Greenhouse gas (GHG) emission, especially CO2 and other GHGs increasing, is a matter of worldwide concern among the scientific communities as they are responsible for global climate change. CO2 is one of the most contributing factors of global warming [1]. Since the late nineteenth century, especially from 1850 to 1900, the global mean temperature increased approximately 0.8°C to 1.3°C, with a best estimate of 1.07°C, and the anthropogenic CO2 is the main contributor [2,3]. At present the global atmospheric CO2 reached into 400 ppm which was 277 ppm in the beginning of the industrial era [4]. The Kyoto Protocol to the United Nations Framework Convention on Climate chang (UNFCC)-2010 and other agreements have specified targets to reduce greenhouse gas emissions to atmosphere. In 2009 global leaders reached an agreement at the 15 Conference of Parties (COP 15) to keep the total global mean temperature rise 2°C by 2050 [2,3]. Besides global warming and climate change, crop production is a new challenge to agricultural scientists across the world. Carbon sequestration in land, specifically agricultural lands, can be the effective way of both issues. The lithosphere holds organic carbon which is almost double than the oceanic and atmospheric carbon [5].

Soil and plants collectively absorb 1012 ton C annually which is more than they emit [6]. Soil organic carbon (SOC) is a potential carbon sequestration sink that helps to mitigate climate change and has an impact on soil sustainability [7]. Soil organic matter (SOM) increasing by 5 to 15% at a depth to 2 m can reduce atmospheric CO2 by 16 to 30% [8]. Besides climate change soil carbon sequestration plays a vital role for crop production and maintaining soil health [9]. The incorporation of C enriched materials such as biochar [10], farmyard manure and vermicompost enhance soil C sequestration as well as soil pysico-chemical and biological activities [11,12].

Biochar is an organic amendment, a carbon-rich by-product of organic material via pyrolysis in an oxygen-depleted environment, which has been proposed as a soil ameliorant to improve soil properties and functions [13]. Black carbon, usually called biochar, derived from plant biomass such as woody materials, straw, husks and manure, obtained under high temperature in the absence or presence of the low amounts of oxygen through pyrolysis process [14]. Biochar can affect the metabolic environment of plants by changing the level of available underground nutrients, thereby improving the ability of plants to assimilate CO2 in the atmosphere. Pyrolyzing of organic materials can accumulate 50 % of carbon in its original biomass [15]. Meta-analysis found that biochar application increased crop productivity by an average of 10%, mainly due to soil liming effects and improved soil water retention capacity and soil nutrient availability [16].

The plant height and biomass increased with the application of a biochar-compost mixture, which was attributed to the increase in the total organic carbon and nitrogen of the soil (Scluz et al., 2013). Also, biochar can sequester up to 2.2 billion ton of carbon every year by 2050, and this carbon will remain in the soil for thousands of years [17]. It is estimated that the use of biochar in the crop field can tie up carbon in soils, which has the potential to reduce current global carbon emissions by 10% [18]. Recent reviews have highlighted the benefits of adding biochar to agricultural soils including improved soil water-holding capacity, enhanced microbial community structure, increased crop production, mitigated greenhouse gas emissions, and the promotion of carbon sequestration [16,19]. However, due to the low level of nutrient content and microbial activity in biochar, vermicompost could also be used as a complementary material.

In recent years, numerous studies have shown that vermicompost amendments can promote soil quality and productivity by improving the structure and chemical properties of the soil, increasing the amount of plant-available nutrients, promoting soil biological activities, and enhancing crop yield [20]. Vermicompost is a product of degrading organic matter through interaction between earthworms and microorganisms [21].Vermicompost enhances decomposition of organic matter and disintegrate the physioco-chemical properties of them [22]. Vermicompost showed the highest amounts of carbon sequestration (29%) than cow-dung, poultry manure and rice residues [23]. However, the excessive use of vermicompost might stimulate nitrogen (N) loss and global warming [24]. Excessive use of vermicompost and organic fertilizers stimulates N loss and global warming through denitrification process where N contents convert to N2O [25]. Nitrogen can also be lost through leaching of nitrate and runoff to groundwater, surface water [26].

Besides biochar and vermicompost, cow-dung is another effective organic amendment for carbon sequestration and crop production. Cow dung is an organic fertilizer derived from the discharge of cow waste [27]. In other words, manure can change soil capability, which becomes the factor that regulates soil fertility [28]. Cattle manures consist of liquid and fibrous materials which contain cellulose, lignin, hemicellulose, and different minerals. These make it a perfect nutrient rich material that can enhance crop production. Additionally, it can be the alternative to chemical fertilizers [29] Cow-dung is an effective component which can sequester 22% of C [23]. Cattle manures enhance hydraulic infiltration, soil quality and structure, and organic matter contents [30]. Although, the consequences of the impact of cattle manures may not be immediate, a consecutive application for long period of time enhances soil quality and crop growth [31]. So, the use of composted and processed cow-dung is more effective than the raw or fresh cow-dung [32]. The other component, sawdust, is a carbon rich material originated from sawmills as a byproduct of wood processing and carpentry workshop [32]. Although sawdust contains little amounts of nutrient essentials for microbial decomposition, high lignin, cellulose, and pectin make it perfect for composting [32]. However, N may be available to occur microbial decomposition in sawdust [33]. Initially, microbes may immobilize N but whenever complete decomposition happens nutrients become available to crops. Saw dust is used as a soil amendment, mulch, and litter and maintains balance between C to N ratio in composting process and also supports by capturing moisture [34].

Organic amendments are widely used in composting process, especially cow-dung. So, cow-dung often in combination with sawdust is the important sources of C [35]. Composting is a sustainable way to convert these materials into nutrient rich amendments for crop growth [36]. In other words biochar could also be an important material to sustainable maintenance, soil organic content, preserving physical nature of the soil, improving soil fertility and crop yield [37].

In this current study, Red Amaranth was selected because it is one of the widely used food crops in Bangladesh, it can easily withstand severe heat, drought, diseases and pest, and moreover it is a highly nutritious crop [38]. Besides Bangladesh, amaranths species have also been cultivated as the widely used food crops in many countries including India, Africa, Nepal, China, South Specific Islands, Caribbean [39]. Amaranth is a fast growing plant which grows rapidly in different agroclimatic conditions under a variety of soils of tropical and sub-tropical regions [39]. Therefore, the aims of the study are to find out the effect of locally available materials such as cow-dung compost, cow-dung and sawdust made biochar as well as vermicompost for the mitigation of CO2 by sequestering organic C in soil and soil productivity for crop production. Additionally, the study aimed to understand the effectiveness of combined application of cow-dung, vermicompost and biochar as soil amendment on plant growth. This study explores the prospects of using these agents to enhance soil organic matter, focusing on their impact on key parameters such as pH, electrical conductivity (EC), and organic carbon. Understanding the mechanisms through which these agents influence soil properties can inform best practices for composting, vermicomposting and biochar operations and contribute to more crop production.

Study Area

The agricultural lands of Tangail District, were selected for the study area (Fig. 1). Tangail District is located from 24°10' and 24°22' north latitudes to 88°46' and 89°59' east longitudes [40]. Tangail has a large area of agricultural land. The soil in this area is sandy loamy, and it is an alluvial land of Yamuna (Brahmaputra) riverine system [41,42]. The sampling points were adjacent to the Dhaleshwari River Basin (A Tributary of the Zamuna River). Agriculture is the main livelihood of Tangail District, and almost 49.53% of the people live on agriculture [43,44]. The experiment was conducted at Soil and Organic Waste Management Lab of Mawlana Bhashani Science and Technology University (MBSTU).

Figure 1: Map Showing The Location Of The Soil Samples Collected From The Study Area In Tangail.

Soil Sample Collection and Experimental Design

Following the instruction from Adekiya et al [37], soil (0-15 cm) was collected for pot experiments from agricultural land (Fig.1). Each of the containers was filled up with 5 Kg of soil which was prepared homogenously. For this experiment, sixteen different treatments of vermicompost, biochar and cow-dung, individually or in combination with each other, including control were used. All the experiments were conducted in triplicate and Completely Randomized Block Designed. Thus, a total of forty-eight pots were used for the experiment based on the number of different treatments. The experimental pots were labeled with treatment ID and rate of amendments. Here, T0 = Control (No treatment), T1 = Vermicompost (1 ton/ha), T2 = Vermicompost (5 ton/ha), T3 = Vermicompost (10 ton/ha), T4 = Biochar (1 ton/ha), T5 = Biochar (5 ton/ha), T6 = Biochar (10 ton/ha), T7 = Cow-dung (1 ton/ha), T8 = Cow-dung (5 ton/ha), T9 = Cow-dung (10 ton/ha), T10= Vermicompost + Biochar (1 ton/ha), T11 = vermicompost + Biochar (5 ton/ha), T12 = Vermicompost + Biochar (10 ton/ha), T13 = Cow-dung + Biochar (1 ton/ha), T14 = Cow-dung + Biochar (5 ton/ha), T15 = Cow-dung + Biochar (10 ton/ha).

Preparation of the Treatments

Biochar was prepared in an electrical furnace in the absence of oxygen at 400°C for 4 hours and then after cooling down the biochar was crashed and sieved through 2 mm Nylon sieve according as [37]. The main ingredients of the biochar were saw dust and cow dung (1:1). Vermicompost was prepared with traditional methods using worm and organic waste along with cow dung. Firstly, sawdust were composted for months with available moisture in a suitable bed and then dried cow dung was added at 1:1 ratio, 14 days later 20 healthy adults Eisenia fetida earthworms were added on the bed for each 2 kg of mixed cow-dung and sawdust and then the bed was kept for 45 days in adequate humid condition (70 to 80 %) in a regular monitoring according to [45].

Cultivation of Red Amaranth (Amaranthus Cruentus) and Plant Harvesting

Fresh and good quality red amaranth (Amaranthus cruentus) seeds were sowed on 10th January 2022. The red amaranth planted in experimental pots were harvested at maturity on 28th February 2022. At the time of harvesting, plants height of each pot was measured by measuring scale. The plant samples were then washed in fresh tap water to wipe out dust, dirt, possible parasites or their eggs and then washed again with deionized water. After harvesting, sample pots with soil remained stable for 3 months to determine soil carbon sequestration in a net house of the university.

Sample Analysis

Soil samples were collected from the homogeneous soil mixture from different experimental pots. Then soil samples were dried at room temperature for 10 days. The dried samples were grained and sieved through a 2 mm mesh sized sieve [46].To analyze the pH and electrical conductivity (EC), 20g of dried powdered homogenous soil added with 50 ml distilled water 1:2.5 (W/V) ratio, then stirring for 30 minutes by automatic stirrer machine and then pH and EC were determined by using HANNA pH and EC meter [46]. Total Organic Carbon (TOC) of soil was analyzed by using Walkley and Black Oxidation Method.

Determination of Soil Carbon Sequestration and CO₂ Potential

Soil Organic Carbon (SOC) Stock in Soil (Ton/Ha)

Total SOC was estimated by using the following formula, as described by [47] and [48] and the amounts were converted into ton per hectare.

SOC= Organic carbon content % × soil bulk density (gm/cm³) × thickness of horizon (cm)

Soil Carbon Sequestration (Ton/Ha Per Year)

The amount of carbon stored during the experiment is calculated using the following formula [72].

Sequestered/Stored C (ton/ha) =

Where, Ce = Carbon stock at the end of the experiment t/ha, Ci= Carbon stock at the initial point of the experiment ton/ha, t = year of the experiment.

CO₂ Mitigation Potential

CO₂ mitigation potential is calculated using the formula [73].

Molecular weight of CO₂, Ma = 44

Atomic weight of C, Mb = 12

CO₂ potential =  = 3.67

CO₂ mitigation potential ? Sequestration ×3.67

Cost Benefit Analysis (CBA)

The benefit-cost analysis was conducted to calculate the gross and net returns to assess the economic feasibility of various soil treatments through the following methods Eqn. (2) and (3) [49].

Here, NR = Net Return (BDT/ha), GR = Gross Return (BDT/ha), CP = Cost of Production (BDT/ha)

Where BDT indicates Bangladeshi taka (1 BDT ≈ 0.012 USD). And CBR = Cost Benefit Ratio.

Statistical Analysis

The statistical analysis of data was carried out using Microsoft Office Excel 2013 software, which was also used to preserve the data. Soil samples data were analyzed by one-way ANOVA further Tukey’s post hoc tests and least significant difference (LSD) test at the 5% level were done by Origin Pro 2025. A significant level was set at p ≤ 0.05. Python code has been applied to determine linear relationship between treatment rates and sequestration as well as their yield. For counting bacterial colony data were log-transformed to log CFU to normalize the data.

Soil pH

The pH value of the control soil was 8.5, which indicates a moderately alkaline soil. The pH range of all treated soil was between 8.1to 8.9 (Table1).The highest pH value was found in mixed biochar and cow-dung 10 ton/ha. All the treated soil with higher rate application slightly increased the pH value compared to the control. Naramabuye and Haynes [50] found that soil pH was increased by the addition of biochar and vermicompost, and the effect was generally greater at the higher rate of addition which is similar to our study. Tian et al. [51] reported that the application of biochar increases soil pH, but its effect depends on the biochar pH and its liming value; the same effect was observed in this experiment, where pH was raised after the application of biochar. It might be happen due to liming effect and the pH of the biochar itself. Pearson’s correlation coefficients showed statistically significant positive correlation between plant height and plant biomass with pH where p < 0.05.

Electrical Conductivity

The EC value of the control soil was found 0.3 mS/cm (Table 1). The application of amendments showed the changes in the EC. Vermicompost with higher doses reduced the soil EC. But the application of biochar at higher rates increased the soil EC. The highest value of EC (0.77 mS/cm) was found in biochar 10 ton/ha (Table 1). Nawaz et al. [52] reported that the application of biochar increases the soil EC. The higher dose of char gradually raises the soil EC. Application of cow dung alone or mixed with biochar at a rate of 1 ton/ha reduced the value of EC in soil compared to the control. But with the increasing rates of treatments 5 ton/ha and 10 ton/ha increased the EC. The application of vermicompost, cow-dung and vermicompost mixed with biochar decreased the soil EC. Previous studies indicated that cow-dung manure reduces the soil EC at a lower rate, but higher doses increase the soil EC [53]. Application of vermicompost decreased the soil EC [54]. Pearson’s correlation coefficients showed statistically significant negative correlation between plant height and plant biomass with EC where p < 0.05. EC on vermicompost relies on feedstock materials applied on soil [55]. The decreasing rate of EC might be stabilization of raw materials [56].

Table 1: Physico-Chemical Properties of Treated Soil and Control.

Total Soil Organic Carbon, Its Carbon Stock and Soil Organic Matter

Soil organic carbon (SOC) can be used for assessing soil quality; it indicates the level of productivity of a given soil. Thus, large amounts of SOC will enhance water retention, prevent leaching of soluble nutrients, serve as source of sink for nutrients and maintain soil fertility [57]. The highest value of total organic carbon was recorded in biochar 10 ton/ha. Application of vermicompost and biochar alone increased up to 56.9% TOC value than the control (Table 1). Manivannan et al. [58] also found that organic carbon had been enhanced in vermicompost-treated soil and a combination of vermicompost mixed with other amended treated soil. Vermicompost provides carbon-rich organic matter, such as humic and fulvic compounds, which undergo partial stabilization through earthworm digestion, with a portion converting into humified forms that are stored as soil organic carbon (SOC) [59]. Soil organic matter (SOM) can be found from organic carbon multiplying with 1.724 [46]. Table 1 shows the SOM variation for different treatments. Biochar mixed with vermicompost and cow-dung showed a significant increase of SOM. Islam and Weil [60] found that at least 2% organic matter should be present in the soil for crop production. But in Bangladesh most of the soil contains organic carbon 1 to 1.7% [61]. The low soil organic matter in Bangladesh is driven by rapid decomposition, continuous removal or burning of crop residues, intensive tillage practices, soil erosion, overgrazing, and the limited use of compost or green manures [62]. A significant difference was found in SOM levels among the treatments where biochar and vermicompost showed the significant difference (p < 0.05). All the treatments increased SOC stock in the treated soil. Our study showed positive results for all the treatments. The higher the rate of treatment was applied, the higher SOC was found. Biochar was observed as having the best output for every application rate 28.02 ton /ha, 56.27 ton/ha and 56.93 ton/ha SOC were calculated for biochar 1 ton/ha (T4), 5 ton/ha (T5) and 10 ton/ha (T6) application, respectively (Table 1). Biochar mixed with vermicompost as well as biochar mixed with cow dung also increased the SOC in soil (Figure 2). Biochar with cow dung at rate of 10 ton/ha (T15) increased 53.25% SOC compared to the control. Vermicompost at 1 t/ha (T1) stocked 19.44 ton SOC, vermicompost at 5 ton/ha (T2) stocked 24 ton and vermicompost at 10 ton/ha (T3) holds 25.97 ton SOC in the experimental soil. Vermicompost supplies carbon-rich inputs to the soil that leads to make abundance of stable organic matter, humic substances, and microbial residues, directly contributes carbon-rich material to the soil, thereby enhancing SOC [63]. A significant difference was found in SOC of the different treatments where biochar and vermicompost showed significant difference (p < 0.05).

                       Figure 2: Treatment Rate.

Microbial Load in Soil

The result of the study showed that the concentration of total bacterial load increased at every treatment as compared to the control treatment. Vermicompost 10 t/ha showed the highest (7.23 cfu/gm) microbial count in all treatments (Table 1). Biochar + vermicompost 10 ton/ha was observed similar value close to vermicompost 10 ton/ha. It denotes that vermicompost with a higher rate alone or combined with biochar places favor for microbial growth. Pathma and Sakthivel [64] found that vermicompost enhances microbial growth because it contains essential micro and macro elements, along with vitamins, enzymes and hormones. Application of biochar and cow-dung alone or in combination with each other increased the microbial growth. Biochar 10 ton/ha addressed 7.02 cfu/g bacterial load (Table 1). Cow dung 10 ton/ha also showed the same value. Wang et al. [65] reported that the biochar with amendment seems to have a higher impact on microbial diversity in nutrient-poor soil than in nutrient-rich soil. Microbial load showed a strong positive correlation with TOC and SOM (p < 0.05). There was no significant difference found between the different treatments of microbial load (p > 0.05).

Soil Carbon Sequestration and CO₂ Mitigation Potential

This study depicted the Soil C sequestration for different treatments. The highest C sequestration was found 73.4 ton/h/yr in biochar 10 ton/ha (Table 1). It can be predicted from this study that increasing amount of biochar treated in soil increased the C sequestration. According to Mao et al., [66] it might be easily happened to sequestrate C in soil treated with biochar. Biochar and vermicompost enhance the microbial activity in soil to decompose organic matter as a result a large portion of C sequestrated with colloidal particles in soil [64,67]. Cowdung also increased soil C sequestration compared to the control. The higher the application rate, the higher C sequestration was found for the cow-dung alone. But there was no significant (p > 0.05) change in C sequestration for the combined treatment with biochar with vermicompost and cow-dung. This might cause the decomposition of C and respiration of microbes as CO₂ emission [68]. The maximum mitigation potential was found in biochar 10 ton/ha which reduced to 269.5 ton/ha CO₂ per year. Vermicompost could mitigate 22.2 ton/ha/year, 41 ton/ha/year and 48.3 ton/ha/year CO₂ for the application of 1 ton/ha, 5 ton/ha, and 10 ton/ha, respectively (Table 1). Vermicompost and cow-dung, alone or mixed with biochar increased CO₂ mitigation potential. Cowdung 10 ton/ha increased 55.8% mitigation potential of CO₂ (Table 1). A significant difference was found in SOC sequestration of the different treatments where biochar and vermicompost showed the significant difference (P < 0.05).

Plant Height

Height of plants varied from 10.7 cm to 23.7 cm and varied on treatments like biochar, vermicompost and cow-dung, alone or combined application (Fig. 3). The minimum plant height was always obtained in the control treatment and the maximum values were obtained with cow-dung, vermicompost and also mixed with biochar at comparative amount (Fig. 3). The height of red amaranth plants increased significantly (p < 0.05) in experimental pots treated with cow-dung and vermicompost, alone or in combination with each other or biochar, as compared to control treatment (Figure 3). Cow-dung alone or combined with biochar at 10 ton/ha showed the highest plant height than other treatments. Cow-dung mixed with biochar at the rate of 5 ton/ha and 10 ton/ha increased 110.9 and 110.6 % plant height than the control, respectively. It was observed that the application of biochar with the rate of 1 ton/ha, 5 ton/ha and 10 ton/ha increased the height of red amaranth by 1.7%, 18.3% and 34.6% accordingly over the control treatment. Vermicompost 10 ton/ha increased 108.4% plant height while vermicompost 1 ton/ha and 5 ton/ha increased 13.6% and 94.4% plant growth over control, respectively. From this study, it was observed that better plant height could be found using biochar at 5 ton/ha with 10 ton/ha mixed with cow dung and vermicompost alone or combined. Carter et al [69] reported that lettuce height increased by 63% after using rice-husk-derived biochar at 50 g/kg. There was no significant difference of plants height among the treatments (p > 0.05).

Figure 3: Average Height of Red Amaranth in Experiment for Control and Treated Soil.

Biomass

The study showed the application of biochar, vermicompost and cow-dung individually or in combination with biochar increased both the fresh weight and dry weight biomass (above ground) of red amaranth in experimental pots when compared with control treatment (Fig. 4). The fresh weight biomass was increased by 20, 30, 75% compared to the control in biochar 1 ton/ha, biochar 5 ton/ha and biochar 10 ton/ha, respectively. Vermicompost alone at the rates of 1 ton/ha, 5 ton/ha and 10 ton/ha showed 42.8%, 128.4%, 188.4% increased of biomass compared to the control, respectively. The biomass red amaranth with the application of cow-dung mixed with biochar 10 ton/ha was 193.7% higher than the control and application of cow-dung alone increased 121.9% biomass compared to the control. Cow-dung as well as cow-dung mixed with vermicompost 1 ton/ha were observed in reduction of biomass about 3.7 and 22.8%, respectively compared with control. It might be caused due to lower amount of cow-dung application and the decomposition of organic matter which may cause scarcity of available nutrients in soil. Hamzah and Shuhaimi [70] reported that very lower and very higher rate of vermicompost application to the soil reduced the heights of oil palm and rubber seedlings due to nutrient deficiency resulted from increasing soil pH. There was no significant difference of plants biomass among the treatments (p > 0.05).

Figure 4: Biomass Weight of Plant (FW= Fresh Weight, DW= Dry Weight).

Total Yield

Fresh weight biomass data are used for the calculation of Red Amaranth yields as ton per hectare, that’s why the value for yields is similar to the value of fresh weight biomass. The result of the research addressed that the application of biochar in soil increased the total yields of red amaranth. The highest 8.9 ton/ha yield was observed in mixed biochar and cow-dung 10 ton/ha. The highest yield was increased by 193.7% in cow-dung + biochar 10 ton/ha as compared to control treatment. Most of the studies showed that the addition of biochar to soil increased crop yields. Chan et al. [71] reported that addition of green waste-derived biochar to soil increased crop yield and improved soil quality. A similar result was found in this research that the addition of vermicompost and cow-dung enhanced red amaranth yield.

Figure 5: Total Yield of Red Amaranth at Different Treatments.

Cost Benefit Analysis

The results of CBA presented under total yield, cost of production, gross return, net return, and cost-benefit ratio (CBR) for the production of Red Amaranth is shown in (Table 2). According to CBR in Table 2, cow-dung + biochar 5 ton/ha treatment showed the highest CBR (7.27) compared to all other treatments. It implies that this treatment will be beneficial for farmers considering the cost of agricultural production and yield. Cow-dung + biochar 10 t/ha treatment’s CBR was the second-best (6.48) which indicates that the use of 5 ton/ha cow-dung and biochar was sufficient to supply essential nutrients for better growth of plants and addition of cow-dung and biochar is excessive cost for plant growth. Among the treatments the worst CBR was found in T10 cow-dung + biochar 1 ton/ha which denotes that application of lower amount of cow-dung and biochar cannot supply proper permissible nutrient for plant growth. The remarkable net benefit returns were noted 104600, 131400, 145000, 100600, 129600, 152800, 155600 in vermicompost 5 ton/ha and 10 ton/ha, cow-dung 5 ton/ha and 10 ton/ha, cow-dung mixed with vermicompost 10 ton/ha, cow-dung mixed with biochar 5 ton/ha and 10 ton/ha respectively. Mamon et al. [49] observed similar result with Red Amaranth cultivation by applying chemical fertilizers.

Table 2: Cost-Benefit Analysis (CBA) Of Red Amaranth Production under Various Treatments.

Linear Regression and Pearson’s Correlation of Different Treatment Rates with Sequestration and Plant Yield

Linear regression and correlation between different treatment rates and their sequestration potential (Fig. 6) showed that cow-dung has Pearson correlation coefficient (r), significant linear relationship (P) and coefficient of determination (R²) values were 1, 0.03, and 1, respectively that means cow-dung maintained a significant relationship between treatment rates and sequestration. Followed by, mixed vermicompost and biochar showed almost similar result. Vermicompost alone showed R² = 0.90 in linear relationship with treatment rates (Fig 7.a). In terms of treatment rates and plant yields there were no significant correlations between them but Vermicompost, Biochar and Vermicompost + Biochar showed a significant linear relationship with the treatment rates where R² values greater than 0.9 (Fig. 7).

Figure 6: Correlation between Different Treatment Rates and Their Sequestration Potential of Vermicompost (A), Biochar (B), Cowdung (C), Vermicompost + Biochar (D), and Cowdung+ Biochar (E).

Figure 7: Correlation between Different Treatment Rates and Their Yields of Vermicompost (A), Biochar (B), Cowdung (C), Vermicompost + Biochar (D), and Cowdung+ Biochar (E).

Correlation among Parameters and Linear Regression

Pearson’s correlation analysis was executed to define the relationships among pH, EC, SOM, Microbial count, Plant growth and Total yield (Fig. 8).

Figure 8: Pearson’s Correlation Matrix among Plant Growth Parameters, C Sequestration and Physico-Chemical Parameters. Here, ** Indicates The Correlation Is Significant At The 0.01 Level (2-Tailed) And * Represents The Correlation Is Significant At The 0.05 Level (2-Tailed).

The current research exhibited that the application of biochar, vermicompost, and cow dung, individually or in combination, significantly improved soil properties, enhanced carbon sequestration, and promoted red amaranth growth and yield. Biochar at 10 ton/ha demonstrated the highest soil organic carbon (SOC) stock (32.2 ton/ha), carbon sequestration (73.4 ton/ha/yr), and CO₂ mitigation (269.5 ton/ha/yr), indicating its excellent carbon sequestration potential. Vermicompost and cow dung also marked up soil fertility by improving total organic carbon (TOC), soil organic matter (SOM), and microbial activity, which were positively correlated with plant height and biomass (p < 0.05). Combined treatments, particularly cow dung + biochar at 5 -10 ton/ha, acquired noteworthy boosting in plant height (over 110%), total biomass (193.7%), and total yield (8.98 t/ha), while retaining conducive pH and EC levels. Cost-benefit analysis (CBR) demonstrated that cow dung + biochar (5 ton/ha) provided the highest return (CBR = 7.27), keeping it up the most economically favorable. Correlation and regression analyses ensured strong positive relationships between amendment rates, SOC sequestration, and crop yield (R² > 0.9). Above all, the results suggest that biochar-based organic amendments, especially in combination with cow dung or vermicompost, can sufficiently enrich soil carbon storage, improve soil health, and enhance agricultural productivity, proposing a eco-friendly and climate-resilient soil management strategy.

Author(S) Statement

Mahmudul Hasan, Shamim Al Mamun, Md. Ariful Islam and Hiroshi Asakura conceptualized and designed the study. Mahmudul Hasan, Misbahul Alam, Mohammad Mahdi Hasan, and Asikha Zannat Rathna performed laboratory Analysis. Mahmudul Hasan and Misbahul Alam conducted the statistical analyses as well as generated figures, graphs, charts, and wrote the first draft of the manuscript. Md. Emran Hossain, Al Mahin and Saleh Mahmud helped in statistical analysis, generating graphs, figures and charts and result discussion parts. Shamim Al Mamun, Md. Ariful Islam and Md. Samrat Hossain revised and finalized the draft of the manuscript. All authors reviewed the results and approved the final version of the manuscripts.

Though the research grants significant vision into the influence of biochar, vermicompost, and cow dung on soil carbon sequestration and plant growth, various constraints should be recognized. At first, the study was conducted in a managed pot environment that may not fully portray the actual representation of field agricultural conditions where some important agricultural factors such as rainfall, temperature and biological activities can considerably dominate outcomes. Secondly, the study period was relatively limited and may not depict long-run carbon sequestration and soil important chemical properties such as nutrient cycle, mainly carbon sequestration and biochar decaying efficiency require a long-term or period of years of observations. Also, there was only one crop species (red amaranth) used which may not depict the nutrient demands and root behaviors of other species. Besides these, the research did not incorporate essential greenhouse gases emission measurement such as CH2 and N2O for a comprehensive evaluation of climate mitigation benefits. Above all, the analyzed materials and pyrolysis provisions of biochar and compost materials were not substantially analyzed, which may affect the results.

Future Pathways

The results of this study can be expanded upon in a number of significant ways in future research. Firstly, validating sustainability, carbon persistence, and crop-specific responsiveness to organic amendments under natural environmental circumstances will be aided by carrying out long-term field trials across various agro-ecological zones. Additional studies should include direct measurements of GHG emissions (CO2, CH2, N₂O) to increase the assessment of climate mitigation potential. It would be more practically applicable to look at how these changes affect other commonly grown crops in Bangladesh, like rice, wheat, and vegetables. Additionally, socioeconomic evaluations of farmer adoption patterns, the cost-effectiveness of organic amendments at various scales, and the viability of the supply chain would aid in directing extension and policy efforts. Overall, further study should focus on incorporating organic amendments into Bangladeshi agricultural systems that are resource-efficient and climate-smart.

Financial Statement

Author(s) did not receive any additional funding from any authority. The costs and resources that were used in this study did not receive any additional funds. Author(s) associated in this study conveyed the costs themselves.

Acknowledgement

The authors express sincere gratitude to the officials and teachers of the Department of Environmental Science and Resource Management of Mawlana Bhashani Science and Technology University (Tangail, Bangladesh) for their valuable cooperation and technical support. The authors are also grateful to all the anonymous reviewers for providing their valuable comments and suggestions to improve the quality of this manuscript.

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