## 4 Discussion
### Plant-Zeolitite Interactions  
N is preferentially allocated in leaves and panicles and the lack of significant differences in N content with respect to the distinct treatments suggests that, notwithstanding the fertilization reductions, plants uptake similar N amounts, reflecting a higher fertilizer recovery or N use efficiency. As reported in literature [65; 66]\(\delta^{15}\)N of leaves is determined by the physiological mechanism within the plants and by the isotopic ratio of external N sources. Moreover, intra-plant N isotopic variations, between roots and leaves can be due to different patterns of N assimilation or reallocation. These intra-plant \(\delta^{15}\)N variations were recognizable in the sorghum plants, where leaves \(\delta^{15}\)N was generally greater (from 2 to 3%o) than that determined in roots, except for 7 CZ where differences were even greater (about 6%o). Many authors used \({}^{15}\)N natural abundance of leaves in order to trace the N source in the soil [65; 66; 67; 68; 69] mostly because leaves represent the primary sink of N and the largest plant N pool [41]. Average \(\delta^{15}\)N of 7 CZ leaves was very close to NEZ \(\delta^{15}\)N and significantly higher than the leaves from the other parcels (Figure 6). The higher isotopic signature of 7 CZ leaves indicated that during the growing season, an uptake from a significantly higher \(\delta^{15}\)N occurred. Assuming negligible fractionation effects during uptake, the main N source was probably represented by NEZ-N. Indeed, 7 CZ is the only parcel where a high \(\delta^{15}\)N input (represented by pig slurry) was added during the experimentation. It has been demonstrated that crops grown under chemical fertilizers (such as urea) have lower \(\delta^{15}\)N with respect to plants grown under organic fertilizers (which have usually higher \(\delta^{15}\)N) [70; 71; 72]. It is important to consider that the other N inputs were urea and di-ammonium phosphate, which have a negative \(\delta^{15}\)N and were applied to all treatments, supporting the hypothesis that NEZ was an efficient slow release fertilizer and retained N even after several months and with no important N losses occurring (from leaching or denitrification). Most N in soil is bound in forms not immediately available to plants [41; 71], so usually the \(\delta^{15}\)N of TN is not a good approximation of the isotopic signature of the N source preferentially used by plants. Anyhow, \({}^{15}\)N natural abundance in leaves of CNTR, 15 NZ and especially 5 NZ reflect the use of N characterized by a lower \(\delta^{15}\)N and related to chemical fertilizers signature. It is important to note that notwithstanding the lower urea application (\(-30\%\) with respect to the CNTR), in 5 NZ and 15 NZ treatments, leaves \(\delta^{15}\)N is similar or even lower with respect to CNTR. In particular, as visible from the mixing model (Figure 6), 5 NZ plants showed the lowest \(\delta^{15}\)N not only in leaves but also in roots, stems and panicles, suggesting a higher efficiency in the uptake of N from a source with lower \(\delta^{15}\)N, i.e., chemical fertilizers.  
The positive yield results of all ZT must be attributed also to the soil physical properties amelioration induced by zeoliticite amendments. In our case, zeolitites can in fact increase soil water retention and, thanks to their coarser size with respect to the fine-grained soil (particle-size analysis performed within ZeoLIFE project are not shown in this work) they can also increase soil permeability and aeration [28; 36].  
Figure 6: \(\delta^{15}\)N vs total nitrogen (TN) mixing diagram for sorghum plant organs. Chemical fertilizers, NEZ and Soil have been chosen as end-members.