Growth Response of Tartary Buckwheat to Plastic Mulching and Fertilization on Semiarid Land

Abstract

Integrated hole-sowing, fertilization, and plastic mulching techniques are common agronomic practices applied to collect rainwater and to improve rainwater utilization in semiarid rain-fed regions. However, little is known about the growth responses of tartary buckwheat (Fagopyrum tataricum L.) to the practices adopted in semiarid areas of Loess Plateau in Northwest China. To address the concerns, a long-term field experiment was conducted in 2015–2017. Four fertilization levels, namely, high fertilization level (N–P₂O₅–K₂O: 120–90–60 kg ha⁻¹, HF), moderate fertilization level (80–60–40 kg ha⁻¹, MF), low fertilization level (40–30–20 kg ha⁻¹, LF), and zero fertilization level (ZF), were applied to hole-sown tartary buckwheat with whole plastic mulching, in comparison to the control with no-mulching and zero fertilization (CK). Several key growth-influencing indicators were measured in the consecutive experimental years, including soil temperature (T_s), soil water storage (SWS), leaf area index (LAI), dry matter (DM), and grain yield. The results showed that in different precipitation years, 2015 (193 ± 23 mm), 2016 (149 ± 19 mm), and 2017 (243 ± 28 mm), the ZF, LF, MF, and HF treatments had the potential to optimize T_s in 0~25 cm soil layers (at 5 cm interval). The four treatments improved SWS in 0~300 cm soil layers by 3.5% and increased soil water consumption in the pre-anthesis period by 22.4%, compared with CK. Moreover, the four treatments shortened the pre-anthesis growth period by 0.4~5.4 d, while extended the post-anthesis growth period by 5.7~10.0 d, giving rise to an overall extension of 0.6~5.0 d for a whole growth period of tartary buckwheat. Furthermore, the ZF, LF, MF, and HF treatments increased LAI by 4.4~225.3% and DM weight by 41.5~238.0%. The rain yield of the four treatments was increased by 14.0~130.4%, and water use efficiency (WUE) was improved by 11.3~102.7%, especially for the LF treatment, compared with CK. The study indicated that the technique of hole-sowing and plastic mulching combined with a low fertilization rate was an effective measure for tartary buckwheat to optimize crop growth and to boost grain yield and WUE on semiarid lands.

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 barren 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 semiarid 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 mulched 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]. Integrated with NPK fertilizers, plastic film mulching was shown to effectively improve nutrition environment of plant rhizosphere [25]. This combined practice significantly promoted crop growth and development, and finally boosted grain yield [26]. There has been research showing that film mulching in combination with reasonable application of NPK fertilizers increased the crop water consumption of spring wheat (Triticum aestivum L.) by 1.5~5.1%, increased grain yield by 7.3~95.3%, and improved water use efficiency by 7.6~87.1%, in Northwest China [27]. It can be concluded that plastic film mulching combined with reasonable NPK application had a positive perspective on not only optimizing crop water consumption but also improving fertilizer utilization efficiency for cereal crops such as tartary buckwheat [28,29].

At present, relevant studies on the effect of hole-sowing with whole film mulching on yield boosting have been reported in terms of dryland wheat [16,18], but little is known about the effect of comprehensive agronomic practice on the soil hydro-thermal status in a tartary buckwheat field under different application levels of N, P, and K fertilizers. To the authors’ knowledge, the mechanism of the technique’s effect on tartary buckwheat growth and yield formation is still unclear. In view of this, a long-term located field experiment was conducted to apply the soil covering with whole film mulching and the hole-sowing technique to the cultivation of tartary buckwheat at the Dingxi Dryland Experimental Station, with a typical semiarid climate in Loess Plateau, Northwest China. By setting whole film mulching and four fertilization levels, with no fertilization and no mulching as the control (CK), this study aims to verify our hypotheses that the comprehensive agronomic practices have great potential to optimize the soil hydro-thermal status, which is beneficial to crop growth and yield boosting. The purpose of this study was to determine an appropriate amount of the fertilizer for soil- and plastic-mulched and hole-sown tartary buckwheat in semiarid areas. The study will provide a theoretical basis for forming high-yield and high-efficiency cultivation techniques for tartary buckwheat planting in semiarid areas of China.

2. Materials and Methods

2.1. Study Area

The field experiment was conducted at the Dingxi Dryland Experimental Station, affiliated to Gansu Academy of Agricultural Sciences (35° 35′ N, 104°36′ E, 1970 m a.s.l.) during 2015–2017. The long-term (2005–2015) annual mean precipitation (Pre) was 415 mm, of which 68% occurred from June to September; the annual mean temperature was 6.2 °C; the solar radiation was 5898 MJ m⁻²; the potential evapotranspiration (ET₀) was 1260 mm; the effective accumulated temperature ≥10 °C was 2075.1 °C; and the frost free period was 140 d. The weather was a temperate continental monsoon climate with a ratio of Pre to ET₀ of 0.33, belonging to a typical semiarid climate. In the consecutive experimental years, seasonal precipitation and mean air temperature were 193 mm and 16.3 °C in 2015 (normal year), 149 mm and 19.1 °C in 2016 (dry year), and 243 mm and 17.8 °C in 2017 (wet year), respectively (Figure 1). The cropping system was one cropping per annum. Most crops were rain-fed due to the lack of water resources, either from wells or rivers. The soil was a loessial soil, a typical soil in the Loess Plateau, Northwest China [30]. Average bulk density in the 0~30 cm soil layer was 1.25 g cm⁻³, and the field capacity and wilting point were 21.2% and 7.2%, respectively [31]. Organic matter content in the 0~30 cm soil layer was 12.0 g kg⁻¹, total nitrogen was 1.16 g kg⁻¹, total phosphorus was 0.73 g kg⁻¹, total potassium was 17.3 g kg⁻¹, available phosphorus was 8.67 mg kg⁻¹, and available potassium was 121.5 mg kg⁻¹ [32]. The chemical trait of the soil was alkaline, with a pH value of 8.35.

2.2. Experimental Design

A popularly used tartary buckwheat variety (cv. Yunqiao No. 2) was used in the experiment. The seed rate was applied at 90 kg ha⁻¹. Four fertilizer application treatments were set as follows: high fertilization level (N 120 kg ha⁻¹ + P₂O₅ 90 kg ha⁻¹ + K₂O 60 kg ha⁻¹, HF), moderate fertilization level (N 80 kg ha⁻¹ + P₂O₅ 60 kg ha⁻¹ + K₂O 40 kg ha⁻¹, MF), low fertilization level (N 40 kg ha⁻¹ + P₂O₅ 30 kg ha⁻¹ + K₂O 20 kg ha⁻¹, LF), and zero fertilization (none of N–P–K, ZF). The abovementioned four treatments were all plastic film mulched, in comparison with zero fertilization with no mulching taken as the control (CK) (Figure 2). All treatments were repeated three times and arranged in a completely random block design. The plot area was 35 m² (5 m × 7 m) in a south–to–north direction. Fertilization and film mulching were carried out in late March each year. Field operations, including hole sowing, film mulching, and fertilization, were completed using an integrated seed drill and mulching machine (Sanniu Agricultural Machinery Manufacturing Co., Ltd., Dingxi, China). The size of the polyethylene plastic film (Golden–soil Plastic Products Co., Ltd., Lanzhou, China) was 0.01 mm thick and 110 cm wide. Urea (N ≥ 46.0%), diammonium phosphate (P₂O₅ ≥ 46.0%, N ≥ 18.0%), and potassium chloride (K₂O ≥ 51.0%) were applied as base fertilizers. Sowing depth was 3~4 cm, with 5~7 seeds per hole. The row spacing was 30 cm, and plant spacing was 12 cm (equivalent to 18 rows in each plot), giving rise to a plant density of 1.8 × 10⁶ plants ha⁻¹. In order to reduce white pollution, the plastic film was recycled after crop harvest, leaving the land uncovered until the second year’s soil preparation. Tartary buckwheat was sown on 6 May 2015, 29 May 2016, and 26 May 2017 and harvested on 31 August 2015, 5 September 2016, and 12 September 2017, respectively. Except manual weeding at the jointing stage, no additional management was conducted during the whole growth period.

2.3. Measurements

2.3.1. Soil Temperature

Soil temperature (T_s, °C) was measured at soil depths of 5, 10, 15, 20, and 25 cm using thermometers (Wuqiang Thermometer Co., Ltd., Hengshui City, China) installed in plant rows between two sowing holes. The readings were manually recorded at 8:00, 14:00, and 18:00 every seven days.

2.3.2. Soil Water Content

The soil water content (SWC, %) was measured using an oven-drying method at the sowing, flowering, and maturity stages of tartary buckwheat. Soil cores were sampled using an auger (5 cm in diameter) to a depth of 0~300 cm at an increment of 20 cm and were oven-dried at 105 °C for 8 h to a constant weight. SWC was calculated by weighing the wet and dry soil cores and was converted to volumetric soil water content by multiplying its corresponding soil bulk density, which was determined using the cutting ring method [28].

2.3.3. Soil Water Storage

Soil water storage (SWS, mm) was calculated in the 0~300 cm soil depth using the following equation:

SWS = 10 × h × a × θ

(1)

where h is soil depth (cm), a is soil bulk density (g cm⁻³), and θ is soil water content by weight (%).

2.3.4. Crop Water Consumption

Crop water consumption was considered equal to crop evapotranspiration (ET_c, mm). calculated using the water balance equation. In the study, ET_c was the sum of the soil water at sowing minus the soil water at harvest plus the seasonal precipitation, which was calculated using the following equation:

ET_c = SWS_s − SWS_h + Pre

(2)

where SWS_s and SWS_h are soil water storage (mm) in the 0~300 cm soil layer at sowing and harvesting, respectively. Pre is seasonal precipitation (mm).

2.3.5. Dry Matter Accumulation

The dry matter weight (DM, g plant⁻¹) was estimated by randomly sampling 10 tartary buckwheat plants from each plot at the seedling, flowering, and maturity stages. These samples were firstly separated by leaves, sheaths, stems, and grains; were oven-dried at 105 °C for 30 min to cease the activity of enzymes and the respiration of plant organs; and then, were oven-dried at 80 °C for 48 h to a constant weight.

2.3.6. Leaf Area Index

Leaf area index (LAI) was measured using a CI–110 plant canopy digital image analyzer (CID Inc., Camas, WA, USA) at 10:00 on a sunny day at the jointing, flowering and mid–filling stages of tartary buckwheat. The LAI was measured on 10 plants with similar leaf ages and uniform growth in each plot, and the average values were calculated to represent the whole plot’s LAI.

2.3.7. Grain Yield and Water Use Efficiency

Grain yield (kg ha⁻¹) of tartary buckwheat was determined by hand-harvesting all plants in each plot at full maturity. The grain yield was calculated on a dry weight basis (14%). Water use efficiency (WUE, kg ha⁻¹ mm⁻¹) was calculated as the grain yield (kg ha⁻¹) produced per unit of crop evapotranspiration (mm).

2.4. Data Statistics

The normality of the data and variance homogeneity were checked before being applied to the statistical analysis. Significant differences in GY, WUE, DM, LAI, and ET_c among treatments were tested through an analysis of variance (ANOVA) using SPSS 19.0 software (SPSS 19.0, SPSS Institute Inc., Chicago, IL, USA). The treatment effects were determined using the Duncan’s multiple-range test (p < 0.05).

3. Results

3.1. Soil Temperature

Soil temperature (T_s, °C) had similar trends to air temperature (T_a, °C) in 2015 and 2017, whereas in 2016, T_s in the seeding–branching period showed different trends compared with T_a. Specifically, T_s was observed to be higher in the sowing–seedling period of 2016 than those in 2015 and 2017 (Figure 3). Compared with CK, mean T_s with the plastic mulching treatments (ZF, LF, MF, and HF) in a normal year (2015) were higher, with an increase by 1.2~2.4 °C (p < 0.05). The values of T_s with mulching treatments from the sowing to branching period were 0.9~2.2 °C (p < 0.05) higher than that of CK and 0.3~2.2 °C lower than that of CK from branching to maturity in a dry year (2016). In a wet year, 2017, T_s with mulching treatments from sowing to flowering was 1.2~1.7 °C higher (p < 0.05). It can be concluded that, during the three precipitation years, plastic mulching generated less abnormal values of T_s in the 0~25 cm soil layers, compared with CK. This was conducive to the growth of root systems during the early growing periods of tartary buckwheat.

3.2. Soil Water Storage

Soil water storage (SWS, mm) in the 0~300 cm soil layers varied differently among the three precipitation years (p < 0.05) (Figure 4). At the flowering stage, the MF and ZF treatments conserved 13.3 and 8.4 mm more SWS than did the CK, while HF and LF consumed 18.4 and 32.8 mm more SWS than did the CK in normal year (2015). No significant differences in SWS were observed at the sowing and maturity stages. However, in a dry year, 2016, SWS with CK was 11.0–32.4 mm higher at the flowering stage and 14.9–39.0 mm higher at maturity, compared with the other treatments. SWS of the ZF, LF, MF, and HF treatments was 39.7, 43.3, 41.9, and 35.5 mm higher than that of CK at the sowing stage in a wet year, 2017, whereas at the flowering stage, only ZF produced 44.3 mm higher SWS compared with CK (p < 0.05).

3.3. Water Consumption by Crops

Because the crops were wholly mulched with plastic films, soil evaporation accounted for a tiny proportion of the crop evapotranspiration (ET_c). Therefore, water consumption by crops was considered equivalent to ET_c. The pre-anthesis ET_c treated with LF and HF was 11% higher than that of CK in a normal year, 2015 (p < 0.05) (Figure 5). The post-anthesis ET_c with the ZF, LF, MF, and HF treatments was 17.3, 2.9, 9.7, and 9.9 mm lower compared to that of CK. The pre-anthesis ET_c of ZF, LF, MF, and HF in a dry year (2016) was 42.6, 26.2, 21.2, and 22.0 mm higher than that of CK (p < 0.05). Except ZF, seasonal ET_c with LF, MF, and HF was 37.3, 30.1, 36.2, and 30.3 mm higher than that of CK in 2016 (p < 0.05). In a wet year, 2017, the pre-anthesis ET_c with LF, MF, and HF was 76.6, 91.3, and 66.0 mm higher, but the post-anthesis ET_c was 26.0, 51.5, and 25.4 mm lower, compared with CK. Consequently, seasonal ET_c with ZF, LF, MF, and HF became 40.6, 50.6, 39.8 and 40.6 mm higher (p < 0.05), respectively, compared with that of CK in 2017.

3.4. Growth Period

Fertilization treatments had significant effects on the growth period of crops (Figure 6). In general, the LF, MF, and HF treatments extended the growth period of tartary buckwheat by 1.6~3.0 d, 2.0~4.0 d, and 2.6~5.0 d, respectively, in 2015–2017, compared with CK. However, the pre-anthesis period was shortened by 3.0~3.4 d, 1.0~5.4 d, and 0.4~2.4 d, respectively, showing noticeable accelerating effects of fertilization on the growing process from the sowing to flowering periods. In contrast, the post-anthesis period was extended by 6.3~9.0 d, 7.0~10.0 d, and 5.7~9.0 d, respectively. Furthermore, ZF showed a similar effect on the growth period, indicating that plastic mulching also regulated crop growth to some extent. In detail, fertilization and mulching sped up the vegetative growth period but extended the reproductive phase of crops regardless of precipitation years.

3.5. Leaf Area Index

In any precipitation year, the LAI values with ZF, LF, MF, and HF at the branching, flowering, and filling stages were significantly higher than those of CK (p < 0.05) (Figure 7). Specifically, LAI with the ZF, LF, MF, and HF treatments was increased by 4.4~78.5% in a normal year, by 77.1~225.0% in a dry year, and by 74.6~218.0% in a wet year, respectively. Among the mulched treatments, LAI with the ZF treatment was significantly lower than that of LF, HF, and MF (p < 0.05). Regardless of precipitation year, LAI at the filling stage was greatest with the LF treatment, showing an increase by 1.8~217.8% compared with that with other treatments.

3.6. Dry Matter Accumulation

Compared with CK, dry matter (DM) weights with the ZF, LF, HF, and MF treatments were increased by 41.5~74.0%, 63.4~78.5%, and 63.7~73.2% at the seedling, flowering and maturity stages, respectively, in a normal year (2015) (p < 0.05) (Figure 8). Similarly, DM weights were increased by 202.0~238.0%, 87.4~145.0%, and 108.2~142.6%, respectively, in a dry year (2016), and by 68.9~177.8%, 52.5~85.4%, 67.9~83.2%, respectively, in a wet year (2017). Among the mulched treatments, DM with the ZF treatment was significantly lower than that of LF, HF, and MF (p < 0.05). Regardless of precipitation year, DM weights at maturity were greatest with the LF treatment, showing an increase by 1.5~142.6% compared with those with other treatments.

3.7. Grain Yield and Water Use Efficiency

On average, grain yield and WUE with the LF treatment were highest in the three years, while CK produced the lowest yield and WUE (Figure 9). Generally, the yield increasing rate with fertilization and mulching treatments was greatest in the dry year (2016), moderate in the wet year (2017), and lowest in the normal year (2015). For example, LF, ZF, MF, and HF had increases by 130.4%, 127.8%, 115.8%, and 76.6% in yield compared to CK in the dry year; by 95.0%, 59.0%, 70.5%, and 51.4% in the wet year; and by 33.6%, 29.4%, 19.7%, and 14.0% in the normal year (p < 0.05). Grain yield with the LF treatment was significantly highest among the treatments in the wet year (p < 0.05). Moreover, LF showed the greatest increasing rate in WUE in the wet year. Compared with CK, the treatments with LF, ZF, MF, and HF produced significantly higher WUE. For example, LF, ZF, MF, and HF increased WUE by 96.6%, 99.2%, 89.4%, and 58.2% in the dry year; by 102.7%, 36.5%, 25.0%, and 30.0% in the wet year; and by 34.9%, 34.5%, 22.5%, and 11.3% in the normal year (p < 0.05).

3.8. Implications of the Results

The present results confirmed that hole-sowing and plastic mulching practices were conducive to the improvement in LAI, DM weight, grain yield, and WUE. Tartary buckwheat grew faster and more vigorously under different fertilization levels with whole plastic mulching. LF treatment generated the highest LAI and DM weight and was shown to be favorable to the growth and development of crops. Because MF and HF treatments made the vegetative growth period much longer due to the higher fertilizer availability, they resulted in a shorter period of reproductive growth and reduced the final grain yield, while LF balanced the vegetative and reproductive growth period well. Our results clearly implied that hole-sown and whole-plastic-mulched tartary buckwheat with low fertilization levels were the best management practices for maximizing yield and WUE of rain-fed tartary buckwheat in semiarid regions of China.

4. Discussion

4.1. Effects of Mulching and Fertilization on Soil Thermal Traits

Whole plastic mulching with the hole-sowing technique has shown great potential to conserve soil moisture and to increase soil temperature (T_s), especially in the early growing period [33,34]. The technique optimized the soil thermal status in the plough layer through the soil heat preservation effect of the whole mulching practice [35,36]. Our results showed that the current cultivation practice had significant regulating effects on T_s in the 0~25 cm soil layer. The heat preservation of plastic mulching was more noticeable in the early growing period due to low air temperature [37]. For example, during the sowing to branching period, T_s was increased by 0.9~2.2 °C for mulching treatments compared to that of CK. The warming effect of plastic mulching promoted seed germination and root vitality and was conducive to the early morphological formation of crops [38]. In the mid–late growing period (flowering to maturity), characterized by its high air temperature and drought stress, ZF and LF showed significant potential to reduce the average T_s. That was probably because the lower nutrient availability of LF and ZF resulted in slower microbial and root respiratory activity, giving rise to a lower energy exchange environment [39,40]. However, excessive nutrient availability by MF and HF accelerated the root respiratory rate and senescence, whereas LF markedly delayed the physiological senescence of roots [41,42], explaining why the grain yield and WUE of LF were the greatest.

4.2. Effects of Mulching and Fertilization on Soil Water Storage and ET_c

The plastic mulching with the hole-sowing technique optimized the soil moisture status by reducing the dissipation of soil moisture and by inhibiting soil evaporation [14]. Reasonable fertilization improved the soil moisture environment for crop growth [43]. The whole mulching technique markedly reduced soil water storage (SWS) in the 0~300 cm soil layer across different precipitation years. LF, MF, and HF significantly decreased SWS in the flowering period of tartary buckwheat, resulting in an increase in ET_c during the entire growing season. The result suggested significant correlation between SWS and ET_c, which was reported in many previous studies [44,45]. In the wet year, enough precipitation was received prior to sowing and whole plastic mulching played a key role in conserving soil moisture. In this study, SWS of the mulching treatments was significantly increased during the sowing to branching period. A similar soil moisture conservation effect was also observed in the published literature [46]. Among the treatments, LF showed a favorable advantage in moisture conservation over MF and HF, which was probably due to the lower plant transpiration and water consumption by LF. The beneficial effect of LF in conserving SWS is conducive to the growth of tartary buckwheat [47].

4.3. Effects of Mulching and Fertilization on Crop Growth

The sowing period of tartary buckwheat was usually dominated by less rainfall and a lower temperature. Hole-sowing and whole mulching technology can optimize the soil hydrothermal environment in the rhizosphere. Together with fertilization application, these techniques will significantly regulate crop growth. It has been reported that the vegetative growth period was shortened, while the reproductive growth period was extended for the mulching and fertilization treatments of wheat regardless of the precipitation year [48]. Moreover, the final grain yield formation and components were associated with crop growth, DM weight, and LAI [49]. The present study proved that regardless of the precipitation year, tartary buckwheat grew faster and more vigorously under different fertilization levels with whole plastic mulching, as indicated by its higher DM and LAI. This was considered conducive to the formation of photosynthetic substances such as carbohydrate [30]. Among all treatments, LF generated the highest DM weights and LAI, especially in the dry year, implying that LF created a relatively favorable nutrient environment for the growth and development of tartary buckwheat, which was conducive to the formation of a high grain yield [50].

4.4. Effects of Plastic Mulching and Fertilization on Grain Yield and WUE

Plastic mulching was shown to improve grain yield and WUE of various cereal crops, but the promotion effects varied across different precipitation years [51,52]. Reasonable fertilization can promote the full use of soil moisture, but excessive fertilization can not only make yield stagnated but also may have a negative effect on yield formation [53]. Previous studies reported that the combined application of fertilizers had a significant impact on the grain yield of buckwheat without mulching [54]. The present study found that hole-sowing and plastic mulching were able to improve the promotion effect of fertilization application on grain yield. However, the degree of the effect varied across precipitation years, among which the promotion effect was greatest in the dry years. Our results confirmed that, in the semiarid areas of the Loess Plateau, grain yield and WUE were jointly affected by natural factors (e.g., precipitation, temperature, etc.) and cultivation practices (e.g., hole-sowing and mulching). This can be verified by the significantly higher grain yield of LF, ZF, MF, and HF treatments in the three consecutive years than that of CK. LF produced the highest grain yield, especially in the dry year. This was because LF generated a shorter vegetative period due to fertilization control, and its soil water extraction depth was observed to be deeper than MF and HF, promoting the use of soil moisture in deep soil layers and improving the utilization efficiency of soil nutrients [55].

5. Conclusions

In the present study, we investigated the growth response of hole-sown and whole-plastic-mulched tartary buckwheat to four fertilizer application levels in Northwest China. In general, the results suggested that a low fertilization level (LF: N 40 kg ha⁻¹ + P₂O₅ 30 kg ha⁻¹ + K₂O 20 kg ha⁻¹) was the best treatment regulating soil temperature, conserving soil moisture, and boosting yield. Moderate and high fertilization (MF and HF) resulted in longer vegetative and shorter reproductive growth due to a higher nutrient availability, bringing an adverse effect on dry matter transportation. The yield-boosting effect of LF was more obvious in dry years because crops with LF stored more water than those with MF and HF, increasing crops’ tolerance to drought. LF balanced the vegetative and reproductive growth well, producing the greatest leaf area index and dry matter accumulation, and a longer reproductive growth provides adequate time for crops to transfer photosynthetic carbohydrates to reproductive organs, which was conducive to grain yield and WUE. The findings improve our understanding of reasonable applications of film mulching and fertilizers to optimize crop growth and to achieve high yield and WUE in the semiarid areas of Loess Plateau, Northwest China.