Valorizing Combustible and Compostable Fractions of Municipal Solid Waste to Biochar and Compost as an Alternative to Chemical Fertilizer for Improving Soil Health and Sunflower Yield

Abstract

Reduced reliance on synthetic chemical fertilizers necessarily requires using renewable biomaterial-derived soil organic amendments (SOAs) in agriculture for sustained retention of nutrients through improvement in the soil organic matter (SOM). SOM replenishment through SOAs derived from wasted materials could help in its valorization by furthering the sustainability prospects of agronomic crop production systems. In the current study, compost (CP) and biochar (BC) were derived as SOAs from combustible and compostable fractions of municipal solid waste (MSW) for their potential valorization by adding SOAs as potential sustainable sources of nutrients as a replacement of chemical fertilizers (CF) for sunflower crops cultivated in potted soils. The experimental design included quadruplicated soil application of MSW-derived BC and CP in discrete and combined forms, each in three doses (% w:w), viz., low (L), medium (M), and high (H), i.e., BC-L, BC-M, BC-H; CP-L, CP-M, CP-H; and BC + CP-L, BC + CP-M, BC-CP-H. The results showed that, compared to the control (soil only), the sunflower growth and harvestable yield were significantly greater in BC + CF with a medium dose and were comparable to the growth and yield obtained in soils with CF. Sunflower growth in the discrete SOAs remained less than in the combined SOAs (BC + CP) and was attributed to the comprehensive soil health improvement rendered by the applied SOAs. The soil health improvement factors included SOM, CEC, and concentrations of total and available NPK. The dose-effect comparison of the SOAs showed highly variable trends, i.e., the sunflower growth did not correspond with the increase in dose of the SOAs. It is concluded that the combined application of BC + CP derived from MSW components at a medium dose could act as a potential alternative to CF. The developed approach resulted in MSW valorization, which improved soil health and yielded a better sunflower crop.

1. Introduction

Continuously increasing population pressure as well as climate change have a negative impact on soil fertility, so it leads to a decline in agricultural production in developing countries [1]. The decline in per capita land allotment and degrading soil quality have resulted in an escalation in the application rates of inorganic synthetic fertilizers for managing crop productivity. The literature generally agrees that most arid to semi-arid agricultural systems undergo more depletion of soil organic matter (SOM) due to a severe disparity between lower inputs than the long-term extractive agricultural approaches; hence, making agricultural systems unsustainable. Deriving organic amendments from waste and their application in soil is imperative to the effective management of SOM.

Overuse of chemical fertilizers in agricultural soils could lead to nutritional imbalances and depletion of soil fertility at the cost of global warming, which would proportionately lead to rapid loss of SOM and depletion of soil organic carbon (SOC) [2]. Chemical fertilizers contribute nearly 20% to the total global greenhouse gas emissions (GHGs) budget. It is crucial to explore cost-effective, environmentally friendly, and sustainable crop management techniques that enhance crop productivity, improve SOM, and lessen GHGs emissions from agricultural systems for potential mitigation of climate change. Such SOM management techniques could help in supporting soil preservation and conserving the long-term productivity potential of agroecosystems by boosting soils’ ability to act as carbon sinks rather than carbon sources. A sustainable agricultural production system could improve per-unit area crop production to confirm food accessibility for the continuously increasing world population.

The role of organic amendments such as biochar (BC) and compost (CP) has been reported as feasible options for addressing the ever-intensifying challenges of sustaining soil health and productivity [3]. Soil organic matter enrichment stands as a vital factor for enhancing soil fertility as well as productivity due to its integrated improvement in soils’ water holding capacity, nutrient availability, mineralization, and activity of soil microorganisms [4]. BC and CP have been some of the major sources of replenishing SOM for enhancing soil productivity potential and crop performance. Using discarded wastes as feedstock for deriving BC and CP could be an added benefit to their sustainable aspects as it could render waste valuable, which has been less reported in the literature in terms of its integration in agricultural crop production systems.

The valorization of wastes such as Municipal Solid Waste (MSW) could be described as its potential use by transforming it into a value-added product(s) coupled with a substantial reduction in its volume. The conversion of MSW into useful derivatives has been the subject of emerging research and development projects in recent years, with a focus on methods and procedures that lower emissions and incorporate life-cycle assessments of technologies and materials. Direct recycling of garbage and residues has been encouraged since the 1990s due to ever-intensifying socioeconomic concerns and the continuous depletion of non-renewable raw materials. Over time, there has been a continuous evolution of techniques that promote waste valorization by lowering GHGs emissions and minimizing the loss of resources as waste. Composting and pyrolysis of organic waste have been techniques for the conversion of waste components into CP and BC [5]. From the defined applications of the CP and BC derived from MSW for their valorization, application as SOAs has been a dominant approach for achieving comprehensive soil health improvement in agricultural crop production systems.

BC derived from MSW has exclusive physicochemical properties with great potential for improving soil health and enhancing plant crop yields [6]. CP derived from MSW through low-tech composting also has unique potential for comprehensive soil health improvement. The performance of BC and CP could become even better when both are applied as SOA by boosting soil microbial activity and refining soil physico-chemical properties such as bulk density, soil structure, water holding capacity, cation exchange capacity (CEC), pH, nutrient bioavailability, SOM, and SOC [7]. Application of BC and CP could also reduce nutrient loss from the soil profiles by minimizing nutrient leaching by storing them in pores with large surface areas and providing unique sites for nutrient sorption [8] and toxic element immobilization in the contaminated soils [9]. CP could make carbon and nutrients available to the crop plants through the integrated activities of its microbial and microbial consortia being stocked in the SOM, even for a short time lasting in the soil after application [10]. In recent times, research on potential synergic SOAs from BC and CP has been much demanded [11]. Hence, the application of BC and CP derived from no longer carried wastes as SOAs for substituting CF in plant crop production systems is worth exploration.

In the current study, BC and CP were derived as SOAs from combustible and compostable fractions of MSW for its potential valorization by adding SOAs as potential sustainable sources of nutrients, replacing chemical fertilizers (CF) for sunflower crops.

2. Materials and Methods

2.1. Collection of Materials

The combustible and compostable fractions of MSW were collected from the open dump location at the Mehmood Booti site in Lahore (Figure 1). The selected open dump site has been receiving fresh batches of unsorted MSW on a regular basis for several decades. The disposed MSW at the selected site mainly contained (≥65%) biodegradable components for potential composting and quite a fraction (≤4%) of combustibles. The composite samples of each of the combustible and compostable fractions of MSW were collected in a statistically representative way. The collected soil was mainly silty loam and was collected from the Botanical Garden, University of the Punjab Lahore-54590, Pakistan.

2.2. Derivation of BC and CP from MSW Components

The combustible fraction of MSW included a mixture of paper, cardboard, textile waste, wood chips, dry leaves, and twigs. After washing and air drying for 24 h, the mixture of components was used as feedstock for deriving BC by loading it in a pyrolysis unit set at 550 °C under a limited supply of oxygen [12]. The process of BC derivation is given in Figure 2. Based on weight differences, the BC yield was determined before being grounded and saved in airtight bags for further analysis and a pot trial experiment.

The compostable fraction of MSW included kitchen waste (fruits and vegetable leftovers) and dry leaves segregated from bulk of MSW sample collected from Mehmood Booti Open dump site Lahore (Figure 3). The compostable feedstock components were chopped (≈2 cm particle size), mixed in a proportion for optimizing cumulative moisture contents (≈60%), cumulative C:N ratio (C:N 25 ± 5:1) [13], and introduced in a pit (3 × 3 m) provided with an aeration assembly (interconnected perforated pipes). The composting pile in the pit was optimized for thermophilic to mesophilic composting with a total duration of 2.5–3 months.

2.3. Characterization of BC and CP Derived from MSW Components

As the stability of BC in CP greatly depended on its surface area, mineral content, and pH, proximate (moisture, ash, and volatile content) and elemental analysis (carbon, hydrogen, nitrogen, and sulfur) were conducted by following the standard procedures [14]. The characterization parameters of BC and CP are given in Figure 4.

2.3.1. Proximate and Elemental Analyses of BC and CP

The proximate analyses of BC and CP included the determination of moisture contents (MC), ash contents (AC), and volatile contents (VC), which were determined by following the standard methods [14]. The fixed carbon contents were determined by subtracting the values of MC, AC, and VC from a total of 100% [15]. The elemental analyses included the determination of C, H, N, and S contents in the feedstock components of MS, and derived BC and CP were determined by using an elemental analyzer [16].

2.3.2. Physicochemical Analyses of BC, CP and Soil

The physicochemical analyses included the determination of pH and electrical conductivity (EC) of the extracts of feedstock components of MSW and derived BC and CP. The extract was prepared by vigorously mixing 1 g of the feedstock components in 20 mL of distilled water [17]. The cation exchange capacity (CEC) of BC and CP was determined by determining the consumption of ammonium acetate solution, as given by Lu (2000) [18]. Bulk density (BD) was calculated by using the pre-weighed 100-mL measuring cylinder filled with the respective volumes of BC and CP and following the equation [19]:

$$Bulk Density\left(BD\right)=\frac{Mass of dried BC \left(or CP\right)}{Vol. of cylinder}$$

The loss-on-ignition (LOI) method was used to quantify the organic matter in soil [20]. The acid digestible (in HCLO₄:HNO₃ in a ratio of 4:1) samples (nearly 100 mL of digestate dilution with distilled H₂O) of BC, CP, harvested plants, and soil were used for the determination of micro- and macro-elements [21].

The soil used for the pot cultivation trial was analyzed for the determination of pH, CEC, ECe, BD, WHC, SOM, total nitrogen (TN), available phosphorous (P), and available potassium (K) by following the protocol used for BC and CP (as mentioned above). The Walkley [22] method was used to determine the SOM. For all the soil analyses, saturation paste was developed by mixing 100 g of soil with 100 mL of distilled water. The water-holding capacity of soil was determined by following the equation:

$$Water holding capacity (\%)=\frac{Retained water in saturation paste}{Total Volume of water}\times 100$$

The TN in the soil was determined through Kjeldahl’s digestion protocol [23]. The Olsen’s P [24] was determined by using a sodium bicarbonate solution and quantification at 700 nm on a spectrophotometer (Cintra 1010 GBC, Melbourne, Australia). The K was determined by running the soil digestate on a flame photometer (PFP7 and PFP7/C Jenway, UK) [25].

2.3.3. Surface and Structural Characterization of BC and CP

The surface and structural morphology of BC and CP were determined by scanning electron microscopy (SEM JEOL, 6480LV, Akishima, Japan) analyses [26]. The samples were put on non-two-edge carbon tape that stuck to an aluminum (Al) stub, and then the complete distribution of micro- and mesopores was determined by SEM. The composition of elements and functional groups associated with the surfaces of BC and CP were determined by energy dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FTIR), respectively. To detect the functional groups, the BC and CP samples were scanned at a specific resolution (4 cm⁻¹) with a wavelength range of 650–4000 cm⁻¹. The IR software was used to visualize the FTIR spectra. The thermal analyzer (model SDT Q600, TA-instruments, New Castle, DE, USA) was used for thermogravimetric analysis (TGA) of BC and CP conducted in an N-rich (100 mL/min) atmosphere up to 1000 °C ambient temperature with a 20 °C/min heating rate [27].

2.4. Crop Performance of Sunflower in Potted Soils Amended with BC and CP Derived from MSW

2.4.1. Sunflower Pot Cultivation Trial in Potted Soils Amended with BC and CP

A mesocosmic sunflower cultivation trial was conducted in pots filled with soil mixed with SOAs. The pot trial experiment was set in the Botanical Garden, University of the Punjab, Lahore-54590, Pakistan. The mesocosmic cultivation containers included PVC pots (height-diameter: 15 × 17 cm, respectively). The experimental layout included quadruplicated soil application of MSW-derived BC and CP in discrete and combined forms, each in three doses (% w:w), viz., low (L), medium (M), and high (H), i.e., BC-L, BC-M, BC-H; CP-L, CP-M, CP-H; and BC + CP-L, BC + CP-M, BC-CP-H (

Table 1. Treatments and their replicates for sunflower cultivation trial in potted soil amended with BC, CP, and their combinations.

Control								Dose	SOAs
Soil only				Soil + CF					BC				CP				BC + CP
Replicates									Replicates
R1	R2	R3	R4	R1	R2	R3	R4		R1	R2	R3	R4	R1	R2	R3	R4	R1	R2	R3	R4
S-R1	S-R2	S-R3	S-R4	S + CFR1	S + CFR2	S + CFR3	S + CFR14	L	BC-L-R1	BC-L-R2	BC-L-R3	BC-L-R4	CP-L-R1	CP-L-R2	CP-L-R3	CP-L-R4	BC + CP-L-R1	BC + CP-L-R2	BC + CP-L-R3	BC + CP-L-R4
								M	BC-M-R1	BC-M-R2	BC-M-R3	BC-M-R4	CP-M-R1	CP-M-R2	CP-M-R3	CP-M-R4	BC + CP-M-R1	BC + CP-M-R2	BC + CP-M-R3	BC + CP-M-R4
								H	BC-H-R1	BC-H-R2	BC-H-R3	BC-H-R4	CP-H-R1	CP-H-R2	CP-H-R3	CP-H-R4	BC + CP-H-R1	BC + CP-H-R2	BC + CP-H-R3	BC + CP-H-R4

Here; BC: biochar; CP: compost; CF: chemical fertilizer; L: low; M: medium; H: high; SOAs: soil organic amendments; Rn: replicate no.

). The statistical design [28] consisted of a randomized complete block design (RCBD) developed for the experimental units based on the treatments and their replicates given in Table 1. Control included soil only and soil applied with the recommended dose of CF for sunflower, i.e., 150:70:50 kg/ha of NPK. The BC + CP consisted of the mixing of both SOAs in a ratio of 1:1 (v:v). The pot mixtures were left to equilibrate for 20 days. Certified sunflower seeds (Orisun oil-yielding variety) were sown in the middle of each pot with 100% germination in each pot. Watering in each pot was performed twice a week to maintain soil moisture according to the pot capacity.

2.4.2. Sunflower Crop Performance in Pot Trial

After 75 days of cultivation trails, sunflower plants were harvested carefully from each pot after observing morphological and agronomic parameters [plant height (cm), disk diameter (cm), number of seeds, and seed weight per disk]. For post-harvest analyses, the soil samples from each pot were collected and examined by following the relevant methods as stated above.

2.4.3. Oil Contents in Sunflower Seeds Derived from Pot Cultivation Trial

For the determination of oil contents, 20 sunflower seeds were selected from each treatment on a random basis. The selected seeds were air dried and ground before oil extraction according to the method given by Abudlrazzaq et al. [29]. The Soxhlet apparatus was used for extracting oil from sunflower seeds by using a solvent of n-hexane at 60–65 °C. Through the distillation process, solvent was recovered from the extract. The mixture of oil and solvent was positioned in a water bath to let the solvent evaporate to obtain purified oil.

2.5. Statistical Analysis

For the allocation of positions to each of the experimental units of the experimental design, the random number approach was used, according to instructions [28] given for RCBD. The descriptive statistics on the quadruplicated observations for each of the variables were applied, mainly including the arithmetic mean and standard deviation. After ensuring normality of data for the selected treatments, the significance of the mean was determined by applying the F-significance test for single-factor experiments, i.e., one-way ANOVA (analysis of variance), after staying strictly compliant with the instructions given for the RCBD of a single-factor experiment [28]. After confirming F-significance (p = 0.05), the mean grouping was executed by computing the least significance difference (LSD_0.05) values for each of the treatment combinations considered for comparisons. For all the descriptive and analytical statistics, the Data Analysis package of MS Excel (Microsoft Office 2016) was used. The same software was used for developing data graphs.

3. Results and Discussion

3.1. Obtained Yield of BC and CP

The BC derived from selected components of MSW showed a 33% yield under the chosen pyrolysis conditions (Figure 5). The reduction in BC yield was mainly due to the thermal degradation of the components of MSW selected as feedstock, which contained components such as lignin, cellulose, and hemicellulose. The reduction in BC yield was also due to the hydroxyl groups present in the selected components of MSW considered as feedstock [16]. An average 70–80% reduction in feedstock contents has been reported for BC derived from components of MSW [29]. An average CP yield was 38% (Figure 5), i.e., the obtained CP was of medium to high quality based on yield. Mostly, yield is inversely proportional to the quality of CP, i.e., the greater the yield of CP, the lower its quality, and vice versa. The low yield and high quality of CP are generally attributed to the high losses during the rigorous maturation and curing phases of the composting process, which have not happened in the case of CP derived over a short span of time.

3.2. Characterization of BC and CP

3.2.1. Proximate and Ultimate Analyses of BC and CP

Proximate analyses of combustible and compostable components of MSW used as feedstock for deriving BC and CP are given in Figure 6. Both BC and CP had low VC (%) contents as compared to their respective feedstocks. The AC (%) contents in the BC and CP were greater than their respective feedstocks. Perhaps it was the high mineral contents that resulted in enrichment after a reduction in the mass of feedstock up to 1/3rd rendered by the pyrolysis and composting processes, respectively. Such an increase in AC (%) contents due to enrichment has been well documented [30,31]. The VC (%) contents in BC could also be lower because of the high temperature provided during the pyrolysis process [32], which induced a great amount of porosity in BC [33]. The AC (%) in the CP had increased due to the increase in organic acid contents rendered by the microbial activity on the biodegradable components of the MSW selected as feedstock. Hence, AC (%) in CP increased due to the accumulation of minerals and organic residues due to the production and accumulation of organic acids [34], especially during the thermophilic phase of the composting process, when temperature had sustained around 65–70 °C for 6–8 weeks.

The ultimate analyses of the BC and CP are given in

Table 2. Ultimate analyses of BC and CP derived from MSW components. Mean ± S.D., followed by difference letter, is considered significantly different according to LSD 0.05.

Elemental Constituents	Units	SOAs
		BC	CP
C	Wt.%	42.86 ± 1.16 a	17.44 ± 1.07 b
H		2.85 ± 0.44 a	1.43 ± 0.18 b
S		0.55 ± 0.02 a	0.36 ± 0.05 b
N		0.90 ± 0.10 b	1.92 ± 0.01 a

Here: SOAs: soil organic amendments; BC: biochar; CP: compost.

. The C, H, and S contents (Wt. %) were significantly greater in BC than CP. However, N contents were significantly greater in CP than BC. The CHS contents in BC were potentially due to mineral enrichment in the pyrolyzed derivatives left after the loss of most of the volatiles from the feedstock selected from MSW. The N contents in the CP were perhaps because of the mineralization rendered by the microbial activity on biodegradable feedstock derived from MSW.

3.2.2. Physico-Chemical and Elemental Analyses of Soil, BC, and CP before Cultivation of Sunflower

The physicochemical and elemental results of BC and CP are given in

Table 3. Physicochemical and elemental analysis of soil, BC, and CP.

Anlyses Type	Parameter	Units	Soil	BC	CP
Physico-chemical analysis	pH (1:20)	-	7.39 ± 0.11	8.73 ± 0.11	7.67 ± 0.17
	ECe1:20	mS cm−1	364.3 ± 2.1	422.3 ± 1.45	553.66 ± 2.33
	WHC	(wt.%)	19.91 ± 0.62	33.5 ± 0.97	24.41 ± 0.22
	SOM		0.55 ± 0.07	7.14 ± 0.78	15.78 ± 1.66
	BD	(g cm−3)	1.25 ± 0.02	1.09 ± 0.11	0.60 ± 0.02
	CEC	(cmolc kg−1)	22.29 ± 1.58	46.69 ± 0.94	43.06 ± 0.74
Elemental analyses	N	(wt.%)	0.17 ± 0.02	0.90 ± 0.10	1.92 ± 0.01
	P		0.06 ± 0.01	1.5 ± 0.01	1.69 ± 0.14
	K		0.03 ± 0.006	1.90 ± 0.49	0.89 ± 0.11
	C		ND	42.86 ± 1.16	17.44 ± 1.07
	H		ND	2.85 ± 0.44	1.43 ± 0.18
	S		ND	0.55 ± 0.02	0.36 ± 0.05
	Ca	ppm	1.003 ± 0.06	1.17 ± 0.04	2.22 ± 0.55
	Mg		1.36 ± 0.07	3.03 ± 0.18	1.52 ± 0.67
	Zn		0.23 ± 0.02	0.49 ± 0.01	0.56 ± 0.01
	Cu		0.02 ± 0.01	0.11 ± 0.01	0.14 ± 0.005

Here; BC: biochar; CP: compost; ND: not detected.

. The BC derived from MSW was alkaline in nature with a pH of 8.73, whereas the CP had an alkaline neutral pH of 7.67. From recent studies, it has been clear that BC mostly has an alkaline pH, which, however, depends upon the temperature and feedstock type [35]. The high pH is generally due to the accumulation of alkaline salts from carbon-based waste [36], while the acidic pH is due to the volatilization of ammonia during the composting process [37].

The high values of ECe and CEC in BC and CP were due to the presence of functional groups (O-bonded) on the surface of SOAs derived from MSW feedstock [38]. The WHC of BC was greater than that of CP, being 33.5 and 24.41%, respectively. It was due to the high porosity of BC, as revealed by its surface properties as determined by SEM results. The porous structure of BC has been responsible for better WHC. The BD of CP was low because of the presence of mineralization of nutrients under aerobic conditions, with a loss of over 60% of the mass of feedstock derived from MSW. The carbon contents in BC were greater than those in CP due to the high cellulosic contents of the selected feedstocks, viz., paper and cardboard. BC has a remarkable amount of mineral nutrients [36]. The K contents were lower in CP than BC because the biodegradable components of MSW selected as composting feedstock had an inherent low concentration of K as well as N and P. During composting, microbial activity typically mineralizes small portions of nutrients, increasing the end product’s nutrient content. So, composting and pyrolysis yield CP and BC as potential sources of nutrient recycling, such as when applied to the soil as SOAs [37,38]. Hence, BC and CP derived during the current study had potential for application as SOAs in the soil for the cultivation of plant crops.

3.2.3. Surface and Structural Characterization of BC and CP

BC derived from MSW components was stable based on its structure, surface functional groups, elemental, and ultimate analyses [39]. The characterization techniques of BC have mostly been based on the determination of its structure, surface functional groups, elemental analysis, and ultimate analysis [40,41] by using different characterization methods, including SEM, EDX, FTIR, XRD, TGA, proximate analysis, and ultimate analysis [16]. The stability of BC under consideration is mainly indicated by its pore structure, pH, minerals, sorption process, particle size, and surface area [22].

FTIR spectra were analyzed to investigate functional groups present on the surfaces of BC and CP, as shown in Figure 7. Several peaks were observed at different wavelengths (3500–1000 cm⁻¹). Medium peaks were detected at 3412.4 cm⁻¹, with the OH functional group being the main component of cellulose (Figure 7A). The hydroxyl (OH) present on the surface of biochar is one of the important functional groups responsible for its adsorption capability [42]. Feedstock type and temperature of pyrolysis have been reported as main factors for the determination of functional groups on the surface of BC [43]. The O-H stretching was associated with the peaks observed at 3748 and 3252 cm⁻¹, due to the dehydration of hemicellulose and cellulose compounds. Thus, the peak at 3412 cm⁻¹ could be allocated to O-H stretching. The aromatic C=O, C=C, and functional groups were associated with 1636 cm⁻¹. Li et al. [22] identified aromatic C=O and C=C peaks at 1636 cm⁻¹ in Eichornia crassipes derived biochar, which might be glossed at a higher temperature. Between 700 and 873 cm⁻¹ C-H stretching peaks were observed. In the current study, the bonds were observed at around 776.2 cm⁻¹ for BC.

As shown in Figure 7B, the FTIR spectra obtained for CP were in the 4000–500 cm⁻¹ range. The peak at 1420 cm⁻¹ characterized the aliphatic C-H stretching. A change in the functional groups was the result of metabolite exchange in MSW due to the biodegradation process. The peaks declined at 1633.74 and 1078.1 cm⁻¹ and were associated with C=C, C=O, and S=O stretching. In the composting process, microbes used aliphatic substances as well as polysaccharides as energy sources, which ultimately enhanced the process of biodegradation and mineralization of nutrients. Similar interpretations about the functional groups of composts have been described in the literature [31].

The surface structures of BC and CP characterized by SEM are given in Figure 8. Various micro- and mesopores were present on the surface of both BC and CP. The results revealed the presence of smooth structures on the surface of CP at lower magnification, but pours and tubular structures were observed in BC at high magnification [44]. SEM of BC depicted that the pores and fibrous structures were present on its surface, as can be seen in Figure 8A. Micro- and macro-pores were present on the surface of BC, which could be responsible for enhancing WHC [45]. Usually, biochar with a greater surface area and high porosity will have a greater adsorption capacity. Thus, pore structure is produced as the result of the dehydration phenomenon during pyrolysis [46]. SEM images of BC showed that various processes and temperatures led to substantial changes in the surface morphology of the original particles while still largely maintaining their visible shape [47]. The enlargement of pores in BC with temperature rises might have greatly enhanced the pore properties of BC. Moreover, it was likely that the rise in the pyrolysis temperature increased the crystallinity of mineral sections, giving rise to highly favorable aromatic structures in BC [32]. SEM images of CP at different magnifications showed a spongy and deposited structure on its surface, as shown in Figure 8B. In the previous study [48], the MSW-derived CP contained fluffy and spongy-type structures, usually due to the microorganisms involved in the degradation process.

SEM-EDX spectral assays of the BC and CP were made to determine the structural configuration of both SOAs (

Table 4. SEM-EDX data achieved from the elemental investigation of biochar and compost derived from their respective municipal solid waste fractions.

	Biochar		Compost
Elements	Weight (%)	Atomic (%)	Weight (%)	Atomic (%)
C	77.73 ± 0.01	87.3 ± 0.006	10.6 ± 0.41	15.31 ± 0.61
O	9.8 ± 0.10	8.36 ± 0008	46.01 ± 2.48	53.59 ± 6.19
Na	0.36 ± 0.005	0.24 ± 0.01	1.75 ± 0.13	1.29 ± 0.07
Mg	0.39 ± 0.005	0.26 ± 0.01	2.33 ± 0.49	2.01 ± 0.54
Si	1.48 ± 0.003	0.76 ± 0.008	22.28 ± 2.53	16.04 ± 2.68
K	2.91 ± 0.003	1.1 ± 0.003	3.31 ± 0.49	2.11 ± 0.54
Ca	12.41 ± 006	4.52 ± 0.01	1.88 ± 0.42	1.12 ± 0.17
Al	0.63 ± 0.01	0.33 ± 0.01	9.50 ± 1.25	6.92 ± 1.22
Fe	-	-	2.27 ± 0.55	0.54 ± 0.03
P	-	-	0.25 ± 0.01	0.14 ± 0.01

and Figure 9), which revealed that carbon contents in BC were significantly higher than CP and O and Si contents were significantly greater in CP than BC. The significant variation in SEM-EDX assays of BC and CP was mainly because of feedstock type and the temperature of processing [49]. The 550 °C processing temperature applied to the combustible fraction of MSW taken as feedstock rendered carbon enrichment per unit mass of the derived BC, resulting in a 2/3rd loss of actual feedstock mass. On the other hand, composting of the compostable fraction of MSW considered as feedstock at 60–65 °C (only during the thermophilic phase) over 2–2.5 months rendered low carbon enrichment per unit mass of the derived CP. The SEM-EDX spectra showed that there was less nutrient residual percentage on the surface of BC, mainly including minerals of Mg, Na, K, Ca, O, B, and Si, and the highest percentage of carbon (Table 4 and Figure 9).

The relationship between the carbon content and the yield of BC is negative, suggesting that carbonization sped up with the rise in temperature in the feedstock chamber of the pyrolysis unit. The decrease in hydrogen and oxygen content at higher pyrolysis temperatures was believed to be due to the breaking of weak bonds in the structure of BC [50]. The SEM-EDX spectra (Figure 9B) revealed elemental composition on the surface of CP, mainly including O, Al, Na, Si, C, and K, being greater than P, Mg, Fe, and Na. Phosphates and dissolved nitrates may be adsorbed into the pores of the fresh BC. However, the mineral deposition on the surface of BC may vary depending on the feedstock type and pyrolysis conditions. For example, K, Na, and Ca minerals were in higher percentages on the surface of BC derived from Wodyetia bifurcata feedstock [51].

The thermal stability as well as heat response of the BC surface are shown in the TGA spectra (Figure 10A), which demonstrated that by increasing the pyrolysis temperature, combustion of feedstock took place, which indicated carbon oxidization and the production of VC (%) at high temperatures [52]. In the case of CP, the TGA spectra showed that there was significantly less carbon oxidation due to the very low feedstock processing temperature compared to BC. There have been reports that composting temperature and air circulation through the feedstock pile directly affect its thermal decomposition behavior and other associated kinetic parameters [53]. Results of the TGA-BC showed that the weight loss was about 27.3% at 1000 °C and remained stable thereafter. TGA-CP (Figure 10B) showed that the weight loss percentage was about 60% at 1000 °C and was stable from 600 to 1000 °C. Results revealed that between 33 and 180 °C, the first weight loss happened with the loss of the main percentage of moisture contents in the sample. About 55–60% weight loss was observed at 600 °C, which was perhaps because of the decomposition of lignin and cellulosic contents in the sample. In the case of CP, there were three phases of degradation of the sample, viz., the 1st between 25 and 120 °C, the 2nd between 200 and 350 °C, and the 3rd between 350 and 700 °C. The TGA spectra have been one of the most commonly used parameters for observing thermal degradation behaviors and associated kinetics of feedstocks during the processing of plant-based feedstock [54]. At the beginning of thermal decomposition, dehydration of the sample occurred, which transited to the thermal degradation of organic acids and volatile contents such as cellulose and hemicellulose, i.e., the breakdown of aliphatic compounds. Weight loss of the sample was proportionate to the loss of moisture contents driven by a continuous increase in temperature, i.e., dehydration of the sample over temperature processing time [11]. Hence, the stability of the derived BC and CP was higher at high temperatures.

3.3. Soil Health Improvement Induced by BC, CP, and Its Combinations

The application of SOAs in the soil, i.e., BC, CP, and their combinations, significantly (p < 0.05) affected its pH, CEC, WHC, and SOM (

Table 5. Physicochemical characterization of post-harvested soil amended with selected treatments after growth period of sunflower.

Treatments		pH	EC (mS cm−1)	BD (g cm−3)	CEC (Cmolc kg−1)	SOM	WHC	N	P
						Wt. (%)
Control	Soil only	7.49 ± 0.03	415.6 ± 1.97	1.22 ± 0.01	20.22 ± 0.52	1.23 ± 0.04	53.72 ± 0.02	0.175 ± 0.04	0.21 ± 0.03
	Soil + CF	8.56 ± 0.09	538.5 ± 0.52	1.12 ± 0.002	28.63 ± 0.66	5.40 ± 0.11	84.55 ± 0.17	1.61 ± 0.02	1.52 ± 0.10
BC-L		8.13 ± 0.05	23.42 ± 0.47	1.16 ± 0.01	23.42 ± 0.47	5.12 ± 0.05	70.41 ± 0.02	0.675 ± 0.02	1.21 ± 0.16
BC-M		8.37 ± 0.09	519.25 ± 0.84	1.14 ± 0.001	25.75 ± 0.50	4.98 ± 0.14	66.57 ± 0.13	0.59 ± 0.03	1.38 ± 0.11
BC-H		8.06 ± 0.01	510.52 ± 0.14	1.11 ± 0.007	27.27 ± 0.45	5.42 ± 0.05	66.41 ± 0.02	0.74 ± 0.01	1.59 ± 0.09
CP-L		8.59 ± 0.02	557 ± 1.44	1.18 ± 0.002	27.16 ± 0.8	8.33 ± 0.12	63.79 ± 1.00	1.18 ± 0.06	1.47 ± 0.17
CP-M		8.05 ± 0.01	563.7 ± 0.09	1.11 ± 0.002	29.44 ± 0.55	8.44 ± 0.11	59.75 ± 0.01	1.66 ± 0.01	1.63 ± 0.1
CP-H		8.65 ± 0.01	548.075 ± 2.5	1.10 ± 0.01	30.38 ± 0.65	7.81 ± 0.40	57.61 ± 0.95	1.73 ± 0.02	1.66 ± 0.09
BC + CP-L		8.63 ± 0.03	536.15 ± 1.88	1.06 ± 0.01	29.32 ± 0.33	8.57 ± 0.16	68.86 ± 0.01	1.78 ± 0.01	1.85 ± 0.01
BC + CP-M		8.24 ± 0.29	583.37 ± 0.13	1.05 ± 0.004	31.90 ± 0.86	9.26 ± 0.23	62.73 ± 0.57	1.82 ± 0.01	1.83 ± 1.0.03
BC + CP-H		86.26 ± 0.01	574 ± 0.075	1.008 ± 0.15	32.31 ± 1.51	13.21 ± 0.43	86.26 ± 0.01	1.85 ± 0.01	1.91 ± 0.02

Here: BC: biochar; CP: compost; CF: chemical fertilizer; L: low; M: medium; H: high; SOM: soil organic matter; EC: electrical conductivity; CEC: cation exchange capacity; WHC: water holding capacity; BD: bulk density of soil.

). The BD of soil got reduced with the increasing dose of SOAs, while soil pH kept significantly increasing with the increasing dose of SOAs. The combined application of BC and CP had resulted in better results than individual applications of either BC or CP in terms of inducing cumulative soil health, which aligned with previously reported findings about the combined effects of BC and CP on soil health improvement [55]. However, an increase in the soil pH was perhaps because of a soluble ionic salt bank deposited on the surface of BC, which got drained into the soil over time, leading to the gradual increase in the pH value over time, as reported by Khan et al. [56]. The actual pH values of BC and CP were 8.73 and 7.67, respectively. On combined application of BC and CP in the basic neutral soil (pH 7.39), the mutual interactions of the soluble salty cations and anions led to an initial increase and then a decrease in the soil pH. The high surface area rendered by the high micro- and microporosity of the BC could have a significant contribution to soil pH fluctuations due to its own surface, electrostatic charge, and the emergence and propagation of microbial communities on the BC surface over time [57]. Hence, the application of BC, CP, and their combination in the soil induced changes in the redox dynamism-related parameters of cumulative soil health, which were more turbulent at the early phase of experimentation (on the addition of freshly prepared BC and CP) than later in the study.

The soil EC of the soil amended with the combined application of BC and CP was significantly higher (583.3 µS cm⁻¹) than the control soil only (415.6 µS cm⁻¹). However, control soil with CF had greater EC than soil alone, which was perhaps because of the mineral salts rendered by CF [58]. The WHC and CEC of the soil improved (86.26% and 32.31 cmol kg⁻¹, respectively) with the combined application of BC and CP. This was perhaps because of the high porosity, mineral, and ash contents in the BC and CP, as observed in EDX analyses. However, the dose effect of both BC and CP had no corresponding effects on the variation in indicators of cumulative soil health, i.e., the increase in dose of BC and CP from low to medium to high did not result in an increase in the values of the soil health parameters; rather, they fluctuated in a variable manner. It was perhaps because of complex interactions between the soluble redox dynamics of the SOAs that an induced shift in the kinetic equilibrium of their physical, chemical, and biological properties was less compatible with the apparent increase in dose of BC, CP, and their combinations.

Crop performance is generally significantly influenced by the variation in WHC, which is influenced by the shift in BD rendered by the types and doses of SOAs applied to the soil. Adding BC and CP to the soil improved its ability to retain water, which improved the soluble mineral contents of the soil [59]. The combined application of BC and CP proved more effective in improving SOM (13.12%) as compared to the control. At high doses of BC + CP, the SOM was the highest. Compost enhances SOM in soil over time if continuous replenishments are made on a recurrent basis. Soils with low SOM require more recurrent and constant replenishment of CP, as it degrades more rapidly than BC. Soils in Punjab (Pakistan) contain very low SOM due to climatic conditions and unhealthy soil management practices [60]. Regular replenishments of SOAs, as developed in the current study, would be helpful in restoring and improving SOM over time. Applying SOAs like CP made from MSW-based feedstock helped improve SOM in the field soil over time [61]. It has also been observed that the long-term application of CF alone in soil without the addition of SOAs could cause a gradual reduction in SOM and a cumulative drop in the crop productivity potential of the soil.

The results of the current study showed that the available N increased in soils applied with BC, CP, and their combinations (Table 5). The highest available N was observed in the soil treated with the combined application of BC and CP, i.e., compared to the control treatment (0.175%), there was up to 1.85%, 1.73%, and 0.74% increase in soil with the application of CP, BC, and its combination, respectively. The BC potentially acted as a slow-release N source [62]. Generally, applying BC along with other fertilizers has more favorable impacts on counteracting the potentially inaccessible N from applied BC [63]. This is why there was more soluble N in soils applied with a combined dose of BC and CP, as observed in the current study. The combined application of BC and CP led to the highest increase in the level of available P in the soil (Table 5). The available K contents at the pre-sowing stage were below the adequate level in soils applied with either BC or CP. However, soil applied with both BC and CP had greater available K contents (1.91%). Overall, application of BC, CP, and their combinations at any time has improved cumulative soil health by stabilizing soil pH, increasing SOM, CEC, and the level of available NPK, which could be attributed to the complex microbial consortium added through CP.

3.4. Crop Performance of Sunflower in the Presence of BC, CP and Its Combinations

Sunflower plant growth and yield performance are influenced by the presence of BC, CP, and their combinations, as shown in Figure 11. Compared to the control, the sunflower dry biomass increased dramatically under the individual and combined effects of BC and CP at different doses. The sunflower yield obtained from soil applied with combined BC + CP was almost equal to the yield derived from soil applied with CF (control). From the dose of SOAs applied, it was observed that BC + CP applied at a medium dose performed noticeably better. This could be due to the provision of NPK by the SOAs to the sunflower crop in a range that is optimal and equal to the recommended NPK in the form of CF. This could also be because of the slow solubilization of NPK in the soil due to the microbial activity of the consortia added to the soil through CP. In soil treated with combined BC + CP at a medium dose, sunflower showed greater disc diameter (13.54 cm) and seed yield (29.64 g), as given in Figure 11c,d. This may be because different organic volatiles deposited on the surface of the biochar caused plants to undergo hormonal changes, leading to early inflorescence and a notable (p < 0.05) increase in seed yield. The activity of microbes rendered by CP in combination with BC has been well documented [64]. However, the soil with CF (control) yielded the highest sunflower dry biomass (32.38 g and 31.42 g). During the current study, at high doses of the SOAs, the sunflower dry biomass decreased, along with a small drop in the seed yield. The high doses of the SOAs might have caused nutrient sorption on the electrostatically active surfaces of the SOAs, which might have compromised sunflower growth. Another potential reason could be an apparently hidden abiotic stress in the soil solution rendered by the salty minerals released by the freshly prepared BC combined with CP. When BC and CP were applied individually, the soil showed better quality; nevertheless, the sunflower seed yield got reduced in comparison to soil with CF (control) but remained better than soil alone (control). Hence, it could be inferred that the combined application of BC and CP at a medium dose could be an effective SOA for improving soil health and yielding sunflower growth and yield comparable to the CF, i.e., the BC and CP derived from the combustible and compostable fractions of MSW have potentially acted as an alternative to the CF for sunflower crop performance.

4. Conclusions

The study concluded that the combined application of BC and CP at medium doses resulted in soil improvement (SOM + 90.7% and CEC + 37.4% compared to the control). The soil health improvement effects of the SOAs were mainly attributed to the significant increases in SOM, CEC, total, and available NPK. The BC + CP at medium dose yielded the highest sunflower dry biomass and seed yield, being 30.8 and 68.9% greater than the control, respectively. The biomass yield in combined SOAs was comparable to the biomass yield in CF, viz., the former being just 4% less than the latter. In the case of the dry biomass of sunflower, the SOAs derived from combustible and compostable fractions of MSW acted as an alternative to the CF. However, seed yield in BC + CP at a medium dose was 28.5% less than in CF. The crop growth performance of sunflower in the presence of SOAs derived from combustible and compostable fractions of MSW being comparable to the CF could create new avenues for developing BC + CF composites in the future. Recycling components from MSW open dumps in developing countries could create avenues for producing SOAs with the benefit of reducing waste stream loads.