Nitrate leaching and potato tuber yield response to different crop rotations

Intensive potato production causes nitrate leaching and contributes to groundwater nitrate contamination. This study investigated the impacts of five potato rotations on nitrate leaching and tuber yield using a plot-based field experiment in combination with numerical modeling in Prince Edward Island, Canada, from 2014 to 2021. The rotations included barley underseeded with red clover-red clover-potatoes (B-RC), three buckwheat-buckwheat-potatoes rotations with buckwheat harvested for seed (BWS), terminated mid-season by disking (BWD) and terminated mid-season by mowing (BWM), and barley underseeded with timothy-timothy-potatoes (B-T). The buckwheat rotations represented typical management options for wireworm control, B-RC represented the local industry standard rotation, and B-RC and B-T acted as controls. Rotation-based total nitrate leaching that occurred from 23 October 2014-20 July 2020 (covering two 3-year rotation cycles) was calculated using a combination of lysimeter sampling and soil drainage numerical modeling. The three buckwheat rotations had comparable effects on total nitrate leaching and marketable tuber yield in most cases. When treated as a group, the buckwheat rotations (BW) were shown to significantly reduce total nitrate leaching by 30% within the two rotation cycles, while significantly increasing marketable tuber yield (mean of 2016 and 2019 potato yields) by 16% compared to B-RC. However, BW produced 58% more total nitrate leaching than B-T and had a comparable marketable tuber yield. Total nitrate leaching and tuber yield were highly correlated (R-2 = 0.95) with nitrogen (N) surplus (calculated as total N supply minus N removal at crop harvest within the total nitrate leaching accounting period), with higher N surplus producing higher nitrate leaching and lower tuber yield. Nitrogen removal accounted for 33%, 36% and 53% of total N supply for B-RC, BW and B-T, respectively, indicating very low N use efficiency. These findings suggest that N surplus was the key explanatory variable for the variations in total nitrate leaching and tuber yield, and adequately accounting for N credits from the preceding rotation crop (especially red clover) is important to minimize nitrate leaching while maximizing potato productivity.