Linking soil N dynamics and plant N uptake by means of sensor support

Monitoring the spatial and temporal plant availability of nitrogen (N) in agroecosystems is a key step to improve the synchronization between N fertilizer application and crop N demand, consequently reducing the risk of N emissions to the environment. Using a winter wheat N fertilization dataset from six site-years, we linked dynamic nitrate data measured in the soil solution to standard soil and crop analyses data and multispectral imagery acquired by an unmanned aerial vehicle. Wheat N uptake was determined as remotely estimated N uptake (REN) from the spectral data with a power regression model (mean absolute error = 17 kg N ha−1). The nitrate-N in the soil solution (NSS), extracted by means of suction cups, was measured with an ion-selective electrode. The REN proved to be suitable for monitoring the accumulation of N in the plants along the season. The NSS was characterized by low values and found of limited use as a direct indicator for potentially plant-available N. The N balances resulted in N surplus in the range of 43–100 kg N ha−1 over the six site-years. The most important contribution to the N balances was the soil N supply (67–143 kg N ha−1; mineralization and atmospheric input). Including this factor in the fertilization strategy was investigated post-season by calculating the ‘adjusted N fertilization norm’, reflecting the current best fertilization practice in Switzerland. The approach suggested lower N fertilization rates in the fields with higher N surplus. However, such static empirical strategies do not allow to react to in-season changes. Sensor-based monitoring could help to overcome this shortcoming.