## 3 Results and Discussion
### Nutrient Slow-Release Index  
The scarcity of the NSRI data limited the calculation of this indicator only to two fertilizers (Figure 6). For this indicator, the uncertainty ranges are particularly narrow (i.e., 0.53% and 0.60%), which directly reflects the low uncertainty (3%) associated with the original data source. This uncertainty level characterizes the results as having high data accuracy, according to the GHG protocol's rating (Table 5). The results of NSRI allow us to assert that both spent mushroom substrate and turkey manure have significantly longer release times compared to the mineral options. This is a logical outcome, consistent with the values presented in Table 4. For instance, it can be seen that 35 days after the application of turkey manure, 74% of the nutrient remains  
Figure 6: Comparison of the spent mushroom substrate and turkey manure based on the nutrient slow-release index. available, whereas only 1% remains when U-PSf is used. It is also aligned with the scientific findings indicating that one of the main advantages of bio-based fertilizers is precisely the prolonged time in which nutrients are released, limiting nutrient loss and leaching [66; 67]. This prolonged nutrient release behavior can be linked to the circular economy in several aspects, including the following:  
* Reduced leaching implies reduced eutrophication risk, contributing to a good stewardship of the environment. * Reducing nutrient loss promotes a better use of resources, reducing extraction needs. * Prolonging nutrient release also prolongs the time between fertilizer applications, in a certain way extending the \"useful lifetime\" of the applied products and reducing overall fertilizing product consumption and its associated costs. The NSRI complemented the other indicators by incorporating the time-related characteristics of bio-based fertilizers into the assessment. However, this was not without limitations, as the necessary data for the five target products were not available during the course of the research. With the available data, no relevant difference was identified between the two bio-based fertilizers, but the inclusion of the NSRI within CBE assessments may contribute to enlarging the available database and its use for managerial purposes. It is important to note that the NSRI represents a first approximation to incorporating the concept of nutrient release speed as a feature of the circular economy. This indicator does so in a straightforward manner, acknowledging that many other variables influence nutrient behavior in soil (e.g., soil microbiomes, moisture dynamics, etc.). More comprehensive frameworks to monitor these dynamics have been proposed by [PERSON] et al. [68], [PERSON] et al. [69], and [PERSON] et al. [70], offering in-depth soil science approaches that provide a more accurate representation of reality. Soil dynamics are particularly complex to model, requiring extensive mathematical and analytical operation, as well as dedicated data collection tools and laboratory analyses. Moreover, each soil-climate setting may respond differently to the same fertilizer, meaning that calculations for a specific location may not be representative of others. This creates a dilemma between achieving high accuracy through a complex indicator--which may be perceived as burdensome by businesses--and having a straightforward indicator that guides general business strategy but acknowledges it does not capture all soil nuances. Currently, tensions are emerging in the European Union between businesses and regulators due to the approval of strict corporate reporting requirements (i.e., the Corporate Sustainability Reporting Directive (CSRD) and the Corporate Sustainability Due Diligence Directive (CSDDD)), which are now in the process of being simplified by an Omnibus package. In this context, a framework intended for business use should aim to reduce complexity to enhance accessibility. The NSRI contributes by integrating the consideration of nutrient release speed into circular economy management practices.