## 3 Results  
### Biomass and Nitrogen Use Efficiency Indexes  
The biomass production (fresh matter yield) of each vegetable crop showed significant differences between the treatments (Figure 1a,b; \(p\) < 0.001). The biomass (FMY) of the lettuce ranged from 3544 g m\({}^{-2}\) in treatment C to \(\cong\)6000 g m\({}^{-2}\) in DG-N85+Ni60 and DG-N170+Ni60. The biomass production of kale ranged between 1175 to 2157 g m\({}^{-2}\) in C and DG-N170+Ni60 treatments, respectively. Overall there is a decrease in the relative yield (Equation (4)) of both crops in the plots with DG application as the only source of N. The two crops showed significant differences in the RY between the treatments (\(p\) < 0.001). However, lettuce and kale responded differently in their RY regarding the application of DG. In the case of the lettuce, the relative yield ranged from 72-74% in DG-N85 and DG-N170+85 to 111-112% in DG-N85+Ni60 and DG-N170+Ni60 treatments. The kale crop showed a more profound decrease of the RY in the DG treatments compared with the lettuce crop. Thus, the relative yield of kale decreased 54-56% in DG-N85 and DG-N170 (in these two treatments, the RY was even lower than in the control) but increased to 128% in the DG-N170+Ni60. We can also observe that the lettuce biomass and RY of the DG-N85 and DG-N170+85 treatments showed statistically similar values to the control and were significantly lower than the Ni85 treatment. The same trend was observed in the DG-N85 and DG-N170 treatments of the kale experiment. It is worth noting that for lettuce, the DG-N170+85 treatment applied same amount of mineral N (N\({}_{\rm m}\)) to the soil as the Ni85 treatment (Table 3). The N use efficiency indexes (NPP, NAE, N\({}_{\rm m}\)AE, NRE and N\({}_{\rm m}\)RE) also showed significant differences between the treatments of both crops (\(p\) < 0.001, Figure 2a-d). The NPP in lettuce ranged between 25.21 gDMY g\({}^{-1}\) N in Ni85 to 6.15 in DG-N170+170 gDMY g\({}^{-1}\) N, and kale ranged between 42.97 10.87 gDMY g\({}^{-1}\) N in the same treatments. The NPP of Ni85 of both crops was significantly higher than the other treatments, and this treatment had values similar to those referred to in other works [32]. The treatments with the significantly lowest NPP were those with the highest amount of DG application, the DG-N170+85 and the DG-N170+170. The NAE was a small proportion of the NPP of each treatment of both crops; for lettuce, NAE was 33% of NPP in Ni85 treatment and 7% in DG-N170+85. For kale, NAE ranged from 34% of NPP in Ni85 treatment to \(\cong\)2% in the DG-N85. In lettuce, the NAE of Ni85 (8.41) and of the DG-N85+Ni60 (6.60) was significantly higher than the NAE of the other treatments. The significantly lowest NAE values of lettuce were observed in the DG-N170+85 (0.50) and the DG-N170+170 (1.39). Regarding the kale crop, the significantly higher NAE values were observed in the Ni85 (14.76), in the DG-N85+Ni60 (10.37) and the DG-N170+Ni60 (9.75). The N\({}_{\rm m}\)AE index showed the same trend as the NAE for both crops. However, for lettuce, the best behaviour was observed for the treatments with the application of DG together with inorganic mineral fertilisation and for kale in addition to those treatments with higher application rates of DG DG-N170+85 and DG-N170+170. Figure 1: Biomass production of (**a**) lettuce (kg FMY m\({}^{-2}\)) and (**b**) kale (g FMY m\({}^{-2}\)). According to [PERSON]’s multiple range test, different letters above the columns indicate statistically significant differences among the treatments (\(p\) < 0.05). The data are presented as mean \(\pm\) SE (\(n\) = 4). The more profound decrease of the NRE index observed in the DG treatments as the only source of N occurred in both crops. The NRE of Ni85 treatment was significantly higher in lettuce (29%), and kale (40%) compared with other treatments and were similar to those found for cabbage in [33].