Impact of Nitrogen Fertilization on Tuber Yield, Sugar Composition and Nitrogen Uptake of Two Yacon (<i>Smallanthus sonchifolius</i> Poepp. &amp; Endl.) Genotypes

The tuberous root crop, yacon, is native to the Andean region and contains high amounts of fructooligosaccharides (FOS) with up to 70% of dry matter. Due to FOS, consumption of tubers may have health-promoting effects. However, regarding the overall cultivation system, no recommendations exist for farmers on nitrogen fertilization and nitrogen (N) uptake of yacon. Therefore, three different N fertilization levels (0, 40, and 80 kg N ha&#8722;1) and two genotypes (brown-shelled (BG) and red-shelled (RG)) were examined in a two-year field experiment regarding their tuber yield, sugar composition, and nitrogen uptake. Tuber yields increased with increasing fertilization level and were highest for B80 and R80 (50 and 67 t FM ha&#8722;1), while significant differences between the genotypes existed. Sugar and the amount of FOS slightly decreased with increasing N fertilization level, and ranged between 36% and 73% and 30% and 58% of dry matter, respectively. An overall decreasing amount of FOS led to a slight increase in the amount of FOS with a higher degree of polymerization. Regarding the N-use efficiency of tubers and the total plant, an N fertilization level of 40 kg N ha&#8722;1 seems to favor tuber yield