Evaluation of variable rate nitrogen fertilization scenarios in cereal crops from economic, environmental and technical perspective

Optimizing variable rate nitrogen fertilization (VRNF) is required to minimize inputs, maximize outputs, and reduce environmental footprints. However, this is not possible in commercial farms, since it would require to intervene in regular agricultural practices with potential economic and environmental consequences and would take years of costly experiments. This study aims at the evaluation of different VRNF scenarios based on simulations that include the crop nitrogen need, soil variability, nitrogen limits set by legislations and the technical specification of variable-rate machinery. A simulation software was designed and developed to allow automatic prediction and mapping of soil and crop properties, and calculate recommendations for VRNF. The software was used to compare traditional uniform rates nitrogen (N) fertilization (URNF) against three VRNF treatments [e.g., map-based (MB), sensor-based (SB) and map-sensor-based (MSB)] in four commercial fields. Under each of the three treatments two different application schemes were evaluated, e.g., applying more N fertilizer to the more fertile zones (Kings scheme - KS) and more N fertilizer to the least fertile zone (Robin Hood scheme - RHS). Simulations were made after imposing N legislation limits and without imposing them. Finally, VRNF application was evaluated for a full boom sprayer, a section control sprayer and a nozzle control sprayer. Results indicated that the VRNF did not exceed the traditional URNF treatment, but only if the N limit by legislation is imposed. The RHS consumed 16.4-118.1 % less N fertilizer than the KS and 33.3-56.2 % than URNF treatment. However, the KS without N limit always exceeded the applied N fertilizer compared to the URNF treatment, generating high risk of N leaching. When imposing the N limit, both KS and RHS consumed less N than the traditional URNF treatment, except for the MSB under the KS, which used the same N as the URNF treatment. Spatial variability can be observed in the MB treatment, temporal variability in the SB treatment and both variabilities in the MSB treatment. Regarding the spatial variability, no significant differences between section and nozzle control options could be observed, whereas variability was minimized when using the full boom control. We concluded that the optimal fertilization scheme is VRNF based on a combination of RHS and section control, which is expected to result in reducing N fertilizer application cost, minimising risk of N leaching, and ensuring N applied is under the set legislation limits. Future work should include the crop response in the analysis to provide a comprehensive evaluation of the optimal VRNF treatment