## 4 Discussion  
The fertilisation with digestate had a positive effect on soil properties. The digestates used in this experiment had a beneficial effect as an organic amendment by increasing the content of the soil organic matter and increasing the soil pH and the exchangeable bases. During the anaerobic digestion, the degradation of more labile fractions (e.g., carbohydrate-like molecules) of the organic materials occurred, determining the relative concentration of more recalcitrant organic molecules (lignin and non-hydrolysable lipids). These modifications of the ingested organic material caused a higher degree of biological stability \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & **pH** & **EC** & **MO** & **Nt** & **K** & **Ca** & **Mg** & **Na** \\ \hline \({}^{1}\) Year & & dSm\({}^{-1}\) & \% & & & cmol\({}_{\rm c}\) kg\({}^{-1}\) & \\ Lettuce-May\_2020 & 6.1 & 0.13 & 6.1 & 2.7 & 1.44 & 7.45 & 1.22 & 0.04 \\ Kale-March\_2021 & 6.3 & 0.09 & 7.6 & 3.2 & 1.03 & 5.53 & 1.17 & 0.01 \\ Significant & \(p<0.001\) & \(p<0.001\) & \(p<0.001\) & \(p<0.001\) & \(p<0.001\) & \(p<0.001\) & \(p>0.05\) & \(p<0.001\) \\ \hline \({}^{1}\) Treatment & & & & & & & & \\ Control & 6.2 cd & 0.09 b & 6.1 c & 2.5 c & 1.23 bcd & 6.50 ab & 0.95 de & 0.020 d \\ Ni85 & 5.8 f & 0.13 a & 5.6 c & 2.4 c & 1.02 e & 5.99 b & 0.84 e & 0.021 cd \\ DG-N85 & 6.3 c & 0.09 b & 6.1 c & 2.8 bc & 1.21 cd & 6.79 a & 1.09 cd & 0.026 bcd \\ DG-N170 & 6.4 b & 0.10 b & 7.7 a & 2.9 abc & 1.35 abc & 6.50 ab & 1.23 c & 0.030 ab \\ DG-N170+85 & 6.5 a & 0.12 a & 7.5 ab & 3.4 a & 1,43 a & 6.76 a & 1.51 b & 0.034 a \\ DG-N170+170 & 6.5 a & 0.13 a & 7.6 a & 3.3 ab & 1.36 a & 6.96 a & 1.72 a & 0.034 a \\ DG-N85+Ni60 & 6.0 e & 0.09 b & 7.2 ab & 2.9 abc & 1.10 de & 6.01 b & 1.07 cd & 0.026 bcd \\ DG-N170+Ni60 & 6.1 d & 0.10 b & 7.7 a & 3.4 ab & 1.21 cd & 6.04 b & 1.14 c & 0.028 abc \\ DG-N170+Ni25 & 6.3 c & 0.10 b & 6.4 bc & 2.8 abc & 1.21 cd & 6.87 a & 1.23 c & 0.028 abc \\ Significant & \(p<0.001\) & \(p<0.001\) & \(p<0.01\) & \(p<0.001\) & \(p<0.001\) & \(p<0.001\) & \(p<0.001\) & \(p<0.001\) \\ \hline Interaction & & & & & & & & \\ Year X Treatment & \(p<0.001\) & \(p<0.001\) & \(p>0.05\) & \(p>0.05\) & \(p<0.01\) & \(p<0.01\) & \(p<0.001\) & \(p<0.001\) \\ \hline \end{tabular} \({}^{1}\)--ANCOVA treatment. Comparisons of the changes in soil properties between the two crop cycles and between the treatments. According to LSD multiple range test, different letters in the columns indicate statistically significant differences among the treatments for each soil property (\(p<0.05\)).  
\end{table} 
Table 5: Changes in the soil properties after the lettuce and the kale crop cycles obtained by the ANCOVA treatment. of the digestate regarding the starting mixture [2; 34]. So, after two crop cycles, the input of this stable organic matter originates an increase of around 26% in SOM regarding the control treatment. The changes in pH after fertilisation can be explained, on the one hand, by the increasing effect on soil acidity of the mineral N fertilisation and the other hand by the decreasing effect on soil acidity provided by the exchangeable bases such as K, Ca and Mg from the digestate. The lowest pH value observed in the mineral N fertilisation was probably due to the application of ammonium-based fertilisers. The ammonium salts acidify soils mainly through nitrification in which the NH4+ is oxidised into NO3- with the release of H+. However, if the NO3- is taken up by crops, there is no net acidification because the nitrate takes up protons. Acidification only occurs when NH4+ is nitrified and NO3- leached [35; 36] or if NH4+ is taken up by the plant before nitrification occurs and in quantities greater than the accompanying anion. In this case, the soil acidity will result from proton release from roots [37]. Both crops were done in the cold and rainy seasons in our experiment. Thus, some NO3- leaching must have occurred in relation to the Ni treatment, and the crops might have absorbed some NH4+.