## 1 Introduction  
Bog bilberry (_Vaccinium uliginosum_ L.) has attracted much interest in the field of food and nutritional sciences due to its beneficial health properties [1]. It has been confirmed that these beneficial properties are mainly related to bog bilberry's high level of phenolic compounds, such as anthocyanins and phenolic acids, since these secondary nutrients possess anti-cancer, anti-cardiovascular, anti-obesity, and anti-oxidative features [2, 3]. However, fresh consumption of bog bilberry is not recommended because of its high organic acid-to-sugar ratio compared to other fruits [4]. Sugar fortification has been widely applied to bog bilberry. Its by-product after the process, bog bilberry syrup, has been generally used as the raw material for bog bilberry syrup wine production since it is rich in functional ingredients derived from bog bilberry fruits [5]. Fermentation is a process by which sugar is metabolized by yeasts into alcohol [6]. Therefore, the replication and growth of yeasts during wine fermentation play a significant role in affecting the fermentation performance [7, 8]. At the early stage of the fermentation process, nitrogen sources in the fruit matrix play important roles in the growth and replication of yeasts. Sufficient yeast population can provide suitable fermentation activity, which can ensure the sugar-to-ethanol conversion efficiency during the fermentation period [9]. It has been reported that nitrogen supplements, including inorganic nitrogen and organic nitrogen nutrients, have been applied to the fruit wine fermentation process [4; 7; 8; 10; 11]. Ammonium salts are the main inorganic nitrogen that is used for wine fermentation, whereas organic nitrogen sources applied to wine fermentation mainly include \(\gamma\)-aminobutyric acid, short peptides, purine, and pyrimidine bases [6]. Compared to organic nitrogen supplements, ammonium salts have been reported to have a more stimulatory effect on the yeast activity [6]. It has been reported that fruit wine fermentation performance could be significantly improved with the supplementation of dibasic ammonium phosphate (DAP) [7; 12; 13]. For example, the addition of high DAP amounts in grape matrix significantly accelerated the rate of fermentation and enhanced the sugar metabolism, resulting in grape wine with a low residual sugar level [12; 14]. Wine sensory quality and nutritional value directly affect its perception and acceptability to consumers [15; 16]. Basically, wine sensory quality consists of its appearance, overall aroma, and mouthfeel [17]. Phenolic compounds in wine mainly determine the color attributes and mouthfeel of wine, whereas the overall wine aroma is attributed to the composition and distribution of aromatic compounds in wine [18; 19]. It has been accepted that fruit type (raw material), yeast strain, and fermentation conditions play important roles in determination of the overall aroma of final wine products [19]. Moreover, nitrogen supplement has also been reported to impact the aromatic compound composition in wine [10; 14; 20; 21; 22; 23; 24]. For example, it has been reported that nitrogen source type and/or addition amount could regulate the fermentation kinetics, which further altered the composition and profile of aromatic compounds in wines [9; 20; 21]. An appropriate level of nitrogen source in fruit material can reduce the accumulation of off-flavor volatiles during wine fermentation, improving the sensory attributes of the final wine products after fermentation [10]. To the best of our knowledge, such investigations have only been extensively conducted in grape wine fermentation. Bog bilberry has a totally different aromatic profile compared to grape [25; 26]. More importantly, our preliminary study revealed that a stuck fermentation occurred in bog bilberry syrup without the supplementation of DAP, whereas a 30 mg/L DAP addition in bog bilberry syrup resulted in a sluggish fermentation (Supplementary Materials Table S1). These indicated that bog bilberry syrup had a nitrogen source deficiency.