Numerical Modeling of Nitrate in a Flood‐Irrigated Pecan Orchard

Core Ideas Pecan is a vitally important specialty crop in the southwestern United States. No study is available on modeling temporal variability of NO3–N transport in pecan orchards. Hydrus‐1D predicted well water and NO3–N variability by depth for a heavy textured soil. NO3–N balance identified denitrification contributing to major loss. Nitrogen management in pecan orchards should take into account on and off years to decrease leaching. Pecan [(Carya illinoinensis (Wangenh.) K. Koch] is an important specialty crop in New Mexico. This research quantifies soil water and soil nitrate‐nitrogen (NO3–N) (mg L−1 of soil) variations with depth, root NO3–N (kg ha−1) uptake, and NO3–N (kg ha−1) balance for the 100‐cm soil profile during two growing seasons in a flood‐irrigated pecan orchard. Nitrate‐nitrogen was determined six times during the growing seasons of 2015 and 2016. The HYDRUS‐1D model was used to optimize the water flow parameters using measured volumetric soil water content (θ). Model calibration and validation for NO3–N included the optimization of reaction parameters for nitrification and denitrification of each soil layer. The results showed that the model simulated θ well (0.44 ≤ d [index of agreement] ≤ 0.73) at different depths during both calibration (2009) and validation (2010) periods. Generally, HYDRUS‐1D simulated soil profile NO3–N concentrations that were correlated with measurements at all depths during both years. Total root NO3–N uptake showed a significant increase of 72% in 2016 compared with 2015. The NO3–N balance showed that ∼40% of applied NO3–N per year was denitrified, which was the main contributor to the NO3–N loss from the soil profile during both years. Nitrate‐nitrogen leaching below the soil profile was 32 and 26% of applied NO3–N in 2015 and 2016, respectively. The fertigation rate was much higher than the plant demand during both years, and it should be decreased to reduce NO3–N losses.