## 4 Discussion  
### Discussion of Methods  
The conditions of the study field result in a low soil N mineralization potential, as demonstrated by analyses in field experiments [7; 30]. The results presented in this paper also show low N mineralization from the soil N pool, which is reflected in the low grain yield and low N uptake in the unfertilized treatment (N 1). This means that the results obtained here regarding the suitability and performance of fertilization systems also apply to these conditions but cannot be transferred to farms with animal husbandry and long-term organic fertilization without further testing. The trial size was relatively small (0.9 ha) to ensure uniform conditions, in contrast to N fertilization experiments in agricultural practice, in which much larger and more heterogeneous arable fields are investigated [18]. In field experiments (such as the present one) with homogeneous soils, it can be determined how accurately a fertilization system determines the correct N fertilization rate for the expected yield. However, this does not prove that the same N fertilization system also performs well in situations with small-scale variations in soil properties and yield potential. The fundamental problem of uniform fertilization systems--overfertilization in low-yield zones and underfertilization in high-yield zones [35; 36]--cannot be analyzed in experiments with homogeneous soils. This means that the potential of systems that apply fertilizer uniformly across the field (e.g., according to GFO) may be overestimated and that of site-specific, sensor-controlled systems underestimated. Nevertheless, the results obtained in the Roggenstein trial are relevant because they show that the tested GFO, BESyD, and sensor-based fertilizer systems are very well adapted to the site conditions and provide very good fertilizer recommendations. With the revision of the GFO in 2020, calculating N fertilizer requirements became mandatory in Germany, whereby the amount determined according to the GFO must not be exceeded [27]. Even if sensor measurements indicate a higher N requirement, fertilization with sensors must comply with the GFO limit in agricultural practice. In this trial, various sensor methods were employed, such as treatment N 28 (sensor with GFO upper limit), which applies sensor-based fertilization but does not exceed the limit defined by the GFO. In other sensor-based treatments, this limit was not applied to test the systems independently. Besides the GFO fertilization system, the BESyD system, which is widely used in farming practice, particularly in eastern Germany with larger farms, was tested. A special feature of the BESyD system is its consideration of crop development and the N content of the biomass in the second and third applications for winter barley using a sensor measuring the plant stand reflectance (e.g., yara N tester) [32; 37]. Also, with the BESyD application, the upper limit of fertilization according to GFO must not be exceeded. However, the N doses determined with BESyD are often considerably lower than according to the GFO [22]. For the sensor-based treatments (N18-N28) in this study, a handheld multispectral sensor was used in combination with fertilization algorithms that were derived and validated in other field trials [7]. In contrast to static methods such as GFO, the sensor-based approach enables real-time adjustments based on plant growth. While handheld sensors have limited practical applications, tractor-mounted sensors, satellites, and drones with multispectral or hyperspectral cameras offer scalable solutions for continuous, large-scale crop monitoring, enhancing N application accuracy over larger areas [38; 39]. An essential prerequisite for the application of sensor-based systems for N fertilization, regardless of the specific technology, is the accurate estimation of the site-specific yield potential, which forms the basis for determining realistic target yields and corresponding N requirements.