Sustainable Intensification of Feed Production Through Intercropping of Cereals and Legumes: The Role of Nitrogen Fertilization in Shaping the Circulation of Micronutrients

In the context of sustainable agriculture and the need to reduce mineral nitrogen inputs, intercropping cereals with legumes is increasingly considered a promising strategy to enhance nutrient use efficiency and improve feed quality. However, the effects of such systems, combined with varying nitrogen fertilization levels, on the dynamics of micronutrients in soil and plant biomass remain insufficiently explored. Field research was conducted in central Poland, in Ciechanów, from 2021 to 2023, during the months of April through July each year. The aim of the study was to analyze the impact of intercropping spring barley and spring triticale with narrowleaf lupin and varying mineral nitrogen fertilization (0–60 kg N ha−1) on the concentration and uptake of Mn, Cu, Zn, and Fe in the soil and green matter intended for fodder. It was shown that both the sowing pattern and the level of N fertilization significantly differentiated the concentration of microelements in the soil and their concentration and uptake with the yield. As the proportion of lupine in the mixture increased, the post-harvest soil showed higher concentrations of Mn (2–8%), Cu (2–9%), Zn (9–33%), and Fe (4–10%), accompanied by a marked increase in their levels in green matter, ranging from 6% to 94% depending on the micronutrient. The highest uptake of micronutrients was obtained in intercropping systems with a predominance of legumes, especially with moderate fertilization (40–60 kg N ha−1), where the growth ranged from 16% to as much as 139%. Compared to single-species crops, the intercropping system was characterized by higher efficiency of soil resource use and better mineral quality of the feed. The results indicate that the integration of legumes with cereals can be an effective tool for improving feed security while reducing the intensity of mineral fertilization, in line with the principles of sustainable agriculture.