Excess nitrogen (N) input is a pressing environmental issue, posing a serious threat to watershed ecosystems. In paddy-dominated watersheds, the management of water and nitrogen in rice cultivation influences N dynamics. In this study, we applied the Net Anthropogenic Nitrogen Input (NANI) framework to quantify the spatial and temporal patterns of nitrogen input and to examine the relationship between NANI and N concentrations in rivers flowing into Lake Hachiro, a eutrophic lake in northern Japan. NANI was estimated by calculating four components: N fertilizer application, atmospheric deposition, agricultural N fixation, and net import of food and feed. The estimation was based on statistical data from nine municipalities within the watershed and local fertilizer application standards. Using GIS, we aggregated NANI values for the entire watershed and for each sub-watershed. River water N concentrations were analyzed using long-term monitoring data from 15 rivers flowing into the lake to obtain monthly and average values. The total NANI for the Lake Hachiro watershed was estimated at 4,390 Mg N yr⁻¹ (49 kg ha⁻¹), with chemical fertilizer as the largest contributor, followed by crop N fixation and atmospheric deposition. Net food and feed import was negative, reflecting the region's agricultural characteristics. Land-use-based analysis showed that surplus nitrogen was mainly distributed across paddy fields and uplands, whereas N from urban areas was largely removed through wastewater treatment. Seasonal variation in riverine N concentrations showed a peak in May, likely due to N runoff associated with irrigation practices in paddy fields. A significant positive correlation was observed between sub-basin NANI values and average total N concentrations in river water (r = 0.77, p < 0.01). These findings highlight the need to prioritize improvements in fertilizer application and water management practices in rice cultivation as a strategy for mitigating N loads in the Lake Hachiro watershed. This poster also introduces ongoing initiatives to reduce N loads in the region’s paddy fields.