## 4 Discussion
On the contrary, the increase in soil pH of the digestates treatments, from 0.1 to 0.3 pH units, compared with the control treatment, can be achieved not only by the primary effect of the Ca and Mg content of the digestates but also by the low N mineralisation rate of the digestates putting into soil solution low amounts of NH4+. Since the experiments were done in a high fertility soil but with N provided through different sources, the changes of biomass and RY observed in both crops will be a consequence of the differences in N availability provided through different sources: mineral fertilisation or digestate application. The decrease of biomass and the relative yield of lettuce and kale with the application of digestate as the only source of N can be explained by the low mineralisation of the digestate. As referred above, the high stability of the OM in the digestate caused a low mineralisation rate and, therefore a low rate of mineral N release. Therefore, the organic N (NO) ratio in relation to the total N (TKN) in the digestate is a relevant index to evaluate its N availability. The ratio NO:TKN of DGL was 0.65, and DGK was 0.81. High NO:TKN values indicate lower N availability from the digestates since more N is in organic forms with a low mineralisation rate. So, the higher NO:TKN of DGK compared with DGL and a low mineralisation rate of both digestates can explain the more profound decrease observed in the kale biomass and RY compared with the lettuce. Thus, the net N release of the digestate was related to their intrinsic properties, such as the mineral N content and size of the mineralisable fraction, as also observed by [PERSON] and [PERSON] [38]. These authors also observed that the capacity of the soil type to process and turnover digestate N is also a factor affecting soil N availability. The N efficiency indexes NPP, NAE and N\({}_{\rm m}\)AE also showed that the soil and the mineral fertiliser provided the primary source of N to crops uptake. The proportion of the NAE regarding the NPP showed a decrease in the DG treatments compared with the Ni and the DG-N+Ni treatments. So, the N availability from the digestate should be impaired by both a low mineralisation rate of the digestate (stable organic matter) and soil N immobilisation of the N\({}_{\rm m}\) from the DG. The significant decrease of the N\({}_{\rm m}\)AE and N\({}_{\rm m}\)RE in the DG treatments as the only source of N to crops strengthens the hypothesis of a net soil immobilisation of the N mineral fraction released by the digestates. This immobilisation will impair the N uptake by the vegetable crops and the NH4+ nitrification and can partially explain the increase in the pH values in the DG treatments. In addition, the N\({}_{\rm m}\)RE showed the higher values in the Ni and DG+Ni treatments and the lowest, even negative values, in the DG treatments. Our hypothesis highlights the poor uptake/availability of the mineral N of the DG even in treatments with the same amount of Nm applied as in the treatment with mineral fertilisation (85 kg ha-). The fact that the two vegetable crops were done in rainy and cold months can also contribute to the decrease of the mineralisation rate of the DG. [PERSON] et al. [8] also observed a decrease in the fertilising capacity of the digestate in autumn-winter compared with spring-summer horticultural crops. They observed that the available N fraction of the digestate in the winter crops could be easily lost by leading, and the low temperature slows the rate of microbially mediated processes. For the reasons mentioned above, the low N availability for the vegetable crops, provided by the digestate, advises the need of supplemented its use with a mineral N fertiliser to maintain the crop yields. This experiment showed that the amount of mineral N fertiliser needed is dependent on the ratio NO:TKN of the digestate. Higher ratios needed more mineral N to maintain the crop yields similar to those obtained through the mineral N fertilisation.