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ISSN : 1598-5504(Print)
ISSN : 2383-8272(Online)
Journal of Agriculture & Life Science Vol.49 No.4 pp.105-115
DOI : https://doi.org/10.14397/jals.2015.49.4.105

Suitability of thermal treated sawdust as replacements for peat moss in horticultural media

Ji Young Jung1,Ji Su Kim1,Si Young Ha1,Ju Hyun Choi2,Jae-Kyung Yang1*
1Division of Environmental Forest Science and Institute of Agriculture & Life Science, Gyeongsang National University,Jinju, 52828, South Korea.
2Department of Textile Design, Gyeongnam National University of Science and Technology, Jinju, 51767, South Korea.
*Corresponding author: Jae-kyung Yang Tel: +82-55-772-1862 Fax: +82-55-772-1869 E-mail:
jkyang@gnu.ac.kr
February 2, 2015 August 10, 2015 August 12, 2015

Abstract

An experiment was conducted to study the potential of thermal treated oak sawdust(steaming and steam explosion) as horticultural medium component in plug seedlings production of Chinese cabbage(Brassica campestris L.). This study involves the chemical, physical characterization and growth test of thermal treated oak sawdust(steaming and steam explosion) in order to evaluate their use as components of horticultural media. A commercial peat moss and oak sawdust were used as control. The total carbohydrate, C/N ratio, pH, phenolic compound, total porosity and water holding capacity were 45.1g/100g dry wt, 425.1, 4.4, 141.8mg/g wt, 82.5%, 47.1% in oak sawdust and 39.2g/100g dry wt, 300.3, 4.7, 131.7mg/g wt, 84.9% and 49.2% in steamed oak sawdust and 30.3g/100g dry wt, 247.8, 5.7, 40.8mg/g wt, 92.3% and 51.7% in steam exploded oak sawdust, respectively. The mixtures of the horticultural media were prepared using different substrate as peat moss, oak sawdust, steamed oak sawdust, steam exploded oak sawdust and perlite to grow Chinese cabbage in a greenhouse. The seed germination, stem height and leaf area were 68%, 2.2cm, 1.1cm2 in OSP(containing 90% oak sawdust and 10% perlite) and 69%, 2.5cm, 1.5cm2 in SMP(containing 90% steamed oak sawdust and 10% perlite) and 87%, 3.0cm, 2.2cm2 in SEP(containing 90% steam exploded oak sawdust and 10% perlite), respectively. The leaf area SEP(containing 90% steam exploded oak sawdust and 10% perlite) was higher than that of PP(containing 90% peat moss and 10% perlite). This research indicates that steam exploded oak sawdust may be utilized as a suitable replacement for peat moss in horticultural media component for Chinese cabbage.


초록


    Korea Forest Service
    No. S211215L020430

    IIntroduction

    The horticultural media include organic materials such as peat moss and pine bark with other inorganic materials such as perlite, vermiculite and sand. Among horticultural media, peat moss has been the most widely used(Ghehsareh et al., 2011, Tehranifar et al., 2012).

    Peat moss is considered a premier substrate, due to its desirable physico-chemical characteristics such as slow degradation rate, high water holding capacity, low bulk density and high nutrient exchange capacity(Bilderback et al., 2005; Nelson, 2011). Peat moss is especially important in the potting media because it is a stable material, light weight, and readily available. However, the natural reserves of peat moss are limited and its widespread use is critical for ecological reasons. It is also cost increased due in part to the increased cost of transportation.

    Many studies have been carried out in search of substitutes for peat moss. The utilization of waste by-products as components of horticultural media has been extensively investigated during the past several decades. Renewable alternatives to peat moss that have been examined include cattle manure (Jayasinghe et al., 2010a), sewage sludge sugarcane trash (Jayasinghe et al., 2010b), municipal solid waste(Herrera et al., 2008), crumb rubber(Zhao et al., 2011), biochars(Vaughn et al., 2013), food waste, sawdust and Chinese medicinal herbal residues(Zhou et al., 2014), rice straw(Hosseini and Aziz, 2013), date-palm wastes(Ghehsareh et al., 2011).

    Sawdust is widely used as a horticultural media component in areas with wood processing industries, because of its low cost, high moisture retention, and high availability. Sawdust has been standard horticultural media for the greenhouse industry(Sawan & Eissa 1996). Usually it forms a constituent normally less than 50% in mixtures rather than being used as a stand-alone growth medium.

    Sawdust has been shown to be a potential substitute for peat moss as a container substrate. However, before the product can be commercialized there seems to be some inconsistencies that need to be addressed. One of these is the presence of toxins. In some studies using sawdust as a medium, it was found that wood contained phytotoxins that affected the growth of crop. Most tree species produce these phytotoxic substances; however, some produce more than others.

    Phytotoxicity depends on the chemical composition of the medium, and may be due to organic or inorganic substances(Bunt, 1988), which cause salinity, nutritional disorders and/or metabolic alterations (enzymatic or hormonal). High C/N ratios and the presence of phytotoxicity(phenolic compounds) are characteristics typical of the lignocellulosic substrate which have been studied(Mastalerz, 1977). Methods such as composting, ageing, washing, mixtures or fertilisation have been used to reduce or eliminate these properties(Marinou et al., 2013).

    The objective of this study was to determine the physical and chemical characteristics of the various thermal treated lignocellulosic substances and evaluate their potential for use as substrate components.

    IIMaterials and Methods

    2.1Horticultural media preparation

    Peat moss used as control(commercial horticultural media; 20~80mesh size sphagnum, Sunshine, Canada). The lignocellulosic substrate included oak sawdust, steamed oak sawdust and steam exploded oak sawdust.

    The steam process was performed as follows. Two hundred grams(oven-dried) of oak sawdust(Quercus mongolica) was put into autoclave. The autoclave was subsequently heated to 120℃ for 30min.

    The steam explosion process was performed as follows. Two hundred grams(oven-dried weight) of the wood chips was placed in an autoclave. After the saturated steam exposure(25kgf/cm2(225℃), 5min) a ball valve at the bottom of the reactor was opened suddenly to bring the reactor to atmospheric pressure rapidly. Steam exploded substrates was then recovered by 10%(v/v) ethanol washing(60℃, 3hour).

    The mixtures of horticultural media were prepared by mixing(v/v) 10% of perlite and 90% of peat moss, oak sawdust, steamed oak sawdust and steam exploded oak sawdust, respectively. Ratios of each component in each substrate are shown in Table 1.

    2.2Chemical and physical properties of thermal treated sawdust containing media

    Substrate was hydrolyzed with 72% H2SO4 as described by ASTM E1721-95. The hydrolysate from the acid treatment was analyzed for carbohydrates to determine the overall sugar composition of the substrate. The analysis was performed on the gas chromatography(YL–6100, Young Lin Ins. Co., Ltd. Korea) equipped with a DB-225 capillary column (15m, 0.25mm ID, 0.25mm film thickness)(J&W Scientific, Folsom, USA). Injection samples were derivatized according to ASTM 1821-96. This method describes a procedure for derivatizing monomers to their respective alditol acetates and tests for the sugars arabinose, xylose, mannose, galactose and glucose. Total carbohydrate content was calculated as the sum of the individual sugar.

    The Carbon(C) and nitrogen(N) content in substrate samples were determined by using CN analyzer(Micro coder JM 10, G-Science Laboratory, Japan).

    The Mineral elements(K, Ca, Mg, Na, Zn, Fe, Mn and P) of substrate were measured using inductively coupled plasma atomic emission spectrometry (ICP-AES, OPTIMA 3300DV) to quantify aqueous constituents following microwave digestion with HNO3-H2SO4-HClO4solution.

    The pH(Sample: distilled water ratio of 1:5) was measured in water extracts of all substrate samples using a Orion pH meter(model 710, Thermo Scientific, USA).

    Phenolic compounds were assayed using the Folin-Ciocalteu reagent, following Singleton’s method slightly modified(Dewanto et al., 2002). Phenolic compound content of substrate(three replicates per treatment) was expressed as mg gallic acid equivalents g−1distilled water through the calibration curve with galli cacid(R2=0.99).

    The porosity and water holding capacity of lignocellulosic substrates were measured by Verdonck and Gabriels(Verdonck & Gabriels, 1992) method.

    2.3Plant growth test

    The plant species used to evaluate the suitability of the horticultural media was Chinese cabbage(Brassica campestris L.). Seeds were surface sterilized by immersion in 10% sodium hypochlorite solution for 10min, rinsed three times with deionized water, soaked in deionized water for 2h, and finally sown in the prepared mixture substrate. Twenty seeds of Chinese cabbage were placed in petri dishes(three dishes per substrate) containing 10g of each mixture substrate(Table 1). The seed was cultivated in the green house at 25℃ for 7days duration in 16h light/8h dark and a 70% relative air humidity. Stem length(cm) and leaf area(cm2) was measured by digital verniercalipers(Byun et al., 2012). Seed germination was recorded every day.

    The percentage of seed germination was calculated as follows:

    Seed germination(%)= Total number of seeds germinated in substrate Total number of seeds in control × 100

    2.4Statistical analysis

    The measurements were carried out for three replicates, and values are an average of three replicates. The results are expressed as mean values±standard deviation(SD). Obtained data were subjected to analysis of variance to determine the horticultural media effects. Statistical analysis had been carried out with SAS statistical software and according to analyzing from ANOVA test and comparing data mean to Duncan test. Duncan’s multiple comparison range test was used to determine significant differences(p<0.05) between the means.

    IIIResults and discussion

    3.1Effect of thermal treatment on chemical and physical properties

    The chemical and physical properties of different substrates are presented in Table 2 - Table 5. The three lignocellulosic substrate(oak sawdust, steamed oak sawdust and steam exploded oak sawdust) as horticultural media were tested(peat moss was a commercial horticultural media used as the control).

    The carbohydrate composition of peat moss, oak sawdust, steamed oak sawdust and steam exploded oak sawdust were presented in Table 2. Total carbohydrate content of lignocellulosic substrates was higher than that of peat moss. The total carbohydrate content differed significantly among lignocellulosic substrates, with ranging from 30.3 to 45.1g/100g. The content of hemicellulosic sugar(arabinose, xylose, mannose and galactose) in oak sawdust was higher than that in steamed oak sawdust, while the amount of glucose was almost the same. The highest glucose content was observed in steam exploded oak sawdust. The proportion of sugars that are decreased after thermal treatment was dependent on the type of hemicellulosic sugar(arabinose, xylose, mannose and galactose). Arabinose, mannose and galactose were completely removed at the steam explosion treatment. Hemicellulosic sugar(arabinose, xylose, mannose and galactose) are amorphous heteropolysaccharides, which are sensitive to heat treatment. The loss of hemicellulosic sugar mainly happened at 200-260°C (Ramos, 2003). Steam under high pressure penetrates the lignocellulosic structures by diffusion. The moisture in the lignocellulosic biomass hydrolyzes the acetyl groups of the hemicellulose, forming organic acids such as acetic and uronic acids. The acids, in turn catalyze the depolymerization of hemicellulose, releasing xylan and limited amounts of glucan(Jeoh, 1998).

    Carbohydrates especially sugar play an important role in nitrogen fixation(Malik et al., 2001). Haahtela et al.(1983) examined twenty compounds(polysaccharides and organic acids) as carbon and energy source for nitrogen activity in semisolid stab cultures, of diazotrophic root assiociated bacteria. The studies revealed that best sugar was hexose. From all above studies it is concluded that diazotrophs require carbohydrates as a source of energy for nitrogen fixation. The amount of nitrogen fixed increse with the increase in source of energy. Therefore, soil can be amended with lignocellulosic substrates and compounds released by action of cellulase and can be utilized by nitrogen fixing bacteria. Also, nitrogenfixing activity can be increased by addition of organic matter(Malik et al., 2001).

    The carbon(C), nitrogen(N) and C/N ratio of peat moss, oak sawdust, steamed oak sawdust and steam exploded oak sawdust were presented in Table 3. The C/N ratio was oak sawdust(425.1)>steamed oak sawdust(300.0)>steam exploded oak sawdust(247.8) >peat moss(55.3). Peat moss showed C/N ratio within the established almost optimal range(50-70). The various thermal treated lignocellulosic substrates showed significant differences in the C/N ratio. During the thermal treatment process, the carbon (48.6~50.5%) and nitrogen(0.1~0.2%) contents increased, while the hydrogen and oxygen contents of torrefied biomass decreased(data not shown). In general, increased heat treatment temperatures result in increased carbon content and decreased hydrogen and oxygen content (Kim et al., 2012). In the result, the C/N ratio decreased in the thermal treated lignocellulosic substrate. The optimal range of C/N ratio must be between 50 and 70. If the ratio is more than 40, it has negative effects on crops and damages seed germination(Ghehsareh et al., 2011). However, the high C/N ratio could cause immobilisation of soluble nitrogen when lignocellulosic substrates were used as a horticultural media for containerised crop production.

    Mineral elements composition of peat moss, oak sawdust, steamed oak sawdust and steam exploded oak sawdust were given in Table 4. Mineral elements showed significant differences between lignocellulosic substrates at 5% level with respect to K, Ca, Mg, Na, Zn, Fe, Mn and P. A decrease of K, Ca, Mg, Na, Zn, Fe and Mn in steamed oak sawdust(K: 922.3mg/100g dry wt, Ca: 1116.9mg/100g dry wt, Mg: 97.6mg/100g dry wt, Na: 245.9mg/100g dry wt, Zn: 2.5mg/100g dry wt, Fe: 18.3mg/100g dry wt and Mn: 12.0mg/100g dry wt) compared to oak sawdust(K: 998.0mg/100g dry wt, Ca: 1196.0mg/100g dry wt, Mg: 105.6mg/100g dry wt, Na: 296.7mg/100g dry wt, Zn: 9.4mg/100g dry wt, Fe: 165.7mg/100g dry wt and Mn: 15.3mg/100g dry wt) was observed. The available K(1348.1mg/100g dry wt), Ca (2037.1mg/100g dry wt), Mg(199.1mg/100g dry wt), Na(417.7mg/100g dry wt), Mn(22.7mg/100g dry wt) and P(80.4mg/100g dry wt) in steam exploded oak sawdust were higher than for other substrates. The presence of high levels of micronutrients(K, Ca, Mg and P) or low levels of toxic elements(Zn and Fe) in steam exploded oak sawdust would be a potentially for horticultural media preparation. Crops acquire mineral elements from their native soil environments (Grattan & Grieve, 1992). Generally, peat moss was poor in mineral element. It would require frequent or regular use of fertilizer if used as substrate. Steam exploded oak sawdust provides and contains a good reserve of micronutrients(K, Ca, Mg and P).

    The pH and phenolic compound content of peat moss, oak sawdust, steamed oak sawdust and steam exploded oak sawdust were given in Table 5. Peat moss(pH 5.0) and steam exploded oak sawdust(pH 5.7) present pH levels close to optimal range(pH 5.5 ~6.5). The pH content differed significantly among lignocellulosic substrates, with pH ranging from 4.4 to 5.7. The pH of steam exploded oak sawdust was higher than for oak sawdust and steamed oak sawdust. Wood sawdust can contain phytotoxic compounds such as phenolic compound and terpenes. The use of wood sawdust as horticultural media components can involve serious problems of toxicity in plant growth(Nichols, 1981).

    An indirect measurement method of phytotoxic compounds is phenolic compound measurement(Park et al., 2014). The phenolic compound content was oak sawdust(141. 8mg/g dry wt)>steamed oak sawdust(131.7mg/g dry wt)>peat moss(98.1mg/g dry wt)>steam exploded oak sawdust(40.8mg/g dry wt). The pH of steam exploded oak sawdust was lower than peat moss. The increase in pH mirrored the decrease in phenolic compound content. The compost process is needed to alleviate the negative effects (High C/N ratio or phytotoxicity) of fresh wood sawdust(Davey, 1953). However, compost process was a long period of time(Ghehsareh et al., 2011). In this study, It was confirmed that phenolic compound content decreased by thermal treatment.

    Physical properties(total porosity and water holding capacity) of peat moss, oak sawdust, steamed oak sawdust and steam exploded oak sawdust were presented in Table 6. Steamed oak sawdust(84.9%) and steam exploded oak sawdust(92.3%) were a significantly higher total porosity than oak sawdust(82.5%).

    The porosity can be significantly increased by removal of lignin and hemicellulose and reduction of cellulose crystallinity, through thermal treatment (Kumar et al., 2009).

    The water holding capacity of the lignocellulosic substrates increased with the thermal treatment, and it was higher for steam exploded oak sawdust(51.7%) than for peat moss(47.0%). The porosity of lignocellulosic substrate affects the water holding capacity. The water holding capacity is characterized as absorbed and detained water. Absorbed water is mainly absorbed by plant roots and is used to maintain the transpiration and growth of the plant. The detained water is stored within the non-capillary pores and has the potential to supplement the absorbed water when needed(Liu et al., 2013).

    3.2Effect of thermal treated sawdust on plant growth

    The growth properties(seed germination, stem length and leaf area) of PP(containing 90% peat moss and 10% perlite), OSP(containing 90% oak sawdust and 10% perlite), SMP(containing 90% steamed oak sawdust and 10% perlite) and SEP(containing 90% steam exploded oak sawdust 10% perlite) are presented in Fig. 1~23. The highest seed germination and stem length were observed in the horticultural media of PP(93% and 3.1cm, respectively). The leaf area of the SEP(2.2cm2) was higher than that of the PP(2.0cm2)(Fig. 3, 4).

    In oak sawdust, toxicity symptoms were observed on seed germination, stem length and leaf area(Fig. 13). Other studies have described delayed or reduced growth properties due to the presence of phytotoxic substances in fresh sawdust(Mastalerz, 1977; Worrall, 1981; Wright & Browder, 2005, Lu et al., 2006; Schaefer, 2009). The reduced growth in OSP (containing 90% oak sawdust and 10% perlite) was accompanied by the oak sawdust in their phenolic compounds content. This finding supports our earlier result(Table. 5) about OSP(containing 90% oak sawdust and 10% perlite) being responsible for the phytotoxicity of the examined substrate.

    The substitution of peat moss by the SEP (containing 90% steam exploded oak sawdust and 10% perlite) in the media affected seed germination (87%), stem length(3.0cm) and leaf area(2.2cm2) positively. Results of this study confirm that steam exploded oak sawdust can be used effectively in horticultural media.

    Figure

    JALS-49-105_F1.gif

    Seed germination of Chinese cabbage (Brassica campestris L.) grown in PP(peat moss:perlite=9:1), OSP(oak sawdust:perlite =9:1) and SMP(steamed oak sawdust:perlite =9:1) and SEP(steam exploded oak sawdust :perlite=9:1).

    JALS-49-105_F2.gif

    Stem length of Chinese cabbage(Brassica campestris L.) grown in PP(peat moss:perlite =9:1), OSP(oak sawdust:perlite=9:1) and SMP(steamed oak sawdust:perlite=9:1) and SEP(steam exploded oak sawdust:perlite =9:1).

    JALS-49-105_F3.gif

    Leaf area of Chinese cabbage(Brassica campestris L.) grown in PP(peat moss:perlite =9:1), OSP(oak sawdust:perlite=9:1) and SMP(steamed oak sawdust:perlite=9:1) and SEP(steam exploded oak sawdust:perlite=9:1).

    JALS-49-105_F4.gif

    Chinese cabbage(Brassica campestris L.) growth in PP(peat moss:perlite=9:1), OSP(oak sawdust:perlite= 9:1) and SMP(steamed oak sawdust:perlite=9:1) and SEP(steam exploded oak sawdust:perlite=9:1).

    Table

    Substrate mixtures used in the study

    Effect of thermal treatment on carbohydrate content of substrates

    1Optimal range in growing media according to Abad et al(2002).
    2Commercial substrate
    3In each column, values with different letters indicate statistically significant differences(p<0.05) by Duncan’s test.
    4Fresh sawdust
    5Steaming condition: 120°C, 30min
    6Steam explosion condition: 25kgf/cm2(225°C), 5min and ethanol extraction(10% ethanol 60°C, 3hour)

    Effect of thermal treatment on C/N ratio of substrates

    1Optimal range in growing media according to Bunt(1988).
    2Commercial substrate
    3In each column, values with different letters indicate statistically significant differences(p<0.05) by Duncan’s test.
    4Fresh sawdust
    5Steaming condition: 120°C, 30min
    6Steam explosion condition: 25kgf/cm2(225°C), 5min and ethanol extraction(10% ethanol 60°C, 3hour)

    Effect of thermal treatment on mineral elements of substrates

    1Commercial substrate
    2In each column, values with different letters indicate statistically significant differences(p<0.05) by Duncan’s test.
    3Fresh sawdust
    4Steaming condition: 120°C, 30min
    5Steam explosion condition: 25kgf/cm2(225°C), 5min and ethanol extraction(10% ethanol 60°C, 3hour)

    Effect of thermal treatment on pH and phenolic compound content of substrates

    1Commercial substrate
    2In each column, values with different letters indicate statistically significant differences(p<0.05) by Duncan’s test.
    3Fresh sawdust
    4Steaming condition: 120°C, 30min
    5Steam explosion condition: 25kgf/cm2(225°C), 5min and ethanol extraction(10% ethanol 60°C, 3hour)
    6Optimal range in growing media according to Abad et al(2002).

    Effect of thermal treatment on total porosity and water holding capacity

    1Commercial substrate
    2In each column, values with different letters indicate statistically significant differences(p<0.05) by Duncan’s test
    3Fresh sawdust
    4Steaming condition: 120°C, 30min
    5Steam explosion condition: 25kgf/cm2(225°C), 5min and ethanol extraction(10% ethanol 60°C, 3hour)
    6Optimal range in growing media according to Abad et al(2002)

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