Research on Composting and Substrate Utilization of Garden Waste

Jiaomeng ZHANG Mei CHEN Mengyao ZHU Wenwen YU Junchi YAO Xiuya XU Xuqin REN

Abstract [Objectives] This study was conducted to investigate how to use garden waste resources.

[Methods]With garden waste as a raw material for composting treatment， the effects of adding starters on the decomposing effects of garden waste were investigated， and the changes in temperature and microorganisms during the decomposing process were analyzed. On this basis， the compost products were used to partially replace peat to make pepper seedling substrates， so as to further confirm the possibility of the use of compost products as substrates.

[Results] Adding organic material starter or biological bacterial fertilizer starter could help garden waste to decompose and accelerate the composting process; and making seedling substrates by using garden waste compost products to partially replace peat could significantly improve the emergence rate， strong seedling index and fresh weight of pepper. The compost products fermented with the two kinds of starters had better substitution effects and higher seedling quality indexes.

[Conclusions]This study provides a scientific basis for the substrate utilization of garden waste.

Key words Garden waste; Compost; Substrate

Received： May 7， 2021  Accepted： July 3， 2021

Supported by Jiangsu Province Industry-University-Research Cooperation Project （BY2020307）; Horizontal Project of School-enterprise Cooperation （Z421A191169）; Undergraduate Practice Innovation Program （202011049050XJ， 202011049079H， 20211104914 202111049143）.

Jiaomeng ZHANG （2000-）， female， P. R. China， major： horticulture.

Mei CHEN （2000-）， female， P. R. China， major： horticulture.

# These authors contributed equally to this work.

*Corresponding author. E-mail： 824332973@qq.com.

Garden waste generally refers to the botanical materials produced by natural withering and falling of landscaping plants or artificial pruning， which mainly include lawn trimmings， fallen leaves， branches， tree and shrub trimmings and discarded flowers and weeds in gardens and flower beds. The main ingredients are organic matter， and the contents of cellulose， lignin and hemicellulose are relatively high[1-2]. At present， there are roughly four ways to treat garden waste： landfill， composting， incineration and recycling[3]. Composting treatment is one of the important ways to realize the resource utilization of garden waste[4].

The widespread application of soilless cultivation and plug seedling technology has made the substrate market rapidly grow， and processing these organic wastes into substrates to replace peat will have far-reaching significance for meeting agricultural production needs and urban ecological environment construction. In this study， garden waste was used as a raw material for composting， and seedling substrates were prepared by using the matured compost products as raw materials to partially replace peat and then adding different organic and inorganic materials， and screened for substrate formulas suitable for pepper. This study provides a scientific basis for the substrate utilization of garden waste.

Materials and Methods

Experimental materials

The materials were taken from the garden waste collected by the campus greening department， including the litter of Cinnamomum camphora， Salix babylonica， Diospyros kaki Thunb and other green plants. After crushing， the diameter was about 1 to 3 cm. The basic physical and chemical indexes of the waste were pH 7.20， EC value 0.513 mS/cm， organic matter 138.57 g/kg， and water content 39.3%.

Two kinds of starters were selected. One was organic material starter， purchased from Junde Biotechnology Co.， Ltd. Its main ingredients were Bacillus， Bacillus natto， Actinomycetes and Trichoderma， etc. The other was a bio-fertilizer starter， purchased from Zhengzhou Wangnongbao Biotechnology Co.， Ltd.， and its main ingredients were Lactobacillus， Bacillus subtilis and Saccharomyces.

Experimental methods

Garden waste composting test

Garden waste was taken as the fermentation raw materials， which was added with a certain amount of urea to make the C/N ratio reach 30： and added with the organic material starter （1 000 mg/kg， Y1） and the biological fertilizer starter （200 mg/kg， Y2）， respectively， with no starter added as the control （Y0）. The method of static high temperature composting and plastic film covering was adopted. Water was added to a content of 60%， and the materials were fully stirred to make them even， and piled into a cone. The materials were composted for a total of 70 d， during which the piles were turned over and added with water for 4 times. The quantity of microorganisms was determined in different fermentation periods.

Seedling substrate formula test

With the matured compost in the garden waste fermentation test as the raw material， various materials were fully mixed according to the volume ratio in Table  with the addition of 300 g/m3 of water-soluble compound fertilizer （20-10-20） and 200 g/m3 of slow-release fertilizer at the same time， and a CK of replacing peat with the matured compost was set.

The above formula substrates were the treatments， and Xianhong No. 1 pepper was used as the test material. 72-hole plug trays were used for sowing and seedling raising， and three trays were sown for each treatment. The seedling emergence rate， the agronomic index and chlorophyll SPAD value at the 35-day seedling stage were determined.

Determination methods

Determination of the quantity of microorganisms

The number of microorganisms in the composting process was determined by the plate counting method[5]. Specifically， beef extract peptone medium was used for bacteria， Martins medium was used for fungi， and Gauzes medium No.1 was used for actinomycetes.

Determination of agronomic indexes of pepper seedlings

A ruler was used to measure the plant height from the root base to the stem growth point. A vernier caliper was used to measure the stem thickness at the base. The whole plant fresh weight was measured. The strong seedling index was calculated according to Strong seedling index = （Stem thickness/Plant height）×Fresh weight of the whole plant. The chlorophyll SPAD value was measured with an SPAD-502 Plus chlorophyll meter.

Calculation of seedling quality index

① According to plant agronomic indicators and chlorophyll SPAD value， the ratios of the above-mentioned traits between the CK and each treatment were calculated. These ratios were used for evaluating the quality of seedlings raised with substrates. The larger the ratios， the worse the quality.

② For the ratios calculated in ①， the following formula was used to express the membership function value of each index of each variety： Y （μ） = 1-（Y-Ymin）/（Ymax-Ymin）， where Y is a certain index of a certain treatment， Ymax is the maximum value of this index in all treatments to be identified， and Ymin is the minimum value of this index in all treatments to be identified.

③ The affiliation values of various indexes of each treatment to be identified were accumulated to get Y （defined as the seedling quality index in this study）. The larger the Y value， the better the seedling quality of the treatment.

Results and Analysis

Changes in compost temperature during fermentation

Temperature is an important indicator of microbial life activities in composting. The rapid rise of the temperature of piles can promote the degradation of organic matter and kill the pathogenic bacteria in them. It can be seen from Fig. 1 that the whole fermentation process of garden waste experienced three high temperature periods. The fermentation progress of Y0 was relatively slow. The first high temperature interval was kept at 55.0-60.0 ℃ for about 10 d; after the stacks were turned over， the second high temperature interval was maintained at 50.0-56.0 ℃ for about 6 d; and the third high temperature interval still maintained a higher temperature， with the highest temperature reaching 48.5 ℃. Adding organic material starter （Y1） could significantly reduce the temperature in the second high temperature interval. After composting for 30 to 50 d， the temperature of the pile body dropped to room temperature， and after turning over the piles， the temperature remained basically the same， and the compost temperature in the third high temperature interval only rose slightly. The biological fertilizer starter （Y2） was similar to organic material starter （Y1）， except that the temperature in the first high temperature interval was higher， and the compost fermented faster and better.

Changes of compost microbial community during fermentation

Changes in the number of bacteria during fermentation

It can be seen from Fig. 2 that the change trend of the number of bacteria first increased and then decreased， and the total amount was basically stable at the end of composting. Compared with the initial stage of composting， the number of bacteria increased to different degrees. Compared with Y0， the change trends of Y1 and Y2 were more obvious. At 30 d， the numbers of bacteria in Y1 and Y2 were 1.34 and 1.25 times the number of bacteria in Y0， respectively， and the numbers of bacteria in the two treatments were still significantly higher than the control at the end of composting. The difference between Y1 and Y2 was not obvious， indicating that Y1 and Y2 had richer microbial communities during the fermentation process， which could speed up the compost maturity. After the fermentation， the numbers of bacteria in the Y1 and Y2 compost products were larger， so that the compost products maintained a better biological environment.

Changes in the number of fungi during fermentation

It can be seen from Fig. 3 that the change trend of the total amount of fungi was similar to that of bacteria， but the number of fungi was 1-2 orders of magnitude lower than the total amount of bacteria. From 0 to 30 d， the total amount of fungi showed an upward trend in all treatments; from 30 to 50 d， the number of fungi decreased rapidly; and from 50 to 70 d， the total number of fungi gradually stabilized and rose slightly， and the number of fungi in each composting treatment reaches the maximum value on the 20th d.

Adding organic material starter （Y1） and biological fertilizer starter （Y2） could help increase the number of fungi and promote the process of compost maturity. At the peak， the numbers of fungi increased by 74.23% and 48.94% compared with Y0， respectively， and the numbers of fungi at the end of composting also increased to different degrees.

It can be seen from Fig. 4 that in the 0-30 d stage， all treatments showed an upward trend， and the number of actinomycetes decreased and stabilized in the 30-50 d stage， and the actinomycetes in each treatment reached their maximum value at about 30 d of composting. It showed that as the temperature of the piles increased， actinomycetes multiplied in large numbers， but when the temperature reached a certain temperature， a large number of actinomycetes died or dormant. As the piles became mature and the temperature dropped， the number of actinomyces gradually stabilized. Adding organic material starter （Y1） and biological fertilizer starter （Y2） to the compost could significantly increase the propagation speed of actinomycetes in the compost， accelerate the progress of compost maturity， and maintain a high number of actinomycetes to improve compost quality.

Screening of pepper seedling substrate

It can be seen from Table 2 that different seedling substrates had significant effects on pepper seedling emergence rate， agronomic traits， strong seedling index and chlorophyll content， but various indexes showed different trends. From the point of view of emergence rate， strong seedling index and fresh weight， the substrate treatments （J J J3） using compost products instead of peat had a significant increase compared with the CK， which might be related to the better biological environment of compost products.

Through further calculation of the membership function value （Table 3）， the pepper seedling quality indexes of three substrate treatments （J J J3） were obtained， and the effects of the three substrates on the quality of seedling were compared. The results showed that the quality indexes of pepper seedlings raised on the three substrates from large to small were in order of J3>J2>J indicating that the garden waste compost products produced by adding organic material starter and biological fertilizer starter were more suitable for making pepper seedling substrate， which might be related to the more complete maturity of these two compost products， and the more abundant microbial communities and numbers in the compost products.

Conclusions and Discussion

As the most important biological factor in the composting process， microorganisms are indispensable in the whole composting process， while taking appropriate measures to regulate the microorganisms in the compost can improve the efficiency of composting and the quality of compost products. Li et al.[6] found that the inoculation of highly efficient composite microbial flora for mixed composting of domestic waste and sludge could increase the biodegradation rate of organic pollutants. Xi et al.[7] also believe that the high-efficiency composite microbial flora could accelerate the degradation of domestic waste and sludge and ensure the smooth progress of the composting process. In this study， by adding different starters to the garden waste compost， the effects of starters on the composting effect were studied， and the results found that adding the organic material starter or biological fertilizer starter could significantly reduce the temperature in the second and third high temperature intervals， and promote compost maturity， which is similar to the research conclusions of Li Guoxue and Xi Beidou.

In the composting process， the most numerous， common and main microorganisms are bacteria[8]. Bacteria are mainly responsible for the degradation of organic matter in the compost base material and the production of heat energy throughout the composting process， and they are an important microbial population in the biochemical decomposition process of composting. According to relevant studies， at least 80% to 90% of microbial activities are derived from bacteria. They can use a variety of enzymes to degrade organic matter and are the dominant population in the composting microbial community[9]. In this study， with the composting process， the number of bacteria first increased and then decreased， which might be because at the initial stage of composting， mesophilic bacteria dominated the decomposition of organic matter， mainly decomposing some easily degradable starch， sugars and other substances， a large amount of heat energy would be released with the decomposition of the material， which would cause the temperature of the piles to rise， and the compost would enter a high temperature period. When the stack temperature rose to the maximum， the number of mesophilic bacteria also dropped to a minimum， so the number of bacteria dropped sharply[10].

Fungi can achieve the purpose of degrading lignocellulose by secreting extracellular enzymes by themselves. Meanwhile， because fungi have a hyphae structure during the vegetative growth stage， they have a certain mechanical interleaving effect， which can produce certain mechanical damage to the compost material and promote the biochemical reaction of organic matter in the reactor[9]. In this study， the change trend of the total amount of fungi was similar to that of bacteria. The number of fungi presenting such change pattern might be due to the fact that most of the fungi in the piles were mesophilic fungi in the early stage of composting， the most suitable growth temperature of which is 25-30 ℃， and with the progress of the composting process， the temperature continued to rise， and the collective death of mesophilic fungi occurred， resulting in a decrease in the total amount of fungi. When the temperature rose to the suitable growth range of thermophilic fungi， their number began to increase， and when the temperature gradually decreased， the number of mesophilic fungi rose.

The number of actinomycetes in waste composting systems is much lower than that of bacteria， but the law of change is similar to that of bacteria[11]， which is similar to the conclusions drawn in this study. Actinomycetes are a type of prokaryotic organisms that grow in the form of hyphae. In the composting process， actinomycetes mainly degrade hemicellulose and can dissolve lignin-like complex organic matter to a certain extent[12]. The number and species of actinomycetes increase during the high temperature stage of composting. They are the main degrading bacterial group in the high temperature stage， such as thermophilic actinomycetes， which grow well at 50-60 ℃， and are better than thermophilic fungi in heat resistance[13]. The common thermophilic actinomycetes in piles are： thermophilic actinomycetes， Nocardia， Micromonospora， Streptomyces， etc.， which are all dominant bacteria in composting systems， and they appear in the high temperature stage， the cooling stage and the maturity stage， respectively[14].

Cultivation substrates in agricultural production is mostly peat[15]， but as a scarce and non-renewable resource， peat has been listed as a restricted production project by the government[16]， and finding its substitutes has become a top priority. Wang et al.[17] composted the four materials of sludge， sycamore leaves， fluvo-aquic soil and urea in different proportions， made them into 9 kinds of cultivation substrates together with soil after they were decomposed， and studied the effects of different cultivation substrates on the growth and development of rape seedlings. The results showed that that the compost product obtained from garden waste could be used as a substrate for plant growth and could partially replace the scarce resource peat. In this study， the compost product obtained from garden waste was used to partially replace peat to raise pepper seedlings， and the conclusion was similar to that of Wang et al.， and suitable substrate formulas for pepper seedlings were selected.

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