## 1 Introduction  
Water and nutrients are two important factors influencing the sustainable development of rain-fed agriculture in arid and semiarid areas [1,2]. Making full use of natural precipitation and improving the utilization efficiency of soil moisture and nutrients are key to domestic food security and national agricultural productivity in China [3]. Tartary buckwheat (_Fagopyrum tataricum_ L.) is an annual herb belonging to Polygonaceae plants. Its grains are rich in protein, vitamins, mineral elements, and flavonoids, which can be used as both medicine and food [4]. The crop also has the advantages of a shorter growth period, and better tolerance and resilience to drought and nutrient deficiency in parent soils, becoming a special crop widely planted in arid and semiarid regions of the Loess Plateau in Northwest China [5]. However, local small householders usually applied traditional farming techniques to tartary buckwheat cultivation [6]. The most common practice for local farmers was the excessive application of nitrogen fertilizer, resulting in an unbalanced fertilization ratio and soil nutrient leaching loss [7]. Consequently, crops such as tartary buckwheat produced less grain yield per unit of water and nutrient, giving rise to a lower utilization efficiency of water and fertilizers [8]. This restricted the development of the tartary buckwheat industry and lowered the profits of local farmers. Therefore, making full use of the limited soil moisture and optimizing the usage of chemical fertilizer have been crucial for sustainable agricultural development [9; 10]. Plastic film mulching plays an important role in the yield formation of dryland crops [10]. Reasonable water and fertilizer management is also critical in improving the utilization efficiency of water and nutrient resources [11; 12]. In previous studies, plastic mulching was shown to increase soil temperature [13] and to improve rainwater harvesting and soil moisture conservation. The optimization of fertilizer application was conducive to regulating the consumption of water by crops [14], promoting crop growth [15], boosting grain yield [16], and increasing water and nutrient utilization efficiency [17]. These agronomic strategies have become an effective means for agricultural production in semi-arid areas of the Loess Plateau in Northwest China. In addition to plastic mulching, the practice of soil covering with plastic mulching in dryland has recently been an innovative cultivation technology for multiched dryland crops due to its capacity to reduce soil evaporation, to absorb solar radiation, and to suppress weeds [13; 14]. The technique was also popularly adopted in tartary buckwheat planting. Local researchers and scholars called this technique a combination of soil covering with plastic mulching plus hole-sowing, and it has been widely extended and applied in semiarid areas in Northwest China. Reasonable fertilization is one of the key measures to increase grain yield and to effectively use soil nutrients in dryland [18]. The combined application of nitrogen (N), phosphorus (P), and potassium (K) fertilizers can make plants fully use soil moisture because of their promotion for root growth and penetration [19]. Moreover, as a commonly used practice in cool and semiarid areas, plastic film mulching has shown beneficial impacts on crop growth. The technique is shown to conserve soil moisture and to improve soil temperature, which promotes the earlier use of soil nutrient [20]. In Loess Plateau, where soil is erosive, plastic mulching reduces soil erosion by limiting soil particle movement with rainwater [21]. It also has other benefits, such as weed and pest pressure, ripening time shortening, and yield boosting [22]. Nevertheless, it brings adverse environmental effects in agriculture known as white pollution [23]. To reduce the impact of plastic mulching on the environment, all films covered were removed and recycled by farmers and film manufacturers [24].