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Agronomy Journa l • Volume 111, I s sue 5 • 2019 1

AbstrActThe responses of soil water and winter wheat (Triticum aestivum L.) to changes in precipitation are crucial for stability of wheat yield and soil water conservation in the Chinese Loess Tableland Region (LTR). A 2-yr (2014-2016) experiment was conducted in the LTR to investigate soil water dynamics and winter wheat performance under three precipitation levels: Rc (normal), R+ (increased by one-third of normal), and R- (decreased by one-third of normal) by a real-time precipitation allocation system. Soil water storage (SWS) at a depth of 0-3.8 m showed positive (R+) or negative (R–) responses to precipitation. The mean SWS in different soil layers followed an order of R+ > Rc > R–. Dur-ing the experiment, the 0-3.8 m SWS decreased by 34.5 mm (R+), 183.8 mm (Rc), and 218.9 mm (R–). The shoot biomass and grain yield varied little among three precipitation levels in 2014-2015, but was reduced by 50% in R- compared with Rc in 2015-2016. The R– and Rc treatments had higher water use effi-ciency (WUE) than R+ in 2014-2015, and Rc was the highest in 2015-2016. The proportions of soil water to water consumption in R+ and Rc increased from 22.5% in 2014-2015 to 42.5% in 2015-2016 on average, and for R– was stable over 2-yr (average 44.5%). Soil water plays a critical role in winter wheat perfor-mance under changes in precipitation, which should guide win-ter wheat production and soil water management in the future.

core Ideas• Precipitation changes soil water storage but does not change the

regime of soil water changing with time.• Response of winter wheat yield to precipitation change has hyster-

esis effect.• Supplementary effect of soil water compensates for the negative

effect of decreased precipitation.• Performance of supplementary effect is determined by precipitation

pattern.

Winter wheat is the dominant cereal crop in the LTR of China and makes an important contribu-tion to regional food security (Chen et al., 2015;

He et al., 2016). The total planted area is approximately 4.3 million hectares, and accounts for 44% of the total farmland use. Historically, wheat production has depended on precipi-tation and soil water for the shortage of water resources, low groundwater table, and no irrigation in the LTR (Nolan et al., 2008; Wang et al., 2009; Fang et al., 2010). With global climate change, the precipitation in this region has decreased slightly, but it has shown greater variability over the past 50 yr (Wang et al., 2013), which has resulted in greater fluctuations in soil water and winter wheat yield than before (Yang, 2001) and further posed risks to regional agro-ecosystem sustainability and food security (Sang et al., 2016). Hence, it is necessary to assess win-ter wheat performance under climate change to promote crop yield stability and conservation of water resources in the region.

The amount, frequency, intensity, and duration of precipita-tion can strongly affect crop growth and yield by changing water status in the soil profile (Wilson et al., 2004; Salve et al., 2011; Liu and Shao, 2015; Liu et al., 2016). There is a positive relation-ship between the amount of precipitation during the growing season and crop yield in a rain-fed agricultural system (Huang et al., 2005). Also, precipitation during the fallow period is critical because the precipitation determines the SWS before sowing. Low precipitation during this period depresses crop growth and causes a reduction in yield (He et al., 2016). In addition, pat-terns of water use by crops, which are shaped by soil water status and crop root properties (Zhang et al., 2004, 2009, 2012), have great effects on crop production by changing soil water avail-ability (Muñoz-Romero et al., 2010; Sang et al., 2016). It is clear that there are complicated interactions among precipitation, soil water, and crop yield.

The LTR, with a semi-humid climate, is a typical rain-fed agricultural region due to limited precipitation and deeply buried groundwater. The goal of local agriculture is to stabilize crop yield and to reduce water consumption (Huang et al., 2003). Consequently, WUE, which is an indicator that reflects

Soil Water Dynamics and Winter Wheat Performance under Precipitation Change in the Loess Tableland Region, China

Yuanjun Zhu* and Yaqiang Cui

Y. Zhu and Y. Cui, State Key Lab. of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A&F Univ., Yangling, Shaanxi 712100, China; Y. Zhu, Univ. of Chinese Academy of Sciences, Beijing 100049, China. *Corresponding author (zhuyj@nwsuaf.edu.cn).

Abbreviations: CWA, the Changwu Agro-ecosystem Experimental Station; LTR, the Loess Tableland Region; PAR, photosynthetically active radiation; PVC, polyvinyl chloride; Rc, normal precipitation; R+, precipitation increased by one-third of normal; R–, precipitation decreased by one-third of normal; SWC, soil water content; SWS, soil water storage; TR, total radiation; WUE, water use efficiency; ∆W, the change in soil water storage.

Agronomy, soIls, And EnvIronmEntAl QuAlIty

Published in Agron. J. 111:1–9 (2019) doi:10.2134/agronj2018.10.0690

Copyright © 2019 The author(s). Re-use requires permission from the publisher.

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the tradeoff between water consumption and crop yield, is used widely to assess this tradeoff (Sadras and Angus, 2006; Su et al., 2007; Zhang et al., 2013a; Guan et al., 2015).

In past decades, many studies have focused on the interactions among precipitation, soil water, and winter wheat yield and their effects on WUE (Huang et al., 2005; Jin et al., 2007; Ren et al., 2008, 2010; Sun et al., 2010; Lv et al., 2013; Zhang et al., 2013b, 2017b; Ma et al., 2015; He et al., 2016). However, most of this research was performed in the laboratory by artificial rainfall simulation, or in the field by long-term in situ observations. These methods have their advantages and disadvantages. First, laboratory experiments can be designed precisely and purpose-fully, but they cannot represent actual field conditions com-pletely. For example, the limited volume of soil usually used for laboratory experiments not only restricts root growth and water infiltration, but it also makes the role of soil water difficult to express. Second, simulated precipitation is not able to mimic the real distribution of precipitation (frequency and intensity). Crop yield is influenced strongly by the frequency and intensity of precipitation (Ines et al., 2011), which leads to the lack of univer-sality of the results under simulated rainfall conditions. Third, although field in situ observational experiments are conducted under natural conditions, it takes quite a long time to obtain the experimental results under the required precipitation range. Thus, there is a need to develop an improved method to remedy the shortcomings of these methods to investigate the relationship among precipitation, soil water, and crop yield under changes in precipitation, which would serve to evaluate the productivity of winter wheat and to promote the rational utilization and conser-vation of soil water resources in the LTR.

We conducted a 2-yr field experiment with winter wheat by using a real-time precipitation allocation system to generate three precipitation levels (i) to characterize the responses of soil water, winter wheat performance, and WUE to 33% higher and

33% lower precipitation compared to current levels; and (ii) to quantify the proportions of precipitation and soil water to water consumption to address the link between soil water and winter wheat performance under precipitation change.

mAtErIAls And mEtHodsstudy Area

This study was conducted at the Changwu Agro-ecosystem Experimental Station (CWA, 35° 14’ N, 107° 40’ E, 1200 m a.s.l.), which is located in the southern LTR (Fig. 1a). The aver-age annual precipitation was 581 mm (1957–2008); 70% of the annual precipitation fell between June and October, and the precipitation during the winter wheat growing season (between late March and late June) only accounted for 46% of the annual precipitation. The average annual temperature was 9.1°C. The annual sunshine duration was 1977 h, and the frost-free period was 171 d. The soil was developed from loess parent material, and the concentrations of soil organic matter, total N, available P, available K, and inorganic N were 14.15 g kg–1, 0.97 g kg–1, 22.1 mg kg–1, 156.7 mg kg–1, and 3.15 mg kg–1, respectively. There was no irrigation in the region. Before experimenting, we randomly selected five sampling sites in the targeted wheat field to obtain soil samples at a depth of 3 m. According to the natural stratification of the sampling profile, the soil profile was divided into five layers to measure soil particle composition and bulk den-sity at each depth (Table 1). The soil has a soil texture of silt loam (soil texture triangle with USDA classification system) (Table 1).

Experimental treatments

The experiments were performed in a winter wheat field dur-ing 2014 to 2016 (Fig. 1b). We established 11 experimental plots and designed three precipitation levels: normal precipitation that served as control (natural precipitation, Rc), precipitation increased by one-third over the normal (R+), and precipitation decreased

Fig. 1. (a) Location of the experimental site (the Changwu Agro-ecosystem Experimental Station, CWA) in the the Loess Tableland Region (LTR), (b) the image of the experimental field, and (c) a sketch map of the real-time precipitation allocation system.

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by one-third below the normal (R–). The Rc treatment had three replicates, and the R+ and R– treatments had four replicates each. During 1956 to 2015, except for an extremely dry year (1995, with an annual precipitation of 296 mm) and an extremely wet year (2003, with an annual precipitation of 956 mm), the annual precipitation was between R– and R+ at a 95% confidence level. Hence, the three precipitation levels covered the range within which 68% (±22.5%) of observed annual precipitation might fall.

The precipitation levels were created by a manmade, real-time, precipitation allocation system (Fig. 1c). The system was com-posed of a precipitation shelter with transparent plastic sheets, a water tank, and gravitational drip irrigation equipment with 12 polyvinyl chloride (PVC) tubes (2.5 cm in diameter) that were placed evenly in each plot. Four plots were treated with this system. The covered area of the precipitation shelter was equal to one-third of each plot area and, consequently, one-third of the precipitation was intercepted and collected in the tank. In this case, the covered experimental plots only received two-thirds of the normal precipitation, which was the R– treatment. The col-lected precipitation in the tank was dispensed into an adjacent plot using the gravitational trickle irrigation pipes, which created the treatment for R+. The Rc treatment was natural precipitation, and these plots contained no precipitation allocation system. The precipitation allocation system was placed in the plots through-out the entire year (the growing season and fallow period).

The plastic sheets inevitably affected the radiation in the plots. To quantify this effect, we measured photosynthetically active radiation (PAR) and total radiation (TR) every hour from 0800 to 2000 h using a portable digital radiometer under

two weather conditions (sunny and cloudy). The PAR in the covered plots decreased by 16.04 W m–2 (10%) and 20.2 W m–2 (16%) under cloudy and sunny days, respectively, compared with the bare plots. The TR in the covered plot was 16.8 W m–2 (6.3%) and 33.1 W m–2 (17%) lower than those in the bare plots under cloudy and sunny days, respectively. Although the transparent plastic affected the radiation, the maximum effect (the reduction in radiation) was <17%. In this study, we did not measure the effect of a reduction in radiation on winter wheat growth, but there have been studies on the effects of radiation on photosynthetic rate, biomass, and yield of winter wheat (Zhang et al., 2017a; Shi et al., 2018). Their results have shown that when the radiation was reduced by <19%, the photosyn-thesis of winter wheat at different growth stages, the number of spikes and the thousand-grain weight were not significantly dif-ferent from normal radiation (without shading) and, therefore, the yield was not affected significantly. Hence, we assumed that the reduction in radiation due to the transparent plastic cover did not have a significant effect on winter wheat growth.

Each plot was 6 m long and 5 m wide. A 3.8-m long aluminum tube was installed vertically in the center of each plot to measure volumetric SWC using a neutron probe (CNC503DR, Beijing Super Power Company, Beijing, China). The PVC boards of 1 by 1 m were inserted at the boundary of each plot to prevent surface flow and lateral water movement between the plots. A local vari-ety of winter wheat cultivar Changhan 58 was sowed at a density of 150 kg ha–1 for each plot. Fertilizer (N–P–K = 20–10–5) was applied before sowing at a rate of 954 kg ha–1. The wheat was sowed on 5 Oct. 2014 and harvested on 25 June 2015 for the first growing season. The corresponding dates for the second grow-ing season were 27 Sept. 2015 and 27 June 2016. The monthly precipitation and mean temperature in 2014 to 2016 are shown in Fig. 2. Precipitation during the two growing seasons was 282.6 mm in 2014–2015 and 215.8 mm in 2015–2016. Snowfall during the experimental years was 14.9 mm (2014–2015) and 15.5 mm (2015–2016), which accounted for 5 to 7% of the precipitation during the growing period. The efficiency of the precipitation allocation system was lower during snowfall than during rainfall, due to evaporation. However, the low tempera-ture in winter and the low volume of snow precipitation resulted

Table 1. Primary physical properties for the 0- to 3-m soil layers in the winter wheat field.

DepthParticle composition

BD†Sand Silt Claym —— kg 100 kg dry soil–1 —— kg m–3

0–0.4 16.6 62.9 20.5 13400.4–1 21.8 54.0 24.2 13401–1.85 20.4 57.7 21.9 12601.85–2.3 18.5 60.2 21.3 12902.3–3 16.3 59.4 24.3 1310† BD = Bulk density.

Fig. 2. Monthly precipitation and mean temperatures during the first (2014–2015) and second (2015–2016) experimental years in the Changwu Agro-ecosystem Experimental Station (CWA).

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in the practical impact of snow on precipitation allocation sys-tem being negligible.

measurements

The SWC was measured from September 2014 to June 2015 in the first experimental year and from June 2015 to September 2016 in the second experimental year. The depth intervals for SWC measurements were 0.1 m within the 0- to 1-m depth and 0.2 m within the 1- to 3.8-m depth. The measurement fre-quency was once a month from January to April 2015 and July to September 2016 and twice a month during the remaining time.

Shoot biomass of winter wheat was measured every 15 d from the re-greening period to harvest time. For each sampling, we randomly selected three locations in each plot and cut approxi-mately 15 wheat samples in each location. These three locations represented three duplicates. The cutting height of wheat was 3 cm from the surface. The fresh weight of the wheat was mea-sured quickly, the number of spikes was counted, and the wheat sample was placed in a paper bag. Winter wheat samples were first dried at 105°C for 1 h and were then dried continuously at 75°C for 72 h. After the harvest, the winter wheat in 1m2 was cut manually and dried at 75°C in an oven for dry grain yield and thousand-grain weight measurements. Shoot biomass of winter wheat per square meter was calculated from the mea-sured biomass and wheat density, and the grain yield of winter wheat over 1 m2 was used to calculate yield (kg ha–1).

SWS was calculated as:

SWS = k × ∑ SWCi × Di [1]

where SWS is the soil water storage (mm), SWCi is the soil water content in the ith soil layer (cm3 cm–3), Di is the depth of the ith soil layer (cm), and k is a coefficient for translating centi-meters into millimeters (= 10 mm cm–1).

The ∆W was calculated from:

∆W = SWSs – SWSe [2]

where ∆W is the change of soil water storage during the growing season (mm), SWSs is the soil water storage at the sowing stage (mm), and SWSe is the soil water storage at the sowing stage next year (mm).

The loess in the LTR was very thick, and the precipitation during the growth period was very limited. It is generally con-sidered that deep percolation is negligible (Huang et al., 2003). With this consideration, water consumption (i.e., the total water used for the winter wheat production during the growing season), can be calculated as:

WC = ∆W + R [3]

where WC is the water consumption during the growing season (mm), and R is the amount of precipitation during the growing season (mm).

WUE was calculated as:

WUE = Y/WC [4]

where WUE is the water use efficiency (kg ha–1mm–1) and Y is the grain yield (kg ha–1).

statistical Analyses

Statistical analyses were conducted using SPSS software (version 19.0, IBM Corp., Armonk, NY). The significance of the compari-sons was evaluated with a least significant difference test (LSD) built into a one-way ANOVA. Confidence level was set to 95%, and a probability (P) ≤ 0.05 indicated a significant difference.

rEsultssoil Water dynamics at different depths under

three Precipitation levelsWe divided the 0 to 3.8 m soil profile into four layers (0–1,

1–2, 2–3, and 3–3.8 m) to calculate SWS at each depth. The change in SWS was relatively small. Here, we took 10% of the SWS of the Rc treatment as the lower limit of relatively signifi-cant change (blue dot lines in Fig. 3). If the absolute difference between the SWS of R+ and R– and that of Rc exceeded this lower limit, it meant that the SWS changed substantively com-pared with the Rc treatment. Compared with the control (Rc), SWS at a depth of 0 to 3.8 m increased in the R+ treatment and decreased in the R– treatment, although most changes in SWS did not exceed the critical threshold on all measurement dates (Fig. 3), which indicated a positive correlation between SWS and precipitation. The change in SWS was related to soil depth, time, and growth period. For the upper three soil layers, the change in SWS during the first growth period showed almost no substantive difference among the three treatments. From the beginning of the first fallow period to the second fallow period, SWS at a depth of 0 to 3 m in the R+ treatment was significantly higher than that in the Rc treatment most of the time, especially during the second fallow period (Fig. 3a, 3b, and 3c). For treat-ment R–, SWS at a depth of 0 to 1 m was significantly lower than that in the Rc treatment most of the time (Fig. 3a), and SWS at a depth of 1 to 3 m significantly decreased only in win-ter. At other times, although SWS at the 1- to 3-m depth was lower than that in the Rc treatment, it did not reach a signifi-cant level (Fig. 3b and 3c). For the soil layer that was 3- to 3.8-m deep, the influence of change in precipitation was small. There was no significant difference in SWS between the treatments of R+ or R– and the control (Rc) during the two experimental years (Fig. 3d). In general, the effect of precipitation on average SWS during the experimental periods was positively correlated to precipitation level for all soil layers (Fig. 4). Higher precipita-tion would make SWS higher as well.

The values of ∆W at a depth of 0 to 2 m were negative in all treatments during the first experimental year (Table 2), which indicated there was a net reduction in soil water (consumption) in these soil layers. The reduction of SWS was negatively related to precipitation. On the contrary, ∆W values were positive in the 2- to 3.8-m soil layers, which indicated a net increase in soil water (recharge). However, during the second experimental year, it was different. First, the soil water increased in the upper 1 m soil layers for all treatments, but a negative ∆W occurred in the 1- to 3.8- m soil layers for R– and Rc treatments. For the R+ treatment, the soil water increased in the 1- to 2-m soil layers and decreased in the 2- to 3.8-m soil layers. By the end of the experiment, the soil water in the 0- to 3.8-m soil layers

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decreased by 34.5 mm (R+), 183.8 mm (Rc), and 218.9 mm (R–) .

Winter Wheat Performance and Water Use Efficiency

During the first experimental year (2014–2015), there was no significant difference in winter wheat shoot biomass from the booting to the mature stages among all treatments except for

significantly higher shoot biomass at the jointing stage in the R– treatment (Fig. 5a). During the second experimental year (2015–2016), the shoot biomass in the R– treatment was signifi-cantly lower (P ≤ 0.05) than the other treatments at each grow-ing stage, and there was no significant difference in the shoot biomass between the R+ and Rc treatments (Fig. 5b). Compared with Rc, the shoot biomass at harvest time in the R+ treatment increased by 201 kg ha –1 (1.6%) in the first experimental year of

Fig. 3. Changes in soil water storage in R+ and R– relative to Rc in different soil layers during 2014 to 2016. Positive and negative values indicate the increase and decrease, respectively, in soil water storage relative to Rc.

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2014–2015 and by 2112 kg ha –1 (18.6%) in the second experi-mental year of 2015–2016. Shoot biomass in the R– treatment decreased by 1455 kg ha –1 (12.2%) in the first experimental year of 2014–2015 and by 6778 kg ha –1 (52.5%) in the second experimental year of 2014–2015 (Table 3).

Performances of grain yield, spike number, and thousand-grain weight were also evaluated (Table 3). For 2014–2015, the average grain yield ranked as R+ > Rc > R–, but the difference in the yield was small among treatments. Spike numbers and thousand-grain weight showed little variation among all treat-ments. For 2015–2016, grain yield was approximately 50% lower (significant level, P ≤ 0.05) in the R– treatment than in the other two treatments, which could be attributed to a significant reduc-tion in spike number in the R– treatment (Table 3). However, the thousand-grain weight still varied little among treatments.

There was a significant difference in WUE among all treat-ments during the first experimental year. The WUE in the R–

treatment was the highest followed by the Rc treatment, and the R+ treatment had the lowest. For the second experimental year, WUE in the Rc treatment was the highest, and the other two treatments had very close WUE values (Table 3).

contributions of Precipitation and soil Water to Water consumption

Precipitation and soil water are two dominant water sources for winter wheat in the LTR. To determine their contributions to the water consumption by winter wheat, we calculated the

values of precipitation amount and ∆W separately in the 0- to 3.8-m soil layers during the two growing seasons and further deduced the proportions they each contributed to water con-sumption (Fig. 6). Generally, the ratios of precipitation to water consumption were higher than that of soil water over the two experimental years regardless of any change in precipitation (Fig. 6a). The proportions of precipitation to water consump-tion in the R+ and Rc treatments were very high (>77%) in the first growing season due to relatively higher reference precipita-tion (i.e., the normal precipitation of 282.6 mm), during this period (Fig. 6b). Their proportions were reduced to <60%, and the proportions of soil water increased to >40% for the R+ and Rc treatments in the second experimental year due to relatively lower reference precipitation (215.8 mm) during this period. However, the proportion of soil water in the R– treatment (45%) was much higher than the proportions of precipitation in the R+ and Rc treatments (22 and 23%, respectively) in the first experi-mental year, and the proportions of soil water in the R– treat-ment were basically unchanged over the two experimental years (45 and 44%, respectively) (Fig. 6b).

dIscussIonThe response of soil water to changes in precipitation was

positive (i.e., SWS increased with the increase in precipitation and decreased with the decrease in precipitation) (Fig. 3). The SWS at a depth of >3 m in the R+ treatment was higher than that in control (Rc) after the first fallow period, which indicated that the increase in precipitation recharged deep soil water. The SWS at the same depth in the R– treatment was basically stable, which indicated that the root system of winter wheat and its utilization of soil water were both mainly within the 3-m depth. The change in SWS at 0- to 3-m depth was related to soil depth, time, and growth period. In the first growth period, SWS at this depth in R+ and R– increased little compared with the control, but the decrease in SWS in R– was larger than that in R+, which suggested that growth of winter wheat had a strong impact on soil water. This impact was probably because (i) winter wheat strengthened the utilization of soil water under water short-age (R–), which led to a relatively large decrease in SWS with this treatment and (ii) winter wheat further utilized the newly formed soil water due to increased precipitation. The influence of changes in precipitation on soil water increased after the first fallow period. The specific manifestation is that compared with the control, soil water at 0- to 3-m depth in the R+ treatment had been in a relatively high surplus state, but that in the R– treatment had been in a relatively large deficit state. In the second growing season, although winter wheat utilized some soil water, SWS at

Fig. 4. Average soil water storage within the 0- to 3.8-m depth under three precipitation levels.

Table 2. F statistics of changes in soil water storage at different depths for the R+ and R– treatments compared with the Rc treatment over 2 yr.†

Depth2014–2015 2015–2016 2014–2016

R+ Rc R– R+ Rc R– R+ Rc R–

m —————————————————————— mm ——————————————————————0–1 -29.9 -56.8 -80.8 14.3 12.2 5.0 -15.6 -44.6 -75.81–2 -41.1 -58.5 -67.8 3.8 -40.2 -29.3 -37.3 -98.7 -97.12–3 21.6 22.7 2.1 -38.0 -52.1 -33.3 -16.4 -29.4 -31.23–3.8 36.1 21.0 10.1 -1.3 -31.8 -24.9 34.8 -10.8 -14.80–3.8 -13.3 -71.8 -136.3 -21.2 -111.9 -82.5 -34.5 -183.8 -218.9† F statistics generated from ANOVA with repeated measures. Significant F statistics (P ≤ 0.05) shown in bold.

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the 0- to 3-m depth in the R+ treatment was still higher than that in control. The SWS at the 0- to 1-m depth in the R– treatment decreased substantively, but the reduction in SWS at the 1- to 3-m depth did not reach the critical threshold level, which indicated that soil water at 1- to 3-m depth was utilized less than that of the upper 1 m. The ranking of mean SWS for the three treatments also reflected the effect of precipitation level on SWS (Fig. 4).

The response of winter wheat yield to changes in precipitation was determined by precipitation and the supplementary effect of soil water. The effect compensated for the negative effect of decreased precipitation on the yield, which led to a hysteresis effect on the yield. For instance, the grain yield in the R– treat-ment did not decrease significantly because of reduced precipita-tion in the first experimental year (Table 3). The performance of the supplementary effect was dominated by precipitation year type. If wet or normal precipitation was followed by a dry year (wet–dry pattern), soil water was consumed less in the last wet or normal year, and the supplementary effect would run well into the dry year. In this case, the grain yield did not decrease greatly for the R– treatment in the first experimental year and the R+ and Rc treatments in the second experimental year (Table 3). However, if there were consecutive dry years (dry–dry pattern), soil water was largely consumed in the last growing season and its supplementary effect was weak in the next growing season due to insufficient SWS. This resulted in a significant reduction in grain yield (e.g., the yield for the R– treatment in the second experi-mental year) (Table 3). Hence, the supplementary effect only worked under an alternating wet–dry pattern and was weak for consecutive dry years. Also, the change in precipitation affected grain yield by acting on the spike number because the wheat

tillered more under high precipitation, and it reduced tillering to cut energy consumption to adapt to drought (Smith, 1925).

Quantifying the contribution of precipitation and soil water to water consumption (direct correspondence to winter wheat yield performance) is valuable for management of water resources. The proportion of precipitation to water consump-tion was higher than that of soil water (Fig. 6), which indicated that the crop always used more precipitation than soil water. The proportion of soil water indicated its supplementary effect. For example, under relatively high reference precipitation conditions (the year of 2014–2015 with a 282.6 mm precipitation), the proportions of soil water were <23% in the R+ and Rc treatments (Fig. 6), which indicated a weak supplementary effect in these two treatments. For the R– treatment, the proportion of soil water was up to 45% (Fig. 6), which showed that a strong supple-mentary effect was involved. In the second growing season, there was relatively low reference precipitation (the year of 2015–2016 with 215.8 mm precipitation). The proportions of soil water in the R+ and Rc treatments increased from 22.5 to 42.5% on aver-age (Fig. 6), which suggested that a strong supplementary effect had occurred. However, the supplementary effect was always weak in the R– treatment because the proportions of soil water were almost unchanged over the 2-yr experimental period (Fig. 6), which agreed with previous results (Chu et al., 2016; Ren et al., 2016). The supplementary effect made soil water critical to the crop, especially when precipitation was relatively low. Hence, soil water is regarded as a precious “reservoir” because of its regu-lating effect on crop production in the LTR (Huang et al., 2005).

There are complicated linkages among soil water, winter wheat performance, and precipitation in a rain-fed agriculture

Fig. 5. Shoot biomass of winter wheat at different growing stages over the first (2014–2015) and second (2015–2016) experimental years.

Table 3. F statistics of winter wheat biomass, grain yield (GY), spike number (SN), thousand grain weight (TGW), and water use efficiency (WUE) for the R+ and R– treatments compared with the Rc treatment over 2 yr.†Year Treatments Biomass GY SN TGW WUE

kg ha–1 kg ha–1 g kg m–3

2014–2015 R+ 12103 ± 280 4680 ± 112 418 42.6 1.04Rc 11902 ± 721 4479 ± 284 394 44.1 1.23R– 10447 ± 251 4263 ± 116 390 40.9 1.32

2015–2016 R+ 15122 ± 1200 5803 ± 366 521 43.2 1.07Rc 13010 ± 747 5548 ± 297 504 43.4 1.53R– 6232 ± 788 2741 ± 276 272 43.0 1.06

† F statistics generated from ANOVA with repeated measures. Significant F statistics (P ≤ 0.05) shown in bold.

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system in the LTR. Our results demonstrated that soil water played a critical role in stabilizing winter wheat produc-tion under the condition of precipitation change through its supplementary effect. For the long term, local farmers look at soil water as a reservoir to stabilize winter wheat production. Therefore, soil water should be reasonably used and conserved in the LTR in the future.

conclusIonsA 2-yr field experiment showed that a change in precipitation

affected SWS at a depth of 0 to 3.8 m in the root zone and win-ter wheat yield under rain-fed conditions in the LTR. There was a positive response of SWS to precipitation, and the variation in SWS depended on winter wheat growth, soil depth, time, and wheat growth period. The average SWS in the four soil layers of 0 to 1 m, 1 to 2 m, 2 to 3 m, and 3 to 3.8 m maintained the order of R+ > Rc > R–. During the experiment, SWS at the 0- to 3.8-m depth in treatments R+, Rc, and R– decreased by 34.5, 182.3, and 218.9 mm, respectively. The shoot biomass and yield of winter wheat showed no significant difference under the three precipitation conditions in the first year, but these two variables in treatment R– decreased by 50% in the second year compared with the other two treatments. The WUE in the Rc and R– treatments were higher than in R+ for the first year, and WUE in the Rc treatment was the highest for the second year. The proportions of soil water to water consumption in the R+ and Rc treatments increased from 22.5% in the first year (2014–2015) to 42.5% in the second year (2015–2016); the proportions in the R– treatment remained stable during the experimental years. Precipitation and winter wheat growth together determined the state of soil water in the root zone and its supplementary effect, which together with the precipitation year type determined the yield and WUE of winter wheat in the following year. Our study further confirmed the importance of soil water on the yield performance of rain-fed winter wheat, which is of great significance to the stability of winter wheat yield and the conservation of soil water in the LTR.

AcKnoWlEdgmEnts

This work was funded by the National Key Research and Development Program of China (2016YFC0501706) and the National

Natural Science Foundation of China (41371242). We thank the editor and reviewers for their comments on improving this paper.

rEFErEncEs

Chen, Y., T. Liu, X. Tian, X. Wang, M. Li, S. Wang, and Z. Wang. 2015. Effects of plastic film combined with straw mulch on grain yield and water use efficiency of winter wheat in Loess Plateau. Field Crops Res. 172:53–58. doi:10.1016/j.fcr.2014.11.016

Chu, P., Y. Zhang, Z. Yu, Z. Guo, and Y. Shi. 2016. Winter wheat grain yield, water use, biomass accumulation and remobilisation under tillage in the North China Plain. Field Crops Res. 193:43–53. doi:10.1016/j.fcr.2016.03.005

Fang, Q.X., L. Ma, T.R. Green, Q. Yu, T.D. Wang, and L.R. Ahuja. 2010. Water resources and water use efficiency in the North China Plain: Current status and agronomic management options. Agr. Water Manage. 97:1102–1116. doi:10.1016/j.agwat.2010.01.008

Guan, D., Y. Zhang, M.M. Al-Kaisi, Q. Wang, M. Zhang, and Z. Li. 2015. Tillage practices effect on root distribution and water use efficiency of winter wheat under rain-fed condition in the North China Plain. Soil Tillage Res. 146:286–295. doi:10.1016/j.still.2014.09.016

He, G., Z. Wang, F. Li, J. Dai, Q. Li, C. Xue et al. 2016. Soil water storage and winter wheat productivity affected by soil surface management and precipitation in dryland of the Loess Plateau, China. Agr. Water Manage. 171:1–9. doi:10.1016/j.agwat.2016.03.005

Huang, Y., L. Chen, B. Fu, Z. Huang, and J. Gong. 2005. The wheat yields and water-use efficiency in the Loess Plateau: Straw mulch and irri-gation effects. Agr. Water Manage. 72:209–222. doi:10.1016/j.agwat.2004.09.012

Huang, M., M. Shao, L. Zhang, and Y. Li. 2003. Water use efficiency and sustainability of different long-term crop rotation systems in the Loess Plateau of China. Soil Tillage Res. 72:95–104. doi:10.1016/S0167-1987(03)00065-5

Ines, A.V.M., J.M. Hansen, and A.W. Robertson. 2011. Enhancing the utility of daily GCM rainfall for crop yield prediction. Int. J. Clima-tol. 31(14):2168–2182. doi:10.1002/joc.2223

Jin, K., W.M. Cornelis, W. Schiettecatte, J. Lu, Y. Yao, H. Wu et al. 2007. Effects of different management practices on the soil–water balance and crop yield for improved dryland farming in the Chi-nese Loess Plateau. Soil Tillage Res. 96:131–144. doi:10.1016/j.still.2007.05.002

Liu, X., Y. He, X. Zhao, T. Zhang, Y. Li, J. Yun, S. Wei, and X. Yue. 2016. The response of soil water and deep percolation under Caragana microphylla to rainfall in the Horqin Sand Land, northern China. Catena 139:82–91. doi:10.1016/j.catena.2015.12.006

Fig. 6. Amounts of precipitation and soil water consumed by winter wheat and the proportions of precipitation and soil water to water consumption in the first (2014–2015) and second (2015–2016) growing seasons.

Agrono

my Soc

iety o

f Ameri

ca

for pr

oofin

g only

Agronomy Journa l • Volume 111, Issue 4 • 2019 9

Liu, B., and M. Shao. 2015. Response of soil water dynamics to precipita-tion years under different vegetation types on the northern Loess Pla-teau, China. J. Arid Land 8:47–59. doi:10.1007/s40333-015-0088-y

Lv, Z., X. Liu, W. Cao, and Y. Zhu. 2013. Climate change impacts on regional winter wheat production in main wheat production regions of China. Agric. For. Meteorol. 171-172:234–248. doi:10.1016/j.agrformet.2012.12.008

Ma, S., Z. Yu, Y. Shi, Z. Gao, L. Luo, P. Chu, and Z. Guo. 2015. Soil water use, grain yield and water use efficiency of winter wheat in a long-term study of tillage practices and supplemental irrigation on the North China Plain. Agr. Water Manage. 150:9–17. doi:10.1016/j.agwat.2014.11.011

Muñoz-Romero, V., J. Benítez-Vega, L. López-Bellido, and R.J. López-Bellido. 2010. Monitoring wheat root development in a rainfed vertisol: Tillage effect. Eur. J. Agron. 33:182–187. doi:10.1016/j.eja.2010.05.004

Nolan, S., M. Unkovich, S. Yuying, L. Lingling, and W. Bellotti. 2008. Farming systems of the Loess Plateau, Gansu Province, China. Agric. Ecosyst. Environ. 124:13–23. doi:10.1016/j.agee.2007.08.009

Ren, X., T. Cai, X. Chen, P. Zhang, and Z. Jia. 2016. Effect of rainfall concentration with different ridge widths on winter wheat produc-tion under semiarid climate. Eur. J. Agron. 77:20–27. doi:10.1016/j.eja.2016.03.008

Ren, X., X. Chen, and Z. Jia. 2010. Effect of rainfall collecting with ridge and furrow on soil moisture and root growth of corn in semiarid Northwest China. J. Agron. Crop Sci. 196:109–122. doi:10.1111/j.1439-037X.2009.00401.x

Ren, X., Z. Jia, and X. Chen. 2008. Rainfall concentration for increas-ing corn production under semiarid climate. Agr. Water Manage. 95:1293–1302. doi:10.1016/j.agwat.2008.05.007

Sadras, V.O., and J.F. Angus. 2006. Benchmarking water-use efficiency of rainfed wheat in dry environments. Aust. J. Agric. Res. 57(8):847–856. doi:10.1071/AR05359

Salve, R., E.A. Sudderth, S.B. St. Clair, and M.S. Torn. 2011. Effect of grassland vegetation type on the responses of hydrological pro-cesses to seasonal precipitation patterns. J. Hydrol. 410:51–61. doi:10.1016/j.jhydrol.2011.09.003

Sang, X., D. Wang, and X. Lin. 2016. Effects of tillage practices on water consumption characteristics and grain yield of winter wheat under different soil moisture conditions. Soil Tillage Res. 163:185–194. doi:10.1016/j.still.2016.06.003

Shi, P.B., J.F. Zhang, T. Li, and Y.B. Shen. 2018. Effect of shading on dry matter and yield and quality of winter wheat. (In Chinese.) Mod. Agric. Sci. Technol. 36(18):5–8.

Smith, R.W. 1925. The tillering of grain as related to yield and rainfall. Agron. J. 17:717–724. doi:10.2134/agronj1925.00021962001700110009x

Su, Z., J. Zhang, W. Wu, D. Cai, J. Lv, G. Jiang, J. Huang, J. Gao, R. Hart-mann, and D. Gabriels. 2007. Effects of conservation tillage prac-tices on winter wheat water-use efficiency and crop yield on the Loess Plateau, China. Agr. Water Manage. 87:307–314. doi:10.1016/j.agwat.2006.08.005

Sun, H., Y. Shen, Q. Yu, G.N. Flerchinger, Y. Zhang, C. Liu, and X. Zhang. 2010. Effect of precipitation change on water balance and WUE of the winter wheat–summer maize rotation in the North China Plain. Agr. Water Manage. 97:1139–1145. doi:10.1016/j.agwat.2009.06.004

Wang, H., Y. Chen, and Z. Chen. 2013. Spatial distribution and temporal trends of mean precipitation and extremes in the arid region, north-west of China, during 1960-2010. Hydrol. Processes 27(12):1807–1818. doi:10.1002/hyp.9339

Wang, Y., Z. Xie, S.S. Malhi, C.L. Vera, Y. Zhang, and J. Wang. 2009. Effects of rainfall harvesting and mulching technologies on water use efficiency and crop yield in the semi-arid Loess Plateau, China. Agr. Water Manage. 96:374–382. doi:10.1016/j.agwat.2008.09.012

Wilson, D.J., A.W. Western, and R.B. Grayson. 2004. Identify-ing and quantifying sources of variability in temporal and spa-tial soil moisture observations. Water Resour. Res. 40:191–201. doi:10.1029/2003WR002306

Yang, W.Z. 2001. Soil water resources and afforestation in loess plateau. (In Chinese.) Ziran Ziyuan Xuebao 16:433–438.

Zhang, X., S. Chen, H. Sun, Y. Wang, and L. Shao. 2009. Root size, distribution and soil water depletion as affected by cultivars and environmental factors. Field Crops Res. 114:75–83. doi:10.1016/j.fcr.2009.07.006

Zhang, L.Y., X.H. Gu, L. Pei, G.J. Yang, L.Z. Wang, X.N. Wang et al. 2017a. The response analysis of photosynthetic rate of winter wheat to nitrogen application rate and light Intensity (in Chinese). Acta Agric. Boreali-Simica 32:122–128.

Zhang, X., D. Pei, and S. Chen. 2004. Root growth and soil water utiliza-tion of winter wheat in the North China Plain. Hydrol. Processes 18:2275–2287. doi:10.1002/hyp.5533

Zhang, X., W. Qin, S. Chen, L. Shao, and H. Sun. 2017b. Responses of yield and WUE of winter wheat to water stress during the past three decades—A case study in the North China Plain. Agr. Water Man-age. 179:47–54. doi:10.1016/j.agwat.2016.05.004

Zhang, S., V. Sadras, X. Chen, and F. Zhang. 2013a. Water use efficiency of dryland wheat in the Loess Plateau in response to soil and crop man-agement. Field Crops Res. 151:9–18. doi:10.1016/j.fcr.2013.07.005

Zhang, X., Y. Wang, H. Sun, S. Chen, and L. Shao. 2012. Optimizing the yield of winter wheat by regulating water consumption during veg-etative and reproductive stages under limited water supply. Irrig. Sci. 31:1103–1112. doi:10.1007/s00271-012-0391-8

Zhang, X., S. Wang, H. Sun, S. Chen, L. Shao, and X. Liu. 2013b. Contri-bution of cultivar, fertilizer and weather to yield variation of winter wheat over three decades: A case study in the North China Plain. Eur. J. Agron. 50:52–59. doi:10.1016/j.eja.2013.05.005