 200°C, the thermogravigrams of all samples show similar weight losses of about 6 to 9%, except for the compost with a 3% weight loss. Weight losses at this temperature range are usually due to dehydration, decarboxylation, and dehydroxylation of phenols (Tan, 1998). Between 200 and 300°C, the weight loss in the manure products increased to between 17 and 30%, and to 10% in the compost, which may be attributed to the decomposition of phenolic acids, mono- and polysaccharides, and partly cellulose. At a range of 300 to 400°C, the weight loss in different manure products increased to 30 to 40%, and to 20% in the compost. At these temperatures, humic acids and lignin decomposed as OH and COOH groups were completely eliminated. Between 400 and 600°C the weight loss, due mainly to the decomposition of aromatic polycondensates with the lowest oxygen content (Wendlandt, 1974; Tan et al., 1986) increased to 40 to 55% for the manures, and to almost 30% for the compost.



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  		Fig. 3. Thermogravimetric analysis (TGA) of the dried organic products (DOP). Each point represents a mean of three replications.

 
The results of DTA are presented in Table 4. Each of the tested organic materials showed two distinct exothermic peaks. The first exothermic peak at 275 to 310°C was attributed to the decomposition of humic acid accumulated during manure and compost processing. The second peak at 440 to 460°C signified the decomposition of the accumulated fulvic acid. According to the literature (Schnitzer and Kodama, 1972; Tan, 1978; Tan et al., 1986), soil humic acids have strong exothermic peaks at 400 to 410°C (i.e., at a temperature range about 100°C higher than that for our organic products). Soil fulvic acids that are slightly more thermostable than soil humic acids usually show strong exothermic peaks at about 500°C (Tan et al., 1986), which is about 50°C more than in the case of our organic materials.


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  		Table 4. Differential thermal analysis (DTA) of test organic materials.{dagger}

 
Despite the temperature difference and despite the different natures and origins of the processed organic products and soil organic matter, the exothermic peaks of these materials seem to correspond to each other. In considering the temperature difference one has to stress that exothermic peaks of some samples of soil humic and fulvic acids showed a significant shift to lower temperatures (340°C and 430°C, respectively) as compared with temperatures cited above (Dupuis, 1971). This was probably because of the presence of cation species such as Ca2+, Ba2+, and Fe3+ that usually reduce the thermal stability of humic acids (Tan, 1978). Schnitzer and Kodama (1972) reported that the divalent and trivalent cations interact with carboxyl and phenolic hydroxyl groups causing a severe strain on the molecular structure of humic acid. As a result, metal–humic acid complexes become less stable against thermal decomposition. Similar findings were reported by Dupuis (1971), who concluded that at high metal to humic acid ratios, the resulting chelates become water-insoluble and less thermostable than the soluble or pseudo-soluble complexes.

As our organic materials were not extracted or purified, their ash content, ranging from 31 to 65%, was relatively high (Table 1). The most abundant cations in the ash were Ca2+, Fe3+, Mn2+, Zn2+, and Cu2+ (data not shown). Consequently, it seems highly probable that these cation species caused shifting of the respective exothermic peaks toward lower temperatures (Table 4) relative to those expected for purified humic and fulvic acids.

Trace Elements and Heavy Metals
Table 5 presents the contents of major trace elements (Fe, Mn, Cu, Zn) and toxic heavy metals (Cr, Cd, Pb, and As) in all of the test organic materials. The concentration of these elements in the municipal compost product (0.7–2720 mg kg–1) was much higher than in any of the manure products (0.4–1500 mg kg–1), which among themselves showed small or no significant differences in either trace element or heavy metal contents (P < 0.05 and 0.01, respectively). A similar pattern for trace element and heavy metal concentrations was determined when the organic products were extracted with 0.1 M NaOH and purified on the Dowex-50 H+ resin (data not shown). The difference in metal concentrations between manure and municipal compost correlated well with the difference in ash contents of these products. The high ash content of the compost (65.4%; Table 1) as compared with that of the manure products (31.0–33.4%; Table 1) was probably due to the diversity of waste materials entering the municipal sewage system. Hoof disinfectants, feed stuffs, feed additives, and mineral fertilizers are the most common sources of trace elements and heavy metals in animal manure (Schultheiß et al., 2003), and probably were the source in the tested organic materials. Another possible source were the emissions from a variety of industrial sites in the Damascus area.


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  		Table 5. Concentration of trace elements and heavy metals in the dried organic products (DOP).{dagger}

 
Degradation Kinetics of Air-Dried Organic Products in Soil
The decomposition of the organic products in soil was studied using only the air-dried materials (biogas manure, farmyard manure, and municipal compost). Biogas manures dried by rotary evaporation or freeze-drying were excluded because these drying methods are not common in large-scale manure processing. Two soils were used in this experiment. Their physicochemical properties are presented in Tables 6 and 7.


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  		Table 6. Physical properties of Aridisol Xerochrepts (Soil A) and Aridisol Calciorthids (Soil B).

 

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  		Table 7. Chemical properties of Aridisol Xerochrepts (Soil A) and Aridisol Calciorthids (Soil B).

 
Carbon Dioxide Evolution Kinetics
The amounts of CO2 released (in mg kg–1 of soil) from Soil A during a 95-d incubation with different organic products are shown in Fig. 4A . The pattern of CO2 evolution from Soil B did not statistically differ from that for Soil A (at P < 0.05 and 0.01), so only the latter is presented. In the first 3 d, CO2 evolution from all samples, including the controls (soil only), quickly increased from 52.4 mg kg–1 (control) and 80.1 to 87.0 mg kg–1 (amended soils) on Day 1 to 273.6 and 321.9 to 349.7 mg kg–1, respectively, on Day 3. Then CO2 evolution steadily decreased to 47.4 mg kg–1 (control) and 82.5 to 89.1 mg kg–1 (amended soil) between Days 4 and 36 to remain at those levels throughout the rest of the experiment. Statistical analysis indicated significant differences (at P < 0.05 and 0.01) in CO2 production between controls (nonamended soils) and all amended samples throughout the whole incubation period. The relatively high CO2 production in controls, especially at the early stages of the experiment, may be attributed to microbial respiration fueled by the indigenous soil organic carbon. Apparently because of that, no lag periods were observed in the startup of CO2 evolution from manure or compost amended soils.



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  		Fig. 4. Periodical (A) and cumulative (B) carbon losses (in the form of CO2) from Soil A incubated with the biogas manure, farmyard manure, and municipal compost. Each point represents a mean of four replications.

 
After the first 2 d of incubation, when there were no significant differences, MWC showed significantly lower CO2 evolution (at P < 0.05 and 0.01) compared with BM and FYM until Day 17; for the rest of the incubation period, BM, FYM, and MCW did not differ in their CO2 outputs (Fig. 4A). The reduced mineralization during the intermediate 15-d period reflected the fact that the stability of MWC assessed on the basis of C content and other parameters (Table 1 and 2) was greater than that of BM and FYM. Reduced mineralization could be anticipated based on the results of spectral and thermogravimetric analyses (Fig. 2 and 3, Tables 3 and 4), showing lower {lambda}max absorption and weight loss for MWC as compared with those for BM and FYM.

Decomposition of Organic Matter
Figure 4B presents the cumulative losses of organic carbon due to CO2 evolution (in mg C kg–1 of soil). The loss of organic carbon in controls (nonamended soils) was significantly slower than that in amended soils (P < 0.01) from Day 2 throughout the rest of incubation period. Also, the decomposition of MWC was significantly slower (P < 0.01) than that of BM and FYM through the whole incubation period except for the first 3 d of incubation. The decomposition of the biogas and farmyard manure did not differ statistically for the initial 10 d, showing small, but statistically significant differences (P < 0.01) throughout the rest of the experiment. The susceptibility of test materials to decomposition decreased in the following order: BM > FYM > MWC > control.

The stability of test materials was evaluated using the following equation, which according to Jenkinson and Rayner (1977) and Reddy et al. (1980) describes the first order kinetics of organic matter decomposition:

where C0 and Ct are mg C kg–1 soil at the beginning of incubation and at time t (d), respectively, and k is the decomposition rate constant (d–1).

The calculated k values for dried test materials (Table 8) ranged from 0.00555 to 0.00618 d–1, which was in agreement with k values shown by a variety of animal wastes (Reddy et al., 1980), such as feedlot deposits (Gilmour et al., 1977), farmyard manure (Gaur et al., 1971), and cattle manure (Ruehr, 1976). On the other hand, k values for the control soil samples (no test material added) were much higher (0.00504 and 0.00508 d–1) than those reported by Broadbent and Nakashima (1974) for a recalcitrant soil organic matter fraction (0.0014–0.0019 d–1), but similar to a k value reported by Reddy et al. (1980) for a labile soil organic matter fraction (0.0058 d–1).


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  		Table 8. Decomposition rate constant (k), half-life time (T1/2), and mean residence time (MRT) of the tested organic materials.{dagger}

 
The respective half-life times (T1/2 = ln 2/k) and mean residence times (MRT = 1/k) of test materials derived from the rate constant k ranged from 112 to 125 d and from 162 to 180 d, respectively (Table 8). The MRT values for manure-amended soils were in agreement with those reported in the literature for manure products (Reddy et al., 1980). However, MRTs for the nonamended control samples were inconsistent with those reported for soil organic matter decomposition under field conditions, which ranged from 50 to 2500 yr (Hsieh, 1992; Wattel-Koekkoek et al., 2003; Jolivet et al., 2003). A similar inconsistency between calculated and actual residence times was pointed out by other authors (Reddy et al., 1980; Beulke et al., 2000; Wattel-Koekkoek et al., 2003). It appears that soil organic matter in the control samples was sufficiently stabilized to defy the first-order decomposition kinetics. Typically, agricultural practices affect the accumulation of organic carbon by influencing the amount of plant residues returned to the soil (Campbell et al., 2000) and the rate at which the residues and organic matter decompose (Gerzabeck et al., 2001). It is widely assumed that long residence times are due to chemical or physical linkages formed between organic matter and soil clay minerals (Martin and Haider, 1986; Theng et al., 1992; Römkens et al., 1998).

The cumulative CO2 evolution from test materials (Fig. 4B) showed similar patterns with an exponential phase during the first several weeks, followed by a plateau. Based on the amount of the decomposed organic carbon, the decomposition index (DI) was calculated for the dried organic matter, using the following formula:

where Ctd is the total decomposition of organic carbon (during 95 d) in soil containing a specific organic material, and Ctc is the total decomposition of organic carbon (during 95 d) in the control (nonamended) soil.

The decomposition index, introduced by Atallah et al. (1995) as the biodegradation index, may be considered a simplified equivalent of the decomposition rate constant k, conveying essentially the same information (about the stability of organic carbon) in convenient integer values: organic materials that decompose faster will show higher DI and k values than less decomposable materials. Based on the DI and k values, organic materials tested in this study can be ordered as follows: BM > FYM > MWC. The DI values for Soil A were 0.61 for BM, 0.57 for FYM, and 0.40 for the MWC (data not shown). The differences were statistically significant (at P < 0.05 and 0.01). Organic products incubated with Soil B showed similar DI values (0.62, 0.56, and 0.37, respectively).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Despite the different origins and processing procedures (aerobic vs. anaerobic), the resulting organic products were quite similar in terms of chemical composition and physicochemical characteristics (Tables 1–4) that determined the stability of these products during incubation with soil (Fig. 4). Aerobic composting of both the municipal waste and cow feces seemed to accelerate the decomposition process and led to a greater biological stability (maturity) as expressed by the DI and k values (Table 8). However, the preparation of these products took 90 and 60 d, respectively, and their quality did not offset the quality of the anaerobically prepared, slightly more degradable product (biogas manure) that required only 25 d for processing. Therefore, it can be concluded that anaerobic processing is a promising procedure in terms of both preparation time and product quality.

The quality of biogas manure proved to be essentially independent of the method of drying. As an added-value waste material, biogas manure is environmentally sound, because it is odorless, and free of pathogenic agents and weed seeds. It can also be a source of clean energy (i.e., anaerobically generated methane). On the other hand, the aerobically processed municipal compost may represent one of the sustainable and renewable organic products in arid zones to minimize the loss of soil carbon via CO2 emission, and at the same time, to enhance the biological activity of soil environments. The relatively high concentrations of heavy metals, such as Cd, As, and Pb (Table 5), in the municipal waste compost constitute a negative factor for growing metal-sensitive crops on compost-amended soils. Therefore, only limited amounts of municipal waste products may be safely applied to the fields, and the application must be strictly controlled.

The data presented may be useful for land managers, who need more insight into the chemical composition and stability of organic products from animal manures and municipal wastes to ensure rational land application of these materials. The focus of this study was the quality of manure and municipal waste products and their biodegradability in arid soils. More information is needed, however, on the nutritional value of these products and the fate of the retained nutrients (especially N and P) in soil. There is also insufficient knowledge of the bioavailability of manure and municipal waste heavy metals, and future studies need to address this issue.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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