Effects of Straw Strip Covering on Yield and Water Use Efficiency of Potato cultivars with Different Maturities in Rain-Fed Area of Northwest China

Abstract

The strip mulch of corn straw planting technique is widely used in the Northwest rain-fed agricultural region of China due to the region’s good soil moisture-temperature properties. However, the hydrothermal properties and yield effects of this planting technique on different potato varieties are not clear. Therefore, a field experiment was conducted for two years (2015 and 2016) in the rain-fed area of Northwest China, to study the effects of different mulching treatments on the yield and water use efficiency (WUE) of potato with different maturation characteristics (potato varieties Longshu 7 and LK99). A split-plot experimental design was used. The experiment included corn straw strip flat cover planting (SMF), plastic film mulching (PMF), corn straw strip ditch mulching and ridge planting (SMFR), straw flat fully covered (SMWF), and uncovered and flat planting (CK). The results showed that straw mulch and plastic film mulch had significant effects on yield and WUE (p < 0.05). Compared with CK, SMF significantly increased the yield by 3.75–63.17% in the late-maturing varieties, and increased the yield by 24.96–79.02% in the early-maturing varieties. Among the mulching treatments, compared with SMF, PMF made no significant difference in the yield and WUE between the late-maturing varieties in the normal year and the early-maturing varieties in the dry year, but the yield and WUE of the early-maturing varieties in the normal year significantly decreased by 17.25% and 18.44%, respectively, and the yield and WUE of the late-maturing varieties in the dry year significantly increased by 27.57% and 29.26% respectively. Compared with SMF, SMFR reduced yield by 1.44–13.62%; SMWF decreased the yield of late-maturing varieties by 25.94% (p < 0.05), and WUE decreased by 7.65–23.44% (p < 0.05). It can be seen that under this experimental condition, SMF is more suitable for early-maturing varieties and PMF is more suitable for late-maturing varieties.

1. Introduction

Potato (Solanum tuberosum L.) is widely planted all over the world, It is the fourth largest grain and vegetable crop after wheat, rice and corn [1]. According to the United Nations Food and Agriculture Organization (FAO) statistics, 157 countries around the world grow potato, which has a total planting area of 19.46 million hm², and an annual output of 370 million tons [2]. China has become the world’s largest potato producer; its planting area and yield are ranked first in the world [3]. Potato is planted in all ecological regions of China [4], especially in semiarid rain-fed agricultural areas, accounting for 36% of China’s total potato land area [5], which plays an important role in ensuring food security [6].

However, the potato yield in the Northwest arid region is low and unstable. Numerous studies have show that plastic or straw mulch is an efficient practice, which can alter water distribution between soil evaporation and plant transpiration [7]. Plastic film mulching can increase crop yield, but long-term use causes soil compaction, plastic film residue affecting emergence and nutrient absorption, which seriously restricts the sustainable development of agriculture [8].

Most studies have shown that in dryland agriculture, straw mulching is considered to be a green and sustainable dry farming cultivation technique [9], because of its improvement of soil structure, fertility, optimization of soil hydrothermal environment, and increase in crop yield [10]. It is sometimes superior to plastic-film mulching cultivation techniques in terms of soil moisture storage and potato yield [11]. However, studies also showed that straw mulching decreased the yield of potato in some areas by decreasing soil temperatures at seedling stage [11].

On the basis of summarizing the advantages and disadvantages of film mulching and straw mulching, our research group put forward the technology of straw strip mulching in dryland (SMF). The SMF consists of two critical components: straw strips mulch and alternating crop planting strips (non-mulch). Our previous studies showed that, compared with the traditional flat planting without mulching, the SMF significantly increased yield of potato on late-maturing by 36.9–61.2% and enhanced water use efficiency (WUE) by up to 74.8% [12,13,14].

The potato varieties planted in the semiarid rain-fed area of Northwest China are diverse, with multiple varieties of early and late maturity. Different mulching methods have different adaptability to different maturity varieties. Previous research by our team had shown that film mulching had positive effects on soil water storage at the early stage of potato growth, and the straw mulching significantly improved soil water storage at the late growth stage [12]. Our previous studies mainly focused on late-maturing varieties. There were few reports on soil hydrothermal characteristics and yield-increasing mechanism of straw-mulching cultivation on different maturation characteristics potatoes varieties. The effect of straw strip-mulching technology on the yield of early-maturing potato varieties is not clear [15].

To this end, the hydrothermal characteristics and yield effects of different maturity potato varieties under different mulching modes were studied, and the soil moisture, soil temperature and yield effects of early-maturing varieties under straw mulching were compared and analyzed. The yield-increasing mechanism of the mulching mode on potato was further investigated, and the best cultivation mode of different maturity potato varieties in the semiarid rain-fed area of Northwest China was sought, which was conducive to enriching local potato dry farming cultivation techniques and improving potato yield. Therefore, we conducted a field experiment to analyze the yield and WUE of different maturity varieties under different mulching methods. Specifically, the purpose of this field study were to: (a) compare the effects of different maturation characteristics potatoes in different straw-mulching methods on soil water storage, evapotranspiration (ET), and soil temperature during the growing season; (b) evaluate the effects of different maturation characteristics potato varieties under different straw-mulching methods on the tuber yield and WUE; (c) investigate the yield potential of different maturity varieties under different mulching modes; and (d) determine the suitable coverage mode of different maturity potato varieties in the semiarid rainfed area of Northwest China.

2. Materials and Methods

2.1. Description of the Experiment Site

Field experiments were conducted from 2015 to 2016 at the Tongwei Modern Dryland Circular Farming Experiment Station of Gansu Agricultural University, Gansu Province, China (35°11′ N, 105°19′ E; altitude 1755 m). This region has an arid inland climate with an average temperature of 7.2 °C, annual mean soil evaporation of 1500 mm, and average annual precipitation of 390.7 mm, about 60% occurring between July to September (Figure 1). In 2015, the effective precipitation during potato growth period was 281.2 mm, which was a normal water year. In 2016, the effective precipitation during potato growth period was 201.8 mm, which was a partial drought year. The number of frost-free days ranges from 120 to 170 d and an average of 2092 h of sunshine each year. The growing season for potato is from late April to mid-October. The total growing season is almost 170 days. The soil type is loess soil, according to the international soil classification system (2022) [16], with an average bulk density of 1.25 g cm⁻³ in 0–200 cm soil depth. Soil nutrients in 0–90 cm soil layer at the experimental field were determined by the soil analysis institute of Gansu Academy of Agricultural Sciences. The soil components (0–90 cm) at the site were as follows: total N: 0.45 g kg⁻¹, available P: 4.70 mg kg⁻¹, available K: 173.58 mg kg⁻¹, soil organic matter 8.30 g kg⁻¹, and pH was 8.66.

2.2. Experimental Design

Two varieties of local main planting early maturity (LK99) and late maturity (Long 7) were used in the experiment. They were provided by the Potato Research Institute of Gansu Academy of Agricultural Sciences. The growth period of LK99 is about 120 days and that of Long 7 is about 170 days. A two-factor split-plot experimental design was used, with the coverage mode as the main plots and the variety as the sub-plots. Each treatment was repeated three times. Four cultivation treatments were set for both varieties in 2015, which were corn straw strip flat cover planting (SMF), plastic-film mulching (PMF), corn mulching on furrow and ridge planting (SMFR), and uncovered and flat planting (CK). The straw fully covered flat planting treatment (SMWF) was added on the basis of 2015 in 2016. See Figure 2 for potato cultivation practices. There were 12 experimental plots in 2015 and 15 plots in 2016. Each plot area was 80 m² (11.1 m × 7.2 m), and 6 ridges and 12 rows were planted in each plot.

Maize straw strip mulching on flat planting (SMF): The width of cover belt and planting belt was 60 cm, and the two belts were arranged in each other. When sowing, the row spacing was 60 cm and the plant spacing was 29 cm. Two rows were planted in each planting belt, and the plant distribution between the two rows of each planting belt was triangular. The straw-mulching amount was about 5.5 × 10⁴ plants/hm², which was equivalent to the straw dry weight of 9 × 10³ kg/hm², in other words, the straw-mulching amount per hectare was about the straw yield per hectare in the semiarid rain-fed area Northwest China (Figure 2a).

Alternating large ridges and small furrows with only the ridges mulched with black polyethylene film (PMF): The ridge width was 80 cm, the ridge height was 15 cm, the furrow width was 40 cm, The ridge surface was covered with polyethylene black plastic film (width 1.2m, thickness 0.01 mm), and a film gap of 5 cm was left in the furrow as a seepage zone, and the film was compacted with soil, each ridge sowing 2 rows potato, and the planting method was the same as SMF (Figure 2b).

Maize straw-strip mulching on furrow and ridge planting (SMFR): The ridge width was 60 cm, the ridge height was 15 cm, the furrow width was 60 cm, Straw covered in the ditch, no straw on the ridge, each ridge sowing two rows of potato, and the planting method was the same as SMF (Figure 2c).

Straw fully covered flat planting (SMWF): The whole ground was covered with corn straw mulching, and the coverage per hectare was the same as that of SMF treatment. The row spacing was 60 cm, and the plant spacing was 29 cm. The planting method was the same as SMF (Figure 2d).

Uncovered flat planting (CK): Traditional flat planting without cover, the row spacing was 60 cm, and the plant spacing was 29 cm. The planting method was the same as SMF (Figure 2e).

2.3. Field Management

The sowing density of each treatment was 5.7 × 10⁴ plants/hm². Corn was planted in the previous stubble of the experimental field, and rotary tillage was carried out on the test site 7 days before sowing. The fertilizer was evenly applied to the test site at one time by the fertilizer applicator, and no fertilizer at the later stage. The fertilization was 326 kg/hm² of diammonium phosphate (N 18%, P₂O₅ 46%) and 261 kg/hm² of urea (N 46.4%). After the land was prepared, film mulching and straw mulching were carried out according to the requirements of each treatment. The field management of each treatment was consistent with the management of local potato conventional planting. The plants in each treatment were harvested at the same time when most of the stems and leaves turned yellow and gradually withered. The sowing date of early- and late-maturing varieties in 2015 was April 23, and the harvest date of early- and late-maturing varieties was August 27 and October 2, respectively. The sowing date of early- and late-maturing varieties in 2016 was April 13, and the harvest date of early- and late-maturing varieties was September 10 and October 18, respectively.

2.4. Sampling and Measurement

2.4.1. Soil Temperature

The HY-1 geothermometer (Hongxing Instrument factory of Wuqiang county, Hebei, China) was used to measure the soil temperature at soil layers of 5, 10, 15, 20 and 25 cm, the geothermometer is buried between two plants in each planting zone, and the soil temperature was read at the fixed place during the whole growth period. The soil temperature was recorded every 15 days after emergence. Each determination was carried out on dry and sunny days. The average daily soil temperature was determined in the morning (6:50–7:10), noon (13:30–14:00) and evening (18:00–18:30). The daily average soil temperature was taken as the average value of the three measurements.

2.4.2. Soil Moisture

Soil moisture was measure and expressed in two ways: soil water storage (SWs, mm) and gravimetric soil water content (SWg%). Soil sample using a 50 mm diam steel-core sampling drill, which was manually driven. The soil samples were taken from between neighboring rows potato plants in each plot at 0–20, 20–40, 40–60, 60–90, 90–120, 120–150, 150–180 and 180–200 cm at sowing and key growth stages (seedling, tuber formation, tuber expansion, starch accumulation and harvest). The soil sample were weighed wet, dried in a fan-assisted oven set at 105 °C for 48 h, and weighed again to determine the SWg. The SWg was calculate by the following formula:

SWg = 100% × (K₁ − K₂)/K₂

(1)

where SWg was the gravimetric soil water content (%), K₁ was the soil fresh weight (g) and K₂ was the soil dried oven weight (g).

The SWs was calculated by the following formula [17]:

SWs (mm) = H (mm)·D(g cm⁻³)·SWg

(2)

where SWs was soil water storage, mm; H was the soil depth, mm; D was soil bulk density, g/cm³; SWg was gravimetric soil water content, %.

The dry land selected in this experiment had flat ground, deep soil layer, uniform soil texture, deep groundwater level, and no water leakage. The water needed for potato growth was mainly provided by rainfall. Therefore, the water consumption ET in potato field was calculated by the following formula:

ET = W₁ − W₂ + P

(3)

where P was effective rainfall (mm) recorded at the weather station near the experimental site, W₁ (mm) was the soil water storage for before sowing, W₂ (mm) was the soil water storage for after harvest.

2.4.3. Daily ET

Daily ET was the daily average water consumption of different treatments at different growth stage. The daily ET was calculated by the following formula:

ET_D = ET_J/DS

(4)

where ET_D (mm) was the average daily water consumption at a certain growth stage, ET_J (mm) was the amount of soil water consumption at a certain growth stage, and DS(d) was the number of days experienced at a certain growth stage.

2.4.4. Measurements of Yield, Tuber Size and WUE

Potato yields were determined at harvest, during harvest we randomly selected the middle row from each plot. We manually excavated 15 plants, the fresh mass of each tuber was counted, and the commercial potato rate was calculated based on the single tuber mass greater than 50 g. The commercial potato rate was calculated by the following formula:

T(%) = (M₁/M₂) × 100%

(5)

where T was the commercial potato rate (%), M₁ was the total tuber yield of single fresh tuber more than 50 g (g), and M₂ was the total yield of fresh potato (g).

The yield per hectare at harvest was converted according to the average value of the actual fresh tuber yield in three replicate plots.

The following equation was used to calculate potato yields’ WUE, in kg ha⁻¹ mm⁻¹:

WUE = Y/ET

(6)

where WUE was the water use efficiency (kg ha⁻¹ mm⁻¹), Y was the potato tuber yield as fresh weight (kg ha⁻¹) and ET was the total water consumption (mm) in the whole growth period, It could be calculated by Equation (3).

2.5. Statistical Analysis

The Excel 2007 software was used for data processing. The SigmaPlot (Ver.14.0, Systat Software Inc., Palo Alto, CA, USA) was used to draw figures and calculate Fisher’s least significant differences (LSD) value between each treatment repetition. The SPSS 20.0 statistical analysis software was used for variance analysis and correlation analysis. The analysis of variances was performed on all the data collected each year. Correlation analyses were also performed to evaluate the relationship between potato yield and soil temperature, soil moisture, and yield components. The mean values between treatments were compared by a LSD at p < 0.05.

3. Results and Analysis

3.1. Soil Temperature

3.1.1. Mean Soil Temperature in 0–25 cm Soil Layer during Whole Growth Period

The cover treatment had significant effects on the average soil temperature in 0–25 cm soil layer of different varieties during the whole growth period (p < 0.05) (Figure 3). Compared with CK, PMF significantly increased the average soil temperature of early- and late-maturing varieties during the whole growth period in both experimental years. (p < 0.05), and the warming range was the largest in late-maturing varieties in 2015, reaching 1.82 °C. The warming range of late-maturing varieties was greater than that of early-maturing varieties, and the warming range of normal water years was greater than that of partial drought years. There was no significant difference between SMF and CK in normal years, but SMF significantly reduced soil temperature of the two varieties by 2.07–2.2 °C in a partial drought year (p < 0.05), and the cooling amplitude of late-maturing varieties was greater than that of early-maturing varieties. SMFR significantly increased soil temperature of the two varieties by 0.87–0.88 °C in normal years compared with CK (p < 0.05), and the warming range was consistent between early- and late-maturing varieties. In a partial drought year, SMFR decreased soil temperature compared with CK, and the cooling rate of early-maturing varieties was higher than that of late-maturing varieties. Compared with CK, SMWF significantly decreased the soil temperature of the two varieties in the whole growth period from 1.93 to 4.48 °C in partial drought years (p < 0.05), and the cooling amplitude of early-maturing varieties was greater than that of late-maturing varieties.

3.1.2. Average Soil Temperature in 0–25 cm Soil Layers at Different Growth Stages

Cover treatment had significant effects on the average soil temperature in 0–25 cm soil layer of different varieties at different growth stages (p < 0.05) (Figure 4). Compared with CK, PMF significantly increased the soil temperature at seedling stage, starch accumulation stage and harvest stage (2.28–3.31 °C) in the two experimental years (p < 0.05). Compared with CK, the temperature effects of straw-mulching cover in the two test years were inconsistent. In the normal year, there was no significant difference between straw mulching (SMF, SMFR, SMWF) and CK in most growth stages. Only SMFR significantly increased the soil temperature by 0.61–2.8 °C in the late growth stage (p < 0.05). In a partial drought year, SMF, SMFR, and SMWF significantly decreased the soil temperature by 0.74–4.96 °C in different growth stages (p < 0.05). In the late-maturing varieties, SMF and SMFR decreased the most in the tuber formation stage (2.76 °C, 1.68 °C). SMWF had the largest decrease at seedling stage (2.84 °C), and SMF, SMFR and SMWF at starch accumulation stage (July 26) had the largest decrease at early-maturing varieties (3.27 °C, 2.84 °C, 4.96 °C).

3.2. Soil Moisture

3.2.1. Mean Soil Moisture in 0–200 cm Soil Layer during Whole Growth Period

The cover treatment had significant effects on the average soil moisture in 0–200 cm soil layers of different varieties during the whole growth period (p < 0.05) (Figure 5). Compared with CK, PMF and SMFR had no significant difference in the two varieties at the normal precipitation year, and had no difference in the late-maturing varieties in a partial drought year. However, PMF and SMFR significantly increased the soil moisture content of 0–200 cm in early-maturing varieties 2.0 and 1.52 percentage points higher than CK in both experimental years (p < 0.05), respectively. Both SMF and SMWF significantly increased the whole growth period soil moisture of the two varieties by 0.54–2.57 and 0.89–2.96 percentage points compared with CK in the two test years. The water storage and soil moisture conservation effects of SMF and SMWF were greater than those of PMF and SMFR. The effect of SMF and SMWF on water storage and soil moisture of early-maturing varieties was greater than that of late-maturing varieties.

3.2.2. Average Soil Moisture in 0–25 cm Soil Layers at Different Growth Stages

There were significant differences in soil moisture at different growth stages of each treatment (p < 0.05) (Figure 6). The difference of soil moisture content at 0–200 cm between different growth stages of each treatment was significantly greater in partial drought years than in normal precipitation years, and the early-maturing varieties were greater than the late-maturing varieties. Compared with CK, PMF decreased the soil moisture content in the two varieties by 0.15–1.13 percentage points in normal precipitation years (p < 0.05), and the decrease was the largest in the tuber expansion stage (0.78 percentage points in late variety and 1.13 percentage points in early variety). Compared with CK, SMF increased the soil moisture content in the two varieties by 0.02–1.31 percentage points at each growth period in normal precipitation years (p < 0.05). The starch accumulation period of late-maturing varieties was the largest, reaching 1.31 percentage points, and that of early-maturing varieties was the largest at seedling stage, reaching 0.88 percentage points. There was no significant difference between SMFR and CK in each growth stage.

Compared with CK, in the late-maturing varieties in a partial drought year, PMF increased soil water content at seedling stage and tuber expansion stage by 0.75 and 0.46 percentage points (p < 0.05), and decreased soil water content at the tuber formation stage and starch accumulation stage by 0.30 and 0.78 percentage points (p < 0.05). Compared with CK, SMF, and SMWF increased soil water content by 0.11–2.22 percentage points and 0.19–2.01 percentage points, respectively. Compared with CK, SMFR decreased soil moisture content by 0.41 percentage points at seedling stage, and increased soil moisture content by 0.14–1.46 percentage points after seedling stage.

Compared with CK, the early-maturing varieties in partial drought years, PMF, SMF, SMFR and SMWF were significantly higher than CK by 0.66–3.84 percentage points, 0.73–3.70 percentage points, 0.62–2.15 percentage points, and 0.85–4.3 percentage points (p < 0.05), respectively, and SMWF had the best effect on water storage and soil moisture conservation, followed by SMF. There was no significant difference between SMF and PMF before seedling stage, but SMF and SMWF significantly increased soil moisture content during the tuber formation and the starch accumulation stage (p < 0.05).

3.3. Soil Water Consumption

Table 1. The daily average water consumption of different treatments at different growth stages.

Year-Variety	Treatments	Sowing to Seedling (mm)	Seedling to Expansiont (mm)	Expansion to Starch Accumulation Early (mm)	Starch Accumulates Early to Late Stage (mm)	The Late of Starch Accumulation to Harvest (mm)	Water Consumption during the Whole Growing Period (mm)
2015-Long7	PMF	2.32 a	2.71 b	5.29 a	0.91 c	1.64 c	2.56 a
	SMF	1.85 ab	2.66 b	4.35 c	0.86 c	2.67 a	2.50 b
	SMFR	1.64 b	2.96 a	4.18 c	1.54 b	2.38 b	2.53 ab
	CK	2.10 ab	2.75 b	4.85 b	2.08 a	1.71 c	2.53 ab
2015-LK99	PMF	2.20 a	3.30 b		2.83 a	0.49 b	2.55 ab
	SMF	1.55 b	3.59 a		1.75 c	3.31 a	2.51 ab
	SMFR	1.65 b	3.29 b		2.30 b	2.72 a	2.46 b
	CK	2.02 a	3.36 ab		1.61 c	3.52 a	2.60 a
2016-Long7	PMF	0.52 b	3.46 a	2.27 a	1.67 a	0.82 bc	1.52 a
	SMF	0.52 b	3.09 b	0.87 b	0.46 bc	2.12 a	1.50 a
	SMFR	0.97 a	2.34 d	2.61 a	0.47 bc	0.75 c	1.33 b
	SMWF	0.70 ab	2.86 c	1.16 b	0.86 b	1.68 b	1.49 a
	CK	0.70 ab	2.72 c	3.06 a	0.13 c	1.26 ab	1.54 a
2016-LK99	PMF	0.09 b	5.69 a	0.90 e	2.20 a	0.19 c	1.47 c
	SMF	0.45 b	0.81 c	6.26 a	0.74 b	1.34 b	1.43 c
	SMFR	0.99 a	1.19 c	3.68 c	1.91 a	1.64 ab	1.65 b
	SMWF	0.29 b	0.62 c	5.22 b	1.77 a	2.42 a	1.57 bc
	CK	1.27 a	2.96 b	2.51 d	1.73 a	1.51 ab	1.84 a
Years		**	**	**	**	**	**
Varieties		NS	NS	NS	**	NS	**
Mulch treatments		**	**	**	**	**	**
Varieties × Years		NS	**	**	NS	NS	NS
Years × Mulch treatments		**	**	**	NS	NS	NS
Varieties × Mulch treatments		*	**	**	*	**	**
Years × Varieties × Mulch treatments		NS	**	**	*	NS	*

Note: Different lower case letters in the same column mean significant differences between treatments at 0.05 level. * and ** represent significant effects on the average daily water consumption in each stage at p < 0.05 and p < 0.01, respectively. NS represents non-significance.

shows the daily average water consumption of different maturity variety and treatments at different growth stages. It can be seen from Table 1 that years, mulching treatment and varieties had significant effect on the daily average water consumption of two varieties during the whole growth period (p < 0.05). The difference in daily average water consumption of early-maturing varieties was greater than that of late-maturing varieties, and the difference in late-maturing varieties was not significant. Compared with CK, the mulching treatment decreased the daily average water consumption of the early-maturing varieties during the whole growth period, and SMF decreased the most in early-maturing varieties in the partial drought year, reaching 0.41 mm. The SMF significantly decreased the daily average water consumption during the early and late growth stages, and significantly increased the daily average water consumption during the tuber expansion stage.

The effects of years and mulching treatments on average daily water consumption were highly significant (p < 0.01). The effect of varieties on the average daily water consumption during the whole growth period and starch accumulation stage was highly significant (p < 0.01), but the effect on the average daily water consumption before the starch accumulation period was not significant. The interaction of mulching treatments and varieties had significant effects on the average daily water consumption of each stage and the whole growth period (p < 0.05).

3.4. Yield Components and Commidity Rate

The yield components (weight and number of potato tubers per plant) were measured after harvesting (

Table 2. Potato tuber grades and weight of per plants under different mulching patterns in 2015 and 2016 in Dongwei, northwest China.

Year-Variety	Treatments	Distribution of Tuber Weight in per Plants (%)				Total (kg)	Commidity Rate/%	Tuber Numbers of Different Grades in per Plants (%)				Total
		Less than 50 g	50–100 g	100–150 g	Over 150 g			Less than 50 g	50–100 g	100–150 g	Over 150 g
2015- Long7	PMF	10.5 bc	36.98 a	25.66 b	26.84 ab	0.69 a	89.48 a	34.52 b	38.02 a	16.16 ab	11.3 a	8.83 a
	SMF	10.04 c	28.83 b	32.64 a	28.48 a	0.64 ab	89.96 a	41.36 ab	28.34 b	19.26 a	11.04 a	8.83 a
	SMFR	15.03 a	36.32 a	26.58 b	22.07 b	0.66 ab	84.97 c	46.59 a	31.10 ab	14.21 b	8.1 b	5.37 b
	CK	12.80 ab	37.72 a	26.24 b	23.25 b	0.62 b	87.21 bc	47.97 a	30.54 ab	13.36 b	8.12 b	9.70 a
2015- LK99	PMF	3.54 c	9.53 a	16.46 bc	70.47 a	0.62 b	96.46 b	22.79 b	17.73 a	18.50 b	40.97 a	4.67 c
	SMF	2.41 d	11.33 a	15.68 c	70.57 a	0.75 a	97.59 a	17.06 c	20.66 a	18.16 b	44.12 a	5.50 a
	SMFR	4.42 b	3.92 b	21.50 a	70.16 a	0.66 ab	95.58 c	29.78 a	6.74 b	22.38 a	41.1 a	5.37 ab
	CK	5.70 a	13.40 a	20.35 ab	60.55 b	0.60 b	94.30 d	27.33 a	22.05 a	19.99 ab	30.64 b	5.00 bc
2016- Long7	PMF	16.80 bc	45.04 a	22.94 a	15.25 a	0.55 a	84.00 a	41.22 bc	39.75 ab	13.41 a	5.62 ab	7.49 a
	SMF	14.34 c	39.63 a	25.24 a	20.79 a	0.43 b	85.66 a	32.18 c	42.69 a	15.81 a	9.33 a	5.62 b
	SMFR	22.77 b	36.49 a	24.95 a	15.79 a	0.42 b	77.23 b	48.62 b	32.65 abc	12.83 a	5.90 ab	6.67 ab
	SMWF	37.62 a	39.08 a	17.25 ab	6.05 a	0.32 c	65.44 c	63.59 a	27.79 c	7.54 b	1.08 b	6.62 ab
	CK	37.79 a	44.91 a	13.21 b	4.09 a	0.26 c	62.21 c	62.97 a	30.65 bc	5.23 b	1.16 b	5.84 b
2016- LK99	PMF	15.05 b	49.82 a	24.66 a	10.47 ab	0.42 a	84.95 a	33.95 c	47.43 a	14.44 a	4.19 ab	6.00 a
	SMF	17.75 b	36.19 a	31.90 a	14.16 ab	0.38 a	82.25 a	45.19 b	32.15 bc	17.34 a	5.32 ab	5.93 a
	SMFR	15.73 b	40.74 a	28.55 a	14.98 ab	0.33 b	84.27 a	37.28 bc	39.55 ab	17.34 a	5.83 ab	4.51 b
	SMWF	13.60 b	41.55 a	26.68 a	18.17 a	0.39 a	86.40 a	38.07 bc	39.77 ab	15.47 a	6.68 a	5.60 a
	CK	53.63 a	41.99 a	4.37 b	0.00 b	0.21 c	46.37 b	71.45 a	27.16 c	1.39 b	0.00 b	5.93 a
Years		**	**	NS	**	**	**	**	**	**	**	**
Varieties		**	**	**	**	NS	**	**	**	**	**	**
Mulch treatments		**	NS	**	**	**	**	**	**	**	**	NS
Varieties × Years		NS	**	**	**	NS	NS	**	**	NS	**	**
Years × Mulch treatments		**	NS	NS	NS	NS	NS	NS	NS	NS	NS	*
Varieties × Mulch treatments		**	NS	**	NS	NS	**	**	*	**	NS	*
Years × Varieties × Mulch treatments		**	NS	NS	NS	NS	**	**	**	NS	**	NS

Note: Different lower case letters in the same column mean significant differences between treatments at 0.05 level. * and ** represent significant effect on the potato tuber grades and weight of per plants under different mulching patterns, years, and varieties at p < 0.05 and p < 0.01, respectively. NS represents non-significance.

). Compared with CK, the coverage treatment of two experimental years significantly increased the weight of potato per plant, the percentage of commercial potato, and the ratio of the mass and quantity of tuber over 100 g (p < 0.05) by 3.43–107.85%, −2.57–83.2%, −1.7–954%, and 3.86–1566.91%, respectively. The effect of mulching treatment on the tuber weight per plant was greater in a drought year than in a normal precipitation year. There were significant differences in total potato weight and commercial potato rate among mulching treatments (p < 0.05). In normal precipitation year, the average tuber weight per plant of SMF was 20.77% and 14.29% higher than that of PMF and SMFR in early-maturing varieties, and the commodity tuber rate was 1.17% and 2.10% higher than that of SMFR. In late-maturing varieties, there was no significant difference between SMF and PMF, and the commodity tuber rate was 5.87% higher than that of SMFR. There was no significant difference between SMF and SMFR and SMWF in a partial drought year, but there was only significant difference between SMF and PMF in late varieties (p < 0.05), and the weight of potato per plant was 21.56% lower than that of PMF.

Through the analysis of the interaction between years, varieties and mulching treatments, it was found that years, varieties and mulching methods had significant effects on the number of tubers, tuber weight per plant and commodity tuber rate (p < 0.05). Years had a significant effect on the tuber number per plant, the tuber weight per plant and the commodity tuber rate (p < 0.01). Variety had no significant effect on tuber weight per plant, but had significant effect on commodity tuber rate and tuber number per plant (p < 0.01). Mulching methods had a significant effect on tuber weight per plant and commodity tuber rate (p < 0.01), but had no significant effect on tuber number per plant. The interaction effects of years and varieties, years and mulching treatment, varieties and mulching treatment had significant effects on tuber number per plant (p < 0.05), but had no significant effects on tuber weight per plant. Varieties and mulching treatment had significant effects on commodity tuber rate (p < 0.01). Therefore, appropriate mulching methods for different maturity varieties can increase potato yield.

3.5. Tuber Yield and Water Use Efficiency (WUE)

Table 3. The effects of different treatment on yield and water use efficiency (WUE) of potato.

Year-Variety	Treatments	Soil Water Storage before Seeding /mm	Soil Water Storage Harvest /mm	Amount of Rainfall in the Growth Period /mm	Crop Water Consumption/mm	Yield /kg•hm−2	WUE/ kg·(mm·hm2)−1	Yield Is More than CK/%	WUE Is More than CK/%
2015-Long7	PMF	446.30	302.06 a	281.10	402.30 a	39,784.40 a	98.89 a	12.50	11.08
	SMF	422.01	306.48 a	281.10	396.63 a	36,689.70 a	92.50 a	3.75	3.90
	SMFR	407.94	296.75 a	281.10	392.28 a	33,645.90 b	85.77 a	−4.86	−3.66
	CK	421.55	305.44 a	281.10	397.21 a	35,363.40 b	89.03 a
2015-LK99	PMF	446.30	338.25 ab	230.60	315.60 a	35,474.70 b	112.40 b	3.41	5.52
	SMF	422.01	341.55 a	230.60	311.06 a	42,867.90 a	137.81 a	24.96	29.37
	SMFR	407.94	333.02 b	230.60	305.52 a	37,495.50 ab	122.73 ab	9.30	15.21
	CK	421.55	330.10 b	230.60	322.04 a	34,305.30 b	106.52 b
2016-Long7	PMF	376.14	292.11 ab	201.80	285.83 a	31,209.30 a	109.19 a	72.26	105.58
	SMF	373.65	285.82 b	201.80	289.62 a	24,463.50 b	84.47 bc	63.17	59.04
	SMFR	370.71	322.63 a	201.80	249.88 b	24,111.00 b	96.49 ab	60.82	81.68
	SMWF	376.46	298.11 ab	201.80	280.14 ab	18,117.60 c	64.67 cd	20.85	21.77
	CK	366.68	286.20 b	201.80	282.28 ab	14,992.20 c	53.11 d
2016-LK99	PMF	376.14	299.91 a	113.10	189.34 b	24,089.40 a	127.23 a	98.01	147.80
	SMF	373.65	302.36 a	113.10	184.39 b	21,778.80 a	118.11 a	79.02	130.04
	SMFR	370.71	271.02 b	113.10	212.79 ab	18,812.40 a	88.41 ab	54.64	72.18
	SMWF	376.46	287.63 ab	113.10	201.93 ab	22,024.35 a	109.07 a	81.04	112.43
	CK	366.68	242.84 c	113.10	236.94 a	12,165.20 b	51.34 b	/	/
Year		/	/	/	**	**	NS	/	/
Varieties		/	/	/	**	NS	**	/	/
Mulch treatments		/	/	/	**	**	**	/	/
Varieties × Year		/	/	/	NS	NS	NS	/	/
Year × Mulch treatments		/	/	/	NS	NS	NS	/	/
Varieties × Mulch treatments		/	/	/	**	NS	*	/	/
Year × Varieties × Mulch treatments		/	/	/	**	NS	*	/	/

Note: Different lower case letters in the same column mean significant differences between treatments at 0.05 level. * and ** represent significant effects on the crop water consumption, potato yield and WUE under different mulching patterns, years, and varieties at p < 0.05 and p < 0.01, respectively. NS represents non-significance.

shows the yield and WUE of potato under different treatments. It can be seen from Table 3 that the potato yield of cover treatment on early- and late-maturing varieties was increased compared with that of CK (p < 0.05). Compared with CK, PMF, SMF and SMFR increased yield in a normal year (2015) by 12.5%, 3.75% and −4.86% in late-maturing varieties, and 3.41%, 24.96% and 9.30% in early-maturing varieties. PMF, SMF, SMFR and SMWF increased yield in the partial drought year (2016) by 72.26%, 63.17%, 60.82% and 20.85% in late-maturing varieties, and increased yield by 98.01%, 79.02%, 54.64% and 81.04% in early-maturing varieties. There was no significant difference between the yield of SMF and PMF in late-maturing varieties in normal years and early-maturing varieties in partial drought years, but the yield of SMF in early-maturing varieties in normal years was significantly higher than that of PMF 20.84%, and the yield of SMF in late-maturing varieties in partial drought years was significantly lower than that of PMF 21.61%. The yield of SMF was higher than that of SMFR and SMWF, and the yield-increasing effect of SMF on early-maturing varieties was higher than that of late-maturing varieties.

There was no significant difference in water-use efficiency between PMF and SMF in most cases. but SMF was significantly higher than SMFR and SMWF in most cases. There was no significant difference in WUE among treatments of late-maturing varieties in normal years, and the WUE of PMF, SMF and SMFR of early-maturing varieties was 5.52%, 29.37% and 15.21% higher than that of CK (p < 0.05). The WUE of PMF, SMF, SMFR and SMWF in early- and late-maturing varieties was significantly higher than that of CK by 147.8%, 130.04%, 72.18%, 112.43%, 105.58%, 59.04%, 81.68% and 21.77% (p < 0.05).

Through the analysis of the interaction between varieties and mulching treatments, it can be seen that years had significant effects on crop water consumption and yield (p < 0.01). Varieties had significant effects on crop water consumption and yield (p < 0.01). Mulching methods had significant effects on crop water consumption, yield and yield WUE (p < 0.01). The interaction between variety and mulching treatment and interaction among years, variety and mulching treatment had significant effects on water consumption and yield WUE (p < 0.01).

3.6. Relationship between Yield and Soil Moisture

Table 4. Correlation Analysis between Yield and Soil-Moisture Content.

Factor	Seedling	Initiation	Expansion	Starch Accumulates Stage	Harvest	During the Whole Growing Period
Yield−2015-long7	0.466	0.023	−0.003	0.490	0.217	0.256
Yield−2015-LK99	0.585 *	−0.281	0.226	0.594 *	0.290	0.138
Yield−2016-long7	0.886 **	−0.173	0.182	−0.237	0.085	0.091
Yield−2016-LK99	0.910 **	0.477	0.727 **	0.767 **	0.857 **	0.822 **

Note: The correlation coefficient in the table is calculated by SPSS 20.0 statistical analysis software. * and ** represent significantly different at p < 0.05 and p < 0.01, n = 12 in 2015, n = 15 in 2016.

shows that relationship between yield and soil moisture. It can be seen from Table 4 that the yield of different maturity varieties in two experimental years was positively correlated with the average soil-water content in seedling stage (r = 0.466–0.910), harvest stage (r = 0.085–0.857), and during the whole growth period (r = 0.091–0.822). The correlation between yield and soil-water content in each stage of early-maturing varieties was greater than that of late-maturing varieties.

3.7. Relationship between Yield and Soil Temperature

Table 5. Correlation Analysis between Yield and Soil Temperature Content.

Factor	Seedling	Initiation	Expansion	Starch Accumulates Stage	Harvest	During the Whole Growing Period
Yield-2015-long7	0.397	0.020	0.342	0.456	0.535	0.511 *
Yield-2015-LK99	0.227	0.334	−0.057	−0.352	−0.397	−0.282
Yield-2016-long7	0.579 *	0.053	0.302	0.447	0.449	0.423
Yield-2016-LK99	0.108	−0.200	−0.280	−0.072	0.062	−0.036

Note: The correlation coefficient in the table is calculated by SPSS 20.0 statistical analysis software. * represent significantly different at p < 0.05, n = 12 in 2015, n = 15 in 2016.

shows that relationship between yield and soil temperature. It can be seen from Table 5 the yield of early- and late-maturing varieties in two experimental years was significantly positively correlated with soil temperature at seedling stage (r = 0.108–0.579), and reached a significant level in late-maturing varieties in 2016 (p < 0.05). The yield of early-maturing varieties was negatively correlated with soil temperature during the whole growth period, and the yield of late-maturing varieties was positively correlated with soil temperature during the whole growth period, which was significant in 2015 (p < 0.05).

3.8. Relationship between Yield and Yield Components

Table 6. Correlation Analysis between Yield and Yield Components.

Factor	Tuber Numbers of Different Grades in per Plant (%)				Total	Distribution of Tuber Weight in per Plant (%)				Total (kg)
	Less than 50 g	50–100 g	100–150 g	Over 150 g		Less than 50 g	50–100 g	100–150 g	Over 150 g
Yield-2015-long7	−0.360	0.165	−0.454	0.494	−0.264	−0.453	0.442	−0.154	0.531	0.586 *
Yield-2015-LK99	−0.692 *	−0.255	−0.431	0.558	−0.383	−0.693 *	−0.103	−0.274	0.716 **	0.732 **
Yield-2016-long7	−0.764 **	0.018	0.613 *	0.449	−0.235	−0.720 **	0.551 *	0.730 **	0.477	0.817 **
Yield-2016-LK99	−0.790 **	0.025	0.543 *	0.683 **	−0.375	−0.722 **	0.457	0.584 *	0.668 **	0.776 **

Note: The yield components are the number and weight of potato tubers per plant, and the number and quality of potato tubers of different grades per plant are the key factors to determine the number and weight of potato tubers per plant. The correlation coefficient in the table is calculated by SPSS 20.0 statistical analysis software. * and ** represent significantly different levels at p < 0.05 and p < 0.01, n = 12 in 2015, n = 15 in 2016.

shows that relationship between yield and yield components. It can be seen from Table 6 that the yield was significantly positively correlated with tuber weight per plant (r = 0.586–0.817, p < 0.01), tuber weight above 150 g (r = 0.494–0.683, p < 0.05), and tuber number above 150 g (r = 0.477 −0.716, p < 0.05). It was negatively correlated with tuber number (r = −0.264–−0.383, p < 0.01), tuber weight below 50 g (r = −0.36–−0.79, p < 0.01) and tuber number below 50 g (r = −0.453–−0.722, p < 0.01).

4. Discussion

4.1. Effects of Mulching on Soil Temperature of Different Maturity Varieties

The results of this study showed that, compared with CK, SMF decreased the soil temperature of 0–25 cm in the whole growth period of potato by 0.05–2.2 °C, and reached the significant indigenous level in a partial drought year. The soil temperature decreased the most at the tuber formation stage, reaching 2.76 °C, and the cooling rate of late-maturing varieties was higher than that of early-maturing varieties, which was similar to the results of previous studies in planting spring maize and spring highland barley under straw mulching [17,18]. SMFR had a warming effect in the normal year, but the soil temperature in the whole growth period was 1.93–4.48 °C lower than that of CK in a partial drought year. The cooling rate of SMFR and SMWF in early-maturing varieties was higher than that of late-maturing varieties. SMFR significantly reduced the soil temperature at tuber formation stage by 1.68 °C, and SMWF significantly reduced the soil temperature at seedling stage by 2.84 °C. The reason for the decrease in soil temperature is that corn straw mulch has low light transmittance and high light reflectivity, which can intercept some direct solar radiation and thermal radiation [19]. The cooling amplitude of SMF was significantly better than that of SMFR, which may be that SMFR covered maize straw in the ditch, and the ridge surface was raised, so that the ridge surface received direct solar radiation longer than that of SMF, and at the same time, it increased the soil light surface area. PMF significantly increased soil temperature, mainly increasing the soil temperature at the early growth stage. The warming amplitude of late-maturing varieties was greater than that of early-maturing varieties, which was similar to the research results of previous studies on plastic film potato and maize cultivation [20,21,22].

4.2. Effects of Mulching on Soil Moisture of Different Maturity Varieties

A large number of studies have shown that mulching treatment can improve the soil moisture content of farmland, especially mulching can improve the soil moisture content at the early growth stage of potato to create good water conditions for the early growth of crops [23]. This study showed that PMF and SMFR significantly increased the soil moisture content of 0–200 cm in early-maturing varieties in the partial drought year, which was 2.0 and 1.52 percentage points higher than CK, respectively. SMF and SMWF significantly increased soil moisture content of 0–200 cm in the whole growth period, 0.54–2.57 percentage points and 0.89–2.96 percentage points higher than CK, mainly increased soil moisture content of 0.11–4.3 percentage points in the tuber formation and the starch accumulation period, which was consistent with the conclusions [12,13]. SMF and SMWF were better than PMF and SMFR in the effect of water storage and soil moisture conservation. The possible reason was that SMF and SMWF were beneficial to rainwater infiltration, and shading and cooling (Figure 3) reduced soil moisture evaporation and reduced daily water consumption (Table 1), which was beneficial to improve soil moisture content. PMF is easy to form low-lying pits and ditches on the ridge surface of plastic film. Due to its own impermeability and evaporation suppression characteristics, reducing rainwater infiltration, especially precipitation greater than 5 mm, is easy to stay on the surface of low-lying film, accelerating water evaporation. SMFR mainly increases the surface area of soil evaporation and soil temperature, accelerates soil water evaporation and reduces soil moisture content. The experiment also concluded that the effect of soil moisture conservation and water storage of mulching was better early-maturing varieties than late-maturing varieties. The main reason is that early-maturing and late-maturing varieties are planted at the same time, but the early-maturing varieties are harvested at the end of August. After harvest, the temperature in the region began to decline, and the stems and leaves of late-maturing varieties of potato were covered on the ground, and the effect of soil moisture conservation and water storage was not obvious. which was consistent with the research conclusion of MA Jiantao et al. [15].

4.3. Effects of Mulching on Yield and WUE of Different Maturity Varieties

The results showed that the potato yield of covering treatment on early- and late-maturing varieties was higher than that of CK (p < 0.05). The SMF was significantly higher than that of PMF, 20.84% in early-maturing varieties. The possible reason was that SMF reduced the soil temperature in late June compared with PMF, which is just consistent with the formation of potato tubers of early-maturing varieties, and is conducive to the formation of tubers. SMF significantly increased the number of tubers and the number of tubers greater than 150 g, Compared with PMF, there was no significant differences in early-maturing varieties in partial drought years, but it was significantly lower than PMF 21.61% in late-maturing varieties. It may be that SMF significantly reduced soil temperature (Figure 3b) and affected yield increase (Table 2, Table 3) at the later growth stage, which was consistent with the conclusions of Fu Qiang. 2014, Zhao Hong, et al. 2014. When the soil temperature at potato growth stage was lower than 18 °C, it was not conducive to potato growth. Previous studies also reported that mulching increased yield on maize (Zea mays L.) [24] and potato tuber [25]. In straw-mulching treatment, the yield of SMF was higher than that of SMFR and SMWF, probably because SMF increased soil moisture content and decreased soil temperature compared with SMFR, creating better growth conditions for the potato. The reason for the low yield of SMWF was that straw-mulching increased soil moisture content, but greatly reduced the soil temperature of 2.84 °C and 4.96 °C at seedling stage and late growth stage (Figure 3b and Figure 4c,d), and affected the increase in potato tuber numbers and large potato numbers (Table 2).

There was no significant difference in WUE between PMF and SMF in most cases, and SMF had the highest WUE in early-maturing varieties in normal years. PMF had the highest WUE in early- and late-maturing varieties in partial drought years, followed by SMF. The possible reason was that PMF and SMF increased soil moisture content during the potato growth period and reduced the average daily water consumption during potato growth period. PMF increased the water consumption of early crops, whereas SMF decreased the water consumption of early crops and increased the water consumption of middle and late stages (Table 1). At the same time, SMF prolonged the potato growth period due to cooling temperature compared with PMF, which created good conditions for the formation and expansion of potato tubers and improved the yield and WUE. This is consistent with the research conclusions of Yu et al., 2018, Zhang et al., 2019 [26,27]. Among straw-mulching treatments, SMF was significantly higher than SMFR and SMWF in most cases. SMFR increased the daily water consumption in the early stage of potato growth due to ridge bulge, which made the soil water deficit in the later stage of potato growth and affected the growth. SMWF increased the soil moisture content in each growth period, but decreased the soil temperature at seedling stage by 2.84 °C (Figure 4c,d), which affected the emergence and later growth of potatoes, which was consistent with the conclusion of Fan Shijie, et al., 2011 [19].

Through correlation analysis, it can be seen that varieties and mulching treatments have significant effects on potato yield components, stage water consumption and WUE. The varieties significantly increased the number of tubers per plant compared with the mulching methods, and the mulching methods significantly increased the weight and yield per plant compared with the varieties.

5. Conclusions

Compared with the treatments without mulching, straw-banded mulching and plastic-film mulching can improve soil water and heat conditions, and increase potato yield and WUE. Straw-banded flat mulching (SMF) increased soil moisture during the potato growth period, significantly reduced soil temperature during the tuber formation period, and the yield effect on early-maturing varieties was higher than that of late-maturing varieties. Plastic-film mulching (PMF) significantly increased soil temperature during the potato growth period and soil moisture at the early growth stage. The plastic-film mulching (PMF) on effect of water storage and soil moisture in partial drought years was greater than that in normal years, and the yield effect on late-maturing varieties was higher than that of early-maturing varieties. Therefore, in the Northwest rain-fed agricultural area, SMF has great potential for increasing yield in early-maturing potato varieties, and PMF has great potential for increasing yield in late-maturing potato varieties.