Site-Specific Nitrogen Response Functions for Maize in Argentina.

Site-specific nitrogen (N) management can contribute to maximize N use efficiency, increase production profits, and reduce environmental impacts. However, a lack of site-specific N recommendations has leaded to a low adoption of variable rate N (VRN) in Argentina. The objectives of this research were to: a) determine the economic optimum nitrogen rate (EONR) for maize (Zea mays L.) and its temporal stability at contrasting management zones (MZ) and b) provide a better understanding of the dynamics of N crop demand and soil N supply across MZ. Fifty N rate trials were conducted in contrasting MZ at four farms in central-west, Buenos Aires, Argentina from 2012 to 2017. Treatments were seven N rates (from 0 to 250 kg ha^-1) applied as broadcasted urea in a randomized complete block design with three replications. Management zones were characterized by elevation, soil apparent electrical conductivity (ECa), and chemical and physical soil testing. Maximum attainable yield and economic optimum N rate (EONR) values varied spatially and temporally across MZ, suggesting that VRN could be economically and environmentally beneficial in central-west Buenos Aires. Results indicated that in general producers from this region are mostly under applying N fertilizer as a result of using fixed conservative N rates or static-type N recommendation tools. The EONR varied between 115 to 260 kg N ha^-1 when precipitation were above normal, and its variability across MZ was partially explained by differences in N soil supply. In contrast, when precipitation where normal or below the average, the range of EONR within fields was much higher (from 0 to 260 kg N ha ^-1). For those years, the relationship between EONR and the yield at the EONR (indicator of crop N demand) tended to be positive. Thus, soil properties related with variations in crop available water that affect crop N demand across MZ (e.g. sand content, organic matter content, and ECa) explained more than 50% of the EONR variability. The understanding of site-specific N soil and crop dynamics and its application into N recommendations will be further explored with the implementation of a crop simulation model (APSIM) calibrated with in-season crop, soil, and weather data. Using a dynamic instead of a static approach based on a crop simulation model will improve site-specific N management in central-west Buenos Aires while sustaining farm economic viability.