Modeling the temporal distribution of water, ammonium-N, and nitrate-N in the root zone of wheat using HYDRUS-2D under conservation agriculture

In the current study, the temporal distribution of both soil water and soil NO₃–N under several conservation agriculture (CA) practices during the wheat crop growth were characterized by HYDRUS-2D model. Treatments comprised of conventional tillage (CT), permanent broad beds (PBB), zero tillage (ZT), PBB with residue (PBB+R) and ZT with residue (ZT+R). Hydraulic inputs of the model, comprising the measured value of Kfₛ, α and n, obtained as the output of Rosetta Lite model were optimized through inverse modeling. Model predicted the daily change in soil water content (SWC) of the profile during the simulated period (62–91 DAS) with good accuracy (R² = 0.75; root mean squared error (RMSE) = 0.038). In general, soil water balance simulated from the model showed 50% lower cumulative drainage, 50% higher cumulative transpiration along with higher soil water retention, in PBB+R than CT. Reported values of the first-order rate constants, signify nitrification of urea to NH₄–N (μₐ) (day⁻¹) nitrification of NH₄–N to NO₃–N (μₙ) (day⁻¹) and the distribution coefficient of urea (Kd−in cm³ mg⁻¹) were optimized through inverse modeling. Later they were used as solute transport reaction input parameters of the model, to predict the daily change in NO₃–N of the profile with better accuracy (R² = 0.83; RMSE = 4.62). Since NH₄–N disappears fast, it could not be measured frequently. Therefore, not enough data could be generated for their use in the calibration and validation of the model. Results of simulation of daily NO₃–N concentration indicated a higher concentration of NO₃–N in the surface layer and its leaching losses beyond the root zone were relatively lesser in PBB+R, than CT, which resulted in less contamination of the belowground water. Thus, the study clearly recommended PBB+R to be adopted for wheat cultivation in maize–wheat cropping system, as it enhances the water and nitrogen availability in the root zone and reduce their losses beyond the root zone.