## 3 Results and Discussion
### Environmental and Agronomic Evaluation  
This study showed that all three BBFs are safe to use in potato growing from an agronomic and environmental perspective (Figure 8). Possibly, the effect can be influenced more strongly by soil type, e.g., soil pH, rather than by quantity and quality of the applied BBF ([PERSON] et al., 2018). Summarized, after soil application, AS has a relatively low N\({}_{2}\)O emission factor. It has highest N release rate and NFRV, and also in the field trial the N concentration in  
Figure 7: Amount of nitrate residue (kg ha\({}^{-1}\) in 0–90 cm soil depth) in post-harvest period for five treatments (MF + M = CAN + manure, LFD + M = liquid fraction of digestate and manure, AS + M = ammonium sulphate solution and manure, only LFD = standalone treatment from liquid fraction of digestate, and only AS = standalone treatment from ammonium sulphate solution). Red dotted line represents the legislative limit (165 kg ha\({}^{-1}\)) according to Flemish environmental standards. the potato tuber, the potato yield, and the NO\({}_{3}\)-N residue in the soil after harvest were comparable with the results of the MF-treated plots. The potato yield of the plot that only received AS was highest of all plots. However, in the Netherlands it is not very likely that AS is applied without manure. AS is tested for being classified as RENURE product [13]. Acidification of the soil is a concern of using AS frequently. This can have negative impacts on mineral composition and biodiversity [34]. The N\({}_{2}\)O emission after the application of LFD was low. The N release rate and the NFRV are lower compared with the highly refined BBF AS, but higher compared with MF. In the field trial, the plots that were treated with LFD showed lowest N concentration in the tuber and lowest potato yield, and performed poorest in the qualitative assessment of the potatoes after harvest. In practice, farmers are restricted to the amount of N from livestock manure they can apply on the field (170 kg N/ha). A meta-data analysis showed that LFD met the criteria of RENURE in only 43% to 58% of the analysed data [42]. Therefore, it is not very likely that LFD will become a RENURE product. However, it can serve as a replacement for slurry manure in areas where the application is restricted by the P regulations because the P content in LFD is lower (0.7 instead of 2.4 g kg\({}^{-1}\)). However, the adoption of LFD will depend on the price farmers receive or pay for the product. Long-term application of LFD can build up nutrients in the soil which can improve the NFRV [22]. It is recommended to investigate the effect of BBF refinement on long-term application or on the use of precision fertigation. NH\({}_{3}\) emissions were not measured, but it is expected that NH\({}_{3}\) emissions are low during the application of AS because of the low pH of the product. The farmer had problems applying the small amounts of AS to the plots and prefers to dilute the product. NH\({}_{3}\) emissions increase when the pH of the product increases above pH 5 and should therefore be taken into account when AS is diluted. The NH\({}_{3}\) emissions during the application of LFD depend strongly on the application method [12]. Confirmed by a fertiliser machinery developer in the Netherlands, machinery for injecting LFD during the potato growing season does not exist. The dispensing nozzles will be clogged by the fibres in the LFD. Therefore, surface application of LFD is unavoidable at the moment. The highly refined BBF KC showed moderate N\({}_{2}\)O emissions. The N release rate is, as expected, the lowest of all fertiliser products tested. The NFRV is higher compared with MF, but lower compared with the less refined BBF LFD.