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Carbon Mineralization of Pruning Wastes Compost At Different Stages of Composting

Posted on: Friday, 7 October 2005, 03:01 CDT

By Benito, Marta; Masaguer, Alberto; Moliner, Ana; De Antonio, Roberto

To study the carbon mineralization of pruning waste compost, four samples originated from pruning waste, leaves and grass clippings were collected each from a different pilot pile at different stage of the composting process: initial nondecomposed material (C1); two- months old at the end of the biooxidative stage (C2); seven months old during the curing phase (C3) and 12 months old at the end of the curing phase (C4). The CO2-C evolution was measured during 56 days of aerobic incubation. The proportion mineralized from the different composts (% of compost TOC) during the incubation period were: 4.54, 2.43, 1.71 and 1.60 for C1, C2, C3 y C4, respectively. Regardless of compost age, C mineralization occurred in two phases: a first rapid phase (corresponding to the decomposition of the most labile products by microorganisms) and a second, slower phase, during which the most resistant organic products mineralized. During the first stage, the model was fitted to a first-order equation, whereas in the second phase the model was a zero-order equation. Because of the similar results obtained for samples C3 and C4, we can conclude that organic matter had similar microbial stability at both stages and the composting process could be shortened by five months.

Introduction

The need to establish adequate levels of soil organic matter that balance losses by mineralization, and environmental criteria that tend to value residues, increased the use of organic residues, composted or fresh, as soil amendments during the last decades. Application of immature or unstable compost to soils may bring about problems such as N immobilization by microorganisms, anaerobic conditions, temperature increase or accumulation of phytotoxic substances (for instance acetic acid, phenolic compounds or ammonia) that could inhibit seed germination or root growth (He et al. 2000) Therefore, it is essential to evaluate the biological stability of compost during the process of composting. On the other hand, the beneficial utilization of organic materials added to soils requires quantitative information on the decomposition rates of different sources of organic C. Compost biological decomposition depends on the degradation rate of a wide range of C compounds present in the sample (carbohydrates, fatty acids, lignin, etc.), as well as on their nutrient content (Bernai et al. 1998).

There is a wide range of research works whose main objective is to assess the amount of CO released during the decomposition of different organic materials such as municipal solid waste compost (Pascual et al. 1998; Mamo et al. 1999), sewage sludge compost (Bernai et al. 1998), composted manure (Hadas and Portnoy 1994) or poultry litter (Gale and Gilmour 1986). However, none of these works study the C mineralization of pruning waste compost. The closest materials to pruning waste compost are green wastes or yard trimmings but they have a much larger amount of leaves and grass clippings.

According to Bernai et al. (1998), the composting process has four important stages: i) the initial stage, when the material starts to decompose; ii) the thermophilic phase, during which the material reaches its maximum temperature (>40C); iii) the end of the biooxidative phase, with a fall of temperature (close to the external temperature); and iv) the curing phase, whose end product is highly stabilized and humified mature compost. A compost should generally undergo these four stages before being applied to soil, in order to avoid the disadvantages associated with the use of unstable or immature compost. However, as a result of high demand for compost, the material is sometimes applied at different stages of maturity and stability. Although the terms stability and maturity are both commonly used to define the degree of decomposition of organic matter during the composting process, they are conceptually different (Benito et al. 2003). Compost maturity refers to the degree of humification of the material and compost stability refers to the level of microbiological activity. Compost stability can be assessed by measuring O uptake rate, CO production or by the heat released as a result of microbial activity (Iannotti et al. 1994; Chen and Inbar 1993; Wu et al. 2000).

The present study was conducted to evaluate C mineralization of compost samples with different degrees of maturity and stability in order to fit kinetic equations to calculate the fraction of potentially mineralizable C and the mineralization rate.

Materials and Methods

Compost Samples

The compost used in the present study was sampled in the Migas Calientes composting facility (Madrid, Spain) from a mixture of pruning waste (70% by volume), leaves (10%, by volume) and grass clippings (20% by volume). Although there are some seasonal variations in the quantities and characteristics of green waste in the city of Madrid, approximately 6070% of the waste volume manufactured is woody material, mainly from pine, plane, smoothleaf elm and horse chestnut and the rest of it varies from leaves to grass clippings. Table I shows the main chemical analysis of pruning waste materials and compost at different stages of the composting process.

Windrow piles 2.5 m high by 30 m long were constructed using shredded material. Forced aeration was used for the first eight weeks (biooxidative phase), followed by a ten-month curing period during which the piles were turned periodically to maintain adequate O levels. During the biooxidative phase of composting, air was blown through the holes of two tubes placed at the base of the pile. The ceiling temperature for continuous aeration was 70C. The O2 saturation level was also controlled and when it fell below 82% the aeration system was turned on. During the curing phase, the pile was turned every 15 days in order to improve both the O level inside the pile and the homogeneity of the material. Pile moisture was controlled weekly by adding enough water to obtain a moisture content of not less than 50%.

TABLE 1.

Chemical analysis of pruning waste materials and compost at different stages of the composting process.

Samples were taken from four piles of compost at different composting phases as follows: Cl: initial nondecomposed material; C2: material at the end of the biooxidative phase (2 months old); C3: material sampled during the curing phase (7 months old); C4: mature compost (12 months old).

From each pile, five subsamples (each of approximately Ikg in weight) were taken, hand-sieved (<10mm), mixed and transferred to the laboratory for analysis. Each subsample was a mixture of grab samples taken from the top to the bottom of the pile at each sampling point.

Total organic carbon (TOC) was analyzed by the dry combustion method at 540 0C (Nelson and Sommers 1982) and for total N (TN) by Kjeldahl digestion (Bremmer and Mulvaney 1982). The pH was analyzed in a 1:10 (w/v) water extract. Water soluble C (WSC) was extracted in a 1:10 (w/v) ratio at room temperature by shaking for 2 hours. WSC in the extract was determined by potassium dichromate and sulphuric acid digestion at 160C for 30 min. A spectrophotometric method was used to measure Cr^sup 3+^ produced by the reduction of Cr^sup 6+^ (1=590 nm) (Benitez et al. 1999). To determine dehydrogenase activity (DH-ase), 3% of 2,3,5-triphenyltetrazolium chloride (TTC) was used as a substrate. The procedure was performed in the dark on 3 g of compost using the methodology proposed by the U.S. Composting Council (1997), which is based on the Thalmann method (1968). The results are given in mg 2,3,5-triphenyl formazan (TPF)/g dry weight per 24 hours, where TPF is the reduced and colored TTC equivalent. The TPF was measured in a spectrophotometer at 485 nm after its extraction with methanol and a final filtration through a Whatman n5 filter paper. The germination index (GI) was determined using seeds of Lepidium sativum L. (Zucconi et al. 1981).

All results reported in the text are the means of determinations made on three replicates and reported on a dry-weight basis (oven dried at 105 C for 24 hours).

Carbon Mineralization

Twenty five-gram samples of compost at different composting times were incubated in 1 L vessels to evaluate C mineralization. Moisture in the compost samples was adjusted to 60% of weight. Prior to incubation, all composting samples were pre-incubated for 3 days at room temperature. The purpose of this pre-incubation period was to ensure that the microorganisms in the compost samples are acclimated to the mesophilic temperature at which the experiment was conducted.

The CO^sub 2^ evolved was absorbed into 10 mL of IM NaOH placed in small glasses (50 mL) on the top of the samples. Three empty glasses with the same capacity as the ones selected for the samples were used as blanks. After 1, 2, 3, 5, 7, 10, 14 d, and then weekly to 56 d, the CO^sub 2^ evolved was measured in a nondestructive set of triplicates by titration of excess the NaOH solution with 1.00 M HCl after BaCl^sub 2^ precipitation of carbonates. Incubation was carried out in darkness and at a controlled temperature (22C 0.5).

Statistical Analysis

To compare curve-fitting results and determine the most adequate one, the Snedecor F test was applied and the residual mean square (RMS) calculated. The least significant difference was used to compare C mineralization resul\ts (p<0.05). All results reported are means of determinations made on three replicates.

Results and Discussion

During the incubation period (56 days), the maximun C mineralization rate occurred, for all samples, during the second day of incubation (Figure 1a). The high concentration of readily degradable C in the freshest material (C1) led to high microbial activity. The great variety of compounds with different degrees of degradability contained in the compost samples might explain a second peak in C mineralization. After the initially high mineralization rate, there was a gradual decrease in all cases before it became fairly constant. The decrease was more pronounced for the sample that had not initiated the composting process (C1). C mineralization rates for samples C2, C3 and C4 were similar, those for C2 being slightly higher. At the end of the incubation period, the C mineralization rate was: 682, 391, 331 and 332 mg C kg-1 day- 1 for C1, C2, C3 y C4, respectively.

FIGURE 1. Rate of CO^sub 2^-C evolution (a) and Cumulative CO^sub 2^-C mineralization (b) during the incubation of composts at different stages of the composting process (Ci); C1: initial material, C2, C3, C4: 2, 7 and 12 months old, respectively.

The total amount of CO^sub 2^-C released after 56 days of incubation from composting material (Ci) are shown in Table 2. According to the least significant difference test (p<0.05), the total amount of C mineralized (mg CCO^sub 2^ kg^sup -1^ compost) after 56 days followed the following trend: C1>C2>C3≥C4. These results showed that the C remaining after seven months of composting was relatively stable to microbial degradation. Even though samples C3 and C4 were taken at different times of the composting process, results seem to indicate that both present the same degree of stability.

TABLE 2.

Carbon mineralized from compost samples after 56 days of incubation (mg C kg^sup -1^ soil) and the proportion mineralized from the different composts (% of compost TOC)

Cumulative C-CO curves with time are shown in Figure 1b and the parameters that define their kinetic equations in Table 3. The mineralization process was best described for all samples by combined two-step kinetic equations. The rapid phase of mineralization always followed a first-order kinetic model (Table 3), showing that the presence of more readily available organic C led to an increase in the microbial activity. The slow phase followed a zero-order equation. Others authors such as Bernai et al. (1998) and Bernai and Kirchman (1992) also used a kinetic model combining a zero-order and a first-order function for soil samples amended with different waste mixtures prepared with sewage sludges, animal manures, plant residues and others city and industrial refuses.

TABLE 3.

Parameter values of the combined first/zero-order equation model, residual mean square (RMS) and F-values for carbon mineralization of compost samples at different stages of composting process. Cm mineralized carbon after 56 d of incubation (% of total organic C, TOC); CR and CS are the rapid and slow potentially mineralizable carbon (% of TOC); and KR and KS the decomposition rate (days^sup - 1^).

The two-step kinetic equations that describe mineralization, suggest that the organic carbon content of pruning waste compost has two fractions with different degrees of biodegradability: a fraction whose labile organic compounds are rapidly mineralizable during the first step, and another fraction with slow mineralization, which is more resistant to microbial attack. This expresses the idea that recalcitrant fractions of residues remain after the initial period of decomposition.

The values of rapidly mineralizable C (C^sub R^) decreased with increasing composting time (Table 3). The greatest differences for this parameter were observed between those of the initial samples (C1) and those sampled at the end the biooxidative phase (C2). The smallest differences in both C^sub R^ and C^sub S^ were found between samples taken in the middle of the curing phase (C3) and at the end of the curing phase (C4), in spite of a difference of five months in their composting time. The high rate constants corresponding to the first step, K^sub R^, reflect the short time required to break down the most labile C fraction.

During the second phase, the percentage of mineralized C (C^sub S^ in Table 3) in all treatments exceeded 96% of the added TOC, demonstrating the preponderance of the slow phase of C mineralization for pruning waste compost. This phase showed lower values of K^sub S^ than the values reported in others studies (Gale and Gilmour 1986; Bernai and Kirchmann 1992) for different organic residues applied to soil, indicating that the composting material used in the present study have a much larger amount of recalcitrant compounds than those materials reported before. In spite of this, values of K^sub R^ were higher than those obtained by the same authors, reflecting that a short time is required to break down the most labile C fraction.

Conclusions

Results obtained for pruning compost samples taken after 7 (C3) and 12 (C4) months were similar what indicated that C remaining after seven months of composting was relatively stable to microbial degradation, showing that the pruning waste composting process can be shortened by 5 months without any problem for its application as soil conditioner.

Mineralization of C for compost samples took place in two steps, demonstrating the existence of two fractions of different degrees of biodegradability. During the first stage, the model was fitted to a first-order equation, whereas in the second step the model was a zero-order equation. The proportion mineralized from the different composts (% of compost TOC) during the incubation period were: 4.54,2.43 1.71 and 1.60 for C1, C2, C3 y C4, respectively with a rate constant ranging from 0.0001 to 0.0003 day^sup -1^.

Acknowledgement

The authors wish to thank the City Hall of Madrid for providing the samples. This study was supported by the Universidad Politcnica de Madrid (Project I+D-A9911). Dr. M. Benito held an assistantship from INIA-Ministerio de Ciencia y Tecnologia (Spain).

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Marta Benito, Alberto Masaguer, Ana Moliner and Roberto De Antonio

Departamento de Edafologia, Escuela Tcnica Superior de Ingnieros, Agronomes, Universidad Politcnica de Madrid, Madrid, Spain

Copyright J.G. Press Inc. Summer 2005

Source: Compost Science & Utilization

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