Abstract Agriculture faces the great challenge of developing strategies to maintain production while simultaneously reducing environmental impacts. The trade‐off between crop production and water quality services is one of the most serious issues facing agriculture, and interest in achieving win–win outcomes through management of ecosystem services is growing. Although wetlands can reduce nitrogen loads, it is unclear whether maintaining and restoring wetlands can ameliorate the trade‐off between crop production and water quality and thereby increase the likelihood of win–win outcomes. We defined the orthogonal residuals from the regression line relating the trade‐offs between two conflicting services as the degree to which the trade‐off was mitigated (mitigation effectiveness score). The more positive the residual, the higher mitigation effectiveness score, and the greater the potential to mitigate the trade‐off. We measured nitrate concentrations, as an indicator of water quality, five times during summer and winter across 49 sub‐watersheds of the watershed of Lake Kasumigaura, which is highly nitrogen‐loaded by agriculture. We quantified the mitigation effectiveness score from the trade‐off relationships between cropland area and nitrate concentrations, and we also identified landscape and environmental factors that affected these scores. Some sub‐watersheds in our study had high cropland cover but low nitrate concentrations. Overall, we found that mitigation effectiveness scores were positively associated with wetland cover at all sampling times. Other factors, including covers of paddy rice fields, abandoned rice fields, and impervious surfaces, and dissolved organic carbon concentrations, had no significant effects on mitigation effectiveness scores, although these factors were considered to increase nitrogen removal. Our findings suggest that maintaining and restoring wetlands might mitigate the trade‐off between crop production and water quality and thereby enhance the likelihood of win–win outcomes in agricultural landscapes. Because wetland area has decreased, flooding or ponding abandoned rice fields may be an important alternative management option. The nitrate concentrations we observed met the water quality standard for drinking water, but the fact that they sometimes exceeded the nitrogen environmental target adopted within Lake Kasumigaura in terms of eutrophication suggests that simultaneous reduction of croplands and fertilizer inputs should still be encouraged.
