###### Abstract  
Various fertilization systems have been developed to optimize nitrogen (N) application, yet their effectiveness remains a topic of debate in both science and practice. This study evaluates the effects of 28 N fertilization treatments on yield, quality, nitrogen use efficiency (NUE), N surplus, and economic optima in two winter barley (_Hordeum vulgare_ L.) varieties--a multi-row and a two-row type--across a three-year field trial (2021-2023). Specifically, it compares the performance of fertilizer requirement calculations based on the German Fertilizer Application Ordinance (GFO), multispectral sensor-based fertilization systems, and fixed N input treatments. Under the trial conditions (highly productive fields without organic fertilization for decades), the GFO system consistently achieved high yields (>10 t ha\({}^{-1}\)) and NUE (up to 88%) for both barley varieties, often near economically optimal N rates and with minimal N surpluses. Sensor-based systems demonstrated promising potential for yield optimization and reducing N input; however, they did not result in significantly higher yields. Further research is needed to assess the performance of these fertilization systems under different conditions, such as sandy soils in regions with early-summer droughs or in systems involving organic fertilization. N +
Footnote †: journal: Applied sciences  
Article  
A Three-Year Study of Fertilization Systems in Southern Germany  
(GFO), calculate N application rates before the growing season starts, making them easy to implement [7]. In contrast, multi-spectral sensors can record the spectral reflectance of the cultivated crops during the growing season [8]. These sensors calculate vegetation indices, which correlate with plant parameters like biomass and N uptake [9; 10; 11; 12]. By redistributing or even reducing the amount of mineral N fertilizer applied depending on weather and soil conditions, an increase in N use efficiency (NUE) can be achieved by using multi-spectral sensors and fertilization algorithms [13; 14]. Despite their positive aspects, the efficiency of these systems remains unclear [15] and their practical implementation is still limited and not widespread in some agricultural areas [16; 17], partly because the benefits for farmers have not been sufficiently researched. Furthermore, errors can still occur. [PERSON] et al. [18] note that variable rate application is not always superior to uniform application, as issues such as physiological leaf yellowing during measurements can lead to inaccuracies. Recent studies with multi-year field trials and crop rotation with winter barley have examined the impact of N fertilization on grain yield. N fertilization enhances winter barley yield, with modern cultivars showing improved efficiency and greater response to higher N levels compared to older varieties [7; 19; 20], though the potential for sensor-based methods to enhance yield remains a subject of active research. In a multi-year fertilization trial, [PERSON] et al. [7] found high NUE with multispectral sensor-based fertilization, but could not determine a clear yield advantage over the basic fertilization method (GFO). Similarly, [PERSON] et al. [21] reported comparable yields between GFO and sensor-based systems, with the latter showing the potential to save N and reduce N surplus. In other field trials, where sensor-based N fertilization was applied using the 'Online + Map-overlay' principle [5], high NUE was achieved and often the need for the last N application was reduced, often without any clear yield effects compared to uniform fertilization [22]. While NUE and yield are critical, economic considerations also play a central role [23]. Efficiency analysis must evaluate not only whether site-specific yield potential is fully utilized but also whether the economic optimum--defined as the fertilization level yielding the highest net N return--is achieved.