Scenarios for precision nitrogen management in potato: Impact on yield, tuber quality and post-harvest nitrate residues in the soil

Potato belongs to the category of the staple food playing an important role in global food security and nutrition. Because potato plants have a shallow root system and stop nitrogen uptake early in the growing season (between 20 and 60 days after emergence), potato fields are more prone to leaching of nitrate to lower soil layers and the groundwater than deep rooting crops, thus posing a higher risk for polluting the environment with nitrate. On the other hand, lowering fertilizer dosages could result in yield loss and reduced tuber quality. To balance the risk of yield and quality loss against the risk of nitrate leaching, the fertilization dose should be adapted to the local production potential, which can vary considerably within a field. However, there is no consensus on how to adapt the fertilization to this variability. Therefore, the objective of this study was to assess various N fertilization scenarios and their effect on the concentration of residual nitrate, total potato yield and tuber quality. Five potato fields located in Flanders with historical spatial variation in crop productivity were selected as potential candidates for implementing site-specific nitrogen management practice. The fields were divided into management zones based on variation visible in soil maps, maps of electrical conductivity, maps of vegetation indices and soil samples. Nitrogen was applied in three different dosages, including the conventional dose, and two reduced fertilization levels. Based on time series of vegetation indices, cumulative vegetation indices and pair-wise correlations between the vegetation index values obtained from Sentinel-2 satellite imagery (FAPAR, NDVI, FCOVER, LAI) only two fields of five showed temporal stability of the spatial management zones, while in three other fields the zone performance shifted depending on the weather conditions during the growing season. In the fields with stable zones, the lowest N fertilization levels in the areas with lower productivity potential resulted in a reduction of the concentration of leachable nitrates while total yield and potato tuber quality (dry matter and nitrogen content) did not change. For this type of fields with temporally stable zones, it was concluded to recommend reducing the nitrogen dosage in the zones with a lower productivity potential and high mineralization levels to limit nitrate leaching. In the fields where better performing zones change their location in wet and dry years, significant differences between treatments in the total potato yield, dry matter and nitrogen content in the tuber were also often detected. However, application of a low nitrogen dose during the first fertilization in one of the zones in such fields is not recommended as it is not yet known how the zones will behave the coming season, leading to a risk for underfertilization of these zones. Therefore, conventional fertilization remains the recommended practice in case the production potential of the zones depends on the weather during the growing season and is not yet known at the moment of fertilization. However, fertilizer fractions applied later during the growing season may be adapted to the crop development and actual weather conditions.