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Article

Assessment of Heavy Metal Distribution and Health Risk of Vegetable Crops Grown on Soils Amended with Municipal Solid Waste Compost for Sustainable Urban Agriculture

1
Amity Institute of Environmental Sciences, Amity University Noida, Gautam Budh Nagar 201301, India
2
Department of Botany, Banaras Hindu University, Varanasi 221005, India
3
Amity Institute of Environmental Toxicology, Safety and Management, Amity University Noida, Gautam Budh Nagar 201301, India
4
Academy of Biology and Biotechnology, Southern Federal University, Stachki 194/1, Rostov-on-Don 344090, Russia
5
Department of Integrated Monitoring and Information Technologies, Kalmyk Scientific Center of the Russian Academy of Sciences, Ilishkina, 8, Elista 358000, Russia
6
School of Environmental Sciences, Jawaharlal Nehru University, Delhi 110067, India
7
Department of Chemistry, Applied Science Cluster, School of Engineering, UPES University, Energy Acres, Bidholi Campus, Dehradun 248007, India
8
Department of Research & Development, UPES, Energy Acres, Bidholi Campus, Dehradun 248007, India
*
Author to whom correspondence should be addressed.
Water 2023, 15(2), 228; https://doi.org/10.3390/w15020228
Received: 17 November 2022 / Revised: 17 December 2022 / Accepted: 28 December 2022 / Published: 5 January 2023

Abstract

:
Rapid urbanization is one of the key factors that leads to defragmentation and the shrinking of agricultural land. It further leads to the generation of an ample amount of municipal waste. Several technologies have emerged in the past for its utilization, and in this regard, composting is one of the conventional approaches gaining popularity in modern agriculture. To overcome the possible criticality of intense urbanization, the concept of urban agriculture is taking shape. Municipal solid waste compost (MSWC) has been popularly explored for the soil amendments and nutritional requirements of crops. With this, the assessment of soil pollution (due to the heavy metals presently found in MSWC) is a required step for its safe application in agriculture. The present study aims at assessing the utilization of MSWC (in different ratios) to amend the soil and its impact on the growth and yield of brinjal (Solanum melongena), tomato (Solanum lycopersicum), and okra (Abelmoschus esculentus). The study also explored the uptake of heavy metals by plants and their risk to human consumption. The findings suggested that MSWC amendments upgraded the physio-chemical properties of soil, including organic matter (OM) and micronutrients, and increased the heavy metal concentrations in soil. Heavy metal analysis underlined the presence of several heavy metals both in soil and crops. Total metal concentration in soil increased with increased MSWC dosage. Concerning metal uptake by crop plants, 25% of MSWC was found to impart metal concentrations within permissible values in edible parts of crops. On the contrary, 50%, 75%, and 100% compost showed higher metal concentrations in the crops. A Health Risk Index (HRI) of less than 1 was found to be associated with soil amended with 25% MSWC. Our study implies that MSWC significantly improved the growth and yield of crops, and it can be considered an alternative to chemical fertilizer but only in a safer ratio (≤25%). However, further studies are required, especially on field conditions to validate the findings regarding metal accumulation.

Graphical Abstract

1. Introduction

Among developing nations, fast-growing populations and urbanization are major challenging issues that are leading to a reduction in agricultural land. In such circumstances, urban agriculture has been recognized as a sustainable method of efficiently producing and supplying locally cultivated fresh crops [1]. Fragmented cultivable lands, backyard farming, kitchen gardens, and rooftop farming are the most common approaches for urban agriculture [2]. The popular culture of cash crops in modern agriculture has reduced the practice of crop rotation, and the inevitable use of chemical fertilizers has also created a nutrient imbalance in the soil, leading to a significant loss in soil fertility [3]. To balance the nutritional profile of the soil and to enhance crop production, the use of organic fertilizers is gaining interest. This will also help reduce the dependency of farmers on chemical fertilizers.
Organic fertilizers such as compost promote the practice of organic farming and are rich in total organic content and macro- and micronutrients required for the soil [4]. It improves the soil aggregate formation, which enables efficient drainage and improved soil aeration to resist compaction [5]. It also helps in maintaining the macro- and microporosity of the soil and slows down the release of nutrients, thereby enhancing soil fertility and health, providing adequate plant nutrition, and supporting the productivity of crops [6,7]. Since organic farming practices are evident in improving soil fertility, plant health, and yield and contribute to better human health, it broadens the application of compost, especially in urban agriculture [8]. The core necessity for the usage of compost over agricultural soils is its degree of stability, which indicates the presence of OM.
The enhanced rate of municipal solid waste (MSW) generation due to rapid urbanization is an emerging challenge for municipalities and waste-managing companies. MSW is laden with a variety of OM that comes majorly from household waste and sewage waste [9]. Hence, compost prepared from MSW is rich in total organic content and macro- and micronutrients [10]. The municipal solid waste compost (MSWC) predominantly benefits soils with less OM content or nutrient-deficient soils because it supports the process of nutrient mineralization and regulates the soil pH following the growth of plants [11,12]. Using MSWC in soil amendment also upsurges the microbial dynamics and respiration rates of soil [13,14]. This improved microbial pool enhances enzymatic actions in the soil to convert the nutrients further and intensify the microbial biomass linked with roots symbiotically [15,16]. This way, using MSWC as a soil conditioner can be used in urban agricultural practices for sustained soil fertility.
Urban agriculture holds immense potential in supporting the United Nations Sustainable Development Goals (UN SDGs) as MSWC can deliver double benefits: firstly, a substantial amount of domestic and municipal waste can be utilized and recycled, and secondly, the resulting MSWC can be used as fertilizers in various modes of urban organic farming [17]. The MSW contains a large variety of wastes and hence has a greater chance of carrying toxic metals in it. Several studies have confirmed the presence of heavy metals (HMs) in MSWC, leading to metal pollution in soil. When HMs accumulation in soil surpasses the safe threshold limits, it disturbs the ecological balance and puts human health at risk [18]. The specific characteristics and conditions of the contaminated soil and the exposure to the contamination have a significant impact on the level of risk. Plants or crops grown in contaminated soil tend to accumulate HMs and pose certain health risks to humans [19]. These risks could be carcinogenic or non-carcinogenic [20,21].
As a result, it is important to broaden the pollution assessment and risk assessment of HMs in soil and crops after the application of MSWC [22]. Risk assessment offers a structured process to characterize the nature and extent of the risks associated with contaminants, assisting in identifying the areas affected and the movement of the pollutants due to occurring natural processes. To evaluate the risks associated with any contamination, it is important to consider all the environmental parameters that may be affected, the hydrogeological features of the soil, the mobility of the pollutants, and the exposure levels of receptors [23].
The present study was carried out to assess the impact of MSWC amendment on soil physicochemical properties, morphology, growth, and yield of the crops brinjal (Solanum melongena), tomato (Solanum lycopersicum), and okra (Abelmoschus esculentus). The selection of the crops was made with the assumption that they are popularly used in cuisine and are grown locally in urban/suburban areas. In addition to this, the main objective of the study was to assess and identify the suitable MSWC/soil ratio for its safe application in urban agriculture with the least associated human health risks due to presence of HMs in it.

2. Materials and Methods

2.1. Study Design, Experimental Layout, and Treatments

The study was conducted using pot experiments at the organic farm premises (28°32′38″ N, 77°19′59″ E) of Amity University, Noida, India, for two years from July 2020 to October 2020 and July 2021 to October 2021, with an average temperature and humidity of 28.57 °C ± 0.44 and 74% ± 3.64, respectively. Twenty-five bulk samples of surface soil (0–30 cm) were collected from different locations of organic farms at Amity University, Noida, India, using a shovel. Vegetation and surface debris were removed from a 2 m radius around each location prior to collection.
A total of four treatments were prepared with soil by amending with two different samples of MSWC obtained from two compost plants, namely IL & FS Environmental Infrastructure & Services Ltd. and Delhi MSW solutions Ltd. (RAMKY), located in Delhi, India. The soil was thoroughly mixed, and dead leaves, weeds, and plant trimmings were removed, followed by the addition of MSWCs. Pots in three replicates for each crop were prepared using both the compost samples obtained and added as an organic amendment by 25, 50, 75, and 100% with respect to soil weight (Table 1). The pot soil that did not receive any amendment with MSWC was considered as control.
The seeds were sown in the summer season, i.e., July 2020 and July 2021, in pots of a volume of 8.23 L (diameter 8 inches, height 10 inches) filled with different combinations of soil and MSWC. Before sowing, soil in pots was tilled manually for proper mixing of MSWCs and to loosen up and break the crusted soil and compost. All the seeds were sown uniformly in rows at about 7–8 cm distance by making deep furrows (3–4 seeds in each pot). Additional seeds were sowed where germination was not observed in August (2020 and 2021). Each pot contained four seedlings distributed equally.
After reaching the germination stage, all the seedlings were observed for their growth patterns. Pots were irrigated every morning with tap water. All the pots were maintained properly to observe growth patterns of crops. The growth of S. lycopersicum and A. esculentus plants were monitored for 90–95 days and S. melongena for 115–120 days. Crops were also sprayed with neem oil to prevent any fungal infection in leaves [24]. No significant variations in findings were obtained for both years, which is why results have been pooled together.

2.2. Physiochemical Analysis Soil and MSWC

The collected soil samples were prepared before the treatments and seed sowing. The soil was observed as vertisols (alluvial soil), as per US soil taxonomy, with sandy texture. The soil was dried using a hot-air oven at 40 °C. Organic carbon (OC), nitrogen (N), phosphate (P), potassium (K), and pH were analyzed and recorded. The Walkley and Black chromic acid wet oxidation approach was used to assess OC and soil organic matter (SOM) [25]. The available N was determined using the Kjeldahl method [26], the available P was determined using Bray’s P-1 method [27], and the available K was determined by ammonium acetate extraction method using the flame photometric method [28]. The pH and EC of the all samples were determined using a digital pH and EC meter (Systronics, pH meter 361) by making compost-to-water suspension in 1:2 (w/v) ratio [29]. The water holding capacity (WHC) was determined by comparing the values of water adhered to the soil to the dried weight of the sample. Moisture content was measured using gravimetric method.

2.3. Plant Health and Yield Assessment

Plants were harvested after maturation and cleaned to remove all the dirt and dust particles by washing them with tap water three times and once with distilled water. The shoot and root length measurements were taken using a Vernier caliper and recorded. Plant wet fresh weight (FW) was estimated using a standard analytical weighing balance (TLISMI TS500). The dry weight (DW) of the plant samples was taken by drying them in a hot-air oven (SISCO India) of 220–230 V at 80 °C for 24 h. The growth parameters such as the plant’s height (PH), length, and width (LL and LW), number of leaves per plant (LN), wet biomass (WB), and yield were observed along with visual monitoring to describe the growth and quality of plants. The LN of every plant was counted at 15-day intervals. The LL and LW of plants were observed using a Vernier caliper at equal intervals of time (15 days) and after harvesting the plants, respectively. Crop plants were monitored for their growth performance from the phenological stage, i.e., the germination stage. Shoot height at vegetative to fruiting stages (until 13 weeks’ growth), and the yield was monitored as fresh fruit biomass post-harvesting.

2.4. Total Heavy Metal Analysis of Soil, Treatments, Compost, and Crops

Heavy metal (HM) analysis was performed for soil, treatments, roots, and edible parts of the grown plants. All plant samples were washed using distilled water and dried at 80 °C in a hot-air oven. After drying, samples were ground to a fine powder using a blender. It was sieved with a mesh size of 2 mm. In the digestion tubes containing 0.5 g dry vegetable sample, a tri-acid mixture (15 mL; 70% HNO3, 65% HClO4, and 70% H2SO4 (5:1:1)) was added [30]. The samples were heated to 120 °C until turning colorless. After cooling, the samples were filtered using Whatman No. 42 filter paper and the filtrate was diluted to 50 mL with distilled water (DW). The aqua regia + HF digestion method was used for elemental measurement in the treatments. Following this step, the samples were treated with a mixture (2mL HNO3 + 6 mL HCl + 2 mL HF) [31].
The HM analysis of liquid samples was carried out with the inductively coupled plasma-optical emission spectrometry (ICP-OES) ICP-OES instrument (Analytik Jena PQ 9000) using the HM standard (Inorganic Ventures IV-STOCK-4). For the calibration curve of each metal, initially different concentration standards were analyzed on the instrument. Following a similar procedure, the unknown concentration of the desired metals was analyzed for all samples.

2.5. Computation of Health Risk

The consequences of consuming vegetable crops grown in MSWC-amended soil leading to health risks were assessed in humans using three key international indicators i.e., bioconcentration factor (BCF), daily heavy metal intake (DMI), and Health Risk Index (HRI) as follows.

2.5.1. Estimation of Heavy Metal Bioaccumulation Potential

BCF is the ratio of HMs within the parts of the plant to the soil. For the HMs accumulation within the roots and edible parts of the plant, the calculation was performed using equations 1 and 2 [32].
BCF for roots,
BCF = Cr / Csoil
BCF for the plant’s edible part,
BCF   =   Ce / Csoil
where Cr and Ce represent the concentration of HMs in the roots and edible parts of the crops under study and Csoil represents the concentration of HMs in soil used for the growth of plants.

2.5.2. Daily Heavy Metal Intake (DMI)

Using equation 3, the DMI (mgkg−1 per day) of HMs from vegetable consumption was determined for every treatment.
DMI = DIV C BW
where C stands for the metal concentration present in the edible portion of vegetables in mg kg−1, DIV is the daily vegetable intake, i.e., 0.342 kg d−1 and 0.232 kg d−1 for adults and children, respectively, and BW is the assumed body weight, which is 16.2 kg for children and 70 kg for adults [33].

2.5.3. Health Risk Index (HRI)

The HRI was evaluated to identify the possible threats to human health by consuming HM-contaminated vegetables grown in different MSWC ratios. The HRI was calculated as the ratio of the recommended oral dose to the daily consumption of metals using equation 4.
HRI = DMI ROD
where ROD is the recommended oral dose (mg kg−1 d−1), i.e., 0.02 (Ni), 0.001 (Cd), 0.004 (Pb), 1.5 (Cr), and 0.04 (Cu) [34] (US EPA IRIS, 2011) [35].

2.6. Statistical Analysis

To assess the statistical significance between the control and all treatments, one-way ANOVA (with Duncan’s test) was performed to identify significant differences between the means of different physiological parameters of crops, metal concentration in soil, and metal concentrations in crop parts with a concentration of applied MSWC. The criterion for significance in the procedures was set at p < 0.05 (significant). The data are presented as arithmetic means with standard deviation attached in the form of bar graphs and pie charts. All statistical analyses were conducted using Microsoft Excel 2010 and Statistical Packages for Social Sciences (SPSS) 23.0 IBM version Software. The figures were produced using Origin Version 8.5 software (OriginLab Corporation, Northampton, MA, USA).

3. Results and Discussion

3.1. Physiochemical Properties of Soil Post Amendmending withMSWCs

The physicochemical characteristics of the soil are significantly dependent on the pattern of agricultural practices and amendments [36]. Such parameters also govern the biogeochemical cycling of nutrients and microbial dynamics. Organic manure or compost has plenty of nutrients useful for improving soil health and enriching the soil microbiome. In our study, we observed that soil amended with MSWC significantly lowered the pH of both treatment groups. It reduced with the increasing ratio of MSWC in treatments. The average pH of MSWC-B (T4-B) and MSWC-O (T4-O) was found to be 13.73% lower than the control.
The EC of the treatment groups increased with the increased proportion of MSWCs. The EC of MSWC-O was found to be 19% higher than that of MSWC-B. This also indicated the presence of a higher number of dissolved salts in MSWC. The presence of total soluble salts in the soil is solely responsible for the soil’s EC, which regulates the availability of plant nutrients and controls the microbial activity in the soil [37]. The observed changes in the physiochemical properties of soil post amendments are given in Table 2.
WHC directly influences the growth of crops, and it helps in maintaining the desired humidity in the soil. The addition of MSWC did not affect the WHC of soil up to a significant level. The average WHC was found to be improved by 4.25% and 10.63% in T2 and T3, respectively, compared to the control, whereas the average WHC of MSWCs was approximately 23% less than the control. It is evident that the MSWC is rich in OM-sustaining soil texture and plays a significant role in regulating the WHC [38]. Amendments of soil with MSWC also prevent the leaching of minerals, thereby improving the WHC significantly [39].
Similarly, the SOM content of the soil increased with the increased application rates of MSWC in both the treatment groups, which is evidence that OM is contributed by MSWC. Studies have reported that the application of compost increases the SOM content, and it can be used as an amendment to restore the OM levels in soils [40,41]. The treatment of compost also improves the soil quality indices by reducing soil compaction, increasing soil aggregation, preventing soil erosion, and stabilizing soil structure [42].
C:N is one of the most crucial factors influencing soil behavior, crop productivity, and the mineralization process. Our study reported the MSWC addition significantly increased the C:N ratio of soils in all treatments. With this, the C:N of MSWC-O was found to be nearly 30% higher than MSWC-B.
NPK are the major micronutrients required for the growth of plants. Nitrogen carries out a photosynthetic process with the enzymatic synthesis of proteins, phosphorus aids crop productivity by improving fruit production and root growth, and potassium is essential for retaining and absorbing water in the soil. The available N and K of soil were found to decrease with the increased rate of MSWCs. The average available N in control soil was found to be 25% more than in MSWCs. Control soil was found to be enriched with available K and available N by 17% and 33% more, respectively, than the studied MSWCs. Available K and N content were found more in MSWC-B compared to MSWC-O. The available P in soil was positively affected by the increasing dose of MSWC application, and the trend was followed as T1-B < T1-O < T2-B < T2-O < T3-O < T3-B < MSWC-O < MSWC-B. The available P in MSWC-B was 2.25% more than the MSWC-O. Similar findings have revealed the increased P dynamics of soil after the addition of MSWC [43,44].
The above findings from the study have significantly improved the soil quality in terms of several physiochemical parameters. Earlier studies that used compost as a soil amendment have reported improvements in pH, SOC, NPK, EC etc. [45,46]. A study by Arrobas et al. 2022 also reported a significantly increased level in soil properties (OC, pH, CEC, and extractable K) after amending soil with MSWC [47,48]. Therefore, our findings are also in line with the previous studies and appear convincing.
Table 2. Physicochemical characteristics of experimental soil amended with different proportions of municipal solid waste composts.
Table 2. Physicochemical characteristics of experimental soil amended with different proportions of municipal solid waste composts.
Treatment GroupsControlMSWC-BMSWC-OStandard Values
ParametersT0T1-BT2-BT3-BT4-BT1-OT2-OT3-OT4-OSWMOCQSMIS
pH8.487.457.357.007.057.307.357.497.586.5–7.55.5–8.55–8
EC (ds/m)0.100.2813.004.610.231.302.305.524.0<4≥6
WHC (%)474649532847495144-
Moisture Content (%)1.471.5011.3215.2623.822.3010.4615.9721.8715–2540–5030–60
Total N (%)0.141.121.351.401.400.901.231.391.120.80 0.5–6
OC (%)1.373.438.951014.502.256.5211.2714.6612.0
SOM (%)2.367.0020.002531.905.00234153.27->3040–60
C:N ratio9.789.809.951010.359.8010.2011.5013.08<20<22
Available P (mg kg−1)7.1010.3112026427114.25135256265-
Available K (mg kg−1)175146147154157143146135143-
Available N (mg kg−1)473936363734363534-
Values are the mean of three replicates, Indian Standards MSWC (SWM) 2016 [49]; Ontario Compost quality standards, 2012 [50]; multiple international standards compiled by Monica, 2017 [51].

3.2. Morphological Response of Crops to Treatments

Compost is known to improve plant health by modifying the soil structure and microbial dynamics and enhancing the process of mineral uptake [52]. PH, LL, LW, and LN are visible parameters of plant growth. The findings of both MSWCs treatment groups revealed that the morphological parameters of the crop plants were found to be increased in a dose-dependent manner compared to the controls (Figure 1).

3.2.1. S. melongena

The plant height significantly increased (p < 0.05) by 72% and 54% in the T1-O (28.3 cm) and T2-O (31.6 cm) groups compared to the control (18.3 cm), whereas in the treatment with higher MSWC ratios, it decreased by 1.81% and 14.54% in the T4-B (18 cm) and T4-O (15.6 cm) groups, respectively, compared to the control. LL increased by 20.8% in T1-B (9.2 cm) and decreased by 16.9% and 20.4% in T3-B (6.3 cm) and T4-O (6.1 cm), respectively, compared to control (T0, 7.6 cm). The LW improved up to 41.6% and 43% in T2-B (9.1 cm) and T2-O (9.2 cm), respectively, and was reduced by 18.2% in T4-O (5.2 cm) compared to control, T0 (6.4 cm). The LN significantly (p < 0.05) increased by 90.2% and 112.2% in T2-O (26) and T2-B (26), respectively, compared to the control (13.6), whereas it decreased by 2.4%, 9.7%, and 46.3% in T4-B (13.3), T3-O (12.3), and T4-O (7.3), respectively.

3.2.2. S. lycopersicum

PH increased with the rate of supplied MSWC. Higher MSWC ratios favored the PH which significantly (p < 0.05) increased by 91.7%, 110%, and 80.95% in T3-B (67.1 cm), T4-B (73.7 cm), and T4-O (63.3 cm), respectively, compared to the control. LL was found to increase in T2-B, T3-B (6.3 cm), and T3-O (6.6 cm) by 28.1%, 28.1%, and 34.2%, respectively. The LW was enhanced significantly (p < 0.05) by 17.2% in T4-B (3.6 cm) compared to the control (3.1 cm). It increased continuously with an increase in MSWC ratio except in T3-B (3 cm). In addition to this, LN was 118.8% and 108.8% more in T4-B (197) and T4-O (188) compared to the control (90).

3.2.3. A. esculentus

It was found to follow a similar growth pattern to S. melongena. PH significantly (p < 0.05) increased by 65% and 83.3% in T1-B (49.5 cm) and T2-B (55.4 cm), respectively, and decreased by 3.3% in T4-B (29 cm) compared to the control. The plant was observed dead in a higher MSWC ratio (T4-B and T4-O). The reduction in the growth period of the plant with the increasing MSWC ratio could be due to possible heavy-metal-induced phytotoxicity. LL and LW were observed to improve by 19.2% and 10% in T2-B (8.4 cm) and T2-B (6.1 cm), respectively, compared to the control (7 cm) and were further decreased with an increasing ratio. However, LN increased significantly (p < 0.05) by 73.3% and 86.6% in T1-B (26) and T2-B (28), respectively, compared to control (15) but decreased by 13.3% in T3-O (13) with no occurrence of leaves in T4-B and T4-O.
Similar reports have also claimed to improve the morphology of plants. Razavi et al. revealed improved morphological parameters, including leaf number and leaf surface of pistachio (Pistacia vera L.) seedlings when soil was amended with MSWC [53]. Analogously, 100% MSWC decreased growth, number of leaves, and biomass of A. esculentus [54], which is similar to our study. In addition to this, the negative effect of higher ratios of MSWC application on crops has also been considered. Application of 100% MSWC on pepper seedlings grown in a nursery decreased height, LN, stem diameter, and fresh weight [55]. These findings support the present study, where growth of S. melongena and A. esculentus are restricted at higher MSWC ratios.

3.3. Biomass and Yield

The wet biomass of S. melongena was significantly (p < 0.05) enhanced by 8.3% (384.6 g) and 7.4% (381.6 g) in T2-O and T1-O, respectively, compared to the control (355.3 g). Higher ratios of MSWC decreased the biomass by 3.9% and 2.6% in T4-O (341.3 g) and T3-O (346 g), respectively. Similarly, improved yield in T1-B (31 g) and T1-O (28 g) by 93.7% and 75%, respectively, were observed with a decrease of 60.4 % and 64.5%, respectively, in higher MSWC ratios, i.e., T3-B (6.3 g) and T3-O (10.5 g) compared to control (16 g). With this, T4-B and T4-O did not support the fruit formation.
In S. lycopersicum, WB increased with an increased dose of MSWCs. It was significantly enhanced by 22.5% and 13.4% in T4-O (571 g) and T4-B (528.6 g), respectively, compared to the control (466 g). T2-B and T3-O favored the maximum yield, which increased significantly by 79.5% and 67.5%, respectively, compared to the control (185 g). With this, the yield was significantly (p < 0.05) raised by 62.2% and 35% in T4-B (300 ± 10 g) and T4-O (250 ± 15 g), respectively. This was evident enough to show that S. lycopersicum can grow in 100% MSWC, unlike S. melongena and A. esculentus.
For A. esculentus, the higher proportions of MSWC (T4-B and T4-O) were found not to support biomass growth. However, it improved by 29.8% (484.6 g) and 21.2% (452.3 g), respectively, in T2-B and T2-O, respectively. The yield significantly (p < 0.05) increased by 61.5% and 75% in T2-B (28 g) and T2-O (30.3 g), respectively, compared to the control (17.3 g), whereas it decreased in higher ratios of MSWC, i.e., T3-B and T3-O, and was observed absent at 100% dosage of MSWC, i.e., T4-B and T4-O, indicating that plants could not survive the high levels of MSWC.
Compost is believed to be rich in a variety of nutrients and minerals and to significantly improve plant growth [56]. The reported findings of the present study reveal improved biomass and yield of crops. A similar study on maize revealed that maximum biomass was found with the regular application of MSWC in soil, replacing mineral fertilizers [57]. A recent study on S. lycopersicum reported similar results. It revealed increased yield with the increased rate of MSWC application; 15 t ha−1 MSWC produced the maximum yield (72.7 t ha−1) when compared to the control [58]. As per Frimpong et al., the addition of MSWC enhanced fruit yield, quality, and height of two A. esculentus cultivars [59].
The MSWC markedly increased total biomass and yield at 25% for S. melongena and A. esculentus and 25% and 50% for S. lycopersicum. The improved properties of the soil induced ideal growth and improved nutritional conditions for the crops, which played a significant role in the rise in yields at 25% and 50% as seen in this study. It suggests that compost has a significant influence on the health of the plants in addition to altering the soil’s characteristics, which increases output.
Our study found that the yield increased in accordance with the applied compost doses but declined at larger dosages, indicating that there is a limit to the usage of compost. This scenario might be explicated by the fact that the quantity of fertilizers brought by the MSWC exceeds the desirable requirement of crops. As a result, the decrease in yield obtained might be due to the antagonistic interactions between nutrients [60].

3.4. Available Heavy Metal Content in the Soil after Amendmending with MSWCs

The usage of compost as fertilizer may introduce metals into the agroecosystem and enter the food chain [61]. There are several studies that reported an increased fraction of metals in soil on the application of MSWC [40,62,63].
In the present study, MSWC amendments enhanced the metal concentration in soil (Cr, Cu, Cd, Ni, Pb) (Figure 2). The metal concentration was found positively correlated with the concentration of MSWC added to the soil (Table 3). Control soil was found unpolluted and HMs concentration within SPL (mg/kg) ranged from 35.3 ± 6.8 for Cr, 146.4 ± 6.3 for Cu, 81 ± 3.6 for Ni, 57 ± 3.6 for Pb, 0.00 for Hg, and 0.86 for Cd [64] (Table 4). In addition to this, significant increased metal concentrations in the treatments were found with increasing ratio of MSWCs.
MSWCs in higher ratios were found to be unfit for their usage in agricultural purposes as the HMs concentrations were found to surpass the SPL. The maximum metal concentrations were significantly associated with the highest proportion of MSWCs, i.e., T4. This dose had a significant impact on the concentration of almost all metals. In comparison to the HMs content in the control group, the average percent elevation for the six measured HMs after four different amendments ranged up to a maximum 742% increase for Cd, 396% for Cr, 179% for Cu, 22% for Ni, and 149% for Pb in 100% MSWCs. By contrast, when compared with FCO standards India, MSWCs were found not to meet the standard requirements and HMs exceeded the standards by a maximum of 45.2% for Cd, 250.4% for Cr, 97.2% for Ni, and 42% and 36.1% for Pb and Cu, respectively (Table 5).
The results discussed above clearly indicate the significant increase in metal pollution in soil post amending with higher ratios of MSWCs. Previous studies have reported the enhanced HMs in soil [65,66,67,68]. Cd, Cr, Ni, Zn, and Pb are the most found metals, depending on the amount of HM materials in the waste [68,69]. These metals can persist in the soil for a long time and using metal contaminated MSWC may increase the HMs mobility in the soil [70]. Major sources of these metals in compost could be due to the presence of electronic items, ceramics, paints, plastic items, printed paper, and vegetables and other food materials in the MSW [71].
A study performed by Ashfaq et al. revealed the increased Se concentration in soil with an increase in MSW [72]. It led to increased Se concentrations in the grown vegetables and in the consumers. The trend followed by HMs in treatment groups was observed as Cu > Cr > Pb > Ni > Cd, where Cu was found dominating. A similar trend of HMs distribution has been identified in a recent study by Singh et al. in the fine fraction of MSW collected from a dumpsite in Mumbai, India [73].
Table 3. Correlation matrix between the heavy metal concentration of MSWC and soil.
Table 3. Correlation matrix between the heavy metal concentration of MSWC and soil.
CdCrCuNiPbMSWC Concentration
Cd1
Cr0.900 **1
Cu0.837 **0.874 **1
Ni0.535 **0.637 **0.433 *1
Pb0.908 **0.925 **0.866 **0.537 **1
MSWC concentration0.950 **0.963 **0.894 **0.639 **0.964 **1
**. Correlation is significant at the 0.01 level (2-tailed). *. Correlation is significant at the 0.05 level (2-tailed).
Table 4. Heavy metal contents and standards of control soils and MSWCs used in the experiment (mg kg−1).
Table 4. Heavy metal contents and standards of control soils and MSWCs used in the experiment (mg kg−1).
HMsSoil/MSWCPermissible Limit
SoilMSWC-BMSWC-OPFA aUSEPA bFCO cNEERI d
Cu89.12 ± 6.39398 ± 6.02408.34 ± 6.39137–27056.5300326 ± 42.3
Ni81 ± 3.6893.2 ± 5.8998.64 ± 5.6175–1507250NA
Pb57 ± 3.60142 ± 6.78137.3 ± 2.9250–500200100NA
Cr35.3 ± 6.88165.6 ± 3.48175.2 ± 5.52NANA5040 ± 151.6
Cd0.862 ± 0.017 ± 1.987.26 ± 2.363–60.4852.66–4.86
HgBDLBDLBDLNANANANA
a Awasthi 2000, b USEPA [74] for soil, c FCO standards, d NEERI standards for MSWC; BDL: below detection limit; NA: not available.
Table 5. Percentage showing exceedance in metal concentrations in MSWC when compared to permissible limits of soil and FCO standard values.
Table 5. Percentage showing exceedance in metal concentrations in MSWC when compared to permissible limits of soil and FCO standard values.
Heavy Metals in MSWCExceedance When Compared to Organic Soil (%)Exceedance When Compared to FCO Standards (%)
MSWC-BMSWC-OMSWC-BMSWC-O
Cd7127424045.2
Cr369396231.2250.4
Ni45.6554.12586.497.28
Pb1491414237.3
Cu346.58358.1932.6636.11
HgBDLBDLBDLBDL

3.5. Available Heavy Metal Content of Crops Grown in the Treatments

Crops are known to grow with enhanced productivity and quality when supplied with organic amendments, but there have been reports of elevated uptake of harmful metals in vegetables from soil amended with organic fertilizers [75].
In the present study, enhanced metal concentrations in crops were observed in MSWC-amended soils (Figure 3). Roots bear higher HMs concentrations than aerial parts of plants. Similarly, HMs concentration in roots was observed more than the edible parts in all the treatments. In edible parts of the crops, Cu concentrations were found within SPL at 25% (T1-B and T1-O), stating that Cu accumulation is low in crops at 25% and thereby increased with increased dosages.
Cr concentration exceeded the SPL of the WHO in S. lycopersicum, even at low MSWC ratios (T2), whereas Ni concentrations in S. lycopersicum and A. esculentus were identified below SPV at 25% and 50% (T1, T2). On the other hand, it surpassed the SPL even at low MSWC doses (T2) in the case of S. melongena. Similarly, Pd and Cd concentrations also exceeded the SPL when amended with MSWCs.
The lowest Cu concentration was found in A. esculentus fruit (1.4 mg/kg), while the highest was in S. lycopersicum (523.5 mg/kg). The maximum and minimum Pb concentrations were observed in S. lycopersicum (196.7 mg/kg) and S. melongena (0.25 mg/kg) in the controls. Even the minimum Pb concentration (56.7 mg/kg) of A. esculentus in T1-B was found exceeding the threshold value, revealing high contamination of soil with Pb. For Ni, the minimum concentration was detected in A. esculentus (1.2 mg/kg), while the maximum (123.4 mg/kg) was found in S. melongena roots.
In addition to this, several studies on MSWC application have revealed the increased metal uptake by crops. A similar work on lettuce and ryegrass revealed increased Cu and Cd concentrations with the application rate of MSWC, whereas Pb was observed following the opposite trend [76]. A long-term study by Escobedo et al. also showed the increased metal content in Solanum tubersoum l. when applied MSWC as an organic amendment [77]. He revealed Pb, Cr, and Ni concentrations in tubers increased with the successive addition of MSWC by up to 3–6 mg kg−1, 15–20 mg kg−1, and 12–20 mg kg−1, respectively. Contrastingly, HMs concentration significantly decreased in Faba bean grains when supplied with MSWC [78]. Our study infers similar findings, stating increased metal concentration in soil at higher MSWC dosages.
Generally, metal accumulation in plants is higher in pots compared to fields. It might be attributed to the restricted environment (soil and space) of the pots, where plant roots are more exposed to soil amendments. This could be the reason for the increased metal concentration in the crops. A similar study conducted by Zhivotovsky et al. revealed that willows grown in the pots were found to have accumulated higher amounts of Pb than in those in the fields [79]. Due to fewer visual effects of phytotoxicity in the analyzed crops with elevated HMs concentration (Pb, Cd), HMs can aggravate a possible risk in the food chain due to being bioavailable to humans via consumption.

3.6. Human Health Risks Associated with Consumption of HMs Contaminated Crops

The Health Risk Index, based on BCF and DMI, reveals that the transfer of metals to edible parts of crops does not cause health risks to adults and children. The BCFs of edible parts were found to be higher than those of the roots. The BCFs for roots and edible parts are represented in Figure 4.
In S. melongena and A. esculentus, the BCFs for the edible parts surpassed 1 for each metal at higher MSWC ratios (>25%), indicating excess metal bioaccumulation by crops. Exceptionally, T1-B and T1-O supported BCF < 1 in the edible parts, whereas in S. lycopersicum, the BCFs > 1 were found for every metal at MSWC amendments >50%.
The obtained BCFs < 1 of edible parts revealed that Cr, Cu, and Ni did not accumulate in the edible part to a higher extent. The accumulation of Cr, Cu, Ni, Pd, and Cd is better explicated when BCFs are estimated from metal-available soil. Cd and Pb accumulation in the crop increased at a higher ratio of MSWCs. The BCF for Cd, Cu, and Cr of the edible parts of S. melongena and A. esculentus were found below 1 at 25% MSWC (T1-B and T1-O), whereas with BCFs less than 1, the concentration of metal was exceeded in the S. lycopersicum at 25% MSWC. This resulted in metal contamination of vegetables against the acceptable limits of the WHO [80].
Consumption of the studied crops grown at >25% MSWC have significant negative effects on the DMI of humans and may pose health risks to both children and adults (Supplementary Materials and Figure 5). Due to greater DMI of children, they are exposed to more HMs by consuming vegetables grown at >25% MSWC. Children’s body weight (16.2 kg) compared to adults’ body weight (70 kg) contributes to children having higher DMI levels [81]. Even when crops were within the threshold DMI level, they were still identified to possess health risks. The DMI for Cd was found to surpass the threshold values (0.046), whereas for Cr, the DMI in children exceeded the permissible values for S. melongena and S. lycopersicum at >25% MSWC. The DMI of Cu was under the limits for crops in every treatment, whereas in Pb, it increased beyond the permissible level, and for Cd it was under safer limits at 25% MSWC.
HRI ≥ 1 indicates that local inhabitants are going to bear a concerning health risk. Figure 6 shows the comparison of HRI for crops grown in control and treatment groups. The HRI values in control groups were under the safe limits for both children and adults. HRI values exceeded 1 for Pb, Ni, and Cd for 25% MSWC (T1-B and T1-O), whereas at >25% MSWC, HRI > 1 was obtained for all identified metals. These findings suggest that intake of Ni, Cd, Pb (T-1), and all metals (T-2, T-3, and T-4) due to the consumption of vegetables produced in these treatments can pose adverse health hazards to people.
That is why it is important to identify the appropriate MSWC ratio (>25%) posing no health risk to be used for amendments. An analogous study by Dada et al. reported higher HRI values of kernels grown in the plots amended with 4 t/ha of MSWC, which may cause children to have the danger of non-carcinogenic risk [82]. Inadequate MSWC quantity may lack sufficient microbes or organic matter to help immobilize metals that may cause health risks. Co-composting may decrease the metal bioavailability and reduce the health risk associated with MSWC application [83]. A similar study by Shah et al. revealed that co-composting MSWC by different means reduced the HRI associated with the consumption of carrot and spinach [81,84]. It is therefore critical that farmers consider and balance the amount and constituents of MSWC so that poses no health risks when used as a soil amendment.

4. Conclusions

Due to rapid urbanization and an expanding population, solid waste management is one of the emerging issues. Improper waste management practices are one of the challenging issues which promote environmental degradation. To combat this situation, the government is also struggling to find an efficient alternative or better management plans. Composting has emerged as a successful environment friendly strategy to utilize the organic portion of waste. However, utilizing this can have adverse effects on soil, water, crops, and human health.
Our study infers that the soil amendments made with 25% MSWC have been able to improve the physiochemical properties of soil, health, and the yield of crops (PH, LN, LL, LW, yield, and biomass). Nevertheless, the negative effects of higher MSWC ratios on crops (S. melongena, A. esculentus) causing severe toxicity (death of the plants) is also evident from the findings. The most favorable ratio of MSWC for soil amendments can be 25% in pot-based studies as BCF was found to be less than 1 in the edible parts. Health risks were found associated with metals (Cd, Pd and Ni) on consumption of vegetables grown at 25% MSWC, even within DMI permissible limits.
The current observations and findings suggest the use of 25% MSWC for the soil amendment; however, it is too early to recommend the least toxic amendment for the crops. Therefore, further toxicity studies such as field-based studies and rigorous toxicological assessment pertaining to HM and persistent organic pollutants also need to be assessed for determining the best-suited amendment of soil using MSWC for growing crops within the recommended HRI (<1). Additionally, studies related to the removal efficiency of crops, the nature of the risks, the hazard quotient index, and organic amendments in MSWC will help to validate and acquire optimum results.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w15020228/s1, Table S1: Daily intake of heavy metals via consumption of studied vegetables (mg kg−1 d−1) under different amendments.

Author Contributions

Conceptualization, A.T. and P.B. (Pallavi Bhardwaj); methodology, P.B. (Pallavi Bhardwaj); soft-ware, P.B. (Pallavi Bhardwaj); validation, A.R., R.K.S., S.W. and A.T.; formal analysis, P.B. (Pallavi Bhardwaj); investigation, P.B. (Pallavi Bhardwaj) and A.T.; resources, S.W.; A.T., P.B. (Prakash Bobde), S.S.M., T.M. and V.D.R.; data curation, A.R. and R.K.S.; writing—original draft preparation, P.B. (Pallavi Bhardwaj); writing—review and editing, P.B. (Pallavi Bhardwaj), R.K.S., A.R. and S.W.; visualization P.B. (Pallavi Bhardwaj) and A.T.; supervision, A.T., U.M. and A.C.; project administration, A.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Science and Engineering Research Board, Government of India, grant number “EMR/2017/004448”.

Institutional Review Board Statement

Not Applicable.

Informed Consent Statement

Not Applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.

Acknowledgments

The authors are highly thankful to Amity University, Noida, for allowing us to conduct the study and the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India, New Delhi, India (Grant No. EMR/2017/004488) for providing the financial assistance to perform the same. One of the authors, S.W., is grateful to the Central Instrumentation Centre (CIC) and the department of Research & Development, UPES, for providing analytical support. The authors, A.R., T.M., V.D.R., are very grateful to the Strategic Academic Leadership Program of the Southern Federal University (“Priority 2030”).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Morphological parameters include plant height, leaf length, leaf width, number of leaves, yield, and biomass of crops at the stage of harvesting when exposed to different concentrations of MSWC. Means with different letters are significantly different at p < 0.05. Each bar represents the mean of three replicates, and the error bars represent SD.
Figure 1. Morphological parameters include plant height, leaf length, leaf width, number of leaves, yield, and biomass of crops at the stage of harvesting when exposed to different concentrations of MSWC. Means with different letters are significantly different at p < 0.05. Each bar represents the mean of three replicates, and the error bars represent SD.
Water 15 00228 g001aWater 15 00228 g001b
Figure 2. Available heavy metal contents of the soil samples with increased dosage of MSWC. Means with different letters are significantly different at p < 0.05. Each point represents the mean of three replicates, and the error bars represent the standard deviation.
Figure 2. Available heavy metal contents of the soil samples with increased dosage of MSWC. Means with different letters are significantly different at p < 0.05. Each point represents the mean of three replicates, and the error bars represent the standard deviation.
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Figure 3. Metal concentration (mg/kg dry weight) in roots and fruits of crops under different MSWC concentrations, noted as 0(T0), 1(T1-B), 2(T1-O), 3(T2-B), 4(T2-O), 5(T3-B), 6(T3-O), 7(T4-B), and 8(T4-O). Means with different letters are significantly different at p < 0.05. Each point represents the mean of three replicates, and the error bars represent the standard deviation.
Figure 3. Metal concentration (mg/kg dry weight) in roots and fruits of crops under different MSWC concentrations, noted as 0(T0), 1(T1-B), 2(T1-O), 3(T2-B), 4(T2-O), 5(T3-B), 6(T3-O), 7(T4-B), and 8(T4-O). Means with different letters are significantly different at p < 0.05. Each point represents the mean of three replicates, and the error bars represent the standard deviation.
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Figure 4. BCF of (A) S. melongena, (B) S. lycopersicum, and (C) A. esculentus under different MSWC concentrations, noted as 0(T0), 1(T1-B), 2(T1-O), 3(T2-B), 4(T2-O), 5(T3-B), 6(T3-O), 7(T4-B), and 8(T4-O).
Figure 4. BCF of (A) S. melongena, (B) S. lycopersicum, and (C) A. esculentus under different MSWC concentrations, noted as 0(T0), 1(T1-B), 2(T1-O), 3(T2-B), 4(T2-O), 5(T3-B), 6(T3-O), 7(T4-B), and 8(T4-O).
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Figure 5. Daily intake of heavy metals via consumption of studied vegetables (mg kg− 1 d− 1) under different amendments. a RDA, 1989; b WHO, 1996; c JECFA, 2003; MTDI—maximum tolerable daily intake.
Figure 5. Daily intake of heavy metals via consumption of studied vegetables (mg kg− 1 d− 1) under different amendments. a RDA, 1989; b WHO, 1996; c JECFA, 2003; MTDI—maximum tolerable daily intake.
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Figure 6. HRI values for the crops grown in MSWC amendments.
Figure 6. HRI values for the crops grown in MSWC amendments.
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Table 1. Percentage composition of prepared treatments using two MSWCs collected from different plants.
Table 1. Percentage composition of prepared treatments using two MSWCs collected from different plants.
MSWC (%)
TreatmentsSoil (%)MSWC-BMSWC-O
Treatment GroupsControlT010000
MSWC-BT1-B75250
T2-B50500
T3-B25750
T4-B01000
MSWC-OT1-O75025
T2-O50050
T3-O25075
T4-O00100
MSWC-Bmunicipal solid waste compost Bawana; MSWC-Omunicipal solid waste compost Okhla.
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MDPI and ACS Style

Bhardwaj, P.; Sharma, R.K.; Chauhan, A.; Ranjan, A.; Rajput, V.D.; Minkina, T.; Mandzhieva, S.S.; Mina, U.; Wadhwa, S.; Bobde, P.; et al. Assessment of Heavy Metal Distribution and Health Risk of Vegetable Crops Grown on Soils Amended with Municipal Solid Waste Compost for Sustainable Urban Agriculture. Water 2023, 15, 228. https://doi.org/10.3390/w15020228

AMA Style

Bhardwaj P, Sharma RK, Chauhan A, Ranjan A, Rajput VD, Minkina T, Mandzhieva SS, Mina U, Wadhwa S, Bobde P, et al. Assessment of Heavy Metal Distribution and Health Risk of Vegetable Crops Grown on Soils Amended with Municipal Solid Waste Compost for Sustainable Urban Agriculture. Water. 2023; 15(2):228. https://doi.org/10.3390/w15020228

Chicago/Turabian Style

Bhardwaj, Pallavi, Rajesh Kumar Sharma, Abhishek Chauhan, Anuj Ranjan, Vishnu D. Rajput, Tatiana Minkina, Saglara S. Mandzhieva, Usha Mina, Shikha Wadhwa, Prakash Bobde, and et al. 2023. "Assessment of Heavy Metal Distribution and Health Risk of Vegetable Crops Grown on Soils Amended with Municipal Solid Waste Compost for Sustainable Urban Agriculture" Water 15, no. 2: 228. https://doi.org/10.3390/w15020228

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