Optimizing nitrogen-water use efficiency for nitrate reduction: spatiotemporal strategies and potential of climate-smart kiwifruit agriculture across SSP scenarios.

The intensive use of nitrogen (N) fertilizers in agriculture has led to surpluses and widespread nitrate pollution in surface water and groundwater. Co-optimizing multiple management practices offers a pathway toward climate-smart and sustainable agriculture, but is hindered by complex interactions among climate, crop, and soil processes across space and time. Here, we developed a hybrid framework integrating machine learning with multi-year (2015-2024) water quality data from the world's largest kiwifruit belt to identify key drivers and synergistic strategies for balancing crop productivity and environmental protection. We found that, over the past decade, nitrate concentration in surface water and groundwater has consistently increased. Average nitrate concentrations reached 9.7 mg N L-1 in surface water, 29.7 mg N L-1 in shallow groundwater, and 7.2 mg N L-1 in deep groundwater, with corresponding increase rates of 0.05, 0.15, and 0.06 mg N L-1 month-1, respectively. Two significant changes in agricultural management (N surplus and irrigation) led to nitrate levels exceeding the WHO standard 21 %, 88 %, and 31 % in surface water, shallow groundwater, and deep groundwater, respectively. The N surplus and precipitation jointly accelerated nitrate pollution in both surface water and groundwater. We predict that, by 2030, crop yield and quality remain unaffected, and under different climate scenarios, single optimization measures improving N and water use efficiency by 20 %-50 % reduced nitrate levels by 4.2 %-18.4 % and 3.4 %-10.1 % of surface water and groundwater, respectively. Combined measures led to a 5.2 %-31.0 % reduction in nitrate levels, while the optimal synergistic combination achieved a 16.1 %-31.0 % reduction. Our work proposes strategies to mitigate nitrate pollution in waterways and to facilitate climate-smart agriculture by optimizing management practices across agroecosystems, thereby highlighting the importance of integrated nutrient and water management to sustain productivity while protecting water quality under changing climatic conditions.