# HYDRUS-1D Simulation of Nitrogen Dynamics in Rainfed Sweet Corn Production
## 3 Results and Discussion
### Nitrogen Leaching  
The details of simulated cumulative N fluxes at 20 and 40 cm depths during the first and second seasons are presented in Figure 10. The trend of vertical fluxes shows their higher accumulation during the second season. The significant amount of N applied during the first season was lost due to runoff, leaving behind a lesser amount of N to be infiltrated in the root zone as compared to the second season. The difference in vertical flux can be also attributed to relatively frequent and low-intensity rainfall events in the second season. A significant impact of rainfall contributed to leaching [9]. The cumulative flux of NO\({}_{3}\)-N and NH\({}_{4}\)+-N continue to increase with time at 20 and 40 cm depths during both seasons. The NH\({}_{4}\)+-N flux decreased with increasing soil depth. The total NH\({}_{4}\)+-N flux at 40 cm dropped to 0.77 kg/ha and 2.55 kg/ha during the first and the second season due to various NH\({}_{4}\)+-N removal processes i.e., nitrification of NH\({}_{4}\)+-N to NO\({}_{3}\)-N and plant root uptake in form of NH\({}_{4}\)+-N. As a result of nitrification, the NO\({}_{3}\)-N concentrations in the root zone were much higher than NH\({}_{4}\)+-N. The NO\({}_{3}\)-N concentrations that remained in the root zone were vulnerable to leaching along with water flux. The higher nitrate contamination was reported for larger precipitation [38]. The drop in cumulative N leaching curves at certain points shows the small additions of N from shallow water table as results of upward water movement due to capillary rise.  
Figure 11 illustrates the daily and cumulative N fluxes at 60 cm depth. As the effective root zone was set at 60 cm, the leaching fluxes have been discussed for the top 60 cm of profile depth. The results of fluxes at 60 cm have been described as a sum of NO\({}_{3}\)-N and NH\({}_{4}\)+-N as the most of the NH\({}_{4}\)+-N vertical flux occurred in the top 40 cm, the amount of NH\({}_{4}\)+-N present at 60 cm was negligible. The observed NO\({}_{3}\)-N and NH\({}_{4}\)+-N concentrations at 60 cm were in the range of 0-24.2 mg/L and 0-4.1 mg/L, 0-18 mg/L and 0-4.4 mg/L during first and second seasons, respectively. The observed concentrations justify the NO\({}_{3}\)-N as a dominant form in leaching water. The nitrogen (N) and water leaching are correlated. The comparison of N leaching between two seasons shows a similar pattern. The simulated N leaching loss accounted for 4.0% (4.8 kg/ha) and 18.5% (22.2 kg/ha) during the first and second seasons, respectively. Considering the heavy soil texture, the N leaching is significant for the second season. A study on corn under continuous furrow irrigation detected 18.3% (55 kg/ha at TNI of 300 kg/ha) of peak NO\({}_{3}\)-N leaching. However, the rates of leaching were lower for low N applications [39]. The NO\({}_{3}\)-N was again a dominating form of N leaching that was around 97% and 96% of total N leaching in first and second seasons, respectively. The NH\({}_{4}\)+-N portion in leaching was negligible. [PERSON] et al. [35] reported 78% of N leaching in the form NO\({}_{3}\)-N. As the NH\({}_{4}\)+-N was either utilized or converted to NO\({}_{3}\)-N at shallower depths the NO\({}_{3}\)-N ratio in the current study is high. The difference in simulated N leaching between two seasons is quite significant. The observed N leaching results were found in good agreement to simulated results.  
Figure 9: Comparison of simulated (Sim.) and observed (Obs.) cumulative N runoff fluxes during the (a) first season and (b) second season.  
Figure 11: Simulated (Sim.) N leaching fluxes, cumulative N fluxes and observed (Obs.) cumulative N fluxes at 60 cm depth during the (**a**) first season and (**b**) second season.  
Figure 10: Comparison of simulated (Sim.) cumulative N fluxes at 20 and 40 cm depth during the (**a**) first season and (**b**) second season.