## 3 Results and Discussion
### Nutrient Fertiliser Replacement Value  
A response yield curve was fitted through the measured crop yields that correspond to different mineral N or K fertiliser application rates. The curve of the mineral N fertiliser fit more nicely in a polynomial trendline (R\({}^{2}\) = 0.99) compared with the yield response curve of the mineral K fertiliser (R\({}^{2}\) = 0.92) (Figure 5). The fresh yield at 40% of the recommended N rate was 0.43 kg pot\({}^{-1}\) for the potato plants that received mineral N fertiliser, and slightly higher (0.47 and 0.49 kg pot\({}^{-1}\)) for the potato plants that received LFD and AS, respectively. The yield at 40% of the recommended K rate was 0.33 kg pot\({}^{-1}\) for the plants that received mineral K fertiliser, and 0.42 and 0.41 kg pot\({}^{-1}\) for LFD and KC, respectively. The corresponding NFRV of N treatments was 1.04 for LFD and 1.13 for AS. For the K treatments, the NFRV was 1.52 for LFD and 1.41 for KC. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Fertiliser Type**} & \multicolumn{2}{c}{**N\({}_{2}\)O**} & \multicolumn{2}{c}{**CO\({}_{2}\)**} & \multicolumn{2}{c}{**CH\({}_{4}\)***} \\ \cline{2-4}  & \multicolumn{3}{c}{**per 100g N Applied**} \\ \hline Liquid fraction of digestate & 0.02 \(\pm\) 0.005 & 53 \(\pm\) 9 & \(-\)0.11 \\ Potassium concentrate from evaporator & 0.05 \(\pm\) 0.03 & 58 \(\pm\) 9 & \(-\)0.06 \\ Ammonium sulphate solution & 0.03 \(\pm\) 0.008 & \(-\)20 \(\pm\) 12 & \(-\)0.14 \\ Calcium ammonium nitrate & 0.11 \(\pm\) 0.01 & \(-\)20 \(\pm\) 7 & \(-\)0.06 \\ Urea & 0.11 \(\pm\) 0.02 & 20 \(\pm\) 6 & \(-\)0.08 \\ \hline \hline \end{tabular}  
* The standard deviations were not significant considering the relatively lower values of CH\({}_{4}\)  
\end{table} 
Table 5: Emission factors of nitrous oxide (N\({}_{2}\)O), carbon dioxide (CO\({}_{2}\)), and methane (CH\({}_{4}\)) expressed per 100 g of N applied for tested bio-based fertilisers and mineral N fertilisers. The potato yield of the plants that received BBFs are in all cases slightly higher compared with the mineral fertiliser, in which it turned out that all NFRVs are above the response yield curve. Other studies showed that often only assessments on long-term NFRV reach yields above the response yield curve [22]. However, because the total potato yield per pot is low, slightly higher yields (in our case between 40 g and 90 g) can result in NFRV > 1. The values should therefore be evaluated with caution. This experiment showed that (i) both BBF perform well in comparison with mineral fertiliser, and (ii) the level of refinement does not seem to influence the NFRV. Other characteristics of the potato also do not show significant differences between the treatments with a highly refined BBF, a less refined BBF, or mineral N or K fertiliser. The size and number of tubers did not differ much between the treatments, but within a treatment sometimes a large variation exists. The '40% K concentration using LFD'-treatment counted, for example, between 1 and 12 potatoes smaller than 3 cm. The total N concentration in the tubers differed significantly, but the total K concentration in the tubers did not differ significantly. The N-content in the potato tubers increased from 6.5 \(\pm\) 0.3 g kg\({}^{-1}\) DM when 0 kg ha\({}^{-1}\) N was applied to 11.3 \(\pm\) 0.4 g kg\({}^{-1}\) DM when 310 kg N ha\({}^{-1}\) was applied. A N application rate of 40% of MF resulted in a total N content in the potato tuber of 8.9 \(\pm\) 0.7 g kg\({}^{-1}\) DM. A N application rate of 40% in the form of LFD and AS resulted in a total N content in the potato tuber of 8.1 \(\pm\) 1.0 g kg\({}^{-1}\) DM and 8.7 \(\pm\) 1.0 g kg\({}^{-1}\) DM, respectively. Calculating the NFRV based on the N uptake in the potato tuber resulted in a NFRV of 0.50 for LFD and 0.60 for AS. Similar to the NFRV resulting from the fresh potato yield, the NFRV resulting from the N uptake in the tuber was slightly higher in the more refined AS product compared with the less refined LFD product.