By Marnix Van den Brande (The Netherlands)
Nitrogen (N) is the most important nutrient for the plant growth and development of the potato plant. However, N is often also the most limiting essential nutrient for potato growth. Therefore, the farmer needs to add nitrogen in large quantities to increase the plant- available amount of N for uptake by the potato crop. A proper rate and timing of N fertilization is crucial for optimizing the potato tuber yield and its quality, in combination with minimizing environmental pollution. However, nitrogen leaching from agricultural sources is a major environmental concern in Europe. The main objective in this study was to develop a nitrogen mass balance for an agricultural parcel of potato crops by using sensing based methods. A potato field located in the South of the Netherlands was used as a case study to develop this N mass balance. The first step was recognizing the parameters influencing the N-mass balance and defining the assumptions. The second step consisted of the actual measurements which were performed for 14 days spread over the entire growing season. On these days we obtained measurements of the plant spectral reflectances, chlorophyll content, leaf area index and the petiole plant sap NO3- concentrations. In addition, we also investigated the nitrogen content in the topsoil (0-20 cm) and the nitrate concentrations in the groundwater and the unsaturated zone at three different depths (25, 50 and 75 cm). These measurements were used to determine the effect of the different treatments of N-fertilizer inputs on the plant status and the nitrogen content in the soil and the groundwater. The spectral reflectances in combination with derived vegetation indices were used to determine the development of the aboveground plant nitrogen content over the growing season. The Fritzmeier Isaria Sensor is used to scale up the aboveground plant nitrogen content from canopy scale to field scale. The aboveground plant nitrogen is used as one of the output parameters in the N mass balance. The other parameters used in the N mass balance are the N fertilizer inputs, N mineralisation inputs and N deposition inputs. These inputs are equalized to the outputs. The outputs are identified as the N content in the aboveground plant plants, the N content in the potato tubers and the closing parameter N leaching, which comprises the leaching losses to the groundwater and gaseous losses via volatilization and denitrification pathways. The outcomes of this study showed that for this field N leaches losses are ranging from 57 to 73% of the total amount of N supplied, in combination with N uptake efficiencies of 28 to 45%. The results of this study showed that the N balance can be quantified by sensing based instruments, however for some parameters further research is necessary.
Keywords: Nitrogen; potato; Nitrogen mass balance; sensing; aboveground plant nitrogen content; leaching.