Towards a Circular Economy: Unlocking the Potentials of Cigarette Butt Recycling as a Resource for Seashore Paspalum Growth

Abstract

The cigarette butt (CB) recycling process yields several byproducts, including cleaned filters, solid debris (mainly paper and tobacco), and wastewater. This study aimed to assess, for the first time, the long-term suitability of these recycled byproducts for turfgrass cultivation. Under controlled conditions, Paspalum vaginatum Swartz was grown in sand–peat substrate, either unmodified (control) or amended with small pieces of uncleaned CBs or solid byproducts from CB recycling at concentrations of 25% or 50% (v/v). In additional tests, turfgrass grown in unmodified substrate received wastewater instead of tap water once or twice weekly. Over 7 weeks, physiological and biometric parameters were assessed. Plants grown with solid debris showed traits comparable to the control. Those grown with intact CBs or cleaned filters had similar biomass and coverage as the control but accumulated more carotenoids and antioxidants. Wastewater significantly enhanced plant growth when applied once weekly, while becoming toxic when applied twice, reducing biomass and coverage. After scalping, turfgrass recovered well across all treatments, and in some cases biomass improved. Overall, recycled CB byproducts, particularly wastewater used at optimal concentrations, can be a sustainable resource for promoting turfgrass growth.

1. Introduction

Since the mid-20th century, smoking has increased due to the introduction of filters in the composition of cigarettes, leading to an annual production of 5.7 trillion cigarettes, 97% of which are filtered cigarettes [1]. A recent global report estimated that about 1.3 billion people used tobacco in 2020 [2], with up to two-thirds of all smoked cigarettes improperly littered on the ground [3]. Moreover, global cigarette consumption varies among the countries; for instance, annual consumption in the European Union is approximately 1.2 trillion cigarettes, with Italy maintaining a stable per capita consumption, while Germany has seen a decline in recent decades [1,4]. Altogether, researchers estimate that 4.5 trillion cigarette butts (hereafter CBs), weighting 1.2 million tons, are generated worldwide every year, with projections of a 50% increase by 2025 [1].

Although filtered cigarettes partly reduce health risks during smoking, CBs have become a major environmental issue due to improper disposal. The composition of CBs includes the cigarette filter, wrapping paper, and remaining tobacco, with the filter being the main component [5]. Made of cellulose acetate, the filter is a slow-degrading material that can persist in the environment for years [6]. However, decomposition can be accelerated in certain environments, such as those with earthworms [7]. The durability of CBs allows various chemicals trapped in the filter, such as nicotine, heavy metals, and cyclic aromatic compounds, to leach into the environment, posing risks to aquatic and terrestrial ecosystems [8]. Therefore, more efforts to raise public awareness and prioritize effective management solutions for reducing CB waste are urgently needed.

Conventional disposal methods, such as incineration and landfill, are unsuitable for CBs, as chemicals in the filters can contaminate air, soil, and groundwater. This has led researchers to identify, propose, and test alternative methodologies. In recent years, CB recycling has gained attention as an environmentally responsible approach aligned with the principles of circular economy and sustainable development [5]. Studies have demonstrated that CB recycling can produce various useful materials, including building materials, acoustic applied materials, cellulose pulp, active carbon, CB-derived chemicals for pest control, biofilm for wastewater treatment, and plant-growing substrate [9,10,11]. Mariotti et al. [11] were the first to envision a potential horticulture application. Collaborating with the Municipality of Capannori, Italy, they raised awareness of the CB waste issue, installed collection bins in public areas, and encouraged community participation, leading to efficient CB collection [12]. Their study demonstrated that after CBs were cleaned, cellulose acetate filters—previously mechanically treated—could be used as a plant-growing substrate, potentially replacing widely used commercial rockwool in soilless cultivation. However, the existing research has focused on early growth stages of ornamental plants, such as seeding or stem cutting establishment, leaving long-term plant growth performance in the CB-based substrate unexplored. Additionally, wastewater generated during the CB recycling process was treated using microalgal-based technologies [13,14]. Another byproduct of the CB recycling process is solid debris consisting of paper and residual tobacco; however, its utilization has not yet been explored.

Previous research has indicated that CBs can have toxic effects [15,16,17,18] or positively influence plant physiology [19,20]. These varied results depend on several factors, such as plant species [19,21,22], treatment conditions and duration [23], and applied methods (e.g., CB pre-treatment) [24,25]. Concerning ornamental plants, precisely turfgrass, only Festuca arundinacea Schreb. [26] and Lolium perenne L. [21] have been studied, showing opposite physiological effects in response to CBs. Paspalum vaginatum Swartz (seashore paspalum) is an economically important grass, particularly valued in the Mediterranean basin [27]. This warm-season perennial requires minimal fertilizers and pesticides and exhibits strong resistance to abiotic stresses such as salinity and oxygen deprivation [28], making it an ideal choice for golf and tennis courts, as well as public parks. Therefore, the present study aimed to assess the extended applicability of CB recycling byproducts—cleaned cellulose acetate filters (F), solid debris (R), and wastewater (L)— in sustainable ornamental plant cultivation. In this context, this study is the first to investigate the long-term growth performance of P. vaginatum grown in substrates containing each CB recycling output. Unmodified soil (control) and soil modified with intact CBs (I) were also tested. Here, physiological and biometrical parameters including chlorophyll fluorescence, photosynthetic pigments, surface coverage, shoot length, and aboveground biomass were evaluated over 7 weeks in order to examine plant health under different treatments. We hypothesized that soil modified with CB recycling byproducts could be used for growing P. vaginatum, as long as byproduct concentrations were adequate to maintain plant health. By assessing the sustainability and viability of CB recycling byproducts in plant cultivation over an extended period, this study not only addresses a critical environmental issue but also represents a crucial next step to validate their horticultural applications.

2. Materials and Methods

2.1. Products of CB Recycling Process

The processes of collecting and cleaning CBs were performed as described by Mariotti et al. [11]. Briefly, collected CBs were cleaned in boiling water (100 g L⁻¹), and the solids were separated from the liquid through filtration. The solid products were manually separated into cleaned cellulose acetate filters (hereafter F) and debris (hereafter R) that mainly consisted of paper and tobacco. Both solid components were air-dried at room temperature, and F was carded into fibers. The wastewater (hereafter L) was cooled to room temperature, divided into 50 mL tubes, frozen, and stored at −20 °C until use. Uncleaned CBs (hereafter I) were cut into small pieces 0.5 cm in length. Characterization of CB recycling byproducts was previously performed by Mariotti et al. [11].

2.2. Preparation of Plant Growing Substrates

Substrates composed solely of sand–peat mixture (80:20, v/v) were designated as the control (Ctr). A sphagnum moss-based peat and volcanic sand were used in this mixture, as described by Pompeiano et al. [28]. The modified substrates tested in this study consisted of sand–peat mixed with I, F, or R at either 25% or 50% v/v, resulting in the following conditions: I25, I50, F25, F50, R25, and R50. Although in a previous study the germination and establishment of ornamental plants succeeded even when grown in 100% cleaned cellulose acetate filters [11], here CB recycling byproducts were less concentrated in order to promote their potential degradation by microbiota when mixed with sand–peat, as referred to previously [6].

2.3. Plant Material and Growth Conditions

Sod plugs (8 cm in diameter, 2 cm in depth, and filled with 91% sand, 6% silt, 3% clay, and 1.2% organic matter) of mature Paspalum vaginatum Swartz (seashore paspalum) cv. Sea Spray were obtained from Bindi PRATOPRONTO NORD (https://www.pratobindi.it/, accessed on 21 July 2025). Sea Spray is a commercial cultivar of seashore paspalum chosen as a seeded cultivar standard in the warm and transition zone of the Mediterranean basin [29]. Individual plugs were transplanted into plastic pots (8 cm in diameter and 9 cm in height) filled with either unmodified or modified substrates (see Section 2.2 for more details). The growth chamber was set to 22 ± 1 °C, 75% humidity and a 12 h photoperiod with 100 µmol m⁻² s⁻¹ photosynthetically active radiation (PAR). Irrigation (20 mL for each pot) was applied twice weekly, using tap water for Ctr, I25, I50, F25, F50, R25, and R50 conditions, while additional turfgrass tests in unmodified substrate received wastewater one or two times per week (20 mL of L1 or L2, respectively, for each pot) in place of tap water. The experiment lasted 7 weeks, after which scalping was performed. All tests then underwent a 5-week recovery phase, with tap water applied twice per week. A recovery period was conducted to provide further insights into the stability of post-treatment acclimation, during which turfgrass may exhibit adverse responses or adaptive mechanisms following the cessation of treatment.

2.4. Chlorophyll Fluorescence

Measurement of chlorophyll fluorescence was conducted weekly and after the recovery phase, prior to sampling, to assess photosynthetic efficiency. The maximum photosystem II (PSII) photochemical efficiency (F_v/F_m) in dark-adapted leaves was measured using a miniaturized pulse amplitude-modulated fluorometer (Mini-PAM; Heinz Walz GmbH, Effeltrich, Germany) as previously described [30]. To ensure proper dark adaptation, chamber lights were turned off for at least 30 min before the measurement.

2.5. Photosynthetic Pigments and Antioxidants

Measurements of pigments and antioxidants were conducted at weeks 1, 3, 5, and 7, as well as at the end of the recovery period. Pigments were extracted and analyzed according to Huarancca Reyes et al. [31], with some modifications. Briefly, 20 mg of fresh leaves were ground with liquid nitrogen and incubated in 1 mL of 95% ethanol at 4 °C in darkness. Then, the absorbance values of the extracts at 470.0, 648.6, and 664.2 nm were measured using a spectrophotometer (UV-1800 spectrophotometer; Shimadzu, Japan) and converted to chlorophyll (Chla+b) and carotenoid (Car) content according to previously derived equations on a fresh weight (FW) basis [32]. The ethanolic extracts were also used to determine total antioxidant capacity (TAC) by the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, as previously described [14]. The results were expressed as nmol Trolox equivalents mg⁻¹ FW.

2.6. Biometric Measurements

Plant height and coverage images were monitored weekly from 4 to 7 weeks of treatment and at the end of the recovery phase. Plant height was measured at three fixed points, marked with woody sticks, by recording the distance from the turf base to the tip of the tallest blade. The average of the three measurements was calculated for each pot. Plant coverage, representing overall turf performance, was assessed through images captured using a digital camera (Pentax X-5; Pentax Ricoh Imaging Company LTD., Tokyo, Japan).

Aboveground biomass was determined by scalping the turf at the end of the experiment and after recovery. FW was measured, and samples were oven-dried at 60 °C until they reached a stable dry weight (DW). The data of FW and DW were presented on the basis of surface area.

2.7. Statistical Analysis

A completely randomized design was used for this trial, with three replicates per treatment. All values presented in the graphs are expressed as mean ± standard error (SE). The obtained data were subjected to one-way analysis of variance (ANOVA), with treatment (Ctr, L1, L2, R25, R50, I25, I50, F25, and F50) as the main factor. Significant differences among means were estimated at the level of p < 0.05 by using Fisher’s least significant difference (LSD) post-hoc test. The assumption of normality was evaluated with the Shapiro–Wilks test. The software STATISTICA version 6.0 (StatSoft, Inc., Tulsa, OK, USA) was used.

3. Results

The analysis of physiological and biochemical traits over time revealed a significant treatment effect (p < 0.05), except where noted otherwise (Table S1).

3.1. Chlorophyll Fluorescence

Values of F_v/F_m showed significant treatment effects throughout the experiment (p < 0.05), except at T1 and T4 (Figure 1). Specifically, a transient drop was registered at T2, with the greatest reductions observed in F25 and F50. At T3, F_v/F_m showed partial recovery, although F50 remained the lowest value (0.66). From T5 to the end of the experiment (i.e., T7), F_v/F_m in L2 conditions was consistently impaired, while in other conditions it remained comparable to Ctr (0.76). The recovery test following scalping (i.e., Rec) revealed that turf under L2, F25, and F50 conditions had significantly lower F_v/F_m values than the Ctr (0.77). Meanwhile, L1 turf exhibited the highest photosynthetic recovery.

3.2. Biochemical Analysis

Although Chla+b did not show significant treatment effects from T1 to T5 (p > 0.05), a general increase was observed from T1 to T3 (Figure 2A). Differently, Chla+b was significantly affected by treatment at T7 and after recovery. At T7, L1 and Ctr turf showed the highest content, whereas turf grown in the other conditions exhibited lower values (Figure 2A). Following scalping, turf grown in R- and I-modified substrates demonstrated superior recovery compared to the other treatments (Figure 2A). Regarding Car content, it showed a similar pattern to Chla+b across the experiment, while significant treatment effects were only found at T5 and T7 (p < 0.05) (Figure 2B). Concerning the Car/Chla+b ratio, treatment had a significant effect across the experimental period (p < 0.05), except at T3 (Figure 2C). At T1, L1 turf exhibited a significantly higher ratio compared to Ctr and the other treatments, while R50 turf displayed the lowest ratio (Figure 2C). At T5, Ctr turf showed the lowest ratio, which gradually increased with higher L concentrations, reaching levels similar to those observed in I and F plants (Figure 2C). At T7, L1 turf exhibited the lowest ratio without significant differences compared to Ctr, whereas L2 turf displayed a significantly higher ratio, comparable to that of R, I, and F (Figure 2C). After scalping, no significant differences in Car/Chla+b ratios were observed among turf grown in Ctr and most other conditions, except for R25, which displayed a significantly lower ratio than Ctr (Figure 2C).

TAC showed significant treatment effects throughout the experiment (p < 0.05), except at T1 (Figure 3). At T3, a general reduction in TAC was observed compared to T1, with turf under L2 and I25 conditions exhibiting the lowest and highest values, respectively, both significantly different from Ctr. At T5, TAC levels were highest in Ctr turf and lowest in F50 plants, with the other treatments generally falling to intermediate levels. At T7, I- and F-modified substrates induced an increase in TAC, while L and R substrates caused a reduction, with Ctr turf showing intermediate levels (Figure 3). In the recovery of scalped turf, the highest level occurred in I25, which was not significantly different from Ctr, L1, I50, and F treatments. Turf under L2 and R conditions exhibited a reduction in TAC, with R showing the lowest levels (Figure 3).

3.3. Turf Performance

Phenotypical images showed that turf grown under L1, I25, I50, F25, R25, and R50 conditions achieved surface coverage comparable to or even surpassing that of Ctr turf at T7 (Figure 4A). Surface coverage was highly compromised in L2, and to a lesser extent in F50, when compared to Ctr (Figure 4A). After scalping, turf grown under L1, I25, I50, R25, and R50 conditions showed better recovery in coverage compared to other treatments (Figure 4A). Concerning plant height, this biometric trait was significantly influenced by treatment throughout the experiment (p < 0.05) (Figure 4B). At T4, turf under L1 and L2 conditions displayed the greatest height, significantly exceeding that of Ctr and other treatments. Turf height at T5 followed similar patterns to T4 across treatments (Figure 4B). At T6, L conditions strongly stimulated turf growth compared to Ctr and other treatments, with L1 turf reaching a significantly greater height than L2 turf (Figure 4B). This pattern continued at T7, while results of recovery after scalping indicated that turf under L1 conditions showed the best performance in terms of height (Figure 4B).

Among treatments, turf grown under L1 and L2 conditions showed the highest and lowest clipping fresh weight at the end of the experiment (i.e., T7), respectively, both significantly different from the Ctr, while turf under other treatments produced fresh biomass similar to Ctr (Figure 5A). After the recovery period, fresh biomass generally decreased by 60% on average compared to T7. Notably, L1 turf retained the highest fresh biomass, whereas turf under L2, F, and Ctr conditions showed the weakest recovery performance (Figure 5A). In contrast to fresh biomass, clipping dry weight at T7 showed fewer differences among Ctr-, L1-, R-, I-, and F-treated turf, while L2 turf yielded the lowest weight per area (Figure 5B). After recovery, dry biomass generally declined by 73% on average compared to T7. Notably, turf under L1 and R25 conditions demonstrated the best recovery in dry biomass, whereas those under Ctr, L2, and F conditions showed the greatest and most significant decrease (Figure 5B). Regarding the dry-to-fresh weight ratio (DW/FW), turf under L1 showed the lowest ratio, while the other treatments exhibited relatively similar values comparable to Ctr (Figure 5C). After recovery, the DW/FW generally declined by 33% on average compared to T7 (Figure 5C). Analysis of the tolerance index (calculated as the DW of treated turf divided by the DW of the Ctr at T7) revealed that turf generally had a notable capacity to tolerate L1, R25, R50, I25, I50, F25, and F50 conditions, with most index values ranging from 0.8 ± 0.02 to 1.14 ± 0.24. However, L2 conditions seemed to be detrimental, with the lowest tolerance index (0.45 ± 0.05).

4. Discussion

The long-term applicability of all products from the CB recycling process—cleaned cellulose acetate filters (F), solid debris (R), and wastewater (L)—for the growth of P. vaginatum in soil is reported for the first time in this study. Among the tested conditions, L1 (i.e., L once per week) was the only one that positively affected turfgrass physiology, stimulating growth within 7 weeks of exposure. In contrast, L2 (i.e., L twice per week) markedly inhibited growth, while the other modified soil substrates (i.e., F and R) showed results comparable to the control (i.e., Ctr), regardless of their concentration (i.e., 25% or 50%).

Given the contrasting effects of L1 and L2 on turf growth over 7 weeks, it is plausible to argue that L may function as a stimulant-like input, enhancing P. vaginatum growth performance in a dose-dependent manner. Concordantly, previous studies have shown that low doses of CB leachate can either positively influence or have no effect on plant growth, depending on the species [18,19,20,24], whereas high doses have no positive effect regardless of species [18,21]. This dose-dependent response to CB leachates may be a sign of hormesis [33], where plants benefit from low doses of chemical compounds released from CB. In previous research, we demonstrated that L contains various chemicals, with nicotine and silicon (Si)-based compounds being the most abundant [11]. In plants, nicotine can serve as defense compound against predation, act as an outcrossing promoter by pollinators, and function as a growth regulator, depending on its concentration [34,35]. Furthermore, the application of exogenous nicotine and other alkaloids can result in their uptake by roots and subsequent transport to shoots and leaves, leading to growth inhibition or hormesis depending on alkaloid concentration and plant species [36,37]. Here, nicotine in L1 seemed to exert a hormesis effect on turf coverage, plant height, and biomass, without affecting F_v/F_m, pigments, or TAC in comparison to the Ctr. This suggested that P. vaginatum may be resilient to nicotine at L1 doses, possibly due to its nicotine catabolism capacity [38] or degradation in the rhizosphere [39], which provides carbon, nitrogen, and energy to support turf growth. On the contrary, nicotine in L2 may have been highly concentrated, inhibiting F_v/F_m, chlorophyll content, turf coverage, and biomass relative to the Ctr. This suggested that nicotine at L2 doses exceeded P. vaginatum’s tolerance threshold, potentially causing chloroplast deformation and irreversible damage to the photosynthetic machinery, ultimately inhibiting turf biomass [40,41]. Interestingly, while L2 inhibited coverage, it still induced turf height, indicating that nicotine in L may regulate endogenous hormone levels, promoting elongation [42]. However, overdosing (i.e., L2 condition) may disrupt hormone balance in new rhizome tissue, affecting turf spread and inhibiting coverage.

Beyond nicotine, Si can activate physical and molecular defense mechanisms in plants such as turfgrass, mitigating numerous environmental constraints [43,44]. Indeed, applying Si-based compounds to P. vaginatum can enhance ground coverage and improve tolerance to abiotic stressors such as salinity and cold, depending on turf species [45,46]. Here, it is possible that Si-based compounds—such as organosilanes present in L—help counteract the negative effects of exogenous chemicals on turf metabolism and growth, including heavy metals potentially present in L [18]. Heavy metals and other chemicals in L can trigger oxidative stress in plants, which Si application may mitigate by activating antioxidative defense mechanisms [47]. Although TAC levels in this study did not change under L treatment compared to the Ctr, Si-mediated stress amelioration may operate through enzymatic rather than non-enzymatic antioxidants. It is not excluded that other chemical compounds detected in L at lower concentrations, such as PAHs and BTEX [11], may also disrupt metabolic processes and, thus, affect plant growth [48,49]. Further studies are needed to elucidate the mechanisms underlying turfgrass growth regulation in response to L treatment.

Results from treatments using a soil mixture containing R, I, and F over 7 weeks showed that, while turf growth under these conditions was comparable to the Ctr, variations in the physiological health status of plants were observed depending on the composition of CB-related products. The F_v/F_m parameter is widely used as a stress indicator [15]. Here, the transient inhibition of F_v/F_m during the first 3 weeks, regardless of treatment, was likely due to sod plug transplantation. However, plants successfully established themselves in the new system, as indicated by the restoration of F_v/F_m at T4. Accordingly, this aligns with the typical establishment period following turf transplantation [50]. Furthermore, the decrease in TAC levels during the first 3 weeks of treatment further supported the notion that this period reflects the establishment phase of P. vaginatum. A closer examination of this period, specifically at T2, revealed that F_v/F_m reached its maximum decline, with some variations depending on the type and concentration of CB-related products. Unexpectedly, the reduction in F_v/F_m was greater in F than in I, suggesting that cellulose acetate filters may be the primary stress source, while smoking chemicals leached from the filters could mitigate the negative effects of cellulose acetate. Additionally, it is not excluded that leaching from residual cleaned tobacco in R may counteract potential solid debris-related physical changes in the modified substrate, as the decline in F_v/F_m in R more closely resembled that in the I-modified substrate. Further studies are needed to identify the effect of each chemical leachate (e.g., organosilanes) from CB recycling byproducts in turfgrass metabolism and assess their potential in mitigating stress.

Filter-induced stress may arise from either the material composition (i.e., cellulose acetate) or changes in the physical properties of the growing substrate. Cellulose acetate, a key cellulose derivative with biocompatible and biodegradable properties, has gained increasing attention for industrial and commercial applications, such as serving as a coating material for slow/controlled-release fertilizer [51]. In fact, studies have demonstrated that this material does not affect plant growth and development and exhibits no phytotoxic effects, highlighting its great potential in agricultural applications [52]. Taken together, these findings indicated that cellulose acetate might not trigger stress response mechanisms in turf under the growing conditions of our system. Regarding changes in substrate physical properties, all modified soils exhibited differences in field capacity (Table S2). Specifically, I25 displayed a similar field capacity to the Ctr (24%), while the others demonstrated significantly higher water retention. This indicated that turf must adapt to varying levels of moderate root hypoxia following transplanting, potentially prompting specific physiological adjustments. Indeed, turf grown in R-, I-, and F-modified substrates experienced moderate root hypoxia stress, initially impairing photosynthetic capacity (i.e., F_v/F_m), particularly in F50, which exhibited the highest field capacity (48%). However, the photosynthetic apparatus ultimately recovered over time. Consistent with this, P. vaginatum is classified as highly tolerant to anoxia, displaying a remarkable ability to restore PSII photochemistry during recovery from stress [28].

The degradation of total chlorophyll (Chla+b) is commonly attributed to the accumulation of reactive oxygen species (ROS) as a result of plant stress, such as waterlogging and anoxia events [28,53]. In response, plants produce antioxidants and other metabolites, such as carotenoids (Car), to maintain redox homeostasis and prevent cellular damage [54]. In this study, Chla+b content in turfgrasses grown in R-, F-, and, to a lesser extent, I-modified soils was lower than in those grown under Ctr conditions at T7. Interestingly, the aforementioned modified soils exhibited higher field capacity than Ctr, except for I25 (Table S2). Altogether, these findings suggested that the reduction in Chla+b may be due to ROS-induced degradation triggered by root hypoxia/reoxygenation cycles during watering period. Additionally, R-, F-, and I-modified soils caused a slight decrease in Car content relative to Chla+b, leading to a slight increase in the ratio between Car to Chla+b. This indicated that Car in P. vaginatum may be insufficient to counteract ROS production caused by root hypoxia/reoxygenation, as previously suggested by Spinoso-Castillo et al. [55]. A trade-off between Car production and other non-enzymatic antioxidants has been demonstrated to protect cells from oxidative damage [56]. Here, a slight increase in TAC was observed in turf grown in F- and I-modified soils at T7, underscoring the protective role of total non-enzymatic antioxidants in maintaining an antioxidant/pro-oxidant balance under root hypoxia/reoxygenation conditions. In contrast, plants in R-modified soil did not exhibit increased TAC, likely due to differences in the biodegradation rates of solid components in R compared to those in I and F, which may lead to changes in field capacity over time. Indeed, cellulose (e.g., paper in R) decomposes faster than cellulose acetate (e.g., filter in I and F) depending on external factors such as soil type [6,57]. Finally, the recovery test after scalping revealed that none of the conditions negatively affected plant growth. In fact, most resulted in improved biomass compared to the Ctr, with a ranking from lowest to highest as follows: I25, I50, R50 < R25 < L1. Together with the minimal changes in F_v/F_m and plant metabolism, these results indicated that turf can effectively adjust its physiology in response to moderate root hypoxia events.

5. Conclusions

This study has provided the first evidence that all byproducts from the CB recycling process, particularly side-stream at certain concentrations, can be sustainably used in the cultivation of P. vaginatum turfgrass. Among treatments, the weekly application of L improved turf growth and physiology, likely due to a hormetic effect of chemical compounds in L, while higher doses impaired photosynthesis and biomass production, indicating toxicity. These findings reveal a dose-dependent response to CB recycling derived side-stream, highlighting the importance of careful dose management for its safe use in ornamental plant cultivation. Further studies are needed to optimize application regimes and evaluate the potential of treated side-stream as an input for plant growth.

Although F and R treatments resulted in turf growth comparable to the control, temporary physiological stress was observed, possibly due to root hypoxia events linked to increased field capacity. However, P. vaginatum exhibited resilience, recovering its physiological status. Differences in some biochemical traits underscore the complex interplay between substrate composition, water dynamics, and plant physiology response.

Overall, this research supports the use of CB recycling byproducts in turfgrass cultivation over an extended period, addressing a critical environmental issue and promoting a circular economy approach in horticultural application. However, to fully demonstrate the real-world applicability of these findings, future work should include greenhouse experiments and nursery-scale trials in systems where ornamental plants are cultivated and stocked before being sold for urban greening and gardening purposes.