## 2 Materials and Methods
### Analytical Techniques  
CEC and exchangeable bases were determined using the Co(NH\({}_{3}\))\({}_{6}\)Cl\({}_{3}\) method [29; 30] on both bulk soil and rhizosphere samples collected in Sep-2013, pH was determined on 1:5 (w:v) extracts using a pH-meter, while carbonate content was determined using the volumetric method. Total nitrogen (TN) and organic C (TOC) of soil and plant samples were determined by an elemental CHNS-O EA 1110 Thermo Fisher Scientific analyzer coupled with mass spectrometry (Delta Plus, Finnegan, Thermo-Fisher). Soil samples were weighted in silver pots and treated with HCl to eliminate the inorganic C, while plant samples were weighted in tin pots. \({}^{15}\)N natural abundance was measured in total and fixed nitrogen pools (TN and Fix N-NH\({}_{4}\)\({}^{+}\)) and in sorghum organs; the natural abundance was expressed in 8% (Equation (1); [31]), where \(\delta\) represents the difference from the standard atmospheric N\({}_{2}\) (0.3663% \({}^{15}\)N):  
\[\delta^{15}\text{N }(\%_{\infty})=\left[\frac{\left(\frac{\text{{}^{15}N}}{ \text{{}^{4}N}}\right)\!\!sample-\left(\frac{\text{{}^{15}N}}{\text{{}^{4}N}} \right)\!\!\!s!t}{\left(\frac{\text{{}^{15}N}}{\text{{}^{4}N}}\right)\!\!\!s!t} \right]}\times 1000 \tag{1}\]  
To evaluate the influence of the N source on the plant N isotopic signature, mixing lines were built between each end-member represented by soil, NEZ and urea. NO\({}_{3}\)\({}^{-}\) was extracted with Milli-Q (Millipore USA) water in a 1:5 (w/v) ratio, the solution was shaken for 1 hour and then filtered. NO\({}_{3}\)\({}^{-}\) was determined by ion chromatography using an isocratic dual pump ion chromatography ICS-1000 Dionex. An AS-40 Dionex auto sampler was employed to run the analysis; Quality Control (QC) samples were run every 10 samples and the standard deviation for all QC samples was better than 4%. Exchangeable N-NH\({}_{4}\)\({}^{+}\) was extracted with 1M KCl in a 1:10 (w/v) ratio, the solution was shaken for 1 hour and then filtered. The solution was diluted and analyzed with an Ion Selective Electrode (ISE) Orion 95-12 connected to a Thermo Fisher Orion 4 star pH-ISE benchtop. Interferences with Cl\({}^{-}\) and K\({}^{+}\) were verified and excluded by comparing measurement made with NH\({}_{4}\)Cl, CH\({}_{3}\)COONH\({}_{4}\) + 1M KCl and CH\({}_{3}\)COONH\({}_{4}\) + H\({}_{2}\)O standard solutions. After the elimination of organic and exchangeable N, fixed N-NH\({}_{4}\)\({}^{+}\) (Fix N-NH\({}_{4}\)\({}^{+}\)) was determined by adding 20 mL of KBrO to 2 g of air-dried soil samples in two replicates. After 2 hours, 40 mL of distilled water was added and a heating cycle of 10 min in a microwave oven was applied. Then, 20 mL of Milli-Q H\({}_{2}\)O was added and samples were left still for 24 hours. Finally, samples were centrifuged at 5000 rpm for 10 min and washed 2 times with 0.5 M KCl and analyzed by elemental analysis for the evaluation of TN and \(\delta^{15}\)N [32]. Total extractable carbon (TEC) was obtained adding 100 mL of 0.1 M NaOH + 0.1 M NaP\({}_{2}\)O\({}_{7}\)-10\({}_{2}\)O to 10 g of air-dried soil. The solution was left in a Dubnoff bath at 60\({}^{\circ}\)C for 24 hours and then centrifuged at 6600 rpm for 15 min and filtered at 0.45 mm (Millipore). Part of this solution was acidified with HCl 1:1 to reach pH \(<\) 2 to allow the precipitation of Humic Acids (HA), which were separated after centrifugation at 7000 rpm for 25 min by the Non-Humic-Substances (NHS) and Fulvic Acids (FA). The separation of FA from NHS was performed by solid chromatography using a polyvinylpyrrolidone (PVP) polymer in acid ambient, which allows the retention of FA on the PVP and lets NHS pass through the column. The FA was then recovered by bringing the pH of the column to alkaline values with 0.5 M NaOH.