## 3 Results
### \(\delta^{15}\)N Natural Abundance in Soil and Sorghum Organs  
TN and Fix N-NH\({}_{4}\)+ \(\delta^{15}\)N differs significantly (\(p<0.05\)) among treatments and time (Table 4). At the beginning of the experiment (Nov-2012), 7 CZ shows a remarkably higher isotopic signature (\(\delta^{15}\)N of 7.46% and 8.27% for TN and Fix N-NH\({}_{4}\)+, respectively) with respect to the other treatments (\(p<0.05\)). Furthermore, without considering 7 CZ treatment, Fix N-NH\({}_{4}\)+ pool showed a high \(\delta^{15}\)N variability (from 4.55% to 6.00%) with respect to the TN pool (from 3.38% to 3.86%). In May-2013, TN \(\delta^{15}\)N is still significantly higher (\(p<0.05\)) in 7 CZ treatment, while no significant differences were observed regarding Fix N-NH\({}_{4}\)+ pool. In Jun-2013 (after the chemical fertilization), all treatments showed a homogeneous \(\delta^{15}\)N in both TN and Fix N-NH\({}_{4}\)+ pools (\(p>0.05\)). In Sep-2013, rhizosphere samples show higher \(\delta^{15}\)N with respect to the bulk soils in both TN and Fix N-NH\({}_{4}\)+ pools (especially in the latter). The isotopic signatures of NEZ, urea and di-ammonium phosphate were: +18.4 (\(\pm\) 0.5) %o, \(-\)1.18 (\(\pm\) 0.06) %o and \(-\)0.89 (\(\pm\) 0.09) %o, respectively (Table 5). Isotopic analysis on the harvested plants indicated that the \(\delta^{15}\)N of plant tissues was influenced by the different agricultural managements employed in the experimentation (Table 5). \(\delta^{15}\)N was significantly higher in 7 CZ plants leaves and significantly lower in all 5 NZ plants organs with respect to the CNTR plants (\(p<0.05\)). In Figure 6, the TN vs \(\delta^{15}\)N of the different plants' organs are plotted together with urea, NEZ and bulk soil. It is evident that the 7 CZ leaves are strongly affected by NEZ contribution, while 5 NZ plants are more influenced by a low \(\delta^{15}\)N N-source.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline  & & **HA** & **FA** & **TEC/TOC** & **HI** & **HD** \\ \hline
**Time** & **Treatment** & **g kg\({}^{-1}\)** & **g kg\({}^{-1}\)** & **\%** & & **\%** \\ \hline Nov-2012 & CNTR & 5.3 & 3.4 & 47.4 & 0.4 & 34.4 \\  & 5 NZ & 4.3 & 5.0 & 46.2 & 0.2 & 37.7 \\  & 15 NZ & 3.9 & 3.7 & 41.4 & 0.5 & 27.8 \\  & 7 CZ & 2.7 & 3.9 & 40.1 & 0.3 & 31.9 \\ \hline May-2013 & CNTR & 7.4 & 3.2 & 63.0 & 0.5 & 43.1 \\  & 5 NZ & 8.6 & 3.5 & 74.4 & 0.6 & 46.9 \\  & 15 NZ & 11.4 & 3.6 & 87.3 & 0.3 & 68.2 \\  & 7 CZ & 5.9 & 2.5 & 48.6 & 0.1 & 43.8 \\ \hline Jun-2013 & CNTR & 4.5 & 3.1 & 52.3 & 0.4 & 38.6 \\  & 5 NZ & 4.3 & 2.9 & 65.0 & 0.8 & 35.5 \\  & 15 NZ & 3.8 & 3.1 & 50.6 & 0.3 & 39.2 \\  & 7 CZ & 3.4 & 3.0 & 43.6 & 0.6 & 27.4 \\ \hline Sep-2013 Bulk & CNTR & 5.9 & 3.2 & 52.9 & 0.2 & 44.2 \\  & 5 NZ & 3.9 & 2.9 & 54.5 & 0.9 & 29.4 \\  & 15 NZ & 8.6 & 3.3 & 70.7 & 0.4 & 51.3 \\  & 7 CZ & 5.9 & 3.5 & 57.9 & 0.4 & 41.2 \\ \hline Sep-2013 Rhiz & CNTR & 8.1 & 3.1 & 58.1 & 0.3 & 43.4 \\  & 5 NZ & 6.9 & 3.2 & 48.5 & 0.4 & 34.7 \\  & 15 NZ & 9.7 & 3.2 & 49.2 & 0.2 & 41.0 \\  & 7 CZ & 7.1 & 3.3 & 48.8 & 0.3 & 37.0 \\ \hline \end{tabular}
\end{table}
Table 3: C content of Humic Acids (HA) and Fulvic Acids (FA), percentage of Total Extractable and Total Organic Carbon (TEC/TOC) ratio, Humification Index (HI) and Humification Degree (HD) according to [PERSON] et al. [33] and [PERSON] et al. [34].