Analyzing the Consequences of Fertilization Management in Increasing Water and Nitrogen Productivity under Rainfed Maize Cultivation: Scenarios Assessment by Using the HYDRUS Model

Nitrogen (N) loss from the agricultural lands is one of the most important point-source of N which pollutes natural aquifers. While N is one of the driving nutrition in fulfilling crop growth cycle, unmanaged fertilization beyond crop demand leads to increased N leaching to groundwater resources and causes environmental unsustainability. This issue is more highlighted in rainfed cultivation due to unexpected rainfall events which might be beyond crop water requirement. Hence, we determined optimal N fertilizer rate under rainfed maize cultivation in Mazandaran province. In this regard, HYDRUS-2Dwas first calibrated and validated for soil hydraulic and chemical parameters based on data collected during a two-year maize field investigation. The model was then used for simulating the amount of nitrate leaching, N uptake and N residual in soil under 9 fertilization levels treatments including no fertilization (only 10 kg ha-1 was considered as initial soil N nitrogen), 50, 100, 150, 200, 250, 300, 350 and 400 kg ha-1. Finally, optimal fertilization rate for rainfed maize was determined based on N use efficiency and the ratio of crop yield to N fertilization rate.While according to RMSE (0.8-1.18 mm for water content, 0.38-7.95 mg l-1 for nitrate concentration, and 3.96-8.96 kg ha-1 for crop N uptake),HYDRUS-2D was capable enough for simulating soil water and solute dynamics, the lower variation in soil water content and solute concentration during the cropping cycle caused the model to be more accurate for simulating soil water and solute dynamics in these layers compared to the surface layer.Based on the simulated results, N uptake will increased by 20 kg ha-1 on average in response to every 50 kg ha-1 increase in N fertilization rate beyond 200 kg ha-1. N leaching below different soil depths increased along with reduced crop N uptake, which led to nutrient removal from the surface soil layers and its aggregation in underlying soil layers at the end of the growing season. Fertilizer use efficiency had its highest value at fertilization rate of 150 kg ha-1, while it remained unchanged at higher N rates. Nevertheless, increased N rate always lead to yield reduction per unit applied fertilizer.