Effects of film mulching regime on soil water status and grain yield of rain-fed winter wheat on the Loess Plateau of China

Journal of Integrative Agriculture 2017, 16(11): 2612–2622 Available online at www.sciencedirect.com

ScienceDirect

RESEARCH ARTICLE

Effects of film mulching regime on soil water status and grain yield of rain-fed winter wheat on the Loess Plateau of China XUE Nai-wen, XUE Jian-fu, YANG Zhen-ping, SUN Min, REN Ai-xia, GAO Zhi-qiang College of Agronomy, Shanxi Agricultural University, Taigu 030801, P.R.China

Abstract Shortages and fluctuations in precipitation are influential limiting factors for the sustainable cultivation of rain-fed winter wheat on the Loess Plateau of China. Plastic film mulching is one of the most effective water management practices to improve soil moisture, and may be useful in the Loess Plateau for increasing soil water storage. A field experiment was conducted from July 2010 to June 2012 on the Loess Plateau to investigate the effects of mulching time and rates on soil water storage, evapotranspiration (ET), water use efficiency (WUE), and grain yield. Six treatments were conducted: (1) early mulching (starting 30 days after harvest) with whole mulching (EW); (2) early mulching with half mulching (EH); (3) early mulching with no mulching (EN); (4) late mulching (starting 60 days after harvest) with whole mulching (LW); (5) late mulching with half mulching (LH); and (6) late mulching with no mulching (LN). EW increased precipitation storage efficiency during the fallow periods of each season by 18.4 and 17.8%, respectively. EW improved soil water storage from 60 days after harvest to the booting stage and also outperformed LN by 13.8 and 20.9% in each growing season. EW also improved spike number per ha by 13.8 and 20.9% and grain yield by 11.7 and 17.4% during both years compared to LN. However, EW decreased WUE compared with LN. The overall results of this study demonstrated that EW could be a productive and efficient practice to improve wheat yield on the Loess Plateau of China. Keywords: soil water status, plastic film mulching, precipitation storage efficiency, winter wheat, the Loess Plateau

Plateau cropland, and it plays a key role in food security (Jin

1. Introduction The 40-million-ha Loess Plateau of China is a vital cereal production area (Huang and Li 2000). Winter wheat (Triticum aestivum L.) is cultivated on 44% of the Loess

et al. 2007). Wang (1994) found that winter wheat requires approximately 480 mm of water for the maximum yield in the eastern portion of the Loess Plateau. Although the mean annual precipitation is about 580 mm in the Loess Plateau, the average precipitation during the winter wheat growing period is only 205 mm (Jin et al. 2007). The continental monsoon climate of the region provides more than 60% of the annual precipitation from July to September, when wheat

Received 18 January, 2017 Accepted 6 June, 2017 XUE Nai-wen, Mobile: +86-13152911786, E-mail: wendyxue [email protected]; Correspondence GAO Zhi-qiang, Tel: +86354-6287226, E-mail: [email protected] © 2017 CAAS. Publishing services by Elsevier B.V. All rights reserved. doi: 10.1016/S2095-3119(17)61706-4

fields are fallow (Jin et al. 2007). Additionally, because of low relative humidity and high temperatures, more than 60% of fallow period rainfall is lost through evaporation before it can infiltrate the soil and be used by plants (Jin et al. 2007). Therefore, storing rainfall during the fallow period to supplement water for the subsequent winter wheat season

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is crucial for increasing grain yields. Any material used to cover the soil surface to protect it from evaporation or solar radiation is called mulch. Today, plastic film mulching is considered one of the most effective mulching methods. Many studies have demonstrated that plastic film mulching can decrease water loss through evaporation (Wang et al. 2008; Li et al. 2013), improve soil water infiltration (Gan et al. 2013), enhance crop yields in both quantity and quality (Ramakrishna et al. 2006; Wang et al. 2008; Luis et al. 2011), and increase water use efficiency (WUE) (Wang et al. 2008; Zegada-Lizarazu and Berliner 2011). Maize and potato using ridge-furrows covered with plastic film mulching had yields enhanced by 28–90% and 57–78%, respectively, with corresponding WUE increases of 26–88% and 62–70%, respectively (Bu et al. 2013; Gao et al. 2014; Zhao et al. 2014). Accordingly, the objective of this study was to investigate the influence of various mulching time and mulching rate treatments during the fallow period on soil water storage, grain yield, and the relationship between these factors on the Loess Plateau of China. Many studies have focused on the effects of plastic mulching during the growth period, but few have examined the specific effects of mulching time, mulching rate, and precipitation storage efficiency in the fallow period. Thus, a field experiment was conducted to investigate the combined effects of varying mulching time with mulching rate on soil water and wheat yield. It was assumed that different mulching times and coverage rates would affect the soil moisture at sowing stage and the soil water storage during the growing season of wheat. Early mulching treatments and whole mulching treatments were expected to increase the soil water storage at sowing stage and growing season, especially the early mulching with whole mulching treatment . In this study, the objectives were to: (1) investigate the effects of mulching time and mulching rate on soil moisture; (2) assess the impacts of mulching time and mulching rate on spike number per ha, grain number per spike and 1 000-grain weight, grain yield, ET, and WUE; and (3) determine a film mulching time and mulching rate regime to achieve the maximum wheat yield on the Loess Plateau.

from July to September and little rain from October to the following June. The field had no irrigation infrastructure or equipment. The growing regime included a summer fallow period during which the field was bare, and this lasted from after the wheat harvest until the next wheat growing season (always from the middle of June to the end of September). Testing conducted on June 18, 2010 revealed that contents of organic matter, total nitrogen, available nitrogen, and available phosphorus of the top 20 cm of soil were 8.57 g kg–1, 0.65 g kg–1, 32.8 mg kg–1, and 20.11 mg kg–1, respectively. Testing conducted on June 10, 2011 revealed that contents of organic matter, total nitrogen, available nitrogen, and available phosphorus of the top 20 cm of soil were 8.72 g kg–1, 0.78 g kg–1, 40.16 mg kg–1, and 19.87 mg kg–1, respectively. Field water holding capacity changed from 25 to 30% during the two growing seasons. Over the past 11 years (2005–2016), the mean annual precipitation at the experimental site was 478.8 mm (Fig. 1), with approximately 52.9% falling during the fallow period (i.e., July, August, and September). The average annual precipitation during the growing period was 253.1 mm. The 11-year annual average air temperature was 13.4°C. The maximum mean monthly temperature is 26.4°C (in July), and the minimum mean monthly temperature is –0.8°C (in January). From 2010 to 2011, the total precipitation was 559.1 mm, which is almost the same as the historical average precipitation. The rainfall from July to September 2010 was 401.5 mm, comprising 71.8% of the total precipitation received. The average air temperature was 13.1°C. From 2011 to 2012, the total precipitation was 673.9 mm, which is 22.5% higher than the average precipitation accumulation. The rainfall from July to September 2011 was 459.9 mm, accounting for 68.2% of total precipitation in that year. The average air temperature was 13.3°C, which is approximately the same as the annual average air temperature. The meteorological data were collected from the local weather bureau. There is an automatic weather station in the local weather bureau, from which we obtained meteorological data. We used Excel 2007 to calculate the monthly precipitation and temperature.

2. Materials and methods

The wheat cultivar Yunhan 20410 was used exclusively in this experiment. The wheat stubble left after harvesting was 20–30 cm high and was plowed 25–30 cm into the soil before mulching. The experiment employed a two-factor split-plot design with three replicates. The heavy rainfall mainly happens in this area 30 and 60 days after wheat harvest. Therefore, we began to mulch the land 30 and 60 days after harvest. The experimental treatments are shown in Table 1. Whole-mulching treatments at early mulching (EW) and at late mulching (LW) applied the plastic film to cover

2.1. Experimental site The field experiment was conducted from July 2010 to June 2012 over the course of two growing seasons, at Qiujialing Village (35°09´N, 110°59´E), Wenxi County, Shanxi Province, China. The experimental site is located along a hill within the Loess Plateau and has a continental monsoon climate, meaning it receives heavy precipitation

2.2. Experimental design and field management

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2010–2011 precipitation 2011–2012 precipitation 2005–2016 average precipitation 300

30

2010–2011 temperature 2011–2012 temperature 2005–2016 average temperature

250

25

200

15

150

10 5

100

Temperature (°C)

Precipitation (mm)

20

0 50

–5

0

–10 July Aug. Sep. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June Month

Fig. 1 Average monthly precipitation and temperature based on 7 years (2005–2012) and 2 agricultural seasons (2010–2012). Table 1 The experimental design Main area Early mulching

Late mulching

the whole area of the plots.

Sub-area Whole mulching Half mulching No mulching Whole mulching Half mulching No mulching

Treatment Early mulching with whole mulching (EW) Early mulching with half mulching (EH) Early mulching with no mulching (EN) Late mulching with whole mulching (LW) Late mulching with half mulching (LH) Late mulching with no mulching (LN)

Half-mulching treatments at

The experiment was carried out in different plots during

early mulching (EH) and at late mulching (LH) used the same

the two growing seasons, and field management was

plastic film to cover half area of the plots, with a 60-cm gap

conducted according to somewhat different time schedules.

between mulched and non-mulched areas. No-mulching

For the 2010 to 2011 season, early mulching was conducted

treatments at early mulching (EN) and at late mulching (LN)

on July 20, 2010, and late mulching was conducted on

did not use plastic film mulching during the fallow period,

August 20, 2010. In addition, shallow tillage (10–15 cm)

and because of this, in theory, EN and LN were the same

and raking were conducted by machine on September 27,

treatment.

2010, and sowing occurred on September 29, 2010. For

Each plot was 6 m wide and 50 m long. The plastic film

the 2011–2012 season, early mulching was conducted on

was designed and produced by Shanxi Agricultural Science

July 15, 2011, and late mulching was performed on August

Institute, China, to a thickness of 0.008 mm and a width of

15, 2011. Shallow tillage at a depth of 10–15 cm and raking

80 cm. Microporous density was 5 000 holes every square

were conducted by machine on September 29, 2011, and

meter. The distance between the holes was 2 cm and the

sowing occurred on October 1, 2011. The mulching plastic

diameter of the holes was 1 mm. The micro-transparent

film was uncovered on the sowing days. According to local

structure makes the film permeable, though it is not

fertilizer use practices, fertilizers were manually incorporated

biodegradable. When rain fell during the mulching time, it

into the soil at rates of 150 kg N ha–1, 150 kg P2O5 ha–1, and

could infiltrate into the soil through the film. Therefore, the

150 kg K2O ha–1 before sowing. Sowing was carried out

soil of whole mulching treatments could receive the same

by mechanical drilling with row spacing of 20 cm. Planting

amount of rain as the half- and no-mulching treatments.

density (seeds) was 225×104 ha–1 before winter.

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2.3. Sampling and measurements Soil at the local site is sandy loam. Some of the dryland wheat roots could grow as long as 300 cm, so we collected soil samples to a depth of 300 cm. Soil samples were collected and analyzed for soil moisture 30 and 60 days after the wheat harvest. Soil samples were collected on sowing day to calculate the soil water storage before sowing. Soil samples at depths up to 300 cm at 20 cm intervals were also collected by using a soil auger to measure the gravimetric water content at overwintering, reviving, jointing, booting, flowering, and maturing stages during the growing season. Soil moisture was measured gravimetrically at depths up to 300 cm at 20 cm intervals after soil sampling with a soil auger. The soil samples were collected in aluminum soil boxes, which were transported in plastic bags. All boxes were weighed immediately to obtain fresh weights. The boxes were dried at 105°C to a constant weight and weighed again to obtain the gravimetric water content. A 300 cm deep profile was sampled from a representative plot after the wheat harvest, and soil subsamples were taken from the top to the bottom of the profile at 20 cm intervals. Then the soil bulk density from 0 to 300 cm was determined by Core Method using the stainless-steel cylinder. Soil water storage (SWS) for each soil layer was calculated using the following eq. (1): SWS (mm)=Wi×Di×Hi×10/100 (1) Where, W (in percent) is the water content of soil by mass, i is the index for each soil layer, D (in g cm–3) is the soil bulk density of layer i, and H (in cm) is the thickness of the layer. Soil water storage estimates at depths up to 300-cm were calculated by summing the soil water storage across all layers. The Di data before sowing of 2010 and 2011 were shown inTable 2. Evapotranspiration (ET) was calculated using the following soil-water balance equation (Zhou et al. 2011): ET=ΔW+P (2) Where, ΔW (in mm) is the soil water depletion (i.e., soil water storage at depths up to 300 cm at sowing stage minus that measured at harvest stage) and P is the precipitation at the experiment site during the growing season (in mm). Precipitation storage efficiency (PSE, in %) was calculated as the change of soil water storage at depths up to 300 cm during the fallow period divided by precipitation during the fallow period (Tanaka and Anderson 1997). Precipitationuse efficiency (PUE, in kg ha –1 mm–1) was calculated as wheat yield (in kg ha–1) divided by total precipitation throughout the whole year. In addition, WUE (in kg ha–1 mm–1) was calculated as wheat yield (in kg ha–1) divided by ET (in mm; Zhao et al. 2014). Spike number per ha, grain number per spike, and 1 000-grain weights were estimated from harvests of 16 m2

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from each plot. The moisture content of grains during the harvest time was 12% in 2011 and 13% in 2012. The moisture content of grains was used to calculate grain yield.

2.4. Statistical analysis Statistical analysis was conducted using the SAS software package (SAS Institute 1990). The difference among treatments were tested using a two-way ANOVA with the least significant range (LSR) method at a significance level of P<0.05. All the figures were drawn with SigmaPlot 12.0 (Systat Software Inc., San Jose, CA, USA). The precipitation and temperature data were calculated in Excel 2007.

3. Results 3.1. Soil water storage at sowing stage and throughout the whole growing season The early mulching treatments (EW, EH, and EN) held more water than late mulching treatments (LW, LH, and LN) during the fallow periods in both years (Figs. 2 and 3). In addition, whole-mulching treatments (EW and LW) conserved more water than half- and no-mulching treatments (EH, EN, LH, and LN). During the fallow periods of the 2010–2011 and 2011–2012 seasons, EW held 84.5 and 92.4 mm more water in the 0–300 cm soil profile than LN, respectively. And EW held 18.7 and 15% more soil water than that of LN, respectively. Precipitation during the growing seasons of 2010–2011 and 2011–2012 was only 28 and 32% of precipitation for the whole year, respectively. Therefore, rainfall during the growing season had a small effect on soil moisture for all treatments. Soil water storage began to increase from 60 days

Table 2 The soil bulk density before sowing for winter wheat in 2010 and 2011 Soil depth (cm) 0–20 20–40 40–60 60–80 80–100 100–120 120–140 140–160 160–180 180–200 200–220 220–240 240–260 260–280 280–300

Soil bulk density (g cm–3) of 2010 1.31 1.29 1.23 1.28 1.21 1.17 1.12 1.12 1.14 1.12 1.12 1.12 1.12 1.12 1.12

Soil bulk density (g cm–3) of 2011 1.29 1.28 1.22 1.29 1.22 1.17 1.12 1.12 1.15 1.12 1.12 1.12 1.12 1.12 1.12

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2010–2011

45

Soil water storage (mm)

40

35

30 EW EH EN LW LH LN

25

20

15

20

40

60

80 100 120 140 160 180 200 220 240 260 280 300 Soil depth (cm)

Fig. 2 Soil water storage at sowing stage from 2010 to 2011. EW, early mulching with whole mulching; EH, early mulching with half mulching; EN, early mulching with no mulching; LW, late mulching with whole mulching; LH, late mulching with half mulching; LN, late mulching with no mulching. Bars indicate SE.

60

2011–2012

Soil water storage (mm)

55 50 45 EW EH EN LW LH LN

40 35 30 25

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 Soil depth (cm)

Fig. 3 Soil water storage at sowing stage from 2011 to 2012. EW, early mulching with whole mulching; EH, early mulching with half mulching; EN, early mulching with no mulching; LW, late mulching with whole mulching; LH, late mulching with half mulching; LN, late mulching with no mulching. Bars indicate SE.

after harvest, reaching its peak at sowing stage (Figs. 4 and 5). Soil water storage then decreased throughout the growing stages. Soil water storage in the 0–300 cm profile for early mulching treatments was generally improved by 41.7 mm in 2010–2011 season and by 36.9 mm in 2011– 2012 season compared with late mulching treatments from 60 days after harvest to the booting stage. EW and LW had

average increases of 11.9 mm (2010–2011) and 16.2 mm (2011–2012) soil water from 60 days after harvest to the booting stage, relative to EH and LH. EW maintained the most soil water from 60 days until the booting stage. EW enhanced soil moisture by 68 mm on average in 2010 and 83.4 mm on average in 2011 compared to LN from 60 days to booting stage.

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2010–2011

550

0–300 cm soil water storage (mm)

500 450 400 EW EH EN LW LH LN

350 300 250 200

30

45

60

75

S

OW

R

Days after harvest (d)

J

B

F

M

Growing stage

Fig. 4 Soil water storage through the whole year (2010–2011). S, sowing stage; OW, over-wintering stage; R, reviving stage; J, jointing stage; B, booting stage; F, flowering stage; M, maturing stage. EW, early mulching with whole mulching; EH, early mulching with half mulching; EN, early mulching with no mulching; LW, late mulching with whole mulching; LH, late mulching with half mulching; LN, late mulching with no mulching. Bars indicate SE.

2011–2012

0–300 cm soil water storage (mm)

800 700 600 500

EW EH EN LW LH LN

400 300 200

30

45

60

75

S

OW

Days after harvest (d)

R

J

B

F

M

Growing stage

Fig. 5 Soil water storage through the whole year (2011–2012). S, sowing stage; OW, over-wintering stage; R, reviving stage; J, jointing stage; B, booting stage; F, flowering stage; M, maturing stage. EW, early mulching with whole mulching; EH, early mulching with half mulching; EN, early mulching with no mulching; LW, late mulching with whole mulching; LH, late mulching with half mulching; LN, late mulching with no mulching. Bars indicate SE.

3.2. Effects of mulching during the fallow period on water consumption Table 3 shows that mulching time significantly impacted on field water consumption during the fallow period, ET during growing stage, and annual water consumption in both years. Except for annual water consumption of 2010–2011,

mulching rate significantly affected field water consumption during the fallow period, ET during growing stages, and annual water consumption in both years. A significant mulching time×mulching rate interaction effect (P<0.01) was only observed in ET during growing stages in 2011–2012. Early mulching treatments conserved, on average, 39.8 and 33.5 mm more soil water than late mulching treatments

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2010 to 2012. In addition, EW promoted the highest annual water consumption from 2010 to 2012; in contrast, LN had the lowest annual water consumption. In 2010–2011, the most soil water was consumed from sowing stage to jointing stage (Table 4). However, the least soil water was used from flowering stage to maturing stage. In 2011–2012, the most soil water was used from jointing stage to flowering stage, while the least soil water was consumed from flowering stage to maturing stage. Early

in 2010 and 2011, respectively. And EW and LW saved, on average, 16.6 and 19.6 mm more soil water than EH and LH in 2010 and 2011. However, early mulching treatments consumed, on average, 70.3 and 57.1 mm more soil water than late mulching treatments in 2010 and 2011, respectively. And EW and LW used, on average, 70.3 and 57.1 mm more soil water than EH and LH in 2010 and 2011, respectively. EW saved more water than LN during the fallow period and used it during the growing period from

Table 3 Effects of mulching during the fallow period on water consumption (mm) Season 2010–2011

2011–2012

Treatment1) EW EH EN LW LH LN Mulching time Mulching rate Mulching time×Mulching rate EW EH EN LW LH LN Mulching time Mulching rate Mulching time×Mulching rate

Field water consumption during fallow period (mm) 217.72 f 235.51 e 251.30 d 258.64 c 273.96 b 291.44 a

ET during growing stages Annual water consumption (mm)2) (mm) 425.62 a 643.34 a 406.00 b 641.50 a 387.42 c 638.72 a 354.91 d 613.56 b 336.71 e 610.67 b 316.38 f 607.81 b

***

***

***

***

***

ns 128.97 e 154.04 d 183.69 b 171.24 c 185.46 b 210.61 a

ns 622.73 a 585.32 b 548.95 c 553.28 c 533.77 d 498.71 e

***

***

***

***

**

ns

**

ns

ns ns 751.69 a 739.36 b 732.64 bc 724.52 cd 719.23 d 709.33 e ***

1)

EW, early mulching with whole mulching; EH, early mulching with half mulching; EN, early mulching with no mulching; LW, late mulching with whole mulching; LH, late mulching with half mulching; LN, late mulching with no mulching. ET, evaporation transpiration. ** and *** indicate significances at the 0.01 and 0.001 levels of probability, respectively; ns, not significant. Average values within the same column followed by the same lowercase letter are not significantly different (P<0.05). 2)

Table 4 Water decrease amounts and relative percentage across different stages of wheat growth under different treatments (2010–2012) Season 2010–2011

2011–2012

1)

Treatment EW EH EN LW LH LN EW EH EN LW LH LN

1)

Sowing–jointing stage Soil water Relative decrease amount percentage (mm) (%) 149.94 a 51.27 c 143.75 b 52.69 b 138.28 b 54.40 a 118.02 c 53.23 ab 108.15 d 53.14 ab 96.30 e 52.56 b 118.48 a 28.93 c 116.51 a 31.31 ab 107.12 b 31.90 a 88.75 c 26.10 d 81.27 d 25.35 d 87.50 c 30.65 b

Jointing–flowering stage Soil water Relative decrease amount percentage (mm) (%) 114.84 a 39.27 a 99.03 b 36.31 b 83.77 c 32.95 d 75.93 d 34.24 c 69.14 e 33.97 c 61.96 f 33.83 c 199.68 a 48.76 b 166.38 c 44.71 c 131.27 e 39.10 d 180.01 b 52.93 a 158.49 d 49.44 b 102.07 f 35.74 e

Flowering–maturing stage Soil water Relative decrease amount percentage (mm) (%) 27.64 c 9.45 d 30.01 b 11.00 c 32.16 a 12.65 ab 27.70 c 12.50 b 26.49 c 13.02 ab 24.87 d 13.58 a 91.37 bc 22.31 d 89.23 c 23.98 c 97.36 a 28.99 b 71.32 e 20.97 e 80.82 d 25.21 c 95.94 ab 33.61 a

EW, early mulching with whole mulching; EH, early mulching with half mulching; EN, early mulching with no mulching; LW, late mulching with whole mulching; LH, late mulching with half mulching; LN, late mulching with no mulching. Average values within the same column followed by the same lowercase letter are not significantly different (P<0.05).

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mulching treatments used, on average, 36.5 mm (2010) and 28.2 mm (2011) more soil water than late mulching treatments from sowing stage to jointing stage. However, early mulching treatments used a smaller percentage of water than late mulching treatments. EW and LW generally consumed 8 and 4.7 mm more soil water than EH and LH from sowing stage to jointing stage, and which used a smaller percentage of water than half-mulching treatments. EW used 53.6 and 31 mm more soil water than LN in 2010 and 2011 from sowing stage to jointing stage. Early mulching treatments consumed, on average, 30.2 mm (2010) and 18.9 mm (2011) more water than late mulching treatments from jointing stage to flowering stage, also held higher water percentages than the late mulching treatments. EW and LW used, on average, 11.3 and 27.4 mm more water than EH and LH from jointing stage to flowering stage, and they held a higher water percentage than the half mulching treatments. EW used 52.9 and 97.6 mm more water than LN in 2010 and 2011, respectively, from jointing stage to flowering stage, which was almost twice the water consumed under LN. EN consumed the most soil water of all treatments from flowering stage to maturing stage in 2010, and it had the highest water percentage. EN and LN used the most soil water of all treatments from flowering stage to maturing stage in 2011, and they also had the highest water percentage.

3.3. Effects of mulching during the fallow period on yield and yield components Mulching time significantly affected yield and yield components in both years (Table 5). Except for grain number per spike and 1 000-grain weight in 2011, mulching rate also had a significant effect on yield and yield components in both years. A significant mulching time×mulching rate (P<0.05) interaction effect was observed in spike number per ha in

2012, grain number per spike in 2011, 1 000-grain weight in both years and the grain yield in 2012. Early mulching treatments increased, on average, by 3.5×105 and 3.9×105 in spike number per ha compared to Late mulching treatments in 2011 and 2012, respectively. EW and LW increased, on average, by 1×105 and 2.3×105 in spike number per ha compared to EH and LH in 2011 and 2012, respectively. Early mulching treatments had a slightly higher grain number per spike than late mulching treatments during harvest in both years. The grain yield of early mulching treatments was 332.1 and 237.6 kg ha–1 higher than that for late mulching treatments in 2011 and 2012, respectively, which was 7 and 4.3% higher on average respectively than in late mulching treatments. The grain yield of EW and LW was improved by 94 and 450.6 kg ha–1 compared to EH and LH in 2011 and 2012, respectively. In addition, EW produced the highest spike number per ha, grain number per spike, and grain yield in both years. In contrast, LN had the lowest spike number per ha, grain number per spike, 1 000-grain weight, and grain yield. In comparison with LN, the EW treatment increased spike number per ha by 13.8 and 20.9% in 2011 and 2012, respectively. It also enhanced the grain number per spike by 3.1 and 1.3%, respectively, and grain yield was ultimately improved by 11.7 and 17.4%, respectively.

3.4. Effects of mulching during fallow period on precipitation utilization Mulching time and mulching rate significantly affected PSE, PUE, and WUE in both years (Table 6). A significant mulching time×mulching rate (P<0.05) interaction effect was observed in PUE in 2011–2012 and WUE in both years. In this study, EW, EH, LW, and LH significantly improved PSE and PUE for both years during the fallow period. However, these treatments significantly decreased WUE. The PSE of early mulching treatments was improved by 9.9% on

Table 5 Effects of mulching during the fallow period on yield and yield components Treatment1) EW EH EN LW LH LN Mulching time Mulching rate Mulching time×Mulching rate 1)

Spike number per ha (no. ha–1) 2011 2012 5.29×106 a 6.55×106 a 5.28×106 a 6.22×106 b 5.00×106 b 6.03×106 d 5.03×106 b 6.17×106 c 4.85×106 c 6.05×106 d 4.65×106 d 5.42×106 e

Grain number per spike (no. spike–1) 2011 2012 28.50 a 26.63 a 28.30 b 26.53 ab 27.93 c 26.56 a 26.89 e 26.42 b 26.97 e 26.20 c 27.65 d 26.29 c

1 000-grain weight (g) 2011 2012 40.85 bc 37.55 a 41.12 b 36.41 d 42.21 a 37.15 b 41.38 ab 36.72 c 40.09 c 36.34 d 39.14 d 36.32 d

Grain yield (kg ha–1) 2011 2012 5 173.64 a 6 227.15 a 5 075.65 b 5 713.20 c 4 941.19 c 5 412.04 e 4 826.47 d 5 861.75 b 4 736.32 d 5 474.60 d 4 631.47 e 5 303.20 f

***

***

***

***

**

***

***

***

**

***

ns

*

ns

**

**

***

ns

***

***

ns

**

*

ns

***

EW, early mulching with whole mulching; EH, early mulching with half mulching; EN, early mulching with no mulching; LW, late mulching with whole mulching; LH, late mulching with half mulching; LN, late mulching with no mulching. * ** , and *** indicate significances at the 0.05, 0.01 and 0.001 levels of probability, respectively; ns, not significant. Average values within the same column followed by the same lowercase letter are not significantly different (P<0.05).

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average in 2010 and 7.3% on average in 2011 compared to late mulching treatments. The PSE of EW and LW was increased by 4.1% in 2010 and 4.3% in 2011 relative to EH and LH, respectively. Early mulching treatments increased PUE by 0.6 and 0.4 kg ha–1 mm–1 on average compared to late mulching treatments in 2010 and 2011, respectively. EW and LW enhanced PUE by 0.2 and 0.7 kg ha–1 mm–1 on average compared to EH and LH in 2010 and 2011, respectively. The WUE of early mulching treatments decreased by 1.6 kg ha–1 mm–1 in 2010 and by 0.6 kg ha–1 mm–1 in 2011 compared to late mulching treatments. The WUE of EW and LW was reduced by 0.4 kg ha–1 mm–1 compared to EH and LH from 2010 to 2011. However, the WUE of EW and LW increased by 0.3 kg ha–1 mm–1 compared to EH and LH from 2011 to 2012. Compared with LN, the EW treatment increased PSE to 45.8% in 2010 and 72% in 2011. Additionally, EW increased PUE to 9.7 kg ha–1 mm–1 in 2010 and 9.6 kg ha–1 mm–1 in 2011. Thus, EW could use much more precipitation from the fallow period than LN, which may be the biggest underlying cause of the yield improvement.

3.5. Stepwise regression between soil water storage at sowing stage, three components of yield, and grain yield The relationship among X1 (soil water storage at sowing), X2 (spike number per hectare), X3 (grain number per spike), X4 (1 000-grain weight) and Y (grain yield) were analyzed by using the stepwise regression model. The P-value of the full model was less than 0.0001, and the determination coefficient (R2) was 0.9266 (data now shown). Therefore, this stepwise regression model was extremely significant and fit the data extremely well. The P-value of spike number per ha (X2) was less than 0.0001 (Table 7), indicating that it had a highly significant influence on grain yield. The optimal regression equation was Y=1 177.40+7.40X2; Y stands for grain yield. Based on this result, spike number per ha was significantly associated with grain yield. EW, EH, LW, and LH had improved spike number per ha during both years, and therefore could obtain better grain yield than EN and LN. EW obtained the best spike number per ha and grain yield in both years.

Table 6 Effects of mulching during the fallow period on utilization of precipitation1) Season 2010–2011

2011–2012

Treatment2) EW EH EN LW LH LN Mulching time Mulching rate Mulching time×Mulching rate EW EH EN LW LH LN Mulching time Mulching rate Mulching time×Mulching rate

PSE (%) 45.77 a 41.34 b 37.41 c 35.58 d 31.77 e 27.41 f

PUE (kg ha–1 mm–1) 9.68 a 9.49 b 9.24 c 9.03 d 8.86 d 8.66 e

WUE (kg ha–1 mm–1) 12.16 e 12.50 d 12.75 d 13.60 c 14.07 b 14.64 a

***

***

***

***

***

***

ns 71.96 a 66.51 b 60.06 d 62.77 c 59.67 d 54.20 e

ns 9.60 a 8.81 c 8.34 e 9.04 b 8.44 d 8.18 f

*

***

***

***

***

***

***

ns

***

*

10.00 c 9.76 d 9.86 cd 10.59 a 10.26 b 10.63 a

1)

PSE, precipitation storage efficiency; PUE, precipitation-use efficiency; WUE, water use efficiency. EW, early mulching with whole mulching; EH, early mulching with half mulching; EN, early mulching with no mulching; LW, late mulching with whole mulching; LH, late mulching with half mulching; LN, late mulching with no mulching. * and *** indicate significances at the 0.05 and 0.001 levels of probability, respectively; ns, not significant. Average values within the same column followed by the same lowercase letter are not significantly different (P<0.05). 2)

Table 7 Parameter estimates of stepwise regression Variable1) Intercept X2 1) 2)

df 1 1

Parameter estimate 1 177.40 7.40

Standard error 247.78 0.44

X2, spike number per ha. Pr, significance probability; T, random variable obeying t-test distribution.

t-value 4.75 16.66

Pr (|T|>|t-value|)2) <.0001 <.0001

Standardized estimate 0 0.926

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4. Discussion Early mulching treatments (EW, EH, and EN) significantly improved water storage between 0 and 300 cm from 60 days after harvest until the booting stage, in comparison to late mulching treatments (LW, LH, and LN) (Figs. 4 and 5), which was in line with a study in maize (Aina 1981). The plastic film on the soil surface served as a physical barrier inhibiting rainwater from running off or evaporating during the fallow period. Therefore, the plastic film diminished evaporation and increased rainwater penetration, which led to an accumulation of soil moisture (Ramakrishna et al. 2006; Chen et al. 2015). Whole mulching treatments (EW and LW) significantly increased soil water storage between 0 and 300 cm from 60 days after harvest until the booting stage compared to half- and no-mulching treatments (EH, EN, LH, and LN) (Figs. 4 and 5). This result was in accordance with research on maize (Wu et al. 2017) and potato (Zhao et al. 2014). The high temperature and bright sunshine during the fallow period evaporated much water from the soil, whereas the larger plastic film mulching could decrease more water losses during the fallow period (Cai et al. 2015). Therefore, the conserved water could relieve the drought stress that occurs at sowing stage and in the earlier stages of wheat growth. EW had the best soil water storage from 60 days after harvest until the booting stage due to the interaction effect of mulching time×mulching rate (data not shown). From 2011 to 2012, EW, EH, LW, and LH conserved much more soil water from 60 days after harvest until the booting stage than the previous agricultural year. This result was a consequence of the higher precipitation accumulation from 2011 to 2012 (114.8 mm more than the precipitation accumulation in 2010–2011). However, EW, EH, LW, and LH kept the lower soil water storage from flowering stage to maturing stage than EN and LN (Table 4). The reason may be that film mulching could promote root growth and development (Fang et al. 2011), and thus the amount and density of wheat roots under EW, EH, LW, and LH may be larger. Therefore, they need to absorb much more water than EN and LN from flowering stage to maturing stage, especially EW. Whole mulching treatments, especially EW, apparently decreased field water consumption during the fallow period and increased ET during the growing stages (Table 3). In addition, the relative decrease in soil water from sowing stage to jointing stage was the lowest under EW (Table 4). The relative soil water decrease from jointing stage to flowering stage was significantly higher under EW than in other treatments (Table 4). These results indicated that mulching could modify water consumption of crops. Mulching reduced early soil evaporation and increased the

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effective plant transpiration. This enabled the limited soil water to be used for stronger growth. Therefore, increased soil water availability drove biological processes (Zhu et al. 2000). During both growth years, early mulching treatments (EW, EH and EN) produced significantly more spike numbers than late mulching treatments (LW, LH and LN) (Table 5). Early mulching treatments also had significantly increased grain yield in both years, which was consistent with a previous study in maize (Aina 1981). Whole mulching treatments (EW, LW) significantly enhanced grain yield compared with half- and no-mulching treatments (EH, EN, LH, and LN) in both years (Table 5), which was in accordance with studies in maize (Wu et al. 2017) and potato (Zhao et al. 2014). EW had the best soil water storage from 60 days after harvest until the booting stage compared to all other treatments in both years (Figs. 4 and 5). Sufficient soil water supply promotes seed germination, tiller growth, and spike differentiation of the winter wheat (Musick et al. 1994; Zhou et al. 2009, 2011), and therefore, EW had the best grain yield in both years. However, the lowest WUE was associated with EW, which contrasted with previous research (Zhao et al. 2014; Wu et al. 2017). The reason for this discrepancy may be that the WUE of rain-fed winter wheat showed a curvilinear relationship with the increase of grain yield and the yield was at the apex of the curvilinear line, which resulted in a decrease of WUE (Zhang and Oweis 1999). EW also had the highest precipitation use efficiency across both years (Table 6), because it conserved a large proportion of rainfall during the fallow period, which comprised almost 70% of total precipitation each year (Fig. 1). Across both years, spike number per ha was the main factor promoting grain yield (Table 7).

5. Conclusion EW had the highest soil water storage from 60 days after harvest to booting stage. EW significantly improved PSE, PUE, and ET during growing stages and decreased soil water amount from jointing stage to flowering stage compared with LN. But field water consumption by EW during the fallow period was significantly reduced compared to that by LN. EW had significantly higher spike number per ha, grain number per spike, 1 000-grain weight, and yield compared to LN. However, EW significantly reduced the WUE in comparison to that of LN in both years. Therefore, the EW treatment was the optimal agricultural practice for conserving rainfall during the fallow period, which was then mainly used by the crop from jointing stage to flowering stage, thereby enhancing grain yield. This soil water conserving practice could potentially help farmers with wheat production if they could be encouraged by governmental

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supplement to apply the mulching plastic film.

Acknowledgements This study was financially supported by the Special Fund for Agro-scientific Research in the Public Interest in China (201303104 and 201503120), the earmarked fund for China Agriculture Research System (CARS-03-01-24), the Key Science and Technology Program of Shanxi Province, China (20140311008-3), the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2015BAD23B04).

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