Formation and characterization of leaf waste into organic compost

In solid waste management, pollution-free disposal of leaf waste in urban areas is still not standardized and adopted. According to the World Bank report, 57% of wastes generated in South East Asia are consisted of food and green waste, which can be recycled into valuable bio-compost. The present study shows a method of leaf litter waste management by composting it using essential microbe (EM) method. Different parameters, such as pH, electrical conductivity, macronutrients, micronutrients, and potentially toxic elements (PTE) were measured at zero to 50 days of composting using appropriate methods. The microbial composting was shown to mature within 20 to 40 days, and its maturity could be evaluated by the attainment of stable pH (8), electrical conductivity (0.9 mS/cm), and C:N ratio ≥ 20. The analysis was also performed on other bio-composts viz. kitchen waste compost, vermicompost, cow dung manure, municipal organic waste compost, and neem cake compost. The fertility index (FI) was evaluated based on six parameters viz. total carbon, total nitrogen, N ratio, phosphorus, potassium, and sulphur contents. The PTE values were used to calculate their clean index (CI). The results showed that leaf waste compost has a higher fertility index (FI = 4.06) than other bio-composts, except the neem cake compost (FI = 4.44). The clean index of the leaf waste compost (CI = 4.38) was also higher than other bio-composts. This indicates that leaf waste compost is a valuable bio-resource with high nutritive value and low PTE contamination, with a favourable prospective to be used in organic farming.


Introduction
Solid waste management has become a challenging task for metropolitan cities, like New Delhi, in developing countries. The World Bank reported that the world generates 0.74 kg of waste per capita per day. In the year 2016, an estimated 2.01 billion tonnes of municipal solid waste were generated, and it is expected to grow to 3.40 billion tonnes by 2050, if necessary measures are not taken up (Kaza et al. 2018). The East Asia and Pacific region generated about 468 million tonnes of waste, which is contributing the most waste generation globally. Out of this, 53% of wastes were comprised of food and green waste, which can be easily bio-composted into valuable compost. The South East Asia generated about 334 million tonnes of waste in the year 2016, which is expected to be doubled by the year 2050. About 57% of waste generated in this region was characterized as food and green waste, which can be up-cycled into valuable soil amendment through composting, but currently, three-fourth of the waste are openly dumped in this region. According to the Central Pollution Control Board report 2021, India generated 160,038.9 tonnes of solid waste per day (TPD). Out of this, only 50% of the waste is treated, and about 18% is landfilled. A large junk of about 32% of the waste generated still remains unaccounted for, which is a significant concern (CPCB 2021). Delhi alone generated about 10,990 TPD of solid waste, and only 5193 TPD (50%) are treated, whereas the majority of 5533 TPD are landfilled. Generally, the most common methods adopted for the treatment and disposal of solid wastes are composting, recycling, pyrolysis, mechanical and biological treatment, gasification, waste-to-energy, and landfilling (Psomopoulos et al. 2009). Composting has been adopted as a means of sustainable waste management in developing countries since it can lead to the production of organic fertilizer. This practice can generate income with low operational cost, with relatively low air and water pollution (Taiwo 2011). Over utilization of chemical fertilizers has caused extensive contamination of the environment with hazardous chemical pollutants (Chauhan 2016). Excessive and disproportionate use of chemical fertilizers led to the depletion of carbon in the soil, soil degradation, compaction, and nutrients leaching. It has also caused nutritional stress in the soil (Lin et al. 2019). Thus, there is a need to shift towards organic farming to some extent, for sustainable agricultural production with less environmental pollution. The prolonged application of organic fertilizers such as compost and manure has been reported to be associated with replenishing organic matter, nutrients, and microbiomes in the soil (Kaur et al. 2005). Applying organic fertilizers can prevent nutrient leaching and soil erosion. It also improves the soil structure, texture, aeration, and water-holding capacity (Chen et al. 2021;Tadesse 2020). It has been reported that the application of organic fertilizers such as compost can bio-remediate heavy metal contamination in the soil (Huang et al. 2016;Irfan et al. 2021;Mohkam-Singh 2022). However, there are some limitations to using organic fertilizers. These are slow starters for plant growth, since the nutrients are not immediately available to the plant as the mineral fertilizers. In addition, preparation of organic fertilizers involves a lengthy period of treatment. The distribution of these organic fertilizers in soil is non-uniform and can lead to pathogen contamination (Alegbeleye and Ana 2020;Rousseaux et al. 2005;Sikora 1998). Additionally, contamination with potentially toxic elements (PTEs), such as arsenic, cadmium, chromium, lead, lithium, and nickel, has been a constant concern about the quality of the compost. This PTE contamination of the compost has been primarily attributed to the non-segregation and contamination of the substrates (Malamis et al. 2017). Compost as organic fertilizer should have high fertilizing potential but should not have heavy metals and pathogen contaminations. Production of high-quality compost with low PTE contamination is a necessity for its application in agriculture. Composting has been shown to be effective in bioremediation of potentially toxic elements (Das et al. 2016). There have been studies to analyse the quality of the composts produced from municipal organic solid wastes and other types of organic waste such as kitchen waste and agro-waste (Devi et al. 2019;Reyes-Torres et al. 2018;Rich et al. 2017). It has also been studied to consider composts as component of the integrated nutrient management, which indicated that they could be the potential candidates for sustainable cultivation, reducing methane emission with increasing productivity (Bharali et al. 2021). Nevertheless, the comparative analysis of different types of compost qualities has not been comprehensively done and reported.
In this study, we focused on the formation and characterization of leaf waste compost and compared its quality with other bio-composts, such as kitchen waste compost, cow dung manure, vermicompost, municipal organic waste compost, and neem cake compost. It will provide valuable information in understanding the formation and maturation of leaf waste compost, which can be used to optimize the composting process and improve the quality of the final product. Leaf waste is a major component of municipal solid waste and is an abundant resource that can be effectively utilized through composting. Organic waste compost has been found to have high organic matter content, essential nutrients, and beneficial microorganisms that contribute to soil health. While there have been many studies on the formation and characterization of different types of compost, there is a lack of research comparing the quality of leaf waste compost with other organic waste bio-composts commonly used in agriculture. The novelty of this research lies in the comparison of the quality of leaf waste compost with other types of bio-composts, which can help farmers and growers to make informed decisions on the selection of the most suitable compost for their specific soil and crop requirement. The quality assessment in this study includes a greater number of macronutrients, micronutrients, PTEs, and trace elements, which were not incorporated in the previous reports. The findings of this study can also contribute to the development of sustainable waste management practice by promoting the utilization of organic waste as valuable resource for soil health improvement. Overall, this research area is important for promoting sustainable agriculture and reducing the environmental impact of organic waste disposal.

Leaf waste composting and sample collection and preparation
The leaf compost was produced from the leaf litter waste at the recycling unit, Daulat Ram College, University of Delhi. There are about 100 varieties of herbs, plant, and trees in the campus, but majority of the leaf litter waste is contributed by the leaf shedding tress such as Indian beech (Pongamia pinnata), scholar tree (Alstonia), fig (Ficus), mulberry (Morus alba), frangipani (Plumeria acutifolia), ashoka (Polyalthia longifolia), and krishna kadamb (Mitragyna parviflora). During the fall of the season, about 450 kg per month or 15 kg per day of leaf litter waste could be collected. The leaf wastes were shredded with mechanical shredders and composted in an experimental-scale bin size of 3 ft × 1.5 ft × 0.5 ft (length × breadth × height), at ambient temperature through EM method. A waste decomposer, which is a consortium of microorganism extracted from desi cow dung, developed by the National Centre of Organic Farming, Govt. of India, was used as microbial inoculum. The initial amount of 4 kg (dry weight) of leaf waste was taken per bin and composted for 50 days with intermittent mixing after every 3 to 4 days. Samples (0.5 kg) of the leaf compost were collected in a ziplock polybag after every 10 days of composting.
Six samples (1 kg) each of other types of matured biocomposts, viz. kitchen waste compost, vermicompost, cow dung manure, neem cake compost, and municipal organic waste compost, were also collected. These biocomposts were produced locally by local producers at Delhi National Capital Region, India. The kitchen waste compost was produced by residential welfare association, by composting kitchen waste for 70 days, in an ambient environment. The cow dung manure was produced by a local farmer at an ambient environment by composting for 100 days. The vermicompost was also produced by local farmer though worm casting of cow dung for 70 days. The municipal organic waste compost was produced by Municipal Corporation of Delhi composting plant, set up near landfill site at Okhla, New Delhi, India. The municipal organic waste compost was produced from the organic fraction of the municipal solid waste by segregating the compostable organic portion from the mixture of solid wastes. The neem cake compost was collected from a commercial site. All these composts were collected only after completion of the composting process.
All the samples were air-dried, gently ground, and sieved through a 1-4 mm sieve. The samples were stored at room temperature or 1 to 4 °C for further analysis. The physicochemical parameters, nutrient contents, metals, and trace element contamination were analysed through appropriate methods as given below.

The pH and electrical conductivity (EC)
The pH and EC of the composts were measured using a pH meter and EC meter. For the analysis, the samples were dissolved in distilled water at a ratio of 1:10 (w/v compost: distilled water), and thereafter, measurements were carried out using pH and EC meter.

The total carbon, nitrogen, and sulphur (CNS)
Macronutrients such as carbon, nitrogen, and sulphur in the composts were analysed by a CHNS analyser (vario EL cube, ser. no: 19171021). Approximately 6-10 mg of the samples were weighed and placed in the sample holder of the instrument. The combustion and reduction temperatures were set at 1150 °C and 850 °C, respectively, and the helium pressure was set at 1200 millibar (Krotz 2017).

Micronutrients, heavy metals, and trace elements
The samples were heat-digested using triacids (hydrofluoric acid (HF), nitric acid (HNO 3 ), and perchloric acid (HClO 4 )) on a hot plate (Shapiro and Brannock 1962). For this processing, samples (0.5 g) were taken in a Teflon crucible, and a mixture of 10 ml of concentrated HF, 5 ml of HNO 3 , and 2 ml of HClO 4 was added to it. These samples were then heated on the heating plate at 90 °C for 4 h with the lid closed. After 4 h, the lids were opened, and heating continued until it was completely dried. A mixture of 5 ml of concentrated HF, 10 ml of concentrated HNO 3 , and 2 ml of HClO 4 was added, and heating was continued until dryness with the lid open. Again, 10 ml of concentrated HNO 3 was added, and heating continued at 90 °C with an open lid to achieve complete dryness. Thereafter, 20 ml of 1 N HCl was added, and heating (90 °C) was continued for a while to bring the samples into the solution with the lid closed. The solution was then transferred to a volumetric flask. The Teflon crucible was rinsed two or three times with 1 N HCl and Milli-Q water and poured into the measuring flask to avoid any sample loss. The final volume was made up to 100 ml by adding Milli-Q water. The diluted solution was filtered using Whatman paper no. 2 or 42. The filtrate was subjected to another filtration using 0.45-micron filter paper under vacuum pressure. Then, the filtered solution was stored at 4 °C before analysis by inductively coupled plasma-mass spectrometry (ICP-MS) for analytical measurements. The filtered solutions were diluted 20 times using 1% HNO 3 before reading. The reference standard (SQC001 (metal in soil, Sigma)) was also prepared in the similar manner and diluted accordingly. The measurements were carried out using an iCAP-Q ICP-MS from Thermo Scientific in KED (kinetic energy discrimination) mode. The calibration curve was obtained using serially diluted solutions of certified reference material SQC001 (metal in soil, Sigma) at different concentrations. The observed value of correlation co-efficient (R 2 ) was > 99.0% for all elements. For data validation, a fixed concentration of CRM-SQC001 was run as unknown samples after every 10 samples. The average % offset for this was less than 10% for all elements analysed. Also, 129 Xe present in argon gas was measured as internal standard. The recovery of 129 Xe was in between 80 and 1 3 120%. The measured data were analysed using QTEGRA ISDS software, and the reported values are the average of three independent measurement runs.

Indexing
The fertilization index (FI) and clean index (CI) were calculated based on the modified model as reported by Saha et al. (Saha et al. 2010). The fertilizing index was calculated based on the score values and weightage factors of six parameters, as given in Table 1. The FI was computed based on the given formula: where "Si" is the score value of the analytical data and "Wi" is the weightage factor of the "i"th fertility parameter.
The clean index was calculated based on the score values and weightage factors of twenty-two metals in the composts instead of six metals as used by Saha et al. (2010), as given in Table 2. The CI was computed based on the given formula: where "Sj" is the score value of the analytical data and "Wj" is the weightage factor of the "j"th heavy metal.

Statistical analysis
The parameters analysed were subjected to the ANOVA analysis using GraphPad Prism 9.0 software to identify the statistically significant variation of the analytical data among the samples. Two-way ANOVA was performed on the leaf waste compost among the control and test, taking the time and treatment as two factors. For the comparison of variations in the parameters among the matured bio-composts, one-way ANOVA was performed along with a post hoc analysis through Tukey's HSD (honestly significant difference) test (P < 0.05). (1)

Formation and maturation of leaf waste compost
Compost formation and maturation from leaf waste as substrate were analysed from 0 to 50 days of composting by measuring parameters such as pH, EC, total organic carbon (TOC), and total nitrogen (TN) and calculating the C:N ratio. The pH of the leaf substrate was neutral before composting, but it turned into a moderately alkaline state after 10 days of composting. By 20 days, it was observed to be almost neutral, and then, it was increased to a moderately alkaline state and stabilized from 30 to 50 days of composting at about pH 7.92. The two-way ANOVA analysis revealed that the "time" factor had a significant effect on the pH levels, explaining 96.26% of the total variation in pH (P = 0.0011). The "treatment" factor does not have a significant effect on the pH level (P = 0.1235). A post hoc analysis through Tukey's multiple comparison test revealed that the mean value for the 20-day control group was significantly different from the mean values for the 10-day control, 10-day treatment, 20-day treatment, 30-day control, and 30-day treatment (adjusted P values < 0.05). Similarly, there were significant differences between other pairs of groups (results in supplementary file). The stability of pH after 30 to 50 days of the leaf waste composting indicates the maturation of the composting process (Cayuela et al. 2008) (Fig. 1).
The EC was observed to steadily increase from 0 to 50 days of composting, from 0.662 to 1.405 mS/cm in treatment. The two-way ANOVA between the control and test revealed that the "time" factor had a significant impact on the EC of the compost (P = 0.0064), but the "treatment" factor had no significant impact (P = 0.1088). Tukey's test revealed that the mean value of EC for the 10-day control was significantly different from the mean values for the 50-day control and treatment (adjusted P < 0.05). Similarly, there were significant differences between other pairs of composts (results in supplementary file). The EC of the Table 1 The score values and weightage factor assigned to each fertility parameter. The table shows the score values given to the range of concentration of the parameters analysed. Since these are essential nutrients, greater score was given if they were present in higher amount. The weightage factor was given based on the biological importance and function of the parameter. The parameter observed to be having higher biological function was given the higher weightage factor  (Avnimelech et al. 1996).
The total organic carbon content of the leaf waste compost decreased as the composting progressed from 0 to 50 days but stabilized after 30 days of composting at approximately 32 to 33%. A decrease in the organic carbon content and its stabilization indicated that the process of composting was undergoing maturation (Goyal et al. 2005). The mean values of the total organic carbon in the control and test were not significant with P value > 0.05.
The C:N ratio of the leaf substrate was initially observed to be 35.77, which steadily decreased below 20 by composting for 10 to 20 days, and then stabilized at approximately 16 after 30 days of composting. A C:N ratio of 10 to 15 is considered ideally mature and stabilized compost, and a C:N ratio ranging from 15 to 20 has also been commonly accepted (Rashwan et al. 2021). According to the FCO guidelines, the compost used as organic fertilizer should have a C:N of less than 20. The variation of the C:N ratio between the control and test was not significant with P > 0.05.
The stability of the pH, EC, and C:N ratio indicated the onset maturity of composting. Depending on the type of substrate, composting organic waste to produce mature compost could take different periods or a number of days (Mahapatra et al. 2022). The maturation of the leaf waste composting was attained within 20 to 40 days of composting, as observed from the stability of the analytical values Table 2 The score values and weightage factors assigned to each element. The table shows the score values given to the parameters analysed. Since their higher values increase toxicity, lesser score was given to the higher value. The weightage factor was given based on the biological function and toxicity of the parameter analysed. The parameter observed to be having higher toxicity was given the higher weightage factor Parameter (in mg/kg)  The graph shows the pH, EC, TOC, and C/N ratios from 0 to 50 days of composting after every 10 days. The stability of the pH as well as EC and attainment of C:N ratio < 20 was observed within 20 to 40 days of composting indicated the maturation of the compost 1 3 of the pH, EC, C:N ratio, and total organic carbon (Fig. 1). Microbial degradation of organic matter into mature compost generally involves different phases. The initial phase is the mesophilic phase, in which the mesophilic microbiome is actively involved in decomposition, followed by the thermophilic phase, in which the temperature is increased above 45 °C due to the increased activity of the microbiome. In this phase, thermophilic bacteria become active, and the temperature-sensitive bacteria begin to die off. In the third cooling phase, when the organic matter was almost degraded, the temperature began to drop and returned to the mesophilic phase, where the mesophilic bacteria began to re-emerge again. The final phase is the maturation or curing phase, in which the compost is allowed to mitigate any pathogens (Biyada et al. 2021;Meena et al. 2021;Nemet et al. 2021).
The analytical values of the other macronutrients, micronutrients, and PTEs in the leaf waste composting change as the composting progresses from 10 to 50 days. There was a statistically significant variation (P < 0.05) between control and treatment in the content of some elements such as P, As, Cr, Mo, Co, Mn, and Sr, as the composting time progresses towards maturation (Table 3).
In general, there was an increase in nutrient mineralization and a reduction in the PTEs as the composting progresses towards maturation (Table 4). About 32% increment of the total nutrients and 16% reduction of PTEs were seen in the matured leaf waste compost as compared to the initial stage of 10-day compost. The macronutrients such as N, P, and K were seen increment of about 17%, 88%, and 18%, respectively, in the matured leaf waste compost, while there were also increment in the micronutrients such as manganese and cobalt, with an increase of about 60% and 41%, respectively. The increments of P and Mn were statistically significant in the leaf waste composting. The increments of C, N, K, and Co were not statistically significant. Bioremediation of PTEs such as arsenic, chromium, nickel, and copper were seen with a reduction of about 29%, 17%, 55%, and 32%, respectively. The reduction of PTEs was not statistically significant, though a decrease in PTEs during the leaf waste composting was observed. These results were in coherent with recent studies, as reported by Paul et al. 2020, that nutrient recycling, mineralization of NPK, and bioremediation of PTEs such as lead, cadmium, and chromium were also observed in vermicomposting of cotton textile sludge and lignocellulosic waste (Paul et al. 2020;Devi et al. 2023). Table 3 Analytical values of the physicochemical parameters, PTEs, and the two-way ANOVA of leaf waste composting in temporal manner. This table shows the comparison of control and test group at 10, 20, 30, 40, and 50 days of composting. The pH, EC, C:N ratio, Mn, and P showed a significant variation with P < 0.01; parameters like As, Cr, Mo, Co, and Sr showed a significant difference with P < 0.05, and rest of the factors were non-significant. The unit of EC is in mS/ cm. C, N, and S are in percentage (%), while the other are in mg/ kg. *Indicates the statistical significance

Comparison of mature leaf waste compost with other types of bio-composts pH and EC
The pH of the composts was uniquely different in all the composts, although mostly alkaline except for the neem cake compost, which was acidic. The kitchen waste compost (pH: 9.16) was highly alkaline. The vermicompost (pH: 7.97), municipal organic waste compost (pH: 8.13), leaf compost (pH: 8.26), and cow dung manure (pH: 8.40) were moderately alkaline. However, the neem cake compost (pH 5.51) was the only compost that had an acidic pH value. The analysis of variance through one-way ANOVA showed that the variation in the pH among the different types of composts was statistically significant (P < 0.0001). A post hoc Tukey's HSD test revealed that the mean value of pH of kitchen compost was significantly different from vermicompost, neem cake compost, municipal organic waste compost, and leaf waste compost. The pH of vermicompost was significantly different from neem cake compost. While, the pH of cow dung manure was significantly different from neem cake compost, and the pH of neem cake compost was significantly different from municipal compost and leaf compost with adjusted P < 0.05. The pH of the compost could be moderately acidic or alkaline, depending on the properties of the substrates used to produce the compost. The compost that is to be used in agriculture should have a pH ranging from 6 to 8.5, which is an indication of its relative stability (Crohn 2016). The lower pH value of the compost is not desirable since organic acids can be phytotoxic to the plants.
The electrical conductivity (EC) of the composts was observed to be within the recommended value of less than 4 mS/cm. The neem cake compost and kitchen waste compost had the highest EC values of 1.07 and 1.06 mS/cm, respectively. Vermicompost has the lowest EC of 0.56 mS/cm. The leaf compost and cow dung manure had similar EC values of 0.66 mS/cm. The municipal organic waste compost had an EC value of 0.97 mS/cm. The variation in the EC among the composts was statistically significant (P = 0.0441). The multiple comparison analysis showed that the mean value of EC of municipal organic waste was statistically different from the vermicompost and leaf waste compost with adjusted P < 0.001. Also, the EC of neem cake compost was significantly different from the leaf waste compost, cow dung manure, and vermicompost (adjusted P < 0.05). A high EC value indicates a high salinity of the compost with high nutrient content of the compost. If EC is too high, it could have a non-specific and specific impact on crop development. High nutrient content can generate a high osmotic gradient that prevents plants from obtaining the nutrients and water they require. Alternatively, absorbing specific non-nutritive ions in excess can be toxic to plants and directly affect crop development. It can also indirectly affect plant growth by displacing more vital nutrients in the plant with non-nutritional ions (Morales and Urrestarazu 2013). As per the FCO 2013 guidelines, the EC of the compost should not be more than 4 mS/cm for an ideal compost (Table 5).

Macronutrients
The leaf waste compost had the highest total carbon content at 31.75%, which could be used as a good amendment for the soil. The neem cake compost had the second highest total carbon content at 29.94%, followed by kitchen waste compost (22.09%), vermicompost (15.26%), and municipal organic waste compost (13.87%). Cow dung manure has the lowest total carbon content at 10.51%. The variation in the total carbon content among the composts was statistically very significant with P value < 0.0001. A post hoc analysis through Tukey's test showed that the mean value of total carbon in leaf waste compost was significantly different from the municipal organic waste compost, cow dung manure, vermicompost, and kitchen waste compost with adjusted P < 0.05. The carbon in the vermicompost was also significantly different from the kitchen waste compost and neem cake compost. The total organic carbon content in the compost to be used as organic fertilizer should not be less than 14% by weight as per the FCO guidelines. Organic carbon promotes soil structure with excellent stability, enhancing soil aeration and water retention and preventing nutrient leaching and erosion. The organic carbon content is also crucial for the chemical composition and biological productivity of the soil (Corning et al. 2016). The total nitrogen contents were detected to be high in all the composts, and their variation among the composts was statistically very significant (P < 0.0001). The neem cake compost had the highest total nitrogen content at 2.86%, and the cow dung had the lowest value at 0.98%. The mean value of total nitrogen in the leaf waste compost was significantly different from the municipal organic waste compost, neem cake compost, cow dung manure, and vermicompost. The total nitrogen in the kitchen compost was also significantly different from the vermicompost, cow dung manure, and municipal organic waste compost. Nitrogen is an essential macronutrient for plant growth and development and is critically involved in many important functions, such as the photosynthetic process, phytohormone activities, and plant biomass production (Crawford and Forde 2002). Nitrogen has a significantly important role in the improvement of root growth, which can ultimately enhance the nutrient uptake, nutrient balance, and dry mass production of the plant (Diaz et al. 2006).
The leaf waste compost had the highest C:N ratio of 14.26, whereas the kitchen waste compost and municipal organic waste compost had the lowest C:N ratios of 8.28 and 8.71, respectively. The other composts have a C:N ratio of approximately 10. The variation in the C:N ratio among the composts was statistically very significant (P < 0.001). The Tukey's test showed that the mean value of C:N ratio of leaf waste compost was significantly varied from the municipal organic waste compost, neem cake compost, cow dung manure, vermicompost, and kitchen waste compost (adjusted P < 0.05). The C:N ratio was also varied significantly between neem cake compost and kitchen waste compost (adjusted P < 0.01). The C:N ratio is essential in determining compost maturity and quality. The compost used as soil amendment should have a C:N ratio of less than 20 (Crohn 2016). The higher C:N ratio leads to the immobilisation of nitrogen, which renders it unavailable to the plant, whereas the lower C:N ratio stimulates the gradual mineralization of nitrogen made available for the plant (Bruun et al. 2006). All the composts analysed in this study had a C:N ratio of less than 20, which is suitable for soil amendment.
The phosphorus (P) content in all the composts ranged from 0.071 to 0.162%. The neem cake compost had the highest phosphorus content at 0.162%, and the leaf waste compost had the lowest value at 0.071%. The analysis of variance showed no significant variation in the phosphorus content among the composts taken together, but a post hoc multiple comparison analysis showed that the mean value of P in the kitchen waste compost and municipal organic waste compost was significantly different (adjust P < 0.05).
The potassium (K) content across the sample ranges from 0.036 to 0.161%. The neem cake compost had the highest potassium content at a concentration of 0.161%, and the vermicompost compost had the lowest potassium content at 0.036%. The variation of K across the samples was statistically very significant (P < 0.01). The Tukey's test revealed that the mean value of K in the leaf waste compost was significantly different from the municipal organic waste, neem cake compost, cow dung manure, vermicompost, and kitchen waste compost. Similarly, there was also a significant variation of K in other pairs of composts (results in supplementary file). Table 5 The pH, EC, and macronutrient contents of different composts. This table shows the analytical values of the parameters along with the one-way ANOVA of different composts. The pH, C, N, and S ratio were significantly different with P < 0.0001; the K was significantly different with P < 0.01, and EC was significantly different with P < 0.05. *Indicates the statistical significance 0.14 ± 0.00 0.13 ± 0.00 0.12 ±0.00 0.16± 0.00 0.11 ± 0.00 0.071± .00 0.1190 > 0.5 K (%) 0.07 ± 0.0 04 ± 0.00 0.09 ± 0.0 0.16 ± 0.0 0.15 ± 0.0 0.14 ± 0.0 0.0026** > 0.5 S (%) 0.59 ± 0.09 0.49 ± 0.13 0.40 ± 0.10 0.67 ± 0.04 0.88 ± 0.14 0.52 ± 0.17 <0.0001**** > 0.5 The sulphur (S) content was high in all the composts, with statistically significant variation. The cow dung manure had the lowest S content, and the municipal organic waste compost had the highest S content at concentrations of 0.401% and 0.886%, respectively. The mean value of S in the cow dung manure was significantly different from the kitchen waste compost, neem cake compost, and municipal organic waste compost. The sulphur in the municipal organic waste also varied significantly from the kitchen waste compost and vermicompost. But the leaf waste compost did not have significant variation of S with respect to the other types of composts. Sulphur is another essential nutrient for plant growth and development. To supplement the sulphur content in the soil, the compost should have a sulphur content of not less than 0.25% of the dry matter (Sullivan et al. 2018). The sulphur contents in the composts analysed were well within the optimal level of 0.25% to 0.8%, according to the FCO recommendation (Table 5).
According to the FCO guidelines, the total nitrogen, phosphorus, and potassium contents in the compost that are used as organic fertilizer should not be less than 0.5% of the dry matter. The composts analysed in this study had P and K contents less than the recommended levels. Deficiencies in P and K can have many adverse effects on plant growth and development, such as seedling growth impairment, chlorosis, reduced plant biomass production, delayed maturity, and fruit development (Atkinson 1973;Thornburg et al. 2020). Such nutrients must be fortified and enriched in the compost to be used as bioorganic fertilizer.
Different methods of composting could affect the mineralization of nutrients in the compost. Composting methods such as windrows composting and vermicomposting were shown to have valorisation effect on the compost produced using different feedstock through nutrients recycling and mineralization (Lopes et al. 2021;Devi et al. 2023). Such a method could be adopted to produce value added bio-compost from organic waste such as leaf waste, lignocellulosic waste, kitchen waste, and other types of organic waste.

Micronutrients
The composts analysed have a sufficient concentration of micronutrients such as boron, manganese, cobalt, molybdenum, and zinc. Neem cake compost had the highest B content (53.01 mg/kg), whereas kitchen waste compost had the lowest B content (10.16 mg/kg). The cow dung manure had the highest Mn content at 482.68 mg per kg, and the leaf compost had the lowest Mn content at 184.03 mg per kg. The municipal organic waste compost had the highest contents of Co, Cu, and Mo at levels of 4.03 mg per kg, 298.46 mg per kg, and 10.63 mg per kg, respectively. The leaf waste compost had the lowest content of these elements at concentrations of 1.10 mg per kg, 25.90 mg per kg, and 2.32 mg per kg, respectively. The kitchen waste compost had the highest Zn content at 504.15 mg/kg, and the neem cake compost had the lowest Zn content at 45.64 mg/kg.
The optimal concentration of micronutrients such as B, Mn, Co, Cu, Mo, and Zn in the compost is essential for the compost if used as a soil amendment. A high concentration of such elements could be harmful to the plant, and a very low concentration could hinder the plant's growth and development. According to the FCO guidelines, the maximum concentrations of Cu and Zn in the compost to be used as bioorganic fertilizer should be 300 and 1000 mg/kg dry matter, respectively. All the composts analysed had CU and Zn contents below the permissible limits, although municipal organic waste compost had a maximal Cu content near the maximum permissible limit (Fig. 2).

Potentially toxic elements (PTEs) and trace elements
The contamination of the composts with PTEs such as arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), lead (Pb), lithium (Li), and nickel (Ni) was analysed. It was observed that the leaf waste compost had the least contamination with PTEs, as their contents were all below the permissible limits according to the FCO guidelines.
The arsenic content was observed to be high in the cow dung manure, kitchen waste compost, vermicompost, neem cake compost, and municipal organic waste compost. The leaf waste compost had the lowest arsenic content at 3.76 mg/kg. The variation in arsenic contents in the different groups of compost was not statistically significant (P > 0.05).
The cadmium content was above the permissible limit in the municipal organic waste compost. The neem cake and leaf waste compost had the lowest Cd content at 0.26 and 0.20 mg/kg, respectively. The variation in Cd content in different composts was statistically significant. A multiple comparison analysis showed that the mean value of Cd in the municipal organic waste compost was significantly different from that of leaf waste compost, vermicompost, and neem cake compost. Also, the mean value of Cd in vermicompost was significantly different from neem cake compost (adjusted P < 0.05).
The Cr contents in all the composts were high except the leaf waste compost. The municipal organic waste compost had the highest Cr content at 318.52 mg/kg. The variation in the Cr content in the composts was not statistically significant.
Mercury was observed to be high in the municipal organic waste compost, neem cake compost, and cow dung manure. The municipal organic waste had the highest Hg content at 0.37 mg/kg, while the leaf waste compost had the lowest Hg at 0.07 mg/kg. The Hg content in different composts varied significantly from one another (P < 0.01). The mean value of Hg in the municipal organic waste compost was significantly different from the leaf waste compost, kitchen waste compost, cow dung manure, and vermicompost (adjusted P < 0.05).
The municipal organic waste compost had the highest contamination with Pb at 95.26 mg/kg. Leaf waste compost and the neem cake compost had the lowest Pb content at 19.21 and 12.40 mg/kg, respectively. The concentration of Pb varied significantly from compost to compost (P < 0.0001). The mean value of Pb in the municipal organic waste compost was significantly different from that of leaf waste compost, neem cake compost, cow dung manure, vermicompost, and kitchen waste compost (P < 0.05).
All the composts analysed had a high lithium concentration. The Li content was highest in the municipal organic waste compost at 15.88 mg/kg, while the leaf waste compost had the lowest Li content at 2.84mg/kg. The analysis of variance revealed that the variation of Li in all the compost was not statistically significant.
The nickel (Ni) content in all the composts was low, and its variation among them was not statistically significant. Among the composts, municipal organic waste had the highest Ni (29.39 mg/kg) while leaf waste compost had the lowest Ni content (3.15 mg/kg) ( Table 6).
In this study, leaf waste compost was seen with the least contamination with PTEs such as As, Cd, Cr, Hg, Li, and Ni, as their concentration was well below the maximum permissible limits according to the FCO guidelines. Due to the homogeneity and purity of the leaf substrate, the compost produced from leaf waste is reported to be less contaminated with PTEs (Dmuchowski and Baczewska 2011). Most of the composts analysed were found to be contaminated with arsenic and chromium. Arsenic and chromium are toxic to plants, affecting various processes from root growth, germination, and biomass production to fertility and fruit production (Finnegan and Chen 2012;Shanker et al. 2005). Mercury is another toxic element found at high levels in all the composts analysed except the leaf compost. It was detected to be above the maximum permissible limit in the municipal organic waste compost and neem cake compost and just near the permissible limit in the kitchen waste compost and vermicompost. Mercury contamination can cause deformities in seedlings, nodules, and ultrastructural development in plants (Mondal et al. 2015). The municipal organic waste compost was also seen to be high in lead, which is toxic to plants, affecting root development (Fahr et al. 2013).
Contamination of compost with potentially toxic elements is a significant concern for its application in agriculture. Many municipal solid waste composts produced in different Indian cities, the USA, and European countries have been reported to be contaminated with PTEs (He et al. 1992;Herity 2003;Saha et al. 2010). The compost used as organic fertilizer should not be contaminated with PTEs since they can drastically affect plant growth and development. Such toxic elements should be remediated from compost before their application in agriculture, or compost contaminated with toxic elements should not be used for agricultural purposes. Composting could help reduce PTE contamination. Recent study by Paul et al. 2020 reported that vermicomposting could greatly help in the bioremediation of the potentially toxic elements such as lead, cadmium, and chromium (Paul et al. 2020). Vermicomposting could be adopted to further sanitized the bio-composts produced from various feedstocks The trace elements such as barium, beryllium, selenium, strontium, antimony, tin, titanium, thallium, and vanadium were seen to be moderately high in all the composts analysed (Fig. 3). The concentrations of trace elements such as antimony, strontium, thallium, and titanium were statistically different in different bio-composts analysed (P < 0.05). The trace elements should not be too high since they can be toxic to plants at high concentrations. However, their concentration should not be too low since they have a potential role in the biological function of plant development.

Fertilizing index and clean index
Neem cake compost had the highest fertilization index, followed by leaf waste compost and kitchen waste compost. The municipal organic waste compost and vermicompost have a medium-range fertilizing index value, whereas cow dung manure has the lowest fertilizing index value. The leaf waste compost had the highest clean index value, which means that it was least contaminated with heavy metals. The neem cake compost had the second highest clean index value, followed by vermicompost and kitchen waste compost. The municipal organic waste compost had the lowest clean index value, followed by cow dung manure.
Based on the values of the FI and CI, composts could be categorized into different quality composts (Table 7). The neem cake compost and leaf waste compost could be categorized as compost having very good quality with high fertilizing potential and low metal contamination. Kitchen waste compost and vermicompost could also be categorized as good-quality compost. The cow dung manure and municipal organic waste compost could be classified as mediumquality compost, having a medium fertilizing potential with some metal contamination. Compost with either a low fertilizing index or a low clean index should not be used for agricultural purposes but only as a soil conditioner (Mandal et al. 2014). Various organic waste composts could be excellent soil amendments and biocontrol agents that can act as organic fertilizers which are more eco-friendly than chemical fertilizers (Joshi et al. 2015). Applying organic Table 6 The content of potentially toxic elements (PTE) in the composts. This table shows the analytical value of the PTEs along with the results of the one-way ANOVA of different composts. The Pb was significantly different with P < 0.0001; Hg was significantly different with P < 0.01, and Cd was significantly different with P < 0.05. The other elements viz. As, Cr, Li, and Ni were not significantly different among the composts * Indicates the statistical significance 1 3 Fig. 3 The content of trace elements in the composts. This figure gives the observed values of the trace elements in different types of composts. Antimony varied significantly among the compost with P < 0.05. The trace elements such as Sr, Tl, and Ti also varied signifi-cantly among the compost with P < 0.01. The other trace elements such as Ba, Be, Se, Sn, and V in different compost were not significantly different among the composts waste composts could effectively promote plant growth and fruit quality compared with other fertilizer treatments in the greenhouse study. It could also improve the quality of the soil when applied as soil amendments (Wang et al. 2017).

Conclusion
Leaf waste could be suitable for producing good-quality compost. This study showed that leaf waste compost has high fertilization potential with low PTE contamination. They were rich in macronutrients (C, N, S), but P and K were lower than recommended. The micronutrients (B, Mn, Co, Cu, Mo, and Zn) were adequate. However, some PTE contamination could be observed in all the composts analysed except the leaf waste compost. The leaf waste compost has the most negligible PTE contamination with no elements above the maximum permissible limits, whereas municipal organic waste compost has the highest PTE contamination. It can be stated that leaf waste can be used as substrate for the production of organic fertilizer, with improved nutrients and lesser PTEs contents, through the process of composting.