Humification and Degradability Evaluation During Composting of Horse Manure and Biowaste

By Desalegn, Getinet Binner, E; Lechner, P

Performing compost quality assessment such as compost stability is quite necessary for rating the quality of horse manure and biowaste composts and meeting specific regulatory requirements on the composition and compost process. The aim of this study was to identify an appropriate feedstock composition for use in the production of high quality compost. The objectives were to (1) identify an appropriate feedstock composition (2) determine organic matter decomposition and humic acids (HA) development in various composts; (3) compare changes in temperature, C/N ratio and respiration activity, occurring during decomposition of organic matter; (4) evaluate phytotoxicity effects of the final compost on cress seed germination and growth. The above changes in physical, chemical and biological parameters were monitored during the laboratory composting process over a period of 20 weeks. Five organic waste blends of horse manure (HM) and biowaste (BW) were used. The results indicated that at the beginning of the experiment, the highest C/N ratio of 59 was recorded in the pure horse manure. This is attributed to the presence of higher amounts of bedding materials (wood chips and wheat straw). The degradation rate of pure HM was slower than that of BW and the degradation had not been completed by the end of the 20 weeks composting period. An ANCOVA with time as the covariate showed that pure BW was more significantly humified than pure HM (p<0.05). Consistently higher contents of HA and lower E4/E6 ratio were obtained in pure BW than in pure HM throughout the processes. Statistically significant difference (p<0.01) among treatments were found for the shoot fresh weight percentage (SFW%). Correlations between the latter and E4/E6 ratios were significant at (p<0.001). Chemical and biological changes indicated an increase in compost quality in correspondence with BW composition increment. Largely the 50/50 blend was not significantly different from other feedstocks (p<0.01) except from pure HM. It was therefore concluded that composting using a combination of HM and BW (50/50) could be used as an alternative method to pure HM composting. Introduction

The number of horses in Austria has increased from 44,858 in 1985 to 72,491 in 1995 (O Statistik Austria 1995). Riding stables face a challenge in handling manure and used bedding (i.e., horse manure, HM) because most horse owners either do not own or have access to land on which to apply untreated manure as occurs in the case of manure produced on traditional livestock farms. Moreover, the horse owners are not inclined to spread HM onto pasture as it may increase incidence of internal parasites and weed seeds and diseases of plants. Composting HM for generation of saleable useful end product offers one potential tool for solving the horse manure disposal use problem, but HM typically requires the addition of complementary feedstock(s) for optimum processing.

Biowaste (BW), authorized for composting by the Austrian Compost Ordinance (O-NORM S 2023 1993), has great potential as a complementary feedstock for HM. Considerable amount of potentially compostable BW is collected annually in Vienna with 90,423 tons generated in 2002 (EU 2003). Composting and reuse of organic waste materials are desirable for agricultural sustainability (Blake and Donald 1992; Golueke and Diaz 1996; Sommer and Dahl 1999) due to pathogen reduction capabilities and the production of beneficial end products (He et al. 1995; Hoitink and Grebus 1994; Inbar et al. 1990; Sela et al. 1998).

Compost quality, as assessed by stability (Adani et al. 2001; Bernai et al. 1998), and maturity must be optimized to ensure confidence in use of the finished product. Application of nonstabilized organic materials to soil could reduce crop quality growth due to the presence of phytotoxic compounds (Butler et al. 2001). Indeed, in numerous cases the key issue is the quality of final compost, which mainly depends on the composition of feedstock and the composting processes (Heal et al 1997) and stability of organic matrices (US Composting Council 1997).

The stability of compost is an important parameter for product quality assessment (Lasaridi and Stentiford 1998). In particular, monitoring changes in humification, C/N ratios and microbial respiration during degradation of organic waste provide an indication of compost quality and stability. Humification is the biochemical process promoting the chemical resynthesis and repolymerization of aromatic and aliphatic moieties produced by the decomposition of fresh organic material. The degree of such organic reconstitution or humification is frequently considered as the most appropriate indicator of compost stability and maturity (Adani et al. 1997; Chen et al. 1996; Ciavatta et al. 1993; Wu et al. 2000). Butler et al. (2001) defined maturity as ‘the degree of humification of the material, qualified this by describing the effects it can have on nutrient availability and plant growth. The E4/E6 ratios for HA in the final composts ranged 3.3 to 8 that suggest that HA in organic wastes are characterized by a low degree of condensation and humification compared to soil HA (Sensi and Brunetti 1996).

The changes in the ratio of the two predominant elements (carbon and nitrogen) have also been considered as an indicator of maturity and stability (Jimenez and Garcia 1989; Watanabe and Kuriharara 1982). Temperature is the easiest parameter to assess as well as the most commonly used parameter to describe the evolution and state of composting processes (Block 1999; Keener et al. 2000). Niese (1963) was one of the first to observe a correlation between temperature and stability. The evaluation of microbial respiration expressed in terms of carbon dioxide (CO2) production rate or oxygen consumption rate provides an accurate and suitable method for assessing compost stability (Hue and Liu 1995). Lasaridi and Stentiford (1996 1998) and Iannotti et al. (1993 1994) measured the oxygen consumption in the headspace above a sealed reactor. Brewer and Sullivan (2001) related stability directly to microbial activity. Adani (1999) determined that stable compost is represented by respiration index values lower than 500 mg O2 kg^sup -1^ volatile solids h^sup -1^ when measured by the dynamic respiration method (i.e. AT4 = 14 mg O2 g^sup -1^ TM in Austria and Germany).

Maturity of compost may be evaluated with the use of the cress seed germination bioassay which is sensitive to excessive salinity or the presence of phytotoxic simple organic acids or phenolic compounds (Handreck and Black 1991; Gajdos 1997). Grebus et al. (1994) demonstrated that the cress (Lepidium sativum) germination and radish (Raphanus sativus) growth were useful indicators of phytotoxicity and maturity. One of the most significant germination tests is that reported by Zucconi et al (1981), and Zucconi et al. (1985), because many later tests are developed from this.

There is scarce information on how the feedstock composition of HM and BW affects decomposition rate during composting and the quality of the final products. The objectives of our studies were to (1) identify an appropriate feedstock composition of HM and BW for use in the production of high quality composts for agronomic / crop production; (2) determine organic matter decomposition and humic acids (HA) development in various composts produced from the combination of various ratios of HM and BW; (3) compare changes in temperature, C/N ratio and respiration activity, occurring during decomposition of organic matter from various ratios of HM and BW; (4) evaluate the maturity of composts as assessed by phytotoxicity effects of the final compost on cress seed germination and growth under the ABF-Technicum Laboratory conditions.

Materials and Methods

Organic Substrate Collection and Preparation

Horse manure (HM) and biowaste (BW) were obtained from a riding stable in Lower Austria and from the Lobau Composting Plant (Vienna, Austria), respectively. All HM and BW materials were shredded smaller than < 20 mm in the M.B.T Type OC 30 shredder (Moschinenhandel Ges.mbH, Vienna, Austria) at BOKU, the Institute of Waste Management. Subsequently, five ratios of HM/BW were prepared on a fresh weight (w/w) basis in a separate plastic bowl and placed in the respective rotting reactors (i.e. R1 = 100/0, R2 = 75/25, R3 = 50/50, R4 = 25/75 and R5 = 0/100) in September 2005. The feedstock or input compositions and particle sizes were determined only for the ratios of 100/0 and 0/100.

Composting Process and Sampling

The aerobic composting process was carried out in the SGAE-H climate chamber (190-cm length x 200-cm width x 195-cm height), Sud- Electric-GmbH, Austria over a 20-week period. Air cycling was provided continuously from the bottom of each reactor (Binner et al. 2002) and by manual turning the composts on sampling dates. The carbon dioxide (CO2) contents in the air from the reactor exhaust regulated air supply manually twice a day. Ideal CO2 concentrations ranged between 10 to 15% (vol). On day 0,7,14,28,49, 84, 112 and 140, about 300 g homogeneous samples were taken from each reactor after thorough mixing of all compost materials in a plastic bowl. The moisture content was measured through drying to constant in an oven at (105 +- 5[degrees]C) at each sampling, and the level ranged from 40% to 65%. The air-dried compost samples at room temperature (23 to 27[degrees]C) were milled in a vibration disk mill (Retsch Type R81, GmbH & Co., Germany) and passed through a 0.63 mm sieve. Total carbon (TC) and total nitrogen (TN) were analyzed in duplicate by combustion in a Variomax CNS analyzer (Kendro Laboratory Products GmbH, Vienna, Austria). Table 1 shows the biochemical composition of 100/0 (pure HM) and 0/100 (pure BW). TABLE 1.

Biochemical characteristics of feedstock

Temperature Pattern

The temperature of the climate chamber was regulated according to the heat produced by the microbes (natural self-heating), being kept however 1[degrees]C lower than the lowest temperature in the reactors (Binner 2002). This was done just to prevent temperature loss from each small reactor by adjusting the climate chamber temperature manually immediately after determining the temperahure in the reactors. No further heat was supplied to the reactors after the self-heating temperature declined to ambient by day 84, at which time no further temperature monitoring was conducted. For the remainder of the study, the compost curing occurred at room temperature of 25[degrees]C.

Organic Matter Degradation

Respirometric Activity

Microbial respiration activity was determined using fresh samples by measuring the oxygen uptake in a Voith Sulzer sapromat (Binner et al. 1998) (H + P Labortechnik GmbH, Germany) for seven days. The oxygen demand for four days (AT 4d) and seven days (AT 7d) were calculated as mg O g^sup -1^ DM (DM = dry matter).

Humic Acids Development

The time course of the humification allowed different rates of organic matter transformation to be detected. Hsu and Lo (1999) showed that the increasing level of HA represents the degree of humification and maturity of compost. The method for extraction and fractionation of humic acids (HA) was according to Gerzabek et al. (1993) adapted by ABF-BOKU using 0.1 M solution of sodium pyrophosphate (pH 10.5). To determine the stability of the compost, HA developments were measured using the two accepted absorbency values at 400 nm and 600 nm by ABF-BOKU with a Shimadzu UV16OU UV- visible spectrophotometer (Hitachi, Ltd, Tokyo, Japan). These data were used for determination of E4/E6, called the color ratio.

Germination Test

Statistical Analysis

All results except the germination test reported in the text are obtained from determinations of a single composite sample. Thus for only the germination test one-way analysis of variance (ANOVA) was carried out using SPSS 11.5 for Windows Statistical Software Package (SPSS 2003). We used one-way Analysis of Covariance (ANCOVA) to determine whether there were differences in C/N ratio, C- degradation, HA, E4/E6 and respiration among feedstock treatments with time as covariate. The least significant difference test at p<0.05 was carried out to compare the means of the triplicates of GR% and SFW% by Tukey's Honestly Significant Difference. The Pearson correlation between HA (E4/E6) and SFW% was determined as well.

Results and Discussion

Feedstock Composition and Particle Sizes

For composting, the composition of feedstock determines the chemical environment, particularly the presence of an adequate carbon (food) / energy source, a balanced amount of sufficient nutrients, appropriate pH, and absence of toxic constituents and physical environment, the correct amount of water and adequate oxygen supply. The smaller the particle size, the greater the available surface area for microbial activity will be, as the majority of aerobic decomposition occurs on the surface of particles. However, smaller particles also reduce the effective porosity of the material, which is essential for microbial activity (oxygen supply). A compromise must therefore be made to create optimum composting conditions. Horse manure was comprised of manure (62%), wheat straw (28%) and wood chips (10%) and biowaste comprised 18% branches, 4% fruit and tubers, 40% grass and leaves and 38% residues.

The particle size fraction present in the highest percentage (35%) was between 2 mm and 0.63 mm (Figure 1). A higher diversity of organic sources with a more or less uniform particle size distribution was detected in the feedstock of 0/100 (pure BW) than 100/0 (pure HM). Composting proceeded more rapidly in pure BW due to its relatively optimal C/N ratio than in pure H.

FIGURE 1. Particle size distribution in the feedstock of pure HM and pure BW.

Changes in Temperature

Temperature in both the pure HM and pure BW composts was thermophilic (above 40[degrees]C) (CCSE 2003 and Madejon et al. 2001) by day 2 of composting and remained thermophilic for 28 days (Figure 2). The temperature reached 60[degrees]C on day 2 of composting, dictating quick establishments of microbial activities in the composting reactor of pure BW. In pure HM, the maximum temperature of 55[degrees]C was recorded on the same date with pure BW. The short-term drop in temperature was due to the turning of the materials on day 7, 14, 28, 49, 84.

FIGURE 2. Changes in temperature of pure HM and pure BW during composting.

C/N Ratio

Choice of raw material determines the C/N ratio and the amounts of easily decomposable nitrogen and carbon compounds. There was statistically significant differences in C/N ratio among feedstock treatments (p<0.05) (Figure 3). The highest initial C/N ratio of 59 was obtained in pure HM due to the presence of a higher content of bedding materials such as wood chips and wheat straw. The ratio was significantly higher than other feedstock (p<0.01) consistently for over 20 weeks. Decreases in C/N ratios from 59 to 30 (50%) and from 26 to 12 (55%) in pure HM and in pure BW were observed, respectively. Hirai et al. (1983) and Savage, (1996) have suggested that the C/N ratio of solid phase should not be used as an absolute indicator of compost maturation due to the large variation depending on the starting materials. However, when the initial value is between 25 and 30 a value equivalent to or lower than 15 can be considered satisfactory (Hirai et al. 1983; Poincelot 1972). The optimal value of C/N ratio could be at the beginning (25-35) and end of the composting process less than 20 (Alessandro et al. 2005). Proper conditions for active composting include a beginning C/N ratio between 20 and 40 (Rynk et al. 1992). Therefore, according to these C/N ratio criteria maturity of pure BW was reached at day 140 while the other four blends needed more time.

FIGURE 3. Changes in C/N ratio during composting of HM and BW and their mixtures (HM/BW). Different letters within the columns indicate significant difference (p<0.01).

Organic Matter Degradation

Treatment at a high initial C/N ratio of 59 (HM) significantly affected the behavior of a number of important parameters during composting of wheat straw and wood chips, none of which are an easily degradable carbon source. Bedding materials affect the physical and chemical properties of fresh feedlot manure and its composted end product (Larney et al. 2002). The contents of total organic carbon (TOC) input material in pure BW declined by 35% with respect to the starting percentage, whilst a decrease of only 10% (47% to 42%) was revealed in pure HM using the calculation with the waste air CO loss balancing the carbon degradation. The degradation of C was not significantly different in each reactor (Figure 4). Nevertheless, the highest carbon loss, equivalent to 55%, was observed in pure BW having lower initial C/N ratio with easily degradable carbon contents. As a result pure BW reached maturity at day 140, while pure HM was only marginally mature. On the other hand, the lowest degradability of carbon (44%) was registered in pure HM, indicating a low degradability of the carbon contents in bedding materials. No marked difference in carbon degradation was observed between 0/100 (55%) and 25/75 mixes (54%) (the two Figures are overlapped). On the whole, there was a general reduction in carbon content in each reactor with time (0/100 > 25/75 > 50/50 > 75/ 25 > 100/0).

FIGURE 4. Changes in sum of carbon degradation during composting of HM and BW and their mixtures (HM/BW).

Humic Acid Development

Composting is believed to be a humification process, thus, concentration of humic substances is expected to increase with composting (Wu, L and L. Q. Ma 2002). The increasing level of HA represents the degree of humification and maturity of compost (Veeken et al. 2000). Adani et al. (1999) contends that there are no scientific data that confirm with any certainty that an increase in humic substances content occurs during composting. Humic substances data are presented on a relative basis. The results obtained in this study demonstrate the HA content of the pure BW was on average greater than that of a pure or / and blends of HM (the regression coefficients for each treatment were positive and were significantly different (p<0.01) (Figure 5). The HA formation in 50/50 blend was not significantly different from both 75/25 and 25/75 mixes (p<0.05). As a whole HA contents increased over time in all reactors during composting indicating progressing of humification (the regression coefficient for time was positive and significantly different from 0). The HA concentration increases and depends upon the nature of the substrate and the composting technique applied (Adani et al. 1999; Witter and Lopez-Real 1987). The lignin theory proposes that humic acids are comprised of modified lignin (Stevenson 1994) and biodegradation of lignin in a compost environment indicated by Tuomela et al., (2000).

FIGURE 5. Changes in HA content during composting of HM and BW and their mixtures (HM/BW). Different letters within the columns indicate significant difference (p<0.01).

The degree of humification process was significantly affected by feedstock composition (p<0.01) and it decreases with increase in HM proportion. The E4/E6 ratios of HA extracted from composts in Figure 6 indicated that HA in the pure BW compost are significantly higher in molecular weights than in the pure HM and 75/25 (HM/BW) (p<0.01). That might be attributed to the presence of higher aromatic moieties and appropriate or optimal value of C/N ratios in pure BW (Alessandro et al. 2005) for humification at this specific composting time too. During the composting of municipal solid waste, the HA aromatic carbon increased by 39% while the aliphatic carbon decreased by 19% (Garcia et al. 1992). Chen et al. (1996) concluded that the raw materials components influence not only the time of composting but also the formation of humic substances. FIGURE 6. E4 to E6 ratios of HA during composting of HM and BW and their mixtures (HM/BW).

Respiration Activity

Microbial activities were seen to be higher in the 50/50 and 75/ 25 mixtures than in the other mixtures at day 28 and 140 in both AT4 and AT7 (Table 2), possibly due to the presence of a more suitable environmental situation for microbes in those reactors. There were no statistical significant differences among treatments with time as a covariate (p < 0.05). In fact, in AT4 the highest and lowest values of microbial activities were observed in pure BW at day zero and at the end of composting period, respectively (Table 2).

TABLE 2.

Changes in microbial respiration activity (AT4 and AT7) on day 0,28 and 140

Germination Test

The plant bioassays performed on final composts indicated that the fresh yields of cress were better increased in pure BW than pure HM and its mixtures. The GR was 100% in each pots treated with 15% and 30% of compost. However, in pots treated with 45% HM compost and its blends less than 85% GR were recorded (Figure 7).

FIGURE 7. Effects of 45% final composts of HM and BW and their mixtures (HM/BW) on cress GR.

Statistically significant differences were observed among treatments (p<0.01) (Figure 7 and 8). Implying how fresh shoot yields of cress were more responsive to pure BW than pure HM. The lowest value of 80% GR and 44% cress SFW were recorded in pots treated with 45% pure HM compost. The immaturity of the pure HM compost might be the main cause of inhibiting effects produced on cress seed growth in these pots. This is indicated in a final C/N ratio above 20 that may not readily release nitrogen for plant.

FIGURE 8. Effects of final composts of HM and BW and their mixtures (HM/BW) on cress SFW.

A bivariate correlation and linear regression analysis was conducted using HA ratios (E4/E6) as a predictor of SFW%. The result indicates the presence of an inverse relationship between the plant growth test and chemical results (SFW% and HA) (p<0.01) when cress seeds were grown in a mixture of 15% compost (Figure 9) and 30% compost (Figure not shown here). Pearson's correlation coefficient (r) of composts was found as 0.99. A significant (p<0.001) coefficient of linear determination (R^sup 2^) of 0.9798 for this model indicated that SFW% could be estimated by this parameter.

Figure 9: Relationship between HA ratios (E4/E6) in the final composts and cress growth (SFW%).

These correlation indicating stable composts produced from pure BW were more beneficial to plants than pure HM composts. This finding adds further support to prove the conviction that the maturity of compost can also be identified by chemical analyses such as humification. The finding verified how stable and suitable compost material for use as a plant growth media component can be produced under the controlled composting trial conditions described here with manual turning. This positive relationship between E4/E6 and SFW to predict compost maturity observed in our studies necessarily is verified by many more studies under various feedstock compositions and composting processes.

Conclusion

At the end of 20 weeks, the maturity and stability of composts increased from pure HM to pure BW (100/0 < 75/25 < 50/50 < 25/75 < 0/ 100). The pure biowaste compost reached maturity and stability with higher humic acids content than HM after 140 days of composting. This would be attributed to the presence of optimum C/N ratio and largely varied organic sources with better particle size distribution along each sieve size that created more favorable conditions for microbial communities in BW than HM. The type and quantity of bedding material present in horse manure, specifically wood wastes that affect N availability, must be taken into account in the development of humidified final compost production. Indeed, this is particularly important in the case of plant growth. Overall the 50/50 blend was not significantly different in C/N ratio, E4/E6 and cress growth from other feedstocks except from pure HM. It was therefore concluded that composting using a combination of HM and BW (50/50) could be used as an alternative method to pure HM composting. In general, a decrease in C/N ratio, E4/E6 ratio and O2 uptake and an increase in plant growth can be taken as a reliable index of compost maturity and stability. Further investigation should be carried out to determine the loading limits of composts that should be annually applied to the soil in order to satisfy crop nitrogen needs and minimize environmental degradation.

Acknowledgements

We thank Ann Farmers for constructive comments and English language correction on the manuscript. Special thanks to Ena Smidit for her technical support and critical comments and suggestions on the manuscript. The invaluable help with the experiments and laboratory analysis provided by Thomas Ebner and Wolfgang Sprinzl is gratefully appreciated.

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Getinet Desalegn, E. Binner and P. Lechner

BOKU-University of Natural Resources and Applied Life Sciences,

Department of Water-Atmosphere-Environment, Institute of Waste Management, Vienna, Austria

Copyright J.G. Press Inc. Spring 2008

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