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Article

Greenhouse Gas Emissions from Co-Composting of Green Waste and Kitchen Waste at Different Ratios

by
Junhao Gu
1,
Suyan Li
1,*,
Xiangyang Sun
1,
Rongsong Zou
2,
Binru Song
1,
Di Wang
1,
Hui Wang
1 and
Yalin Li
1
1
The Key Laboratory of Comprehensive Utilization of Green Waste in Hebei Province, College of Forestry, Beijing Forestry University, Beijing 100083, China
2
Comprehensive Experimental Center in Yellow River Delta of Chinese Academy of Forestry, Dongying 257000, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(17), 8041; https://doi.org/10.3390/su17178041
Submission received: 28 July 2025 / Revised: 29 August 2025 / Accepted: 4 September 2025 / Published: 6 September 2025
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Abstract

With the rapid expansion of urban green spaces and the increasing amount of domestic waste, efficient and sustainable treatment of green waste (GW) and kitchen waste (KW) has become a pressing issue. Co-composting offers a green and low-carbon solution, yet a systematic understanding of its greenhouse gas (GHG) emission dynamics remains lacking. This study aims to investigate the impact of varying GW:KW ratios on GHG emissions during composting, in order to identify optimal mixing strategies and sup-port the development of low-carbon urban waste management systems. Six treatments with different GW:KW ratios (10:0, 9:1, 8:2, 7:3, 6:4, and 5:5) were evaluated under continuous aeration for 42 days. Results showed: (1) All treatments exhibited a typical composting temperature profile (mesophilic, thermophilic, cooling, maturation), with final seed germination index (GI) > 95% and significantly reduced E4/E6 ratios, indicating maturity. (2) When kitchen waste (KW) was ≤20%, cumulative GHG emissions slightly increased; KW ≥ 30% led to net reductions, with the 6:4 treatment (A4) achieving the highest decrease (17.44%) in total CO2-equivalent emissions. In conclusion, maintaining KW at 40–50% optimally balances compost maturity and emission reduction, providing a viable strategy for the high-value utilization of urban organic waste and carbon mitigation.

1. Introduction

With the rapid advancement of urbanization and the ongoing development of ecological civilization, the expansion of urban green spaces has led to a substantial increase in the generation of green waste (GW), including pruned branches, fallen leaves, and dead twigs. In China, the annual generation of GW is estimated at 70 to 100 million tons, making it the second-largest category of solid waste after municipal solid waste [1]. Traditional disposal methods such as incineration and landfilling not only waste valuable resources but also cause adverse environmental impacts, hindering the sustainable development of eco-cities [2].
As a clean and renewable resource, GW can reduce carbon emissions through composting and energy recovery, while improving soil quality and promoting agricultural sustainability [3]. At the same time, the treatment of kitchen waste (KW) has become increasingly pressing. In this study, kitchen waste (KW) refers specifically to pre-consumer, uncooked organic residues such as vegetable leaves and fruit peels, primarily sourced from household and commercial food preparation [4]. The production of urban domestic waste in China is approximately 249 million tons, of which kitchen perishable organic waste accounts for up to 50% [5]. KW is characterized by high moisture content, abundant organic matter, and a low carbon-to-nitrogen ratio (C/N), making it prone to anaerobic conditions and odor during composting, thus limiting its resource utilization [6]. Conversely, GW has low moisture content, high C/N ratio, good biodegradability, and environmental compatibility, but its long composting cycle and greenhouse gas (GHG) emissions remain concerns [7,8]. Therefore, co-composting GW with KW, taking advantage of their complementary properties, offers an effective pathway for the sustainable utilization of both waste streams.
Co-composting improves the physicochemical properties of the pile, enhances composting efficiency, and reduces nitrogen loss. Previous studies have shown that properly mixed GW and KW can significantly increase fermentation temperature, improve humification efficiency, reduce nitrogen volatilization, and enhance compost quality [9,10]. Some GW-based compost mixtures exhibit strong carbon sequestration potential [11,12]. Key parameters such as temperature, pH, and C/N ratio critically influence carbon stabilization during co-composting [13]. However, microbial degradation during composting inevitably produces gases such as CO2, CH4, N2O, and NH3 [14], which not only contribute to global warming but also reduce the nutrient content of the final product. Carbon loss in the form of gases can reach up to 60% [15], with CO2 accounting for approximately 31.4–57.9% of total initial carbon (TC) [16]. CH4, though emitted in smaller quantities (generally <10% of TC), has a global warming potential (GWP) 25–28 times that of CO2 [17]. Similarly, nitrogen losses as gases can exceed 20%, primarily as NH3 and N2O. While N2O emissions are only 1–6% of total nitrogen (TN), their GWP is 265–298 times that of CO2 [18].
At present, research on greenhouse gas emissions and humification processes during co-composting of green waste (GW) and kitchen waste (KW) remains limited. The majority of research efforts to date have centered on individual gaseous pollutants such as methane (CH4) or nitrous oxide (N2O), falling short of providing a holistic, multi-faceted evaluation of greenhouse gas (GHG) emissions and ammonia (NH3) discharge throughout the entire composting process. This study uses GW and KW (primarily cabbage leaves, excluding post-meal residues such as oil and salt) as raw materials to conduct a series of aerobic co-composting experiments under controlled mixing ratios (10:0, 9:1, 8:2, 7:3, 6:4, and 5:5). The objective is to systematically investigate the emission patterns of greenhouse gases (CO2, CH4, N2O) and ammonia (NH3), as well as the progression of humification under different mixing conditions.

2. Materials and Methods

2.1. Experimental Materials and Composting Equipment

The green waste (GW) used in this study was collected from Xiangshan Park, located in Haidian District, Beijing, China. Prior to composting, the GW was shredded into pieces approximately 2 cm in diameter. The kitchen waste (KW), primarily composed of cabbage leaves and free from oil and salt residues, was sourced from the cafeteria of Beijing Forestry University, was also shredded to approximately 2 cm in diameter. The composting experiment was conducted at the Sanqingyuan greenhouse in Haidian District, Beijing, using six sealed composting reactors with a capacity of 100 L each. Each composting reactor consisted of a filter frame, filter disc, reaction tank, vent pipe, and air pump. Continuous aeration was applied at a rate of 0.3 L/min, which has been reported to minimize water loss while ensuring aerobic conditions during composting [19].

2.2. Experimental Design

Six composting treatments were established. The control group (CK) consisted of green waste alone. The remaining treatments, labeled A1 through A5, were mixed with kitchen waste at mass ratios of 9:1, 8:2, 7:3, 6:4, and 5:5, respectively. An effective microorganism (EM) solution was sprayed onto each pile at 1.5% of total pile mass. According to the supplier, the EM solution primarily contains bacillus subtilis at a concentration of 2 × 108 CFU/mL. The initial moisture content of the composting material was adjusted to approximately 60% using distilled water. Pile turning was performed on days 0, 3, 7, 14, 21, 35, and 42.
After each turning, a 500 g sample was collected from each pile using the five-point sampling method. Water was added as needed based on the moisture content. Each sample was divided into two portions: one fresh portion was stored at −4 °C for the analysis of moisture content, pH, electrical conductivity (EC), E4/E6 ratio, and seed germination index (GI); the other was air-dried, ground, and used for analyzing organic matter, ammonium nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), and other physicochemical parameters. All measurements were conducted in triplicate for each treatment. The basic physicochemical properties of the raw composting materials are presented in Table 1.

2.3. Methods for Physiochemical Indexes

Temperature was measured daily at fixed times using a thermometer. Water extracts were prepared at a solid-to-liquid ratio of 1:10, and the filtrate was used to determine pH and electrical conductivity (EC). For germination index (GI) analysis, 5 mL of the above water extract was added to a Petri dish lined with filter paper and containing 20 Chinese cabbage seeds, and incubated in the dark at (25 ± 1) °C for 48 h. Deionized water was used as the control. The GI value was calculated according to Equation (1). After each turning, water was added as needed to maintain the compost at approximately 60% moisture. Organic matter content was measured by the potassium dichromate oxidation method (Walkley-Black method). The absorbance ratio at 465 nm and 665 nm (E4/E6) was measured using ultraviolet-visible (UV–VIS) spectrophotometry. Ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3−-N) were extracted with 1 mol/L KCl (solid-to-extractant ratio of 1:5, w/v), and determined by the indophenol blue method and dual-wavelength method (220 nm and 275 nm), respectively.
G I = S e e d   g e r m i n a t i o n   r a t e   o f   t r e a t m e n t % × R o o t   l e n g t h   o f   t r e a t m e n t   ( m m ) S e e d   g e r m i n a t i o n   r a t e   o f   c o n t r o l % × R o o t   l e n g t h   o f   c o n t r o l   ( m m ) × 100 %

2.4. Gas Sampling and Analysis

At 8:00 a.m. each sampling day, 30 mL of gas was collected from the outlet of each composting reactor using a polypropylene syringe and injected into a 12 mL vacuum vial for storage. Gas samples were collected daily during the first two weeks and every other day thereafter. Concentrations of CH4, CO2 and N2O were measured using gas chromatography (Trace 1300, Thermo Fisher Scientific, Waltham, MA, USA).
Ammonia (NH3) emissions were measured daily by connecting a gas collection bottle, which contained a 2% boric acid absorption solution mixed with a methyl red–bromocresol green indicator, to the gas outlet of the composting reactor. Each day, the time required for the indicator to change color from pink to blue was recorded. Subsequently, the solution was titrated with a standard 0.01 M sulfuric acid solution to quantify the amount of ammonia absorbed [20].

2.5. Statistical Analysis

Statistical analyses of the experimental data were performed using Microsoft Excel 2021. One-way analysis of variance (ANOVA) was conducted using SPSS 27.0 software. When significant differences were indicated by ANOVA (p ≤ 0.01), multiple comparisons were performed using the Least Significant Difference (LSD) post hoc test. Graphs were created using Origin 2024 and python 3.13, and redundancy analysis (RDA) was conducted using R software version 4.5.1.

3. Results and Discussions

3.1. Physicochemical Parameters

Temperature is a critical parameter reflecting microbial activity, metabolic rate, and organic matter degradation during composting [21]. As shown in Figure 1a, all treatments exhibited four typical composting phases: mesophilic, thermophilic, cooling, maturation [22]. All treatments exhibited a rapid temperature increase within the first 1–3 days, entering the thermophilic phase with temperatures exceeding 50 °C. Among them, treatments A2 and A3 entered the thermophilic phase earlier (on day 2) and maintained temperatures between 50 and 56 °C for up to 8 days. Treatments A4 and A5 reached higher peak temperatures, with A4 recording a maximum of 64 °C on day 3 and sustaining thermophilic conditions for 5 consecutive days. This may reflect rapid early-stage microbial activity and substrate depletion [9]. After day 10, all treatments entered the cooling phase, during which temperatures gradually declined and stabilized around 40 °C. By the end of composting, all treatments had returned to ambient temperature, indicating compost maturity [23].
pH is a key indicator of microbial activity during composting and directly influences microbial metabolism [24] (Figure 1b). At the start, all treatments exhibited near-neutral to slightly alkaline pH values (7.6–8.0). During the thermophilic phase, treatments with higher kitchen waste content (A4 and A5) maintained neutral to slightly alkaline conditions, while treatments with lower kitchen waste (CK, A1, A3) developed acidic conditions, with pH dropping to 5.4–5.7. This difference is attributed to the higher nitrogen content in kitchen waste. Microbial degradation of organic nitrogen released free ammonia, elevating pH and buffering organic acids [25,26]. Previous studies have shown that the ammonification of organic nitrogen consumes protons, thereby contributing to pH buffering during composting processes [27]. In contrast, treatments with higher carbon content and limited nitrogen accumulated organic acids, leading to pH decline [23]. As composting progressed, organic acids were consumed, and ammonia volatilization and nitrification increased, resulting in gradual pH stabilization [28]. All treatments reached final pH values between 7.06 and 7.97, within the optimal range (7.5–8.5), confirming compost stability.
Electrical conductivity (EC) is an important physicochemical indicator that reflects the concentration of soluble salts in the composting matrix. Changes in EC illustrate the dynamics of organic matter mineralization and transformation, and are crucial for evaluating the agricultural safety of the final compost product [29]. Initial EC values increased with the proportion of kitchen waste (Figure 1c). For instance, A5 had an initial EC of approximately 0.98 mS/cm, significantly higher than that of the control (CK), which was 0.69 mS/cm. In the early composting phase, EC temporarily decreased, possibly due to the volatilization of NH3 and organic acids, resulting in reduced salt concentrations [30]. By the thermophilic phase, a sharp increase in EC was observed across treatments, possibly associated with the release of soluble salts during organic matter mineralization [31]. In particular, A4 and A5 (with higher kitchen waste ratios) exceeded 1.00 mS/cm, whereas A1 and CK (with lower kitchen waste ratios) remained around 0.70 mS/cm. In the mid to late composting stages, EC levels declined again and gradually stabilized, indicating further utilization and transformation of soluble salts [32]. At the end of composting, EC ranged from 0.22 to 0.51 mS/cm, showing a trend of higher EC in treatments with more kitchen waste (A5 highest, CK and A1 lowest). Mature compost should generally have EC values below 3–4 mS/cm to avoid phytotoxicity [33]. In this study, all final EC values were well below this threshold, indicating that the compost products were safe in terms of salinity.
As shown in Figure 1d, the organic matter (OM) content of all treatments exhibited an overall declining trend with fluctuations throughout the composting process. For instance, in the CK treatment, OM content decreased significantly from an initial value of 710.01 g·kg−1 to 639.13 g·kg−1 by day 42. This reduction reflects the rapid release of degradable organic carbon following microbial hydrolysis of substrates, a process that was accelerated under thermophilic conditions (40–60 °C) which enhanced the decomposition rate of organic matter [34,35]. Notably, some treatments showed transient fluctuations or slight increases in OM content during the middle phase (days 14–28), which may be attributed to improved aeration following turning, redistribution of moisture, and accumulation of microbial metabolites—common dynamic features observed during composting [36]. In addition, treatments A2 to A5, which contained higher proportions of kitchen waste, had relatively higher initial OM content due to the abundance of soluble carbon sources [37]. These labile substrates not only boosted microbial activity but also prolonged the stabilization phase, promoting the formation of more stable organic matter over time [38,39]. By the end of composting, all treatments showed a net decrease in OM content compared to initial values, indicating the sustained transformation of organic material and the progression toward compost maturity.
Changes in ammonium nitrogen (NH4+-N) content reflect nitrogen transformation processes and NH3 volatilization during composting [40]. In the initial stage (Figure 1e), NH4+-N concentrations were very low in all treatments, with the exception of A5, which reached 0.05 g·kg−1 due to its abundant nitrogen source from food residues. Although all compost piles entered the thermophilic phase (54–58 °C) within 3 days, NH4+-N did not spike sharply. This is attributed to rapid NH3 volatilization caused by high temperatures and strong aeration, which removed newly generated NH4+ from the piles, as well as to microbial assimilation, which consumed part of the ammonium [41,42]. As a result, NH4+-N remained below 0.06 g·kg−1 across all treatments from days 3 to 7. When the temperature dropped to 40–45 °C and pH returned to 7.5–8.0, a surge in nitrogen mineralization occurred [43]. In treatments with medium to high proportions of kitchen waste (A2–A5), NH4+-N concentrations exhibited a pronounced accumulation peak on day 21, followed by a rapid decline. In contrast, the peak in the control (CK) and low-addition treatment (A1) was delayed until day 28, after which NH4+-N levels also declined. A key observation is that the fluctuations in NH4+-N concentrations across all treatments were not accompanied by a corresponding increase in NO3−-N concentrations (Figure 1). Simultaneously, gas monitoring data indicated no significant emission peaks for ammonia (NH3) or nitrous oxide (N2O) during this phase. Based on these findings, a plausible explanation is that microbial assimilation was the primary pathway for NH4+-N consumption at this stage [44]. During the mid-to-late composting phase (days 14–28), although easily degradable carbon sources had been partially depleted, the ongoing breakdown of complex organic matter—such as cellulose and lignin—likely continued to support microbial activity and created sustained nitrogen demand. As NH4+-N was mineralized, it may have been rapidly taken up by active microbial communities for the synthesis of cellular biomass, thereby accounting for the sharp decline in NH4+-N concentrations without a corresponding increase in NO3−-N.
At the early stage of composting (Figure 1f), NO3−-N concentrations ranged from 0.06 to 0.07 g·kg−1, indicating that a small degree of nitrification had already occurred during the raw material pretreatment process [45]. As the compost entered the thermophilic phase and pH temporarily acidified, the activity of nitrifying bacteria was suppressed [46]. Consequently, by day 7, NO3−-N levels in all treatments had declined sharply, falling below 0.02 g·kg−1 [47]. When temperatures decreased to 38–42 °C and pH stabilized in a neutral to slightly alkaline range, nitrifying bacteria regained activity. By day 21, NO3−-N concentrations had rebounded and stabilized within the range of 0.01–0.02 g·kg−1 across all treatments. From that point until day 42, only minor fluctuations were observed, suggesting that a dynamic balance between nitrification and denitrification had been established [48,49]. During this phase, nitrogen was retained primarily in the form of nitrate or stable organic nitrogen, indicating that the composting system had entered a stage of sustained nutrient release and nitrogen stabilization [48].

3.2. Greenhouse Gases and Ammonia Emissions

As shown in Figure 2a, CO2 emissions in all treatments peaked during the initial composting stage (days 1–3), with values reaching approximately 39,907 mg·kg−1·d−1 in CK and 77,133 mg·kg−1·d−1 in A1. These high early rates reflect the rapid aerobic decomposition of easily degradable organic matter under thermophilic conditions [50]. The initial CO2 peak may be attributed to the low C/N ratio and abundance of labile carbon compounds (e.g., sugars and lipids) in the kitchen waste (KW). These readily degradable substrates are rapidly metabolized by microorganisms, resulting in a surge of CO2 release. Similar patterns have been observed in composting studies using food waste with C/N ratios between 15 and 20 [51]. By day 7, CO2 release in all treatments decreased sharply, as the pool of readily available carbon was largely exhausted following the thermophilic phase [52,53]. After pile turning on day 7, oxygen was reintroduced and the compost structure was disrupted, resulting in a transient rebound in CO2 emissions [54]. By day 14, electrical conductivity (EC) had reached its peak, indicating the accumulation of ionic metabolic byproducts, and the temperature had dropped into the 40–50 °C range. These factors, combined with the presence of local aerobic–anaerobic transition zones within the pile, led to minor secondary CO2 peaks observed during weeks 3 and 4 [55]. After day 28, CO2 emission rates across all treatments gradually stabilized at low and steady levels [56]. This indicates that, in the later composting stages, most of the remaining carbon existed in more recalcitrant forms that were difficult to mineralize. Correspondingly, microbial respiration decreased, reflecting compost stabilization [57,58]. In terms of cumulative emissions, significant differences were observed during both the thermophilic phase (p = 0.0152) and the maturation phase (p = 0.0114). Treatments A4 and A5 exhibited lower total CO2 emissions compared to CK, A1, and A2, with A4 showing the greatest reduction. Specifically, A4 reduced cumulative CO2 emissions by 22.35% compared to CK, indicating a strong potential for emission mitigation through optimized co-composting ratios. We speculate that this may be attributed to a more stable composting environment in the A4 treatment. Specifically, it exhibited smaller pH fluctuations, higher germination index (GI) values during the cooling phase, and an earlier transition into the cooling stage. These factors collectively indicate a more balanced microbial metabolic process and a more efficient carbon transformation pathway. Such environmental stability may have enhanced microbial carbon assimilation efficiency, thereby reducing excessive carbon mineralization and lowering CO2 losses.
As shown in Figure 2b, during the thermophilic phase, intense microbial respiration rapidly depleted oxygen in the core region of the compost piles, resulting in the formation of anaerobic zones. However, these early conditions were not favorable for methanogens: the high temperatures well above the optimal 35–45 °C range for many methanogenic archaea likely suppressed CH4 production during the initial days [24,52,59]. As a result, CH4 emission rates remained relatively low up to day 3, despite anaerobic regions forming. After entering the cooling phase, temperatures dropped to 40–50 °C, which has been associated with an increase in CH4 emissions [43,60]. In this study, CH4 release was primarily concentrated during the cooling phase, likely due to the higher moisture content and compaction of the kitchen waste (KW), which may have impeded oxygen diffusion and led to the formation of localized anaerobic zones [61]. Compared to peak variability, cumulative CH4 emissions more effectively reflect the mitigation potential across different composting ratios. Among all treatments, only A5 (GW:KW = 5:5) showed a 23.49% reduction in cumulative CH4 emissions compared to CK, while all other treatments exhibited higher cumulative CH4 emissions than CK. This indicates that A5 represents the most effective co-composting ratio for CH4 mitigation within the tested systems [62].
As shown in Figure 2c, N2O emissions were highest during the heating and thermophilic stages of composting. In all treatments with added kitchen waste, an explosive emission peak occurred around day 3. For instance, treatment A4 reached 111.33 mg·kg−1, whereas the control (CK) showed only 2.30 mg·kg−1. This significant difference is directly associated with the high nitrogen content introduced by kitchen waste (KW), which provides abundant substrates for denitrification and thereby facilitates rapid N2O release [63,64,65,66]. This observation is consistent with the early-stage peak in NO3−-N concentrations detected during the composting process. During the cooling phase, N2O emissions in all treatments dropped significantly to levels between 0.10 and 1.90 mg·kg−1·d−1, and by the end of composting, emissions fell below 1.00 mg·kg−1·d−1 across all groups, indicating a decline in nitrification-denitrification activity and stabilization of nitrogen pathways [67]. In terms of cumulative emissions, statistically significant differences were observed across all composting stages: the heating phase (p = 0.015), the cooling phase (p = 0.0302), and the maturation phase (p = 0.0382), While A5 emitted a total of 34.74 mg·kg−1—higher than CK’s 14.47 mg·kg−1—it had the lowest cumulative emission among all kitchen waste-added treatments. This suggests that mixing ratio 50% kitchen waste to 50% green waste co-composting suppressed N2O generation. Therefore, treatment A5 demonstrated the optimal potential for greenhouse gas mitigation in co-composting systems involving kitchen and green waste.
As shown in Figure 2d, a significant increase in NH3 emissions was observed when the kitchen waste (KW) ratio exceeded 30%. In treatment A5 (GW:KW = 5:5), NH3 emissions surged on day 3, reaching 18.74 mg·kg−1, while no detectable NH3 was observed in the control (CK). A sharp emission peak occurred on day 3, followed by a rapid decline; by day 7, NH3 emissions had nearly dropped to zero. This early NH3 peak is primarily attributed to thermophilic conditions beginning as early as day 3, which shifted the NH4+/NH3 equilibrium toward gaseous NH3. Additionally, the elevated pH levels (7.8–8.3) further enhanced NH3 volatilization [68]. After day 3, the rapid depletion of easily volatilizable ammoniacal nitrogen and enhanced aeration due to pile turning on day 7 contributed to the decline [49,69]. From day 7 onward, NH3 emissions in all treatments were minimal (<0.10 mg·kg−1·d−1). This trend indicates that although adding kitchen waste increases the mineralizable nitrogen in the early stage, the combined effects of high temperature (≥60 °C), elevated pH (>7.5) promote rapid NH3 volatilization, which is then curtailed by substrate exhaustion and aeration. These findings are consistent with existing conclusions that such environmental conditions lead to burst-phase ammonia release early in composting [68]. Cumulative emission data further confirmed statistically significant differences across all composting phases (p < 0.05). Treatment A5 had the highest cumulative loss (25.60 mg·kg−1), indicating more rapid and substantial nitrogen loss compared to other groups. A clear positive correlation was observed between the amount of kitchen waste added and cumulative NH3 emissions, suggesting that KW addition significantly influences nitrogen volatilization.
Gaseous carbon and nitrogen losses were estimated based on cumulative emissions of CO2, CH4, NH3, and N2O (Table 2). Across all treatments, carbon losses ranged from 113.50 to 151.81 g/kg, and nitrogen losses ranged from 0.014 to 0.100 g/kg. Treatment A4 showed the lowest carbon loss (113.50 g/kg) and the highest nitrogen loss (0.100 g/kg). In contrast, A5 had higher carbon loss (131.98 g/kg) but lower nitrogen loss (0.053 g/kg), despite its higher proportion of kitchen waste. In treatment A5, the markedly higher NH3 emissions observed on day 3, along with a moderate decline in NH4+ concentrations, may suggest that a considerable portion of NH4+ was volatilized as NH3 under thermophilic and alkaline conditions. This could have reduced the substrate availability for microbial nitrification, thereby potentially limiting the subsequent denitrification process and resulting in lower N2O emissions. Consequently, N2O emissions, although present, were lower than in treatments with less KW addition [63].

3.3. Analysis on the Change in Greenhouse Effect

To thoroughly assess the potential impact of greenhouse gas emissions from the co-composting of green waste and kitchen waste on global warming, this study uses the global warming potential (GWP) values proposed in the IPCC Sixth Assessment Report, methane (CH4) has 27.9 times and nitrous oxide (N2O) has 273 times the warming effect of carbon dioxide (CO2) [70]—to uniformly convert CH4 and N2O emissions into carbon dioxide equivalents (CO2-eq) (Table 3).
During the composting process, significant differences in greenhouse gas (GHG) emissions—including CO2, CH4, and N2O—were observed among the treatments. In the control treatment (CK, green waste alone), the cumulative CO2 emission reached 547.05·kg−1, while CH4 and N2O emissions were 2.96 and 6.02 g·kg−1 CO2-equivalent (CO2-eq), respectively, resulting in a total GHG emission of 556.03 g·kg−1 CO2-eq.
In contrast, co-composting treatments A1 (GW:KW = 9:1) and A2 (GW:KW = 8:2) exhibited significantly higher emissions of CO2, CH4, and N2O compared to CK. Their total GHG emissions increased to 589.05 and 584.35 g·kg−1 CO2-eq, representing 5.94% and 5.09% increases relative to CK, respectively. These results indicate that the addition of small amounts of kitchen waste notably intensified GHG emissions during composting.
Conversely, treatments A3 (GW:KW = 7:3), A4 (GW:KW = 6:4), and A5 (GW:KW = 5:5) significantly reduced total GHG emissions, by 3.13%, 17.44%, and 10.09%, respectively. All three treatments also showed lower CO2 emissions than CK, indicating that excessive addition of kitchen waste effectively reduced CO2 output. Notably, CH4 emissions in A5 (2.26 g·kg−1 CO2-eq) were also lower than in CK, suggesting that higher kitchen waste ratios help suppress CH4 production.
Previous studies have shown that the addition of biochar to digestate and garden waste composting can significantly reduce N2O emissions, but it had no significant impact on CO2 or CH4 release [71]. In contrast, the introduction of the fungal agent SQ (Trametes sp.) into garden waste compost reduced CH4 and CO2 emissions by 23.52% and 4.43%, respectively, while accelerating organic matter degradation and shortening the composting cycle [72]. In kitchen waste composting, biochar was found to reduce cumulative N2O and CH4 emissions by 47.7% and 47.9%, respectively, while zeolite reduced cumulative CO2 emissions by 28.4% [24].
In comparison, our A4 treatment (GW:KW = 6:4) reduced cumulative CO2 emissions by 24.06% and total greenhouse gas (GHG) emissions by 17.4% relative to the control. These results demonstrate that an optimized mixing ratio of GW and KW, even without exogenous additives, can achieve GHG reduction effects comparable to those obtained with advanced amendments, while also improving compost maturity and nitrogen retention.

3.4. Germination Index and E4/E6 Ratio

The germination index (GI) is a key biological indicator for assessing compost maturity and phytotoxicity. It reflects the presence of plant-inhibiting substances through changes in seed germination rate and root elongation when exposed to compost water extracts. As shown in Figure 3a, GI for all treatments dropped sharply during the initial composting phase, coinciding with intense microbial activity and the release of phytotoxic intermediate metabolites. As composting progressed and toxicity declined, GI rebounded significantly. Treatments with added kitchen waste exhibited extremely low germination index (GI) values in the early composting stage—for example, GI in A5 dropped to just 4% on day 7. This coincided with a decrease in pH observed on the same day, suggesting that the accumulation of organic acids may have contributed to phytotoxicity during the initial phase of composting [73,74]. By day 42, all treatments achieved GI above 95%, indicating that the final compost products were non-toxic and fully matured. This far exceeds the commonly accepted thresholds of GI > 80% for non-toxicity and GI > 100% for composts beneficial to plant growth [75,76]. Co-composting with kitchen waste led to a faster increase and higher final GI, which can be attributed to enhanced humification and more complete nitrogen transformation [77]. These results demonstrate the effectiveness of kitchen waste addition in improving compost maturity and reducing phytotoxicity.
The E4/E6 ratio is commonly used to characterize the stabilization and transformation of humic substances during composting. A lower E4/E6 value generally indicates a higher degree of aromaticity, a more complex molecular structure, and a greater level of compost maturity [78]. As shown in Figure 3b, initial E4/E6 values for all treatments ranged from 3.2 to 4.2. During composting, most treatments exhibited a trend of increasing followed by decreasing E4/E6 values. In the thermophilic phase, E4/E6 temporarily increased in treatments such as CK and A1—for example, A1 reached a peak value of 6.79—likely due to the decomposition of easily degradable organic matter into low-molecular-weight humic substances [79]. As composting entered the cooling phase, a marked decrease in the E4/E6 ratio [77]. By day 42, the organic matter in the compost had become relatively stable. The final reduction in E4/E6 reflects the polymerization and molecular aggregation of humic substances [80].

3.5. Redundancy Analysis (RDA) of Gas Emissions and Physicochemical Properties

RDA results (Figure 4) revealed that CO2 and CH4 emissions were significantly positively correlated with organic matter content, indicating that organic carbon mineralization promoted the synchronous increase in these gases [81]. In contrast, CO2 and CH4 showed strong negative correlations with NO3−-N and EC, suggesting that oxidative and salt stress inhibited anaerobic decomposition pathways [82,83].
NH3 emissions were positively correlated with NH4+-N and pH, indicating that ammonium is prone to volatilization under alkaline conditions, making it a major pathway for nitrogen loss [84]. N2O emissions showed positive correlations with NH4+-N, reflecting the provision of nitrification substrates under high ammonium concentrations [85,86]. Meanwhile, N2O was significantly negatively correlated with NO3−-N and EC, implying that nitrate was rapidly reduced and emitted as N2O under low salinity and micro-oxic conditions [87,88].
Variance inflation factor (VIF) analysis was conducted to assess multicollinearity among explanatory variables. The results showed that all variables had VIF values below the threshold of 5, specifically: E4/E6 = 4.24, pH = 2.87, Temperature = 2.32, NO3−-N = 2.01, EC = 2.01, GI = 1.44, OM = 1.31, and NH4+-N = 1.22. These results indicate a low degree of multicollinearity, thereby ensuring the statistical independence of variables and the robustness of the redundancy analysis (RDA).
Overall, ≤30% kitchen waste addition effectively suppressed NH3 volatilization. A 30–40% ratio increased NH3 emissions to some extent, but also accelerated compost maturity and improved GI. Therefore, this ratio represents an optimal trade-off between emission mitigation and compost quality.

3.6. Economic Benefit Analysis

This section presents an economic analysis of composting systems utilizing kitchen waste (KW) and green waste (GW). According to previous studies [89], the cost structure for commercial composting is as follows. The collection of raw materials is estimated at approximately USD 20.47 per ton, while the pretreatment process—which involves crushing, impurity removal, and size reduction—costs around USD 17.00 per ton. During the composting process, utilities (electricity and water) contribute an additional USD 20.00 per day, and labor for routine maintenance and monitoring is estimated at USD 13.60 per person per day. Furthermore, composting operations often require the use of digesters, where materials must be turned and hydrated weekly, along with routine maintenance of the digester system, which adds approximately USD 20.02 per month. To enhance microbial activity and accelerate decomposition, commercial microbial inoculants are typically applied twice, costing USD 15.80 per application. Given that composting generally requires 90 to 120 days to reach maturity, the total economic cost for producing one ton of compost ranges from approximately USD 1622 to USD 2124.
In this experiment, adopting a composting ratio of 40% kitchen waste (KW) and 60% green waste (GW) resulted in a raw material collection cost of USD 50.47 per ton and a pretreatment cost of USD 20.00 per ton. Daily utility and labor costs were USD 20.00 and USD 13.60, respectively. Importantly, the composting cycle was shortened to 40 days, significantly reducing overall composting time. The total cost per ton of compost produced under this ratio was approximately USD 1444. Compared to conventional commercial composting, this represents a cost saving of 10% to 32%. Beyond the economic benefits, the final compost produced under this optimized KW:GW ratio exhibited superior quality compared to compost produced via traditional commercial methods.

4. Conclusions

This study demonstrated that adjusting the mixing ratio of kitchen waste (KW) and green waste (GW) significantly influences nutrient retention and greenhouse gas (GHG) emissions during composting. All treatments successfully produced mature compost, as evidenced by germination index (GI) values exceeding 95% and decreasing E4/E6 ratios. In terms of GHG emissions, treatments with low KW content (10–20%) exhibited slightly higher cumulative emissions than GW alone (CK), with total GHG emissions increasing by 5.09–5.94%. However, when the KW proportion exceeded 30%, net GHG emissions were reduced. Notably, the GW:KW ratio of 6:4 (A4) achieved the greatest overall GHG reduction, with CO2-equivalent emissions 17.44% lower than those of the CK treatment, while also reducing CH4 emissions. Overall, the 6:4 GW:KW co-composting ratio provided a favorable balance between GHG mitigation, compost stability, and nutrient conservation. These findings offer a practical basis for optimizing composting strategies in urban organic waste management. Future research should further investigate the performance of this ratio under different environmental and operational conditions and explore the underlying microbial and biochemical mechanisms that contribute to its effectiveness.

Author Contributions

Formal analysis, software, writing—original draft preparation, J.G.; methodology, investigation, B.S.; Conceptualization, writing—review and editing, S.L.; supervision, X.S.; data curation, R.Z.; visualization, project administration, D.W.; validation, Y.L.; resources, H.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Fundamental Research Funds for the Central Universities, grant number BFUKF202508. This research was funded by the Research and Demonstration of Key Technologies for the Application of Organic Mulch in Landscape Construction (2019-KJC-02-13).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made. available by the authors upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
GHGGreenhouse gas
GWGreen waste
OMOrganic matter
KWKitchen waste
GIGermination index
RDARedundancy analysis

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Figure 1. The changes in temperature (a), pH (b), EC (c), OM (d), NH4+-N (e) and NO3−-N (f) during the composting process. The results obtained are the average of the three results with an error of standard deviation. CK: GW:KW = 10:0; A1: GW:KW = 9:1; A2: GW:KW = 8:2; A3: GW:KW = 7:3; A4: GW:KW = 6:4; A5: GW:KW = 5:5.
Figure 1. The changes in temperature (a), pH (b), EC (c), OM (d), NH4+-N (e) and NO3−-N (f) during the composting process. The results obtained are the average of the three results with an error of standard deviation. CK: GW:KW = 10:0; A1: GW:KW = 9:1; A2: GW:KW = 8:2; A3: GW:KW = 7:3; A4: GW:KW = 6:4; A5: GW:KW = 5:5.
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Figure 2. Measured gas emissions during the composting process. (a–d) are daily emissions of CO2, CH4, N2O and NH3, respectively. The results obtained are the average of the three results with an error of standard deviation. CK: GW:KW = 10:0; A1: GW:KW = 9:1; A2: GW:KW = 8:2; A3: GW:KW = 7:3; A4: GW:KW = 6:4; A5: GW:KW = 5:5.
Figure 2. Measured gas emissions during the composting process. (a–d) are daily emissions of CO2, CH4, N2O and NH3, respectively. The results obtained are the average of the three results with an error of standard deviation. CK: GW:KW = 10:0; A1: GW:KW = 9:1; A2: GW:KW = 8:2; A3: GW:KW = 7:3; A4: GW:KW = 6:4; A5: GW:KW = 5:5.
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Figure 3. The changes in Germination index (a) and E4/E6 ratio (b) during the composting process. The results obtained are the average of the three results with an error of standard deviation. CK: GW:KW = 10:0; A1: GW:KW = 9:1; A2: GW:KW = 8:2; A3: GW:KW = 7:3; A4: GW:KW = 6:4; A5: GW:KW = 5:5.
Figure 3. The changes in Germination index (a) and E4/E6 ratio (b) during the composting process. The results obtained are the average of the three results with an error of standard deviation. CK: GW:KW = 10:0; A1: GW:KW = 9:1; A2: GW:KW = 8:2; A3: GW:KW = 7:3; A4: GW:KW = 6:4; A5: GW:KW = 5:5.
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Figure 4. Redundancy analysis (RDA) of gas emissions and physicochemical properties.
Figure 4. Redundancy analysis (RDA) of gas emissions and physicochemical properties.
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Table 1. Basic physicochemical properties of composting materials.
Table 1. Basic physicochemical properties of composting materials.
ParameterGreen WasteKitchen Waste
pH7.34 ± 0.154.75 ± 0.01
Electrical conductivity/(mS·cm−1)0.65 ± 0.031.28 ± 0.02
Organic Matter content/(g·kg−1)718.96 ± 15.13808.49 ± 6.58
Table 2. Estimated Carbon and Nitrogen Losses via Gaseous Emissions in Each Treatment.
Table 2. Estimated Carbon and Nitrogen Losses via Gaseous Emissions in Each Treatment.
TreatmentC Loss (g/kg)N Loss (g/kg)
CK149.420.014
A1150.080.083
A2151.810.055
A3135.390.090
A4113.500.100
A5131.980.053
Table 3. Greenhouse gas emissions and global warming potential of different treatments.
Table 3. Greenhouse gas emissions and global warming potential of different treatments.
TreatmentCO2 (g/kg)CH4 (g/kg) CO2-eqN2O (g/kg) CO2-eqTotal (g/kg) CO2-eqEmission Rate
CK547.052.966.02556.03——
A1549.294.5135.25589.05+5.94%
A2555.555.5423.26584.35+5.09%
A3495.474.8538.28538.60−3.13%
A4415.433.2440.40459.07−17.44%
A5483.232.2614.41499.90−10.09%
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Gu, J.; Li, S.; Sun, X.; Zou, R.; Song, B.; Wang, D.; Wang, H.; Li, Y. Greenhouse Gas Emissions from Co-Composting of Green Waste and Kitchen Waste at Different Ratios. Sustainability 2025, 17, 8041. https://doi.org/10.3390/su17178041

AMA Style

Gu J, Li S, Sun X, Zou R, Song B, Wang D, Wang H, Li Y. Greenhouse Gas Emissions from Co-Composting of Green Waste and Kitchen Waste at Different Ratios. Sustainability. 2025; 17(17):8041. https://doi.org/10.3390/su17178041

Chicago/Turabian Style

Gu, Junhao, Suyan Li, Xiangyang Sun, Rongsong Zou, Binru Song, Di Wang, Hui Wang, and Yalin Li. 2025. "Greenhouse Gas Emissions from Co-Composting of Green Waste and Kitchen Waste at Different Ratios" Sustainability 17, no. 17: 8041. https://doi.org/10.3390/su17178041

APA Style

Gu, J., Li, S., Sun, X., Zou, R., Song, B., Wang, D., Wang, H., & Li, Y. (2025). Greenhouse Gas Emissions from Co-Composting of Green Waste and Kitchen Waste at Different Ratios. Sustainability, 17(17), 8041. https://doi.org/10.3390/su17178041

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