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Coastal ecosystems are increasingly affected by water pollution from anthropic activities in coastal catchments, even though these ecosystems are important from an environmental, social and economic perspective. Diffuse-source water pollution abatement costs differ across agricultural sectors, given the spatial heterogeneity in agro ecological and bio-physical conditions as well as the available range of best agricultural practices (BAPs) for water quality improvement. Key BAPs for dissolved inorganic nitrogen (DIN) water quality improvement include reductions in single, split and slow-release nitrogen (N) fertilizer application rates. In this study we assess the cost-effectiveness of these BAPs across two agricultural land uses (corn and vineyards) in the Crtima catchment (Portugal). Hence, we use the Soil and Water Assessment Tool (SWAT) to estimate diffuse-source water pollution abatement cost functions across agricultural land uses based on a stepwise adoption of identified BAPs for water quality improvement and, corresponding, SWAT-based estimates for agricultural production, agricultural incomes, and water pollution deliveries. In corn production, it is shown that: i) reductions in single N-application rates of up to 20-30% provide private benefits and water quality improvements, ii) split nitrogen application only provides private benefits and water quality improvements at N-application rates of 100% and 90%, and iii) slow release nitrogen application provides neither private benefits nor water quality improvements. In vineyard production it is shown that: i) reductions in N-application rates always come at a cost under all nitrogen application regimes, and ii) split and slow release nitrogen application regimes provide significant private benefits and water quality improvements.