The response of celeriac yield, quality and soil nitrate residue to nitrogen fertilization and supply

Despite major efforts to reduce nitrogen (N) losses, nitrate (NO₃^-) concentrations in surface and groundwater remain too high, especially in field grown vegetable production regions. Stringent N fertilization rates stay the best strategy to increase N efficiency. We collected data from 4 Flemish celeriac fields where crop and soil measurements were done during the cropping period. We also re-analyzed 9 celeriac field experiments with various N fertilization rates and examined total and tuber yield and quality and soil NO₃^--N contents at harvest. The seasonal patterns demonstrate that the first months after planting celeriac grows slowly and takes up very little N but growth and N uptake continue till the end of October/November. Concurrently the soil NO₃^--N content keeps reducing. On highly fertile soil an N supply (= applied effective N + NO₃^--N at planting (0-60 cm)) <100-150 kg N ha^‑1, corresponding to <50-100 kg applied effective N ha^‑1, can limit the tuber yield or quality parameter leaf color. Thanks to the extra leaf production at high N supply and a good developed horizontal root system, the risk of a high NO₃^--N residue at the harvest is low. Fractionated N fertilization can abate the NO₃^--N leaching risk below rooting depth caused by heavy rain in spring when N uptake is low and also minimizes the risk of N rich crop residue by improving the N mineralization amount estimation. Due to impact of management and weather the N mineralization amount can differ largely between celeriac fields and is difficult to calculate, especially for hired fields. A field database with soil measurements, fertilization information and crop rotations would facilitate the N fertilization rate calculations and would stimulate farmers to work on soil quality.