Agricultural nitrogen (N) pollution has become the major pressure on inland and coastal aquatic ecosystems (Reusch et al., 2018;Van Meter et al., 2018). Meanwhile, agricultural intensification remains one of the main strategies to satisfy increasing global food demand (Chukalla et al., 2020;Schils et al., 2018). Overarching policies for sustainable development are increasingly calling for better reconciliation of agricultural interests with environment protection needs and further development of cost-effective and targeted measures, especially under changing climatic conditions (EEA, 2020a(EEA, , 2020bReusch et al., 2018).In agricultural research, budget methods are commonly used to assess nitrogen use efficiencies and potential environmental impact (Cassman et al., 2002). The latter is often indicated by N surplus (input minus output) and assessed based on readily available data from the farm gate to national scales (Eurostat, 2013). The budgeting methods provide a clear evidence base of complex relationships between agricultural activities and environmental responses and therefore are particularly favorable for communicating to policy makers (Cherry et al., 2008;Zhang et al., 2020). However, in addition to potential uncertainty embedded in data acquisition (Oenema et al., 2003), it is well recognized that budget calculations only provide estimates of potential N losses, and their credibility Abstract Diffuse Nitrogen pollution from agriculture maintains high pressures on groundwater and aquatic ecosystems. Further mitigation requires targeted measures that reconcile agricultural interests in environmental protection. However, the agriculture-related processes of catchment N modeling remain poorly defined due to discipline-specific data and knowledge gaps. Using field-experimental data, crop N uptake responses to fertilizer management were parsimoniously conceptualized and integrated into a catchment diffuse-N model. The improved catchment modeling further facilitated integration with agricultural budget-based assessments. The integrated analysis in a mesoscale catchment disentangled contrasting agri-environment functional mechanisms in typically flashy chemodynamic and transport-limited chemostatic export regimes. Moreover, the former was actively responsive to interannual climatic variability and agricultural practices; the latter exhibited drought-induced enhancement of N enrichment, which could likely be mitigated through reduced fertilization. This interdisciplinary integration of data and methods provided an insightful evidence base for multi-sector targeted measures, especially under cumulative impacts of changing climate and fertilizer-use intensities.Plain Language Summary Due to intensive nitrogen fertilizer use in agriculture over recent decades, excessive nitrogen has largely accumulated in soil and groundwater systems and is gradually being released to surface waters, polluting aquatic environments. Considering the changing climate and growing food demand, future environmental mitigation should purs...