## 3 Results and Discussion
### Biodegradable Content  
Regarding the BC, the variation among products is smaller. This is logical, as all the products are bio-based, meaning that their main components are biodegradable. The high BC scores of all the products are presented in Figure 4.  
While in other sectors, such as packaging or textiles, biodegradability must comply with strict standards and regulations to be validated, fertilizers are, by definition, integrated into the soil upon application, becoming part of the so-called humus material.  
The relatively low score obtained by the spent mushroom substrate--although not being significantly different from the others--is caused by the use of gypsum and lime in its manufacturing process. Although being non-persistent and soil-assimilable, gypsum and lime are not considered biodegradable _stricto sensu_, as they are inorganic molecules that are not disintegrated by the action of microorganisms. The BC indicator allows us to identify such material consumptions, prompting us to evaluate alternatives to substitute them with biodegradable materials.  
Figure 3: Results for circular index for the five bio-based products and their average.  
The lowering of the compost score is due to the use of iron hydroxide (Fe(OH)\({}_{3}\)) as a hydrogen sulfide (H\({}_{2}\)S) inhibitor during anaerobic digestion, which is afterwards not removed.  
Nonetheless, given the uncertainty level of BC results (between 3.16% and 8.17%) and their overlapping, no significant differences can be established among the products under study. The comparisons presented above are based on the known characteristics of the manufacturing processes, which are indeed reflected in the results, despite falling within the margin of uncertainty. Uncertainty levels, however, convey good and high data accuracy, according to the scale presented in Table 5.  
The BC results helped to shed light on the biodegradability of materials, emphasizing that even in the manufacturing of bio-based products, certain non-biodegradable inputs may still be required. However, four key limitations must be acknowledged:  
1. Unlike the CI, the BC indicator does not function as a standalone metric; rather, its value lies in complementing other frameworks by highlighting aspects that may otherwise go unnoticed.
2. The degree of uncertainty associated with BC calculations may hinder businesses from identifying clear pathways for improvement or innovation.
3. Material substitution is not always straightforward (e.g., substituting gypsum, lime, or Fe(OH)\({}_{3}\) by bio-based inputs), which points to the existence of a potential \"circularity ceiling\"--a limit beyond which improving BC becomes increasingly complex or unfeasible.
4. Operationalizing the concept of biodegradability can be challenging: in some sectors, such as packaging, it may require formal certification (e.g., EN 13432 [64]), while in others--such as fertilizers--it raises questions regarding materials like gypsum or lime, which are soil-assimilable but not biodegradable stricto sensu.  
Despite these challenges, the BC indicator provides important insights into material flows and degradation pathways, reinforcing the need for nuanced, context-specific assessments of circularity in bio-based value chains. Its contribution goes beyond the mere calculation of an index, as it encourages critical questioning and research during the data sourcing phase, which may reveal opportunities for improvement.  
Figure 4: Results for biodegradable content for the five bio-based products and their average.