Research on Composting of Garden Waste and Its Application in Cultivation Substrates

Abstract

To achieve the resource utilization of garden waste, in this study, we used garden waste as the main raw material and conducted static composting with high-temperature aerobic treatment and forced ventilation by adding appropriate external additives. Our results showed that during the composting process, the pH was weakly alkaline, and the electrical conductivity was between 1.42 and 1.50 mS/cm. The E4/E6 (an important indicator of the quality or degree of condensation and aromatization of humic acid) gradually decreased, and nitrogen, phosphorus, and potassium contents increased. The germination index gradually rose and ultimately exceeded 80%. Treatment (T2) with a final C/N ratio of 25:1 and the addition of 0.3% bacterial agent resulted in the highest nutrient content and the best degree of compost maturity. All indicators met the requirements of the Chinese “Technical Requirements for Urban Landscape Waste Resource Recycling and Deep Processing (GB/T 40199-2021)”. When using a composite substrate of garden waste and other horticultural substrates for planting, a membership function was executed for comprehensive evaluation. The V (T2):V (peat):V (vermiculite):V (vermiculite) = 135:135:30:00 substrate treatment resulted in optimal lettuce growth and quality. In summary, combining the compost products of garden waste with traditional cultivation substrates at a specific ratio shows favorable applicative prospects.

1. Introduction

The field of garden greening has rapidly expanded with the construction of ecological cities in China, becoming an important element in promoting urban development and progress while injecting new vitality into urban economic growth. However, the plants in urban greening possess their own metabolic processes, and their maintenance and greening processes are inevitably accompanied by the production of waste such as fallen flowers, leaves, and pruned branches, placing pressure on the urban environment [1]. China produces more than 30 million tons of green waste per year in the form of fallen leaves and tree trimmings, creating a great demand for disposal [2]. This green waste is also an important part of the ecological cycle, as it is principally composed of elements such as carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S). Garden waste material reflects the advantages of being widely sourced, readily accessible, easily transportable, renewable, and biodegradable, and as a result, this waste is not a long-term burden on the environment. The resource utilization of garden waste is an important initiative for environmental protection and sustainable development. Incineration power generation and organic mulch have been gradually transformed to composting treatment for the resourceful treatment of garden waste from an original landfill, and this is now applied to the soil.

The composting of garden waste refers to the decomposition of organic matter into nutrients that can be absorbed and utilized by plants through collection, crushing, mixing, composting, fermentation, and maturation. The resulting compost is rich in organic matter and micronutrients [3]; it promotes the growth of plants [4] and accelerates the ecological cycle. However, traditional composting methods are time-consuming, labor-intensive, produce low-quality compost, are prone to unpleasant odors, and are often accompanied by nutrient loss. In this study, we addressed these issues, taking China’s national environmental protection “13th Five-Year Plan” as a directive for our work. Adhering to the environmental protection concept of “waste utilization, green regeneration”, we herein investigated the comprehensive disposal of garden waste through “resource utilization, reduction, harmlessness, and art” and transformed garden waste originally regarded as useless into reusable resources so as to realize the goal of zero waste for garden materials. This study was aimed at transforming waste materials into valuable resources and thus followed the principles of “holistic, coordinated, circular, and regenerative” to realize the green regeneration of the environment. Landscape waste compost is a type of organic fertilizer. The composting and decay of landscape waste not only effectively eliminate harmful organisms present in the waste but also significantly reduce the size of the waste, allowing the compost to be used as a high-quality soil conditioner and plant growth medium. The organic matter and nutrient contents of garden waste—including nitrogen (N), phosphorus (P), and potassium (K)—are significantly increased after the garden waste decays through composting, while pests and diseases are reduced. This composted product can be used as a soil remediation material and a high-quality cultivation substrate, providing plants with a stable and growth-friendly water, gas, and fertilizer structure [5]. This process increases the soil organic matter content [6], protects the soil environment and ecosystem, and improves the soil’s ability to retain fertilizers, thus promoting the sustainable development of agriculture [7]. This resourceful use of garden waste reduces dependence on chemical fertilizers and achieves a win–win situation between ecological protection and economic development.

The application of garden waste composting has been found to augment the chlorophyll content and stem thickness of tomato plants [8], shorten the number of days to flowering after transplanting, and increase the stem thickness and fruit weight of chili peppers [9]. In addition, research on potato showed that garden waste composting increased the potato yield, number of branches, and weight of the tubers while facilitating potato sprouting [10]. Landscape waste exhibits a high C/N ratio and is difficult to decompose due to high lignin and cellulose content [11]. As a result, landscape waste is difficult to biodegrade, requires a lengthy period of time to use, and produces low-quality compost. There is, therefore, an urgent need to screen improved C/N ratios and biofungal additives suitable for the composting of landscape waste [12]. The substrate application of garden waste compost products presently focuses on the cultivation of ornamental plants, and research on its application in the field of vegetable cultivation remains insufficient. Lettuce, as one of the most widely grown vegetables in China, is highly favored for its high yield, excellent quality, and rich nutritional value. In this study, different amounts of urea were added as a N source and different amounts of microbial agents were added to the garden-waste-composting substrate. The pH, electrical conductivity (EC), nutrient content, and decomposition parameters were then applied as evaluation indices to screen the optimal C/N ratio and microbial agent dosages. The garden waste compost was ultimately compounded with other horticultural substrates to explore the effects of the various compounded substrates on the growth and development of lettuce in order to improve compost quality and promote the resource utilization of garden waste. In summary, the contribution of this study to the field of waste resource utilization was as follows: First, we adopted membrane-covered aerobic fermentation technology that prevented the influence of weather factors such as rainwater and strong winds on the composting process; this effectively prevented gas volatilization and significantly reduced the negative impact of odor on the surrounding environment. Second, we defined an optimal formula for lettuce cultivation, providing an effective means for the resource utilization of garden waste that recognized local resources of regional waste.

2. Materials and Methods

2.1. Test Materials

The garden waste used in this test was provided by Jiangsu Sanshan Environment Co., Ltd., Yangzhou, China. The garden waste was derived from the 2022 Yangzhou municipal garden waste and primarily contained tree waste such as ginkgo, sycamore, and magnolia waste and various types of flower and grass clippings that were chopped to sizes of 2–5 cm using a mechanical grinder (the physical and chemical properties of each type of waste are shown in

Table 1. Basic properties of composting raw materials.

Raw Materials	pH	TC g/kg	TN g/kg	C/N	TP g/kg	TK g/kg
Garden waste	7.45	412.91	8.42	49.04	1.60	4.31
Balsam camphor waste	6.96	432.37	9.21	46.95	2.42	4.67

pH, potential of hydrogen; TC, total carbon; TN, total nitrogen; C/N, carbon–nitrogen ratio; TP, total phosphorus; TK, total potassium.

). We used a commercially available putrefactive microbial agent purchased from Jiangyin Peng Harrier Lianye Biotechnology Co., Ltd., Wuxi, China for our tests. The microbial agent was chiefly composed of Bacillus subtilis, Bacillus amyloliquefaciens, Saccharomyces cerevisiae, and Aspergillus oryzae, with an effective number of live bacteria (cfu) ≥ 0.5 billion/g and protease activity exceeding 15.0 U/g. The microbial agent was principally wielded during aerobic composting and the fermentation of organic materials. Urea was purchased from Shanxi Jinfeng Coal Chemical Co., Ltd., Xinzhou, China with a total nitrogen (TN) content ≥46.0%. We used a desktop pH meter (pHS-2F, Yidian Scientific Instrument Co., Ltd., Yichang, China) to measure pH values, and total carbon (TC) and TN were tested with a CHNS analyzer (Beijing Yingsheng Hengtai Technology Co., Ltd., Beijing, China). Total phosphorus (TP) was determined using the molybdenum antimony colorimetric method, and total potassium (TK) was measured using the flame photometer method.

2.2. Experimental Design

2.2.1. Composting of Garden Waste

The experiment was conducted at the test base of Yangzhou University, Shatou Town, Yangzhou City, Jiangsu Province, where garden wastes were collected and pulverized. The initial C/N ratio was adjusted by adding different doses of urea and mixing. Microbicides were then added in varying ratios to the total mass, and the initial moisture content was adjusted to approximately 60%. The bottom of the waste pile was equipped with an aeration pipe, and the ventilation volume was 30 m³/min. We included five treatments in the experiment. The specific In Proceedings of the in

Table 2. Composting treatments.

Experimental Treatment	Raw Materials	Adjusted C/N	Fungicide Addition
CK	Garden waste	/	/
XZ	Balsam camphor waste	25:1	0.3%
T1	Garden waste	40:1	0.3%
T2	Garden waste	25:1	0.3%
T3	Garden waste	25:1	0.15%

. The materials used in each treatment were mixed well, and a static fermentation system with a molecular membrane was used to conduct the high-temperature aerobic and forced-ventilation static stack composting.

2.2.2. Substrate Application of Compost Products

To produce the decayed garden waste compost product, the C/N of garden waste was adjusted to 25:1, and 0.3% microbial fungus agent was added to ferment the composting product. The compost was crushed manually to a size of approximately 1 cm. Vermiculite was purchased from the Shuyang County Weisha Horticulture Company in Suqian City, Jiangsu Province, China. We purchased earthworm manure from Taizhou, Jiangsu Province, China Chunguang Ecological Agriculture Development Co., and grass charcoal was obtained from the Bizhou Seedling Substrate Processing Factory in Tonghua, Jilin, China. Fruit and vegetable plants were lettuce with stems and a variety is red hibiscus. This variety exhibits a growth cycle of about 80 days, is hardy, and can safely overwinter. With respect to lettuce growth, we watered regularly and quantitatively according to 60% of the water-holding capacity of the field.

The experiment was conducted from November 2023 to February 2024 in the Shatou Town solar greenhouse in Yangzhou. The experiment included six different treatment substrates and two fertilization levels (no fertilization and regular fertilization) for a total of twelve treatments. There were five pots for each treatment, and each treatment was replicated three times. The control (CK) treatment included soil alone, while the S0 treatment included mixed and decayed garden waste, grass charcoal, and vermiculite at a ratio of 45:45:10 (v/v). Treatments S1, S2, S3, and S4 were obtained by compounding S0 with vermiculite manure at different v/v ratios, and fertilization was performed using N fertilizer only (the specific experimental design scheme is shown in

Table 3. Lettuce pot experiment.

Experimental Treatment	Substrates	Different Levels of Fertilization
		No Fertilizer (g/pot)	Conventional Fertilization (N) (g/pot)
CK	Soil	/	20
S0	Garden waste–charcoal–vermiculite = 45:45:10	/	20
S1	S0–earthworm manure = 1:9	/	20
S2	S0–earthworm manure = 2:8	/	20
S3	S0–earthworm manure = 3:7	/	20
S4	S0–earthworm manure = 4:6	/	20

).

2.3. Measurement Items and Methods

The fermented samples were collected from the upper, middle, and lower parts of the plants on days 1, 8, 11, 14, 21, 28, 35, 45, and 55 to form a mixed sample, which was then divided into two parts. One part was stored at 4 °C for the determination of the pH, E4/E6, EC, and seed germination index (GI); the other portion was naturally dried, crushed, and sieved for the determination of total organic C (OC), TN, TP, and TK contents. During the composting process, the indicators were measured according to the “Agricultural Industry Standards of the People’s Republic of China (NY/T 525-2021)” [13]. After 50 days of plant growth, 10 plants were randomly selected from each treatment for the determination of growth and quality indices.

The morphologic plant indicators were then obtained. The natural height of the plant from the base of the stem to the natural highest point measured at harvest time was considered the plant height. The stem thickness was measured at 3 cm from the root using vernier calipers. For the investigation of plant leaf morphology, data were usually obtained from measurements of the largest rosette leaf. The leaf length was measured from the base of the petiole to the tip of the largest rosette leaf, while the leaf width was measured at the widest point of the largest rosette leaf blade. For the investigation of plant bioaccumulation, each plant was divided into above-ground and below-ground parts after the lettuce was harvested, and the fresh weight of each part was measured using an electronic balance with an accuracy of 0.001 g. The soluble protein content was determined using the Coomassie Brilliant Blue method, the chlorophyll content was assessed with the 95% ethanol-extraction method, and the vitamin C content was measured by applying the 2,6-dichlorophenol indophenol method. Digestion with concentrated H₂SO₄-H₂O₂ was conducted to analyze the N, P, and K contents of lettuce stems and leaves.

2.4. Data Analysis

We employed Microsoft Excel 2019 for all data processing. Origin 9.60 was applied to plot the graphs, and SPSS 20.0 software was used to analyze the data for analysis of variance (ANOVA) and multiple comparisons. We implemented a comprehensive analysis system for morphologic plant indicators to assess the morphologic characteristics of the plants, and the comprehensive evaluation value of plant morphology was positively correlated with plant growth according to Equation (1):

$$\mathrm{U}(\mathrm{X}\mathrm{i})={\displaystyle \frac{\mathrm{X}-{\mathrm{X}}_{\mathrm{m}\mathrm{i}\mathrm{n}}}{{\mathrm{X}}_{\mathrm{m}\mathrm{a}\mathrm{x}}-{\mathrm{X}}_{\mathrm{m}\mathrm{i}\mathrm{n}}}}.$$

(1)

This equation can be used to calculate the function values of the morphological indicators of the plants in each treatment, where X is the measured value of an indicator under a certain substrate condition, X_max is the maximum value of the indicator measured, and X_min is the minimum value of the indicator measured. The weight value of each composite indicator can be derived using Equation (2) as follows:

$$\mathrm{W}\mathrm{i}={\displaystyle \frac{\mathrm{P}\mathrm{i}}{{\sum }_{\mathrm{i}=1}^{\mathrm{n}}\mathrm{P}\mathrm{i}}},$$

(2)

where Wi is the weight value attributed to the ith composite indicator, and P is the contribution rate attributed to the ith composite indicator. Equation (3) is expressed as follows:

$$\mathrm{D}={\sum }_{\mathrm{i}=1}^{\mathrm{n}}\left[\mathrm{U}\left(\mathrm{X}\mathrm{i}\right)\times \mathrm{W}\mathrm{i}\right].$$

(3)

where D is the comprehensive evaluation value of lettuce under the 6 different substrate treatments with comprehensive indexes.

3. Results and Discussion

3.1. Influence of Different Treatments on the Composting Effect of Garden Waste

3.1.1. Effects on Physical and Chemical Properties

Researchers have demonstrated that pH induces distinct effects on the composting process, with pH levels that are too high (pH > 9) or too low (pH < 4) affecting microbial activity during composting [14]. The activity of microorganisms is crucial to the high-temperature aerobic composting process, and a weakly alkaline pH is the most suitable for the fermentation of flora [15]. As shown in Figure 1a, the pH after the composting of garden waste exhibited an overall tendency to decrease and then increase, with the final pH higher than the initial value and weakly alkaline. The increase in pH may have occurred because the microorganisms did not degrade the organic N completely but rather released N in the form of ammonium salts [16]. When the pH is between 7.5 and 8.5, the microorganisms involved in composting grow the fastest, which can promote the degradation of organic matter in the compost and achieve maximal composting efficiency. At the end of our composting, the pH was within the range of 7.00–8.05, which is suitable for agricultural applications [17]. As shown in Figure 1b, the EC was usually adopted to indicate the concentration of soluble salts, but it was also used to indicate the concentration of soluble ions in the fertilizer or growing medium. In addition, the EC can reflect the toxic effects of certain compost products on plants [18]. The EC values in this experiment first increased and then decreased slightly before rising again and stabilizing. The increase in conductivity values was largely due to the degradation of organic matter, which subsequently led to an increase in the concentration of soluble salts [19]. Compost reaches maturity when the EC of the composted material is <4.0 mS/cm [20], and excessively high EC values can lead to physiologic drought and plant death and can also hinder the bacterial inhibition of the compost product itself [21]. In this study, the EC values of the five composting experiments ranged from 1.42 to 1.50 mS/cm. The compost products were all within the safe range of plant growth and development, and they exerted no inhibitory effect on seed germination.

N is an important nutrient required by microorganisms to support their life activities, as well as an essential element for incorporation into macromolecules such as proteins, amino acids, and nucleic acids during cell growth [22]. The quality of compost and its finished product is closely related to the transformation of N during the degradation process. This change is caused by the joint influence of microbial forces and temperature conditions. In this experiment, the N content of the compost first increased, then decreased, and then slowly increased again, exhibiting a general upward tendency. The increase in the TN content may be attributed to the reduction in dry matter caused by the mineralization and decomposition of non-nitrogenous organic matter during the composting process. In the subsequent process of composting, the volatilization of ammonia N and the denitrification of nitrate N have been reported to cause the loss of gaseous N [23]. In the garden-waste-composting treatments (T1 through T3), the higher N content in the T2 treatment was due to the use of an appropriate C/N ratio and microbial agent addition, leading to the continuous decomposition of organic matter into CO₂ and H₂O, a decreasing amount of dry matter, and a relative increase in TN content [24]. As key nutrients involved in plant growth, P and K play indispensable roles in promoting plant protein synthesis and enhancing fruit quality. The TP and TK contents in compost products can therefore be framed as important indicators of their fertility. At the end of composting, the P content was augmented in all five treatments. The highest P and K contents in the garden waste compost treatments (T1, T2, and T3) were found in the T2 treatment. This may have been due to the C/N and fungicide additions in this treatment. The absolute content of phosphorus and potassium in the compost pile was constant, and these minerals were not easily lost through volatilization. As the volume and weight of the compost pile continued to decrease, the nutrient “concentration effect” occurred, increasing the TP and TK content of each treatment. However, due to different composting efficiencies, this increase was not consistent (Figure 2).

3.1.2. Impact on Spoilage Indicators

The GI is an important bioindicator of the degree of decomposition of reactor compost. The GI reflects the toxicity of the compost material, affects seed growth, and is a sensitive parameter that influences the overall performance of seed germination [25]. When the GI reaches 80%, the compost product is usually considered to be phytotoxin-free and has reached maturity [26]. The GI values of all treatments except for the CK group manifested a general upward trend and eventually reached greater than 80%. During the organic-waste-composting process, microbial decomposition often produces potentially toxic compounds such as organic acids that may adversely affect seed germination and growth. The GI values of the T2 treatment increased the fastest and were the highest at the end of the garden-waste-composting treatments (T1, T2, and T3) as the T2 treatment (with a C/N ratio of 25:1 and 0.3% bacteriophage addition) was more suitable for garden waste composting, and this was in agreement with previous findings [27]. The E4/E6 value is usually employed as a key indicator of the humus content in compost, which can then be applied to evaluate the degree of humification of compost as a whole, with smaller E4/E6 values reflecting high humus content in the compost. E4/E6 is commonly used to characterize the degree of polymerization of the C-skeleton of the benzene ring within a substance, which reflects the degree of condensation of the aromatic ring of humic acid and its molecular weight [28]. In the process of aerobic composting, large amounts of fulvic acid, humic acid, and complex minerals are produced [29]. As the decomposition of compost proceeds, the mineralization of the material is weakened, the amount of humic acid that is easily converted to degradation is gradually reduced, the unsaturation degree of humic acid gradually increases, and the degree of condensation gradually rises; thus, the E4/E6 value is inversely proportional to the level of condensation of the aromatic ring in the molecule, the degree of aryl conformations, and the molecular weight. In this experiment, the E4/E6 values gradually decreased in each compost pile. At the end of the experiment, T2 exhibited the lowest E4/E6 value, indicating that this treatment resulted in the highest degree of humification at the conclusion of composting (Figure 3).

At the end of the composting process, among the garden waste compost products of the T1, T2, and T3 groups, all five treatments were weakly alkaline in pH, and the EC values were less than 4.0 mS/cm. N, P, and K were all elevated, and the T2 treatment had the optimal nutrient content, with the highest final N content, the best N-vegetable retention, and the most elevated TP and TK contents. The E4/E6 values decreased in all treatments, and the T2 treatment exhibited the lowest E4/E6 value. The GI of the T2 treatment was the highest, reaching 102.39%, and the T2 treatment reached a high degree of decomposition. The T2 treatment, with a C/N of 25:1 and the addition of 0.3% bacteriophage, was the most effective in retaining various nutrient contents during the composting process, increasing the degree of decomposition of the compost, and reducing the toxicity of the composted products to plants. In summary, this experiment revealed that the T2 treatment was the best option for composting garden waste.

3.2. Effects of Different Ratios of Garden Waste and Other Horticultural Products on Lettuce Growth

3.2.1. Effects of Different Treatments on the Growth Pattern of Lettuce

In this experiment, both decomposed garden waste compounds and substrates (decomposed garden waste + grass charcoal + vermiculite) compounded with vermicompost enhanced lettuce growth and provided favorable conditions for lettuce growth and development. As shown in Figure 4, in the absence of fertilizer, the treatments exhibited plant heights ranging from 38.70 to 50.30 cm, lettuce stem thicknesses ranging from 19.10 to 34.23 mm, maximal leaf lengths of 22.33–28.37 cm, and maximal leaf widths of 5.07–6.53 cm. Although the plant height, stem thickness, and maximal leaf length of each treatment were significantly higher than those of the CK treatment, the maximal leaf width did not differ from that of the latter. There were no differences in the plant height, maximal leaf length, or maximal leaf width among the treatments with different ratios of compounded substrate; however, stem thickness was significantly greater in the S1, S2, and S3 treatments relative to the S0 treatment. The lettuce grew best under the substrate treatment (S3) with garden waste composite substrate–earthworm manure = 3:7, and the plant height, stem thickness, and maximal leaf length and width were significantly higher than those of the controls.

As can be seen from Figure 5, the root length of each treatment ranged from 16.42 to 23.21 mm, the root surface area ranged from 252.38 to 640.73 cm², the root projected area was between 80.34 and 203.96 cm², and the average root diameter was between 50.38 and 87.44 mm. The root length, root projected area, root surface area, and average root diameter of the compound matrix of the garden wastes were significantly higher than those in the CK treatment. Among the different treatments, the S3 treatment was the best, with 41.35%, 153.88%, 153.88%, and 73.57% increases in the root length, root projected area, root surface area, and average root diameter, respectively, compared to CK. There were no differences in the root length, root projected area, root surface area, or root mean diameter between treatments with different ratios of substrates.

3.2.2. Effects of Different Treatments on Lettuce Quality Indexes

The composite substrate of well-decomposed garden waste (well-decomposed garden waste + grass charcoal + vermiculite) with vermicompost also enhanced the quality of vegetables, including their chlorophyll content, mineral element, soluble protein, and vitamin contents, all of which are essential to the human diet [30]. While there was no difference in chlorophyll content between the S0 and CK treatments, the chlorophyll content was significantly higher in the S1–S4 treatments than in the CK treatment. Under different ratios of substrate, the chlorophyll content with the S2 and S3 treatments was higher than that using the S0, S1, and S4 treatments, although the difference was not significant. The soluble protein content of lettuce under each treatment ranged from 0.68 to 1.25 mg/g fresh weight (FW), and the soluble protein content of lettuce under the garden-waste-composting treatments (S0–S4) was higher than that of the CK treatment. The vitamin C content of lettuce stems ranged from 71.4 to 79.53 mg/gFW, and the vitamin C content of lettuce stems ranged from 1.5 to 1.5 mg/gFW. The vitamin C contents of lettuce in the compost treatments (S0–S4) were all higher than that of the CK group, with the S3 treatment exhibiting higher vitamin C content than the remaining groups, with an enhancement of 11.39% relative to CK (

Table 4. Chlorophyll, soluble protein, and vitamin contents of lettuce grown in different matrices.

Treatment	Chlorophyll (mg/kgFW)	Soluble Protein Values (mg/gFW)	Vitamin C (mg/kgFW)
CK	0.79 ± 0.02 b	0.71 ± 0.02 cd	71.4 ± 1.21 a
S1	0.81 ± 0.02 b	0.85 ± 0.04 abc	79.86 ± 7.82 a
S2	1.08 ± 0.12 a	0.93 ± 0.10 a	79.41 ± 10.88 a
S3	1.16 ± 0.06 a	0.92 ± 0.02 ab	89.72 ± 9.65 a
S4	1.14 ± 0.11 a	0.64 ± 0.02 d	68.02 ± 3.15 a
S5	0.99 ± 0.06 ab	0.74 ± 0.08 bcd	76.37 ± 1.79 a

Different superscript letters (a, b, c, d) indicate significant differences at p < 0.05 among the different substrate treatments.

).

Altogether, as regards the edible parts of lettuce, the composted products of garden waste formulated with the substrates improved the quality of lettuce stems and enhanced their health benefits for human consumption.

3.2.3. Comprehensive Evaluation of Lettuce Growth under Composted Garden Waste Product Composite Substrate

To comprehensively assess the growth of lettuce in the substrate, principal component analysis (PCA) was performed for the 15 indicators of plant height; stem thickness; maximal leaf length; maximal leaf width; stem fresh weight; root length; root projected area; root surface area; root average diameter; and chlorophyll, protein, vitamin C, N, P, and K contents. The eigenvalues and contribution rates obtained via PCA thus provided the basis for the selection of principal components (the eigenvalues and contribution rates of each principal component are presented in

Table 5. Principal components of characteristic roots.

Ingredient	Initial Eigenvalue		Contribution of Cumulative Variance
	Total	Contribution of Variance
1	10.83	67.76	67.77
2	2.39	14.94	82.71
3	1.29	8.07	90.78
4	0.94	5.89	96.67
5	0.53	3.33	100.00

). We selected the first three principal components according to the principle that the contribution rate was greater than 85%, and the cumulative contribution rate of the first three principal components reached 90.781%. The initial value of the affiliative function of the three lettuce plants, Xi, was obtained from PCA, as shown in

Table 6. Value of the membership function for a single metric.

Treatment	X1	X2	X3
CK	−0.81	0.53	−0.37
S0	−0.34	−1.70	0.50
S1	0.35	1.19	−0.21
S2	0.43	0.50	1.77
S3	1.10	−0.15	−0.89
S4	0.26	−0.37	−0.80

, as were the integrated affiliative function values of each index U (the D values of different treatments were 0.144, 0.425, 0.742, 0.790, 0.835, 0.611, respectively, and the larger the value of the affiliation function, the stronger the promotive effect on lettuce growth, as shown in

Table 7. Membership function value of each comprehensive index and the lettuce growth ranking.

Treatment	U (X1)	U (X2)	U (X3)	D	Rank
CK	0.00	0.77	0.20	0.14	6
S0	0.51	0.00	0.52	0.43	5
S1	0.74	1.00	0.26	0.74	3
S2	0.77	0.76	1.00	0.79	2
S3	1.00	0.54	0.00	0.84	1
S4	0.71	0.46	0.03	0.61	4

) [31]. The S3 treatment exerted the strongest effect on the growth of lettuce, and treatments S1–S4 were all greater than 0.5, indicating that these four treatments promoted lettuce growth. In terms of lettuce growth promotion, the various treatments were ranked in descending order as follows: S3 > S2 > S1 > S4 > S0 > CK.

4. Conclusions

Based on the high-temperature aerobic composting of disparate garden wastes, we herein investigated the changing patterns of key physicochemical properties and nutrients during the composting process by adding appropriate exogenous additives, providing different C/N ratios and microbial additives, and continuously monitoring a series of representative physicochemical indicators. Using high-temperature and high-efficiency composting technology with garden waste as the primary raw material, the most suitable ratios of bacteriophage and urea were screened to effectively promote the composting process and improve the overall quality of the composted products. Our experimental results revealed that the optimal composting effect was achieved by adjusting the C/N ratio of garden waste to 25:1 and adding 0.3% fungicide (the T2 treatment). The indicators of pH, EC, E4/E6, TN, C/N, GI, TP, and TK after composting met the Chinese “Technical Requirements for Resouree Recovery and Further Processing of Urban Garden Waste (GB/T 40199-2021)” [32]. We executed PCA and the fuzzy mathematics affiliation function method to analyze the impact of garden waste composite substrate ratios on lettuce growth, and the promotive effects were in the order S3 > S2 > S1 > S0 > CK, from strong to weak. This demonstrated that the substrate using a ratio of 135:135:30:700 for decomposed garden waste (the T2 treatment)–charcoal–vermiculite–earthworm manure (S3 treatment) was most conducive to lettuce growth.

The resource utilization of garden waste can reduce the pressure on the environment and play an important role in the rationalization and maximization of resources. We posit that the results of this study will serve as technical guidance for the composting of garden waste and other horticultural substrates into a novel type of substrate so as to provide a theoretical and practical basis for the resourceful use of garden waste and that adoption of our work will reduce resource waste and environmental pollution.