| Summary: | Nitrogen (N) in a maize crop is a determining yield factor, but its negative impact on the environment is also known. Therefore, it is necessary to propose mitigation strategies that allow an improvement in the N fertilizer efficiency (NFE), such as the use of cover crops (CC) and the adjustment of the fertilizer dose. The objective of the study was to determine NFE using <sup>15</sup>N isotopic techniques and nitrate (NO<sub>3</sub><sup>−</sup>) leaching in a maize–fallow versus a maize–CC rotation with optimal and excessive doses of N in the Mediterranean area of Chile. The treatments were a combination of crop rotation (maize–fallow versus maize–CC of <i>Lolium multiflorum</i>) with the optimal dose of N (250 kg ha<sup>−1</sup>) or excessive dose (400 kg ha<sup>−1</sup>). We found that the optimal dose of maize–CC rotation contributed to reducing the losses of N by leaching and improving the NFE. Using the optimal dose decreased the dissolved inorganic N (DIN) emission intensity by 50% compared to the excessive doses. Even if grain yield was higher (19 t ha<sup>−1</sup>) when applying the excessive N dose, the NFE (28%) was lower than when applying the optimal dose (40%). In the maize–CC rotation with optimal dose, yield was 17 Mg ha<sup>−1</sup>. The excessive N dose generated higher DIN content at the end of the maize season (177 kg N ha<sup>−1</sup>). In conclusion, replacing the traditional autumn–winter fallow in the maize monoculture with a CC with optimal N dose contributed to improving NFE and reducing N leaching in a Mediterranean agricultural system. Consequently, it is a strategy to consider as it has positive advantages in soil and N management, helping to reduce diffuse pollution of surface and groundwater bodies.
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