Background and Aims: Adequate nitrogen (N) supply, often met through biological nitrogen fixation (BNF) in soybean plants, has been widely studied in North and South America and parts of Europe, but remains largely unexplored in Northern Europe. This study was conducted over two cropping seasons in the central Jutland region of Denmark to investigate the effectiveness of dual inoculation strategy with Bradyrhizobium japonicum for soybean, where local production faces challenges such as climate adaptation. Methods: Building on previous greenhouse findings, we assessed the impact of inoculation treatments on soybean yield, nodulation, root traits, and nitrogen fixation in soils with no prior history of soybean cultivation. Results: Variability in rainfall over the cropping seasons influenced pod development and N accumulation. In 2022, pod development was well-established, whereas the wetter conditions in 2023 promoted vegetative growth and nodule formation. The combination of seed inoculation plus additional inoculation at the V3 growth stage (third node) increased nodule number by 82.3%, nodule dry weight by 20.6%, and the quantity of N derived from the atmosphere by 30.2% compared with seed inoculation alone. Conclusion: Efficient nodulation is crucial for ensuring N ₂ fixation, highlighting that optimized inoculation practices, rather than N fertilizers, may offer a sustainable strategy to meet soybean’s N demands during the growing season were weather conditions challenge growth dynamics.

Irrigation unintentionally delivers reactive nitrogen to croplands via nitrate-rich water, yet this input is rarely quantified and remains largely absent from nitrogen budgets and fertilization policies. Here we compile over 1,300 field observations and upscale them using machine learning to quantify the nitrogen delivered through irrigation and map its contribution across irrigated croplands for 2010–2019. While the median input was 19 kg N ha⁻¹ yr⁻¹, 10% of observations exceeded 100 kg N ha⁻¹ yr⁻¹. Globally, we estimate that irrigation supplied 14 Tg N yr⁻¹, equivalent to 13% of synthetic fertilizers used on croplands. Important clusters appear in regions with intensive irrigation and high nitrogen inputs, indicating that irrigation represents a substantial but underused nitrogen source. Our findings expose an overlooked component of agricultural nitrogen budgets that can reduce fertilizer overuse, improve nitrogen-use efficiency and promote nitrogen circularity in irrigated croplands.