1 Introduction

An adequate freshwater supply is essential for sustainable agricultural development (Ai et al. 2020). Almost 70% of the total underground freshwater is used for irrigation in farmlands (Grafton et al. 2018; Lu et al. 2016). However, the food production under excessive water supply remains inadequate for the growing world population. Approximately 820 million people around the world could not acquire enough food in 2018 (FAO et al. 2019). By 2050, the world population is predicted to reach 9.7 billion, and more people will suffer from hunger (United Nations 2019). With the rapid population increase and the food security crisis, the agricultural water supply faces unprecedented challenges (Grafton et al. 2018; Lu et al. 2016; Zheng et al. 2020). Moreover, improper management practices such as flood irrigation and excessive fertilization have destroyed ecological balance, leading to groundwater level decline and aggravated water pollution (Fang et al., 2010a; Liu et al., 2011a; Sun et al. 2006). Optimizing management practices to improve crop water use efficiency (WUE) could be a possible way to manage this challenge (Lu et al. 2016). Nevertheless, a change in crop WUE through management practices may lead to reduced crop yield or increased water loss (Grafton et al. 2018; Liu et al., 2018a; Zhang et al. 2018a).

In the last decades, people have been paying attention to the positive effects of sustainable management practices on crop growth and development (Fig. 1) and advocated several sustainable management practices to ensure food security and improve WUE (Bai et al. 2020; Liu et al. 2021; Lu et al. 2021; Struik and Kuyper 2017). Water-saving irrigation practices have been widely acknowledged to increase crop yield and improve the crop WUE (Sun et al. 2019a; Yu et al. 2020, 2021). The application of straw return technology can also indirectly improve the crop WUE by reducing field evaporation, reducing irrigation amount, and effectively increasing the soil organic carbon (Berhane et al. 2020; Gao et al., 2019a; Meyer et al. 2019; Wang et al. 2018a). In summary, optimized management practices can increase the ability of crops to use water, change the temporal and spatial distribution of water in farmland systems, reduce water loss, and hence optimize grain yield and the WUE, which may provide an opportunity for developing sustainable management practices.

Fig. 1
figure 1

The growth of winter wheat under sustainable management practices (a), and improper management practices (b) in the North China Plain. (Photographs by the authors.)

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However, most studies only focus on the impact of one or two management practices (such as irrigation or nitrogen application) on crop water use (Kan et al. 2020; Liu et al. 2019a; Peng et al. 2018; Wang et al. 2018a). Hence, the impact of tillage and other management practices on water use has been ignored in previous studies. In addition, inconsistent results of similar experiments in the same regions are also questionable in terms of sustainable management practices. For instance, one study showed that the optimal irrigation amount of winter wheat is 240–330 mm (Peng et al. 2018). However, in another study performed by Wang et al. (2018b), an irrigation amount of 75 mm was recommended for winter wheat production. The apparent conflicts in the conclusions about the frequency and time of irrigation drawn from the winter wheat experiments in the North China Plain (NCP) are even more concerning (Liu et al., 2011a; Sun et al. 2006; Zhang et al., 2004a). Moreover, the results regarding WUE from a single site cannot be extrapolated for application to the whole region. Conclusively, a comprehensive analysis of the impact of management practices on crop water use on a regional scale is crucial to formulate sustainable management practices.

The WUE is a pivotal index to assess the plant’s ability to use water to produce dry matter (Ai et al. 2020; Lu et al. 2016). Current studies have demonstrated that climatic conditions, physical and chemical properties of the soil, management practices, and many other factors may affect the absorption and utilization of water resources in crops (Ai et al. 2020; Fang et al., 2010a; Li et al., 2018a; Liu et al., 2018a, 2019b). However, it is challenging to evaluate the effects of multiple factors on the WUE in a single experiment. In particular, researchers might focus on a single practice at a particular experimental site to investigate the WUE, such as optimizing irrigation practices to reduce water loss by adjusting the amount and frequency of irrigation, changing traditional tillage methods to increase soil water holding capacity, or improving the capacity of the crop to absorb water by their roots (Guan et al., 2015a; He et al., 2019a; Li et al., 2018a; Liu et al., 2011a, 2019a; Mei et al. 2013). However, single field location experiment cannot fully explain the change rule of the crop WUE by meteorological indicators and soil fertility. Sustainable agriculture is a systematic project, which needs to consider multiple influencing factors (Anderson et al. 2016; German et al. 2017; Struik and Kuyper 2017). A systematic study of the factors affecting the crop WUE is of substantial significance to accurately understand sustainable agriculture development.

The NCP is a water-scarce area with lower crop yield and WUE but a vital grain production region in China (Liu et al. 2019a; Sun et al. 2006; Zhang et al. 2018a). Although the total water resources in the NCP only comprise 4% of the country’s total amount, wheat and maize yield comprises 81% and 30% of its total output, respectively (National Bureau of Statistics of China 2020). In addition, the spatial and temporal precipitation in the NCP is unevenly distributed. Summer precipitation comprises 70–80% of the mean annual precipitation, while the seasonal precipitation only meets 25–40% of the wheat water demand (Fang et al., 2010a; Yang et al. 2015). Researchers have conducted various studies on the correlation of crop production and WUE in the NCP, but the majority only focused on either winter wheat or summer maize (Liu et al., 2018a; Peng et al. 2018; Wang et al. 2018a; Zheng et al. 2020). However, as the main cropping system in the NCP, the winter wheat-summer maize cropping system is an indivisible whole. Studies on the water use of single-season crops cannot fully explain the essential concerns of agricultural water use in the NCP. Therefore, it is crucial to consider the crop water cycle from the wheat-maize system perspective to further explore the key measures to improve crop water use in the NCP.

Meta-analysis is an effective analytical method that performs comprehensive statistics on multiple independent experiments or studies on the same topic (German et al. 2017; Wang et al. 2020; Zheng et al. 2020). German et al. (2017) used meta-analysis to analyze multiple factors affecting sustainable agriculture. This approach is also a perfect way to achieve a quantitative analysis of the impact of different factors on WUE. More importantly, meta-analyses have been successfully implemented in various studies to explain the effects of management practices such as fertilization, irrigation, tillage, and straw management, on yield, nitrogen use efficiency, WUE, greenhouse gas emissions from farmland, and soil carbon sequestration in the NCP or Northern China (Berhane et al. 2020; Wang et al. 2020; Xu et al., 2017a; Zheng et al. 2020). However, few studies focused on the impact of multiple management practices on crop water use.

Therefore, we conducted a meta-analysis on the yield and water use of a winter wheat-summer maize cropping system based on peer-reviewed studies in the NCP to (i) investigate the current situation of water use in a winter wheat-summer maize cropping system, (ii) identify the factors that affect the WUE of winter wheat, and (iii) propose sustainable management strategies to optimize the winter wheat production in the NCP.

2 Materials and methods

2.1 Study object

The North China Plain (32°–40°N, 114°–121°E) is the second-largest plain in China, with a mean annual temperature of 8–15°C and a mean annual precipitation of 500–600 mm (Xiao et al. 2021). The primary soil type is aeolian loam (Xiao and Tao 2016). The winter wheat-summer maize cropping system is the main cropping system in this region (Guan et al., 2015a). Winter wheat is sown in October and harvested in June of the following year. Farmers used rotary tillage before sowing and repeated flood irrigation during the growth period. Summer maize is sown in June and harvested in October. Farmers use no-tillage and irrigation before sowing of maize.

2.2 Data collection

This study investigated the effects of four management practices (nitrogen input, irrigation, tillage, and straw return) on the water use of a winter wheat-summer maize cropping system in the NCP. All published peer-reviewed scientific journal articles on the water use of winter wheat-summer maize cropping systems were collected from the China National Knowledge Infrastructure (1979–2019, http://www.cnki.net/) and Web of Science (1979–2019, http://apps.webofknowledge.com/). Different combinations of search terms that included “North China Plain” or “Huang-Huai-Hai Plain” and “winter wheat” or “summer maize” or “summer corn” and “crop product*” or “yield” or “output” were used for the selection of published studies. Data were obtained directly from the articles and the graphs using the GetData Graph Digitizer (http://getdata-graph-digitizer.com/). The following criteria were used during the process of selecting publications: (i) the research object must be field experiments conducted in the NCP, excluding pot and laboratory experiments and model simulations; (ii) the cropping system must be the winter wheat-summer maize cropping system; (iii) the studies must be about one or more of the four leading management practices (e.g., no nitrogen versus nitrogen input, no irrigation versus irrigation, rotary tillage versus other tillage methods, and straw removal versus straw return, with the former as the control and the later as the treatment); and (iv) the data must include the number of trial replications, crop yield, WUE, or seasonal crop evapotranspiration (ETc). The studies that met the criteria and could be used for meta-analysis are shown in the meta-analysis references.

Our data set covered 286 publications, of which 209 were on winter wheat, 124 were on summer maize, and 59 were on the winter wheat-summer maize cropping system. The data set of winter wheat included 77 publications on nitrogen experiments (1030 comparisons), 81 publications on irrigation experiments (795 comparisons), 40 publications on tillage experiments (171 comparisons), and 36 publications on straw return experiments (198 comparisons). The following data were compiled from the selected publications: experimental sites, experimental time, experimental treatments, number of replicates, crop yield, WUE, ETc, and 15 variables that may affect crop water use, including irrigation amount and nitrogen application rate. Detailed explanations of these variables are given in Table 1.

Table 1 Definition of variables used for the meta-analysis.
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2.3 Statistical analysis

2.3.1 Descriptive statistics

The water supply of crops was determined by precipitation, irrigation, runoff, the change in soil water storage, upward capillary flow into the root zone, and downward drainage out the root zone (Lu et al. 2016; Wang et al. 2018b). Most irrigation experiments included in the database ignored the effects of runoff, upward capillary flow into the root zone, and downward drainage out the root zone on the crop ETc. Therefore, during the growth period, the crop ETc can be calculated according to Eq. (1) (Guan et al., 2015a; Sun et al. 2006):

$$ ETc=P+I+\Delta s $$
(1)

where ETc (mm) is the seasonal crop evapotranspiration; P (mm) is the precipitation; I (mm) is the irrigation; and ∆s (mm) is the change in soil water storage of 0–200 cm.

We then used Eq. (2) to analyze the contribution ratio of irrigation, precipitation, and soil water storage changes to the crop ETc.

$$ Cr=X/ ETc $$
(2)

where Cr is the contribution ratio, ETc (mm) is seasonal crop evapotranspiration, and X (mm) could be any factor, including precipitation, irrigation, and change in soil water storage.

Based on the irrigation experimental database, descriptive statistics were conducted using SigmaPlot 14.0 (Systat Software Inc., San Jose, CA, USA) to calculate the average and 95% confidence interval of the following indicators for wheat, maize, and the wheat-maize system: yield, ETc, WUE, precipitation, irrigation, the change in soil water storage, and the ratio of contribution of precipitation, irrigation, and the change in soil water storage to the crop ETc.

2.3.2 Meta-analysis

The random-effect meta-analysis (Hedges et al. 1999; Scheithauer 2003) was used to analyze the data set of wheat water use. The MetaWin 2.1 software (State University of New York at Stony Brook, USA) was selected for this meta-analysis (Hedges et al. 1999). We used the natural logarithm of the response ratio (R), calculated by Eq. (3), as the effect size to reflect the water utilization capacity of winter wheat (Adams et al. 1997).

$$ \ln\ R=\ln \frac{\overline{Xt}}{\overline{Xc}} $$
(3)

where \( \overline{Xt} \) is the mean of the indicator in the treatment group (t) and \( \overline{Xc} \) is the mean of the indicator in the control group (c).

Each set of data used for the meta-analysis had a different contribution to the total data set. In this meta-analysis, we chose the number of experimental replicates to calculate the weight (w) of each effect size in ln R through Eq. (4) (Adams et al. 1997).

$$ w=\left({n}_c\times {n}_t\right)/\left({n}_c+{n}_t\right) $$
(4)

where nc and nt are the number of experimental replications for the control and treatment groups, respectively.

We obtained the 95% confidence interval (CI) and weighted mean effect size (ln R++) by bootstrapping (4999 iterations; Pittelkow et al. 2015). Heterogeneity between subgroups (Qb) was assessed using randomization procedures with 4999 replications. The significance of Qb indicated whether the detected factors were affected by data grouping. To directly analyze the effects on management practices and the indicator, we used Eq. (5) to convert the weighted mean effect size (ln R++) into the percentage change (E).

$$ E=\left(\exp\ \left(\ln\ {R}_{++}\right)-1\right)\times 100\% $$
(5)

If the 95% CIs did overlap with zero, no significant difference between the treatment and the control groups was observed for the indicator of winter wheat. If the 95% CIs were right of zero, the experimental treatment significantly improved the indicator of wheat (p < 0.05). If the 95% CIs were left of zero, the experimental treatment significantly reduced the indicator of winter wheat (p < 0.05).

2.3.3 Regression analysis

Based on the data of winter wheat irrigation experiments, linear regression analysis in SigmaPlot 14.0 was used to study the relationships between effect sizes of WUE under irrigation and other indicators. Furthermore, we discussed the factors affecting the WUE of winter wheat by analyzing the p-value and coefficient.

2.4 Data grouping

As shown in Table 2, according to the distribution characteristics of meteorological indicators, mean annual temperature was categorized as < 13 and ≥ 13 °C, mean annual precipitation was categorized as ≤ 625 and > 625 mm, seasonal precipitation was categorized as < 140 and ≥ 140 mm, and annual precipitation was categorized as ≤ 580 and > 580 mm. According to the suitable growth conditions of winter wheat and the pH of farmland soil in the NCP, soil pH was categorized as ≤ 8.0 and > 8.0. According to the requirement of soil bulk density for winter wheat growth, bulk density was categorized as < 1.4 and ≥ 1.4 g cm−3. According to the distribution of farmland soil fertility in the NCP, soil organic carbon concentration was categorized as < 7.0, 7.0–9.0, and ≥ 9.0 g·kg−1; soil total nitrogen content was categorized as ≤ 0.85 and > 0.85 g·kg−1. Experimental duration was categorized as 1, 2, and > 2 years. According to the traditional management practices and recommended application practices in multiple studies, nitrogen application rate was categorized as < 220, 220–250, and ≥ 250 kg·ha−1; irrigation amount was categorized as < 80, 80–160, and > 160 mm. Nitrogen application times were categorized as 0, 1, and ≥ 2; irrigation times were categorized as 1, 2, and > 2. According to the ratio of topdressing and base application of nitrogen fertilizer, the basal-topdressing ratio was categorized as 0, 0–1, and > 1. Seasonal irrigation amount plus precipitation was categorized as ≤ 240 and > 240 mm.

Table 2 Data grouping of winter wheat under different conditions.
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3 Results and discussion

3.1 Water utilization status of a winter wheat-summer maize cropping system

Due to the higher photosynthetic efficiency of C4 plants compared to C3 plants under suitable conditions (Daryanto et al. 2016), the wheat yield in the NCP averaged 6540 kg·ha−1, which was 21% lower than the maize yield (8293 kg·ha−1, Fig. 2a). The wheat yield had reached 82% of its potential (8000 kg·ha−1, Lu and Fan 2013), but the obtained maize yield was only 75% of their potential (11,000 kg·ha−1, Ton et al., 2015). We observed that the growth potential of wheat production was less than that of maize. Our study also suggested that the wheat ETc (381 mm, Fig. 2b) was roughly equal to 50% of the wheat-maize system ETc (766 mm) and the wheat WUE (1.83 kg·m−3, Fig. 2c) was lower than the wheat-maize system WUE (2.10 kg·m−3). This result also confirmed the previous conclusion that C4 plants generally have higher WUE than C3 plants (Daryanto et al. 2016; Kukal and Irmak 2020).

Fig. 2
figure 2

An overview of a winter wheat-summer maize system: (a) yield, (b) evapotranspiration, and (c) water use efficiency in the North China Plain. The solid and dotted horizontal lines inside each box of the box plots represent the median and average values of the data set; the upper and lower ends of the box represent the 75th and 25th percentiles; the 5th and 95th percentiles are represented by whiskers, and the dots represent outliers.

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While the water utilization capacity of wheat in the NCP is relatively lower, a massive potential for the improvement of WUE under water shortage conditions was observed in wheat (Bai et al. 2020; Liu et al. 2021; Sun et al. 2019a). The key to improve crop WUE is planting high-efficiency water use varieties and optimizing management practices (Bai et al. 2020; Liu et al. 2019a; Mei et al. 2013; Yu et al. 2021). It is challenging to make breakthrough progress in breeding technologies aiming at high-efficiency water use over the short term. However, compared with the planting of maize, farmers in the NCP generally adopted intensive management practices such as flood irrigation and excessive nitrogen application in the wheat season, which can be optimized by management practices to improve WUE (Guan et al., 2015a; Li et al., 2018a; Peng et al. 2018; Sun et al. 2019a; Wang et al. 2018b). The optimization of irrigation and other management practices significantly reduced the wheat ETc (Guan et al., 2015a; Peng et al. 2018; Sun et al. 2006). Irrigation in the wheat season would affect the water and nitrogen balance of the subsequent crops (Lu et al. 2021). Therefore, we need to regulate the spatio-temporal distribution of water through management practices of winter wheat to obtain higher WUE of the entire wheat-maize system.

3.2 Water balance of a winter wheat-summer maize cropping system

The water balance reflects the ability of crops to use water from different sources (Ai et al. 2020; Lu et al. 2016; Zhang et al., 2017a). From the perspective of water balance composition, the seasonal precipitation should meet the water demand of a crop in order to achieve sustainable water use of the agro-ecosystem. The contribution of precipitation to the maize ETc (285–361 mm, 72%–88%, Fig. 3) and the wheat-maize system ETc (447–519 mm, 58%–67%) was much higher than that of wheat (144–151 mm, 39%–41%), due to an uneven precipitation distribution (Peng et al. 2018; Wang et al. 2018a; Yang et al. 2015). Therefore, sustainable development of agriculture in the NCP cannot be achieved with the current wheat-maize system. This conclusion is consistent with most studies (Fang et al., 2010a; Sun et al. 2019b). However, the wheat-maize system has played an essential role to meet the dietary needs of China’s population, and it is imperative to optimize the management practices with the goal of food sustainability (Li et al., 2018a; Sun et al. 2019b; Wang et al. 2018b; Zhang et al. 2018b).

Fig. 3
figure 3

Field water balance of different crops in winter wheat-summer maize cropping system in the North China Plain. NO, not analyzed.

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Reducing soil water loss is a top priority in optimizing agricultural management practices in the NCP (Li et al., 2018a; Zhang et al. 2018b). Our study suggested that the growth and development of wheat (132–142 mm, 34%–37%, Fig. 3) was much more dependent on soil moisture storage than maize (−17 to 34 mm, −6% to 8%) and the wheat-maize system (96–183 mm, 12%–20%). If winter wheat had been continuously planted according to previous management practices, the downward trend of the groundwater level in the NCP would have become more prominent. Research by Li et al. (2018a) and Yang et al. (2015) also confirmed this view. Besides, the study also found that the change in soil water storage of the summer maize season exhibited a negative value, which indicated that the summer maize season could replenish the groundwater (Wang et al. 2018c; Yang et al. 2015).. Liang et al. (2019) and Lu et al. (2021) also reported that changes in management practices can affect soil water storage.

3.3 Effect of management practices on the water utilization of winter wheat

Bai et al. (2020) reported that the synergistic effects of water and nitrogen increased the importance of nitrogen input to improve crop water use. Our study showed that nitrogen input could significantly improve the yield (70%) and WUE (42%) of wheat but only increased the ETc by 6% (p < 0.05, Fig. 4). Similar to the studies by Lu et al. (2021) and Wang et al. (2020), our result illustrated that rational nitrogen application should be considered when developing management practices to improve the wheat WUE.

Fig. 4
figure 4

Effects of management practices on the water use of winter wheat. (a) Yield, (b) evapotranspiration, and (c) water use efficiency. The numbers in parentheses represent the number of comparisons used in the meta-analysis. The error bar represents 95% confidence intervals (CIs). When the error bar did not cross zero, the experimental treatment could significantly improve (or reduce) the WUE of winter wheat at p < 0.05.

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Numerous studies have demonstrated that different irrigation amounts can significantly affect the crop WUE relative to rain-fed area (Sun et al. 2019a; Yu et al. 2021; Zhang et al. 2018b). However, only a few studies have comprehensively evaluated how irrigation affects crop water use. Our study found that the wheat ETc increased by 31% under irrigation (p < 0.05, Fig. 4b), while no significant reduction in the WUE was observed (p > 0.05, Fig. 4c). Furthermore, compared with rain-fed, irrigation significantly increased the wheat yield by 30% (p < 0.05, Fig. 4a). These findings are similar to with Ai et al. (2020), who reported that the WUE of irrigated farmland was generally lower than that of rain-fed farmland. One explanation for this observation is that excessive irrigation amount might have resulted in a decreased yield and increased ETc (Wang et al. 2018b; Zhang et al. 2018a). Subgroup analysis verified the hypothesis described above and found that the wheat WUE significantly increased when the irrigation amount was 80–160 mm (p < 0.05, Fig. 5m).

Fig. 5
figure 5

Water use efficiency of winter wheat response to irrigation compared with rain-fed in different categories. (a) Mean annual temperature (°C), (b) mean annual precipitation (mm), (c) seasonal precipitation (mm), (d) annual precipitation (mm), (e) soil pH, (f) bulk density (g·cm−3), (g) soil organic carbon concentration (g·kg−1), (h) soil total nitrogen content (g·kg−1), (i) experimental duration (yr), (j) nitrogen application rate (kg·ha−1), (k) nitrogen application times, (l) basal-topdressing ratio, (m) irrigation amount (mm), (n) irrigation times, (o) seasonal irrigation amount plus precipitation (mm).

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Yu et al. (2021) reported that improving soil fertility is one of the most effective measures to alleviate drought. Tillage methods can change the physical and chemical properties of the soil (Guan et al., 2015a; He et al., 2019a), but the water use of wheat responded differently to changes in tillage methods (p < 0.05, Fig. 4). In this study, compared with rotary tillage, no-till significantly reduced the yield and ETc of wheat (p < 0.05, Fig. 4a and b) but had no significant effect on WUE (p > 0.05, Fig. 4c), which was in accordance with Kan et al. (2020). This was attributed to the fact that no-till minimized soil disturbance and conserve more water but resulted in the compaction of the upper soil, which was not conducive to root development, and affected crop yield and water use (Guan et al., 2015a; Kan et al. 2020). The same response of crop water use was observed to plow tillage and subsoiling with rotary tillage (Fig. 4). Compared with rotary tillage, subsoiling not only significantly improved the yield (12%, p < 0.05, Fig. 4a) and WUE (13%, p < 0.05, Fig. 4c) of wheat but also significantly reduced ETc (5%, p < 0.05, Fig. 4b). Consequently, subsoiling is the most suitable tillage method, creating a win-win situation for wheat yield and water use. This was also recently demonstrated in studies from He et al., (2019a) and Liang et al. (2019).

It has become a consensus that straw return can reduce soil water evaporation and improve crop WUE (Meyer et al. 2019; Wang et al. 2018a; Xiao et al. 2021). Our study suggested that straw return led to a 9% increase in the WUE and a 4% decrease in ETc (p < 0.05, Fig. 4). However, the impact of straw return on wheat yield was still unclear (Wang et al. 2019; Xiao et al. 2021). The meta-analysis results showed that straw return could significantly improve wheat yield by 4% (p < 0.05).

3.4 Factors that influence the WUE of winter wheat

This study showed that the mean annual temperature (r = 0.274, p < 0.001, Table 3) and mean annual precipitation (r = 0.162, p < 0.001) were positively correlated with wheat WUE. Xiao et al. (2020) reported that both temperature and precipitation would increase in the NCP in the future. The increase in temperature would accelerate the growth of winter wheat, reduce the wheat ETc, and positively impact wheat yield, thereby increasing the wheat WUE (Xiao et al. 2020). The increase in precipitation would also reduce the irrigation amount of wheat.

Table 3 Linear relationship between effect sizes (irrigation/rain-fed) of water use efficiency and influencing factors. N, the number of data sets; r, correlation coefficient in the linear regression; coefficient, coefficient of linear regression equation.
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The improvement in the physical and chemical properties of the soil also could promote the development of crop roots (Guan et al., 2015a; He et al., 2019c). However, the increase in the bulk density was not favorable for the growth and development of wheat roots. Especially under water shortage conditions, crop growth was negatively affected when the bulk density was higher than optimal (Yu et al. 2021; Zhang et al., 2012a, 2017a). Moreover, we found that the bulk density (r = 0.222, p < 0.001, Table 3) was negatively correlated with the ln Rs of WUE.

To some extent, seasonal precipitation, irrigation amount, irrigation times, and seasonal irrigation amount plus precipitation could change the spatial and temporal distribution of soil moisture. The level of soil water supply determined the wheat WUE (Li et al., 2018a; Wang et al. 2018b; Zhang et al., 2004a). Despite severe water shortage during the wheat season (Peng et al. 2018; Xiao et al. 2020), unbalanced precipitation might lead to more water loss. Our study indicated a significant negative relationship between the effect sizes of WUE and seasonal precipitation (r = 0.213, p < 0.001, Table 3), different from mean annual precipitation (r = 0.162, p < 0.001). Proper irrigation can effectively alleviate water deficiency in the winter wheat season while mitigating the less WUE induced by excessive irrigation (Li et al., 2018a; Ma et al., 2015a; Mao et al., 2017a; Sun et al. 2019b), indicated by a significant negative relationship between the effect sizes of WUE and irrigation amount (r = 0.234, p < 0.001), irrigation times (r = 0.162, p < 0.001), and seasonal irrigation amount plus precipitation (r = 0.330, p < 0.001).

Zhang et al. (2017a) reported that the increase in nitrogen supply significantly improved the plant WUE. Hence, we further analyzed the relationship between nitrogen indicators and the wheat WUE. Results also proved that with the increase of nitrogen application rate (r = 0.193, p < 0.001, Table 3), the wheat WUE showed a significant positive trend. Meanwhile, a significant negative relationship was observed between management practices, such as nitrogen application times (r = 0.110, p = 0.008) and the basal-topdressing ratio (r = 0.153, p < 0.001) and the ln Rs of WUE. Previous studies had shown that appropriate nitrogen application times and basal-topdressing ratio could regulate the root development of wheat and then affect the wheat WUE (Liu et al., 2018a; Zhang et al. 2018a).

3.5 Sustainable management practices to optimize the WUE of winter wheat

Mean annual temperature, mean annual precipitation, seasonal precipitation, bulk density, nitrogen application rate, nitrogen application times, basal-topdressing ratio, irrigation amount, irrigation times, and seasonal irrigation amount plus precipitation could significantly affect the wheat WUE (p < 0.05, Table 3). Furthermore, subgroup analysis was applied to identify the optimum management practices for the wheat WUE. However, significant heterogeneity was observed only between the mean annual temperature, seasonal precipitation, nitrogen application rate, nitrogen application times, irrigation amount, irrigation times, and seasonal irrigation amount plus precipitation subgroups in the subgroup analysis (p < 0.05, Table 2).

Our study demonstrated that when the mean annual temperature was ≥ 13°C, irrigation significantly increased the wheat WUE by 5% compared with rain-fed wheat (p < 0.05, Fig. 5a). Xiao et al. (2020) reported that the overexploitation of groundwater in the north of the NCP, where the mean annual temperature was relatively low, while in the south region, where the mean annual temperature was relatively high, faced less water resource pressure. In addition, higher mean annual temperature promoted the early maturity of winter wheat and reduced ETc, which was conducive to the timely sowing of summer maize and the increase of solar radiation (Xiao et al. 2020, 2021; Xiao and Tao 2016). Therefore, a reasonable irrigation strategy should be developed according to the actual water requirement of winter wheat in regions with higher mean annual temperature or in future scenarios.

Studies had shown that the water deficit of winter wheat in the NCP was about 200–300 mm (Fang et al., 2010a; Guan et al., 2015a). Deficit irrigation improved the water use of winter wheat and essential for reducing nitrogen leaching in summer maize (Lu et al. 2021; Yu et al. 2021). In this study, a significant increase in the WUE was observed when the irrigation amount was at a level of 80–160 mm (p < 0.05, Fig. 5m). The optimal irrigation scheme obtained by many studies was in the range of 80–160 mm (Liu et al., 2018a; Lu et al. 2021; Peng et al. 2018; Wang et al. 2018b). Compared with flood irrigation (irrigation amount > 400mm), commonly used in winter wheat production in the NCP, the new irrigation practice could save at least 240 mm of water (Wang et al. 2018b). The determination of the actual irrigation amount should be combined with the comprehensive judgment of seasonal precipitation. We also found that when the seasonal irrigation amount plus precipitation was ≤ 240 mm, irrigation significantly increased the wheat WUE compared with rain-fed (p < 0.05, Fig. 5o).

Similar to the studies of Liu et al. (2018a) and Zhang et al. (2018a), two irrigations can significantly increase the wheat WUE by 5% (p < 0.05, Fig. 5n), compared with no irrigation. In the past, farmers usually irrigated winter wheat 5–6 times during the growing season to ensure crop yield, but this did not lead to a significant increase in yield while using more water (Qiu et al. 2008; Sun et al. 2006). Although the current irrigation times were reduced to two times, it can still maintain the high yield of winter wheat, reduce groundwater use, save a lot of labor and costs, and contribute to the sustainable development of agriculture (Sun et al. 2019a). In addition to the irrigation amount and irrigation times, the application of sprinkler irrigation, drip irrigation, and other irrigation forms can also significantly improve the WUE of crops. However, the higher cost would reduce the profit for small farmers. Therefore, there was no in-depth discussion in this study.

Anderson et al. (2016) reported that management practices are one of the keys to increase rain-fed grain yield in the future. The change of single management practice cannot fully stimulate the potential of crop production, and the combination of multiple management practices can effectively reduce the yield gap and WUE gap. Our study also showed that when the nitrogen application rate was at a level of 220–250 kg·ha−1, a significant increase of 6% in the WUE was observed (Fig. 5j). Liu et al. (2018a) also demonstrated that when the nitrogen application rate was at 240 kg·ha−1 and irrigation was applied twice, winter wheat yield, WUE, and nitrogen use efficiency improved simultaneously. Relative to the farmers’ practices (Ju et al. 2009), a significant reduction of at least 75 kg·N·ha−1 was observed with recommended practices mentioned above (220–250 kg·N·ha−1).

In summary, the wheat production in the NCP will effectively save water, reduce the environmental threat, and enable sustainable development of regional agriculture through the combination of sustainable management practices, including nitrogen application, irrigation, subsoiling, and straw return. Certainly, these sustainable management practices of reducing water and nitrogen are also applicable to agricultural production in warm and dry regions of the world with high nitrogen application rate, such as northern India and eastern Pakistan. Moreover, Mahajan et al. (2012) and Hammad et al. (2012) demonstrated the feasibility of the sustainable management practices of reducing water and nitrogen in arid regions by optimizing the combination of water and nitrogen and stimulating the synergistic effect between water and nitrogen.

Here, we show the effects of management practices on the wheat WUE for the first time. This work provides a theoretical basis for realizing sustainable agriculture at a regional scale in the future. However, we did not observe that a higher WUE is always better. The increase in the WUE can lead to a decrease in yield (Grafton et al. 2018; Liu et al., 2011a; Peng et al. 2018), which is also true for the use of other resources, such as nitrogen and carbon. In the future, studies on sustainable agricultural management practices should adopt innovative models to comprehensively consider the use of natural resources and other factors to propose an ideal model for sustainable agricultural management practices.

4 Conclusion

A regional meta-analysis was conducted to evaluate the water use of the winter wheat-summer maize cropping system in the NCP and formulate corresponding solutions. Our results show that the summer maize WUE (2.29 kg·m−3) was much higher than the winter wheat (1.83 kg·m−3). The potential for improvement of WUE of maize was inferior to that of wheat. Compared with maize and the wheat-maize system, wheat was more dependent on soil moisture storage. Under the wheat-maize system, continuous planting of winter wheat could lead to a decrease in the groundwater levels, but summer maize could restore the soil moisture storage to some extent. Therefore, winter wheat was the limiting factor for improving agricultural water resources in the NCP. Significant correlations were observed between the WUE of wheat and climatic conditions, physical and chemical properties of soil, and management practices. To identify techniques for sustainable agricultural development, we comprehensively evaluated the effects of management practices on the WUE of wheat for the first time. Differences in management practices had varying effects on the WUE of wheat. The optimal WUE could be obtained by reducing water and nitrogen application, which may help to alleviate the water shortage of NCP. In the future, an ideal model of sustainable management practices should be developed by considering additional factors.