Enhancing Sustainable Waste Management Using Biochar: Mitigating the Inhibitory of Food Waste Compost from Methane Fermentation Residue on Komatsuna (Brassica rapa) Yield

Composting is a natural biological process where organic materials such as twigs, grass, flowers, and remains of fruits and vegetables are transformed into humus-rich soil. This serves as a valuable resource for replenishing nutrients and bio elements in the soil [20]. The quality of compost, including factors like reliability, should be evaluated prior to its application in the field. Unstable or immature compost can have a detrimental impact on seed germination, vegetative growth, and the soil habitat due to reduced oxygen stock, limited available nitrogen, or the presence of phytotoxic compounds [21].

A previous study by [4] reported that combining compost from methane fermentation residue with animal manure could enhance the of quality of compost. In this study, we also utilized a combination of compost derived from methane fermentation residue with biochar, clay, and weeds in germination tests and pot experiments, demonstrating its suitability for potted plant growth. One method to evaluate the quality of compost concerning germination is by examining radicle length during germination. Assessing radicle length during the germination process is vital for determining the nutritional efficacy of the provided fertilizer for seed germination. The growth of radicle length during germination provides evidence for identifying potential growth-inhibiting factors in the plant. According to [22], the extension of the radicle through the surrounding embryo structure signifies the completion of germination and the beginning of seedling growth. Consequently, the measurement of radicle length is crucial in this research to ascertain the effectiveness of the germination process.

In the present study, notable disparities in the percentage of Komatsuna seed germination were observed between the use of FW compost, with a germination rate of 40%, and the FW + BC combination, which achieved a significantly higher germination rate of 76% (Figure 3). This variance can be ascribed to the low anion content, particularly chlorine, in FW compost, with FW containing 0.5 g kg⁻¹ of chlorine (Table 2). Underscored the significance of chlorine as an essential micronutrient for plants, positively influencing crop yields and quality when adequately supplied. However, an excess of chlorine can contribute to salinity stress and plant toxicity [23].

Furthermore, FW compost exhibited a high ammonia content, reaching 0.7 g kg⁻¹ (Table 2). Figure 2 presents an analysis illustrating the relationship between germination percentage, germination index rate, and ammonia concentration. Research by [24] on orchid seeds indicated that ammonia content at a concentration of 63.7 mg/L stimulated growth and development, but higher concentrations gradually led to a decrease in germination, with total inhibition observed at 510 mg L⁻¹. Excessive ammonium accumulation beyond the requirements of seed metabolism can be toxic, impeding seed growth and development [25]. Ammonium toxicity results from the disruption of the cell membrane’s charge gradient, allowing ammonia to penetrate the membrane and inhibit photophosphorylation [26]. Ammonium also affects root growth, gravitropism, and auxin transporters in plants [27]. At a concentration of 1 mM, ammonium has been shown to inhibit primary root growth, reducing both elemental expansion and cell production. It also hampers the length of the elongation zone and the maximum elemental expansion rate, while decreasing the apparent length of the meristem and the number of dividing cells without affecting the cell division rate. Additionally, ammonium reduces the number of root cap cells but does not seem to impact the status of the root stem cell niche or the distal auxin maximum at the quiescent center. Furthermore, it inhibits root gravitropism and concomitantly down-regulates the expression of two pivotal auxin transporters, AUX1 and PIN2 [28]. The germination outcomes of compost derived from methane fermentation indicate the presence of growth inhibitors within this compost. This observation correlates with the elevated ammonia content, identified as a growth-inhibiting factor in the compost. Consequently, alternative compost mixing methods show the potential to mitigate these growth-inhibiting factors. This observation aligns with the germination results across all treatments in the study, where consistent improvements in germination, including radicle growth, were observed compared to compost derived solely from methane fermentation. The optimal results observed in the germination test of this study revealed that blending compost produced through methane fermentation with biochar effectively countered the growth inhibitors present in the methane fermentation-derived compost. This indicates that such a combination can balance the nutrient profile of the compost, providing farmers with a more accessible and relatively affordable alternative.

In the Komatsuna growth test conducted in a greenhouse using pots, it was observed that the combination of FW compost with biochar significantly increased Komatsuna yields, as evidenced by the post-harvest yield increase. This improvement in yield is also reflected in Figure 4, which demonstrates that the quality of compost derived from methane fermentation can be enhanced when applied to plants in combination with biochar. Furthermore, nutrient deficiencies in methane-fermented fertilizer can be effectively mitigated when combined with other organic materials [4]. Notably, the more compost combination fertilizers derived from waste and biochar are used, the higher the plant growth rate compared to using FW as a single fertilizer. Additionally, the addition of biochar can reduce the presence of other growth-inhibiting species (inhibitors) in the soil, thereby increasing yields when compared to unamended soil systems [29].

Figure 5 presents soil carbon data, indicating statistical significance within compost, input rate, and biochar factors. However, it’s important to note that no statistically significant interaction was observed among all three factors. Instead, statistical significance was found solely in the interaction between compost and input rate concerning soil carbon levels. As reported by [30], higher biochar application rates lead to a greater increase in total carbon, ranging from 28.9% for rates ≤ 1% to 140% for rates > 5%. Therefore, soils with less than 1% total C before biochar implementation showed a higher percentage enhancement than soils with high initial total C (>2%).

Regarding carbon costs, as shown in Figure 6, a notable interaction between input levels and compost is evident in relation to carbon costs across all treatments. Conversely, the introduction of biochar has a significant influence on carbon costs, albeit in specific aspects of the treatment. Additionally, there is an absence of statistically significant interaction among the three variables: input rate, compost, and biochar. Remarkably, carbon costs exhibit a substantial increase, reaching up to 28 USD per unit in the combination of FW and HM at an input rate of 100 g pot⁻¹.

When it comes to carbon cost analysis, the results mirror the levels of carbon in the soil. Notably, the FW + HM combination at all input rates shows a significant increase in soil carbon levels, leading to considerable carbon costs (Figure 6). In particular, the FW + HM combination at an input rate of 50 g pot⁻¹ and the single FW at 100 g pot⁻¹ both display substantial carbon costs. These costs are notably higher when compared to the FW + HM combination at a lower input rate of 25 g pot⁻¹. However, it’s important to note that no significant interaction was observed in any of the treatments across all input rates.

All treatments applied to Komatsuna plants had a significant impact on the nitrogen content percentage. The highest nitrogen content percentage was observed when using a fertilizer rate of 100 g pot⁻¹. Furthermore, when considering treatments based on the type of fertilizer, the highest yield was achieved with the single FW compost (refer to Table 3). Similarly, in Komatsuna media treated with a fertilizer dosage of 100 g pot⁻¹, the highest nitrogen percentage was achieved, with the treatment based on the type of fertilizer also yielding the highest output, specifically with the single FW compost (refer to Table 3).

The data illustrated in Figure 7 reveal that the maximum uptake of N is achieved when fertilizer is applied at a rate of 100 g pot⁻¹. However, this uptake varies depending on the type of plant. The combination of FW + HM compost, when applied at the same rate, proved to be the most efficient fertilizer in terms of N uptake. Figure 8, on the other hand, demonstrates that the efficiency of NUE, when evaluated based on the fresh weight of the plant, showed optimal results with Komatsuna plants. These plants were treated with a combination of FW and HM compost, but at a lower rate of 25 g pot⁻¹. It’s crucial to understand that the effectiveness of methane-fermented fertilizer is not solely due to its inherent properties. The addition of manure significantly influences its efficacy, which subsequently impacts crop yields. Moreover, the quality of methane-fermented fertilizer can be affected by the type of feed given to livestock, a point further expounded by [4].

This study found that the most effective dose for mixing fertilizer was 100 g pot⁻¹ in all treatments, except for single FW compost. This finding is consistent with the research conducted by [4], which suggests that growth inhibition can occur due to the high N content in both fertilizers. The nutritional composition of compost offers highly favorable nutrition for plants. Elevated levels of nutrients, particularly N, exert a significant influence on plant growth, posing the risk of stunting and impacting nutrient accumulation in the soil. This aligns with findings reported [31], which emphasize the critical importance of the release rate or availability of nitrogen. The evaluation of in situ nitrogen mineralization is proposed as a means to enhance NUE [32]. reported an increase in N input through high N-fixation rest, which resulted in a higher availability of N for the crop and improved NUE. Nitrogen plays a crucial role in plant growth as a structural element. In roots, these elements exist as proteins and enzymes that facilitate the absorption of water and nutrients for plant needs. It’s important to note that an excessive application of N-containing fertilizers does not necessarily lead to increased plant growth. The N content in plants varies across species, but typical concentrations range from 1.5% to 6% of the dry weight of many plants. Optimal values are typically found between 2.5% and 3.5% in leaf tissue [33,34].

Figure 9, which illustrates the cost performance of fertilizer, underscores the impact of biochar on the cost-effectiveness of fertilizer. It’s been observed that the unit cost of commercially available biochar is higher than that of homemade biochar. This difference significantly affects the FCP when using commercial biochar (Figure 9B) as compared to homemade biochar (Figure 9A). The FCP analysis in this research adopts a long-term economic perspective by examining fluctuations in fertilizer costs within a specific area. This includes assessing the quantity of fertilizer input applied to the land, the prevailing unit price of fertilizer in the market, and the corresponding harvest outcomes achieved in each treatment.

Figure 10 presents a comparison of the BC of B/C ratio and economic costs for homemade (Figure 10A) and commercial (Figure 10B) products. In the analysis of economic implications associated with the integration of biochar, a simulation of agricultural conditions is utilized. This comprehensive analysis considers the utilization of both commercial and homemade biochar, taking into account various factors such as costs, revenues, and cash flows involved in farming practices. The calculation of cost factors involves aggregating costs per planting season per hectare unit. This calculation encompasses material costs, daily labor expenses at an hourly rate, and transportation expenses for materials per planting season. There is a significant difference between these two, which is influenced by the profit margins and total costs associated with commercial biochar products. The total cost of commercial biochar itself is influenced by various factors, including material and electrical costs, which contribute to higher variable costs compared to homemade biochar.