## 3 Results and Discussion  
### N Release Rate of Bio-Based Fertilisers  
At the start of the incubation, all tested products were applied at the same rate of 170 kg total N ha\({}^{-1}\) and most of the mineral N was present in the form of NH\({}_{4}\)+-N. However, due to nitrification, the concentration of NH\({}_{4}\)+-N was heavily reduced for all the treatments at day 20 and went down further below detection limits by day 40. As the concentration of NH\({}_{4}\)+-N was negligible after day 40, the total mineral N followed the same trend as the production of NO\({}_{3}\)\({}^{-}\)-N.  
The average net N release (N\({}_{\text{rel,net}}\) calculated using Equation (1)) amounted to 140 \(\pm\) 20% for AS, 113 \(\pm\) 24% for LFD, 54 \(\pm\) 15% for KC, and 105 \(\pm\) 15% for CAN throughout the incubation (Figure 3). Net N release of AS and LFD was in general comparable with the one of CAN as a result of the high NH\({}_{4}\)-N/total N ratio of these BBFs which amounted to 100% and 82%, respectively [12]. As AS solution is 100% in mineral N form, the observed net N mineralisation is seen as a result of the positive priming effect of AS on OM present in the soil [26], hence resulting in N-release value \(>\)100%. A similar result was observed in the LFD treatment from day 80 until the end of the experiment when N release values above 100% were measured. The average net N mineralisation (N\({}_{\text{min,net}}\), calculated using Equation (2)) from amended treatments throughout incubation duration was 40 \(\pm\) 14% for AS, 36 \(\pm\) 12% for LFD, and 20 \(\pm\) 3% for KC (Figure 3). In general, the values were statistically different between the products (_p_ < 0.05). For instance, on day 100, the values of KC and CAN are significantly different from AS and LFD (Figure 3).  
In the case of KC, a positive net mineralisation is observed due to mineralisation of organic N present in the product. However, the net N release was inhibited throughout the incubation period. This is due to the C/N ratio > 7 and NH\({}_{4}\)+-N/total N < 50%. This relatively high C/N ratio and lower NH\({}_{4}\)+-N/total N promotes a very slow release of total N applied, reaching a stagnant release after some days [17; 27]. This hypothesis could be cross confirmed from Appendix B, where Pearson correlation analysis shows a strong inverse interaction between % N released and C/Ntotal ratio (r = -0.999, \(p\) = 0.03). Additionally, it is also mentioned in previous studies that the C/N and NH\({}_{4}\)+-N/Ntotal ratios of applied products typically influence the N\({}_{\text{rel,net}}\) and N\({}_{\text{min,net}}\) (%) [20; 28; 29; 30]. However, in this case, even if the Pearson correlation effect of NH\({}_{4}\)+-N/Ntotal and N\({}_{\text{rel,net}}\) (or N\({}_{\text{min,net}}\)) (%) is strong (r = 0.999, \(p\) = 0.023), suggesting a direct relation between variables, note that there  
Figure 3: Net N release (in % of total N applied) for applied fertilisers over a time span of 120 days. Legend: AS—ammonium sulphate, LFD—liquid fraction of digestate, KC—potassium concentrate, and CAN—calcium ammonium nitrate. Lower case letters (a–d) denote statistically significant differences in means (Tukey’s Test for \(p\) < 0.05) among the products for each sample time (t = 20, 40, 60, 80, 100, and 120).  
can be some bias caused by the relatively high initial NH4-N concentrations of AS and LFD compared with the NH4-N concentrations of K [31].