ZHANG Ming-ming , DONG Bao-di QIAO Yun-zhou SHI Chang-hai, YANG Hong , WANG Ya-kai ,LIU Meng-yu

1 Key Laboratory of Agricultural Water Resources, Hebei Laboratory of Agricultural Water-Saving, Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Shijiazhuang 050021, P.R.China

2 University of Chinese Academy of Sciences, Beijing 100049, P.R.China

3 Qingdao Agricultural University, Qingdao 266109, P.R.China

1. Introduction

Water and fertilizer are the two most critical factors affecting wheat yield (Wanget al.2014; Suiet al.2015;Monet al.2016). In the North China Plain (NCP), one of the most important agricultural areas in China (Sunet al.2015), the winter wheat growing season extends from approximately October to the following June. During this period, an average of 120 mm of precipitation in the NCP, is considerably less than the required amount for winter wheat,i.e., 400–500 mm (Donget al.2011). Thus, to achieve a high grain yield in the NCP, there has been over-extraction of groundwater, which has resulted in a rapid decline of the groundwater table in the region (Moiwoet al.2010; Sunet al.2015). Traditional high-yield cultivation practices for wheat in the NCP include full irrigation and a large input of N fertilizer. Three or more irrigations per wheat cycle is still a routine practice in this region. However, in the NCP, sharp rising temperatures in late May and early June shorten the grain- filling period of winter wheat, resulting in a shortfall of potential yield under full irrigation (Shaoet al.2011). In addition, farmers rely heavily on high rates of N fertilization in their attempts to obtain high crop yields(Suiet al.2015). However, excess N fertilization does not contribute to a significant increase in crop grain yields (Juet al.2009; Coventryet al.2011; Suiet al.2015). Survey data show that the amount of fertilizer (particularly nitrogen fertilizer) used remains at a high level in the NCP, with an average of 500 kg N ha–1per year, of which fertilization of winter wheat accounts for over 55% (Zhanget al.2011). In the Beijing region of the NCP, nitrogen application of 290 kg ha−1is the local conventional level of fertilization (Suiet al.2015). Over-extraction of groundwater and excessive use of nitrogen fertilizer cause serious environmental problems,such as ground subsidence, degradation of soil, nitrate pollution of groundwater, acid rain, ammonia volatilization,and other forms of air pollution (Zhanget al.2003; Yanget al.2006; Juet al.2009; Moiwoet al.2010; Miaoet al.2011; Duanet al.2014; Sunet al.2015).

With an increase in irrigation and fertilizer application,wheat water use efficiency and agronomic efficiency are generally decreased and grain yield is not increased (Qiuet al.2008; Zhanget al.2010, 2011; Shaoet al.2011).Two rounds of irrigation (60 mm each time), one at the jointing and the other at the anthesis stage, during the wheat-growing season have been found to be associated with the highest water use efficiency, and resulted in a grain yield that was not significantly different to that obtained with three rounds of irrigation (Chuet al.2009). The application of 120–240 kg N ha−1, in combination with traditional flood irrigation methods, is sufficient to maintain high yields of winter wheat (Juet al.2009; Suiet al.2015). Under experimental conditions, the wheat yield did not increase significantly at N rates above 200 kg N ha–1(Fanget al.2006). However, these previous conclusions are mainly based on short-term (1–2 year) studies, which are not always consistent. Weather conditions during most studies were normal, and there have been fewer studies that have investigated cropping in successive drought years. Studies on nitrogen fertilizer management in the NCP have generally been carried out under traditional irrigation with relatively large irrigation amount and high irrigation frequencies. In addition, the optimal amounts of irrigation and nitrogen in past studies may not be the same as those currently used at the local level, particularly under the circumstance of accumulated soil fertility that has resulted from excessive fertilizer use (Fanget al.2006; Zhanget al.2011; Wanget al.2015).

The aim of this study was to define a suitable irrigation regime and fertilization rates for wheat production in the drought-prone NCP. We performed experiments over four years, focusing on the effects of different irrigation approaches and fertilizer application regimes on grain yield,water use efficiency, fertilizer agronomic efficiency, and economic benefit under field conditions. We hypothesized that amounts of irrigation and nitrogen applied at the local level currently are excessive, and lower amounts would be satisfactory for wheat growth, improving yield, as well as have economic and ecological bene fits.

2. Materials and methods

2.1. Experimental site

Field experiments were carried out during four growing seasons from 2012 to 2016 at the Luancheng Agro-Ecosystem Experimental Station of the Chinese Academy of Sciences (37°53´N, 114°41´E; elevation 50.1 m).Luancheng Agro-Ecosystem Experimental Station lies in the northern part of the NCP, which has a continental monsoon climate, and is representative of typical NCP high-production areas. Mean annual rainfall in the region is 482 mm, with more than 70% of precipitation concentrated in the summer season. Mean precipitation during the winter wheat growing season for the period 1990 to 2010 was approximately 120 mm (Wuet al.2012). During the present study,precipitation in the first, second, third, and fourth growing seasons was 92, 49, 77, and 70 mm, respectively. According to Fuet al.(2014), all four experimental growing seasons would be considered drought years. Meteorological data were recorded at a weather station approximately 500 m from the experiment field (Table 1).

Soil at the study site was a loam with a field capacity of 38% (v/v), a permanent wilting point of 13% (v/v), and a bulk density of 1.53 g cm–3for a 2-m pro file. Soil fertility was measured from the soil surface to a depth of 30 cm prior to the first growing season (2012–2013). Soil organic matter was 16.3 g kg–1, total nitrogen was 1.2 g kg–1,and available phosphorus and potassium were 38.0 and 158.5 mg kg–1, respectively.

2.2. Experimental design

In this study, the winter wheat cultivar Shixin 828 was used in a maize-wheat rotation system over four growing seasons (2012–2016). It is an efficient and high-quality winter wheat cultivar widespread extended in northern China (Xuet al.2013). The experiment was carried out during the winter wheat growing seasons for the years 2012–2016 in the same field using a completely randomized block design. The size of each experimental plot was 4 m×6 m. The experiment included 12 treatments that consisted of each combination of four levels of fertilization and three levels of irrigation. Each treatment was replicated three times. The application of 300 kg N ha−1and three rounds of irrigations is the local conventional practice. The fertilization treatments used in the present study were 0, 100, 200, and 300 kg N ha−1for N0, N1, N2,and N3, respectively. All the treatments incorporated fertilization with 172 kg P ha−1. Nitrogen fertilizer was added to the soil of each plot in two stages: 70% was added at sowing and 30% at the jointing stage. P was applied as 100% basal. Detailed protocols are shown in Table 2. The three irrigation treatments were one(W1), two (W2), and three (W3) irrigations based on the wheat development stage (Table 2). For each irrigation, 60 mm of water was applied to the soil by border irrigation using a 7-inch hose pipe connected to a pumping well. Water meters were used to record the irrigation applied to each plot. In each year, pre-sowing water was applied to ensure the same soil water content before sowing.Seeds were sown on October 9 using a 2BJM Bevel-type Precision Seeder (Laiwu Hualong Machinery Factory, China) at a seeding rate of 195 kg ha–1and row spacing of 15 cm. Winter wheat was harvested on June 8th, 7th, 11th, and 10th in 2013, 2014, 2015,and 2016, respectively. Other field management practices were conducted using the same standards as those used in local fields to ensure good crop growth throughout the growing season.

2.3. Measurements

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Plant height, aboveground biomass, grain yield, and yield componentsAt harvest, 20 winter wheat plants were selected at random from each plot to measure the plot’s average plant height.Plants were harvested manually and three 1-m2samples were selected at random from each plot. All plant samples were air-dried to a grain water content of approximately 11%. The spike number(SN) and aboveground biomass (BM) per 1 m2were recorded.Plants were then threshed using a stationary thresher to obtain the grain yield. The 1 000-grain weight (GW) was measured using the average of three samples of 1 000 grains. The grain number (GN)per spike was estimated using SN, GW, and grain yield.

Soil moisture and water use efficiencyNeutron access tubes were installed in each plot to measure soil water content. Soil volumetric water content was measured using a neutron probe(CNC503B; Beijing Aozuo Ecology Instrumentation Ltd., China)down to a soil depth of 160 cm at 20-cm interval before sowing and after harvest. The water content in the top 20 cm of soil was measured using the oven-drying method.

Total water use or evapotranspiration (ET) during the winter wheat growing season was calculated using the following waterbalance formula modified from that described by Zhanget al.(2012):

Table 2 Experimental design

Where, ET (mm) is the total water consumption during a given growth period, I (mm) is the amount of irrigation,P (mm) is precipitation, and ΔSWC (mm) is the initial soil water content minus final soil water content.

Water use efficiency at the grain yield level (WUEy) was de fined as the grain yield (kg ha–1) divided by ET (mm), and water use efficiency at the BM level (WUEbm) was calculated as BM (kg ha–1) divided by ET (mm):

Agronomic efficiencyAgronomic efficiency (AE) is an index used to estimate the grain yield increase from fertilizer application (Duanet al.2014). AE was calculated using the following formula (Monet al.2016):

Where, YF(kg ha−1) is the grain yield in a fertilized plot,Y0(kg ha−1) is the grain yield in an unfertilized plot, and AF(kg ha−1) is the total amount of fertilizer applied to the plot.

Statistical analysisData were analyzed using an analysis of variance (ANOVA) to determine differences between treatments. ANOVA was performed using IBM SPSS 22.Differences among mean values were calculated using least significance differences (LSD) at the 5% level.

3. Results

3.1. Plant height and aboveground biomass

Plant height responded to both irrigation and N levels, but the effect of N was not as strong as that of irrigation (Table 3;Fig. 1). For all four years, plant height for a given N level tended to increase with increasing amounts of irrigation. The plant height with treatment W3 was 4.0–7.1%, 2.2–4.9%,5.2–12.9%, and 1.9–2.7% higher than that with W1 for the same N treatment during the 2012–2013, 2013–2014,2014–2015, and 2015–2016 seasons, respectively. The plant height with treatment N0 was significantly lower than that with the three N fertilized treatments. However, plant height within each irrigation level did not differ significantly among N1, N2, and N3. The N×Irrigation interaction for plant height was significant (Table 3). The higher N level,the higher was the plant height with an increasing number of irrigations (Fig. 1). The lower plant heights recorded for the 2014–2015 season may probably be attributed to low precipitation before April 2015 (11 mm).

Total BM of the wheat varied among the different irrigation regimes, N levels, and years (Table 3; Fig. 2). BM varied with irrigation level under each N level, except N0 in 2015–2016 (Fig. 2). Total BM under W2 and W3 conditions was similar and higher than that in the W1 treatment for the same N treatment. On average, over the four growing seasons,total BM was 13.1, 16.9, 15.6, and 12.8% greater under W2 than that under W1 for N0, N1, N2, and N3, respectively.The BM of the N0 treatment was significantly lower than that of all three N fertilization treatments. N fertilization resulted in an increase in BM by 14.5–55.7% over the N0 treatment.Maximum BM differed across the 4 years with values of 15 509, 16 337, 17 193, and 18 315 kg ha–1in 2013, 2014,2015, and 2016, respectively (Fig. 2).

3.2. Grain yield and yield components

Grain yields among the different treatment groups showed significantly different responses to irrigation and N levels in all four growing seasons (Fig. 3).F-values were similar among irrigation and N treatments, and the N×Irrigation,Year×Irrigation, and Year×N interactions were alsosignificant (Table 3). The yield of the W1 group was very low, regardless of N level, and was significantly lower than that of the W2 and W3 groups. The yield of W2 was high and similar to that of W3. The yield was greater with the addition of N than with the N0 treatment; however, there were no significant differences in grain yield among the different levels of N fertilization. Overall, grain yield showed a year-after-year rise over the course of the investigation.The pattern of variation in yield was similar for all 4 years.The higher grain yields for the years 2013, 2014, 2015, and 2016 (7 500, 8 800, 8 800, and 9 200 kg ha–1, respectively)were achieved using the W2N1 treatment (Fig. 3).

Table 3 F-values of analysis of variance combined across year, nitrogen and irrigation on winter wheat height, BM, yield, SN, GW,GN, ET, WUEy, WUEbm and AE1)

Fig. 1 Plant height in 2012–2013 (A), 2013–2014 (B), 2014–2015 (C), and 2015–2016 (D) for winter wheat under different irrigation and nitrogen application conditions. W1 indicates irrigation once at jointing stage; W2 indicates irrigation once at jointing and once at heading stage; W3 indicates irrigation once at jointing, once at heading, and once at filling stage. N0, N1, N2, and N3 represent nitrogen fertilization at 0, 100, 200, and 300 kg N ha−1, respectively. Different letters indicate significant differences at P=0.05.Error bars indicate standard deviation.

Fig. 2 Aboveground biomass in 2012–2013 (A), 2013–2014 (B), 2014–2015 (C), and 2015–2016 (D) for winter wheat under different irrigation and nitrogen application conditions. W1 indicates irrigation once at jointing stage; W2 indicates irrigation once at jointing and once at heading stage; W3 indicates irrigation once at jointing, once at heading, and once at filling stage. N0, N1, N2, and N3 represent nitrogen fertilization at 0, 100, 200, and 300 kg N ha−1, respectively. Different letters indicate significant differences at P=0.05. Error bars indicate standard deviation.

Fig. 3 Grain yield in 2012–2013 (A), 2013–2014 (B), 2014–2015 (C), and 2015–2016 (D) under different irrigation and nitrogen application conditions. W1 indicates irrigation once at jointing stage; W2 indicates irrigation once at jointing and once at heading stage; W3 indicates irrigation once at jointing, once at heading, and once at filling stage. N0, N1, N2, and N3 represent nitrogen fertilization at 0, 100, 200, and 300 kg N ha−1, respectively. Different letters indicate significant differences at P=0.05. Error bars indicate standard deviation.

Table 4 Yield component factors of winter wheat as affected by irrigation and nitrogen during four growing seasons

As the yield of winter wheat is positively correlated with SN, GW, and GN, fertilizer and irrigation can improve yield by regulating these components of yield. Measurements associated with yield components (SN, GW, and GN) for each treatment group are presented in Table 4. Irrigation level had a significant effect on all yield components(Table 3). Although SN and GW were significantly affected by N level, GN did not show a similar response (Table 3).F-values were larger for GW than for SN and GN, and irrigation had a larger effect than N (Table 3). As shown in Table 4, GW increased significantly with an increasing number of irrigations. In the 2012–2013 growing season, the mean GWs for the W1, W2, and W3 groups were 29.5, 33.2,and 35.1 g, respectively. The corresponding values in the 2013–2014, 2014–2015, and 2015–2016 growing season were 39.5, 40.5, and 42.9 g; 32.3, 37.5, and 38.6 g; and 32.0,32.1, and 34.1 g, respectively. Lvet al.(2013) indicated that sunshine duration plays a key role in determining grain weight, and that longer daylight hours at the filling stage are beneficial for an enhancement in grain weight. In the present study, the values of GW in the 2013–2014 season ranked the highest, followed by those in 2014–2015, 2012–2013,and 2015–2016, with 8.7, 8.4, 6.3, and 6.3 h of sunshine in May, respectively (Table 1). The W2N1 treatment resulted in higher SN and GN and moderate GW, and ultimately produced a higher yield. This result was likely mainly attributable to the higher mobilization efficiency and selfadjustment mechanism among the three yield components of the grain (Zhanget al.2008; Lvet al.2013).

3.3. Evapotranspiration and water use efficiency

ET was more strongly affected by irrigation than by N treatment (Table 3). ET increased significantly with the number of irrigation events per season (Fig. 4). With the exception of the 2014–2015 growing season, N fertilization increased ET to a greater extent than that observed in the N0 treatment. However, there was no significant difference among N-fertilization treatments. Among the different irrigation treatments, the highest ET values were obtained in the W3 groups for each N treatment and growing season.

WUEyvaried among years and treatments (Tables 3 and 5). With regards to irrigation treatments, the WUEyvalues of the W1 and W2 groups were higher than those of the W3 groups, and some of the differences reached a significant level during the four years. There were no marked differences between WUEyvalues of the W1 and W2 groups.In terms of N application, with the exception of N0, WUEydid not differ significantly among N fertilization treatments within the same growing season. Lower values of WUEywere obtained in the N0 treatments than in N-fertilized treatments under the same number of irrigations. In all years, the WUEyvalue in the W2N1 treatment were performed at the highest level, being 25.4, 29.6, 34.4, and 28.5 kg ha–1mm–1for the 2012–2013, 2013–2014, 2014–2015, and 2015–2016 seasons, respectively.

The pattern of differences in WUEbmamong treatment groups was nearly identical to that for WUEy(Tables 3 and 5). With an increasing number of irrigations, there appeared to be a downward trend in WUEbm. With the exception of N0, no significant differences were observed among N treatments. There was a clear linear correlation between WUEyand WUEbmover the four-year experimental period(Fig. 5). These results indicate that the high WUEbmof winter wheat may contribute to high WUEy, and this may ultimately result in a high yield.

3.4. Agronomic efficiency

AE was significantly affected by both irrigation and N(Table 3). At the same N level, AE generally increased with an increasing number of irrigation events (Table 6). At all irrigation levels, the rate of fertilizer application negatively affected AE, AE decreased with increasing fertilization rates.

Fig. 4 Evapotranspiration (ET) in 2012–2013 (A), 2013–2014 (B), 2014–2015 (C), and 2015–2016 (D) for winter wheat under different irrigation and nitrogen application conditions. W1 indicates irrigation once at jointing stage; W2 indicates irrigation once at jointing and once at heading stage; W3 indicates irrigation once at jointing, once at heading, and once at filling stage. N0, N1,N2, and N3 represent nitrogen fertilization at 0, 100, 200, and 300 kg N ha−1, respectively. Different letters indicate significant differences at P=0.05. Error bars indicate standard deviation.

Table 5 Water use efficiency at the grain yield level (WUEy) and at the aboveground biomass level (WUEbm) of winter wheat under different irrigation and nitrogen treatments during four growing seasons

The AE values of N1 groups were higher than those of N2 and N3 groups for all irrigation treatments. Averaged over the four growing seasons, AE values under W2 irrigation conditions were 32.6 and 82.1% greater under N1 than under N2 and N3, respectively.

3.5. Economic efficiency and ecological sustainability

Fig. 5 Linear regression between water use efficiency (WUE) at the grain yield level (WUEy) and at the biomass level (WUEbm) of winter wheat in 2012–2013 (A), 2013–2014 (B), 2014–2015 (C), and 2015–2016 (D).

Table 6 Agronomic efficiency (AE) of winter wheat as affected by irrigation and nitrogen application during four growing seasons

The economic bene fit of different treatments was analyzed using the average grain yield of the four years (Table 7).Results indicated that within the different N treatments, W2 groups showed the greatest bene fits. With the exception of treatment N0, net income decreased with increasing N fertilization treatments under the same irrigation treatment.Compared with the traditional cultivation practice in local fields (W3N3), the net income of W2N1 was higher by 12.3%, and the output-input ratio was 19.5% higher.

From the standpoint of environmental sustainability, the W2N1 treatment economizes on irrigation and fertilizer usage. Compared with the traditional practices of irrigation and fertilization, using the W2N1 regime could save 600 m3ha–1of irrigation water and 200 kg N ha–1without impairing farmers’ benefits. Given that irrigation water is becoming less available in many areas and that the effects of global climate change (e.g., global warming, haze) are becoming increasingly evident (Jiru and Van Ranst 2010), the W2N1 treatment employed in this experiment meets the acute water challenges facing those who farm in the NCP, ful fills the requirements for crop production, and is environmentally friendly.

4. Discussion

In the present study, we determined that the optimal fertilization rate and irrigation regime for wheat production in the drought-prone NCP was 100 kg N ha−1coupled with two irrigations of 60 mm each at the jointing and heading stages,respectively. This W2N1 treatment consistently produced the highest yield in all four years of the study. Similarly, Donget al.(2011) reported that two irrigation events of 75 mm in a drought year would result in optimal WUEyand yield in the NCP. Wanget al.(2015) reported that fertilization with 210–240 kg N ha−1under supplemental irrigation at the jointing and anthesis stages produced the highest grain yield. In contrast, the results of our study indicate that there were no marked increases in yield with increasing nitrogen fertilizer application in excess of 100 kg N ha–1under each of the irrigation treatments. Lvet al.(2013)showed that ET increases gradually when irrigation water supply is increased and that ET values with four irrigations were the highest, being 25–113 mm higher than with two irrigations. Consistently, we showed that the ET of the W2N1 treatment was approximately 30 mm lower than that obtained with local traditional practice (W3N3). Excessive application of irrigation water and nitrogen cannot increase crop yields further, and thus represent a considerable waste of resources (Shiraziet al.2014). Under the conditions in the present study, the optimal W2N1 treatment yielded the maximum bene fits in terms of economizing the use of irrigation water and nitrogen fertilizer.

Across all years in the present study, grain yields under W2N1 were similar to those under W3N3. Many studies have shown that biomass and the harvest index (HI) are the two main factors determining grain yield (Sayreet al.1997; Shearmanet al.2005; White and Wilson 2006).The W2N1 treatment produced the same biomass and HI as the W3N3 treatment (W2N1, HI=0.53; W3N3, HI=0.52;P＞0.05), with lower water and nitrogen inputs, which resulted in maintenance of a higher grain yield, WUE, and AE. In the present study, W2N1 resulted in a higher WUEbmand WUEythan W3N3 did. It has been shown that it is possible to simultaneously achieve both satisfactory yield and WUE by regulating the number of irrigations (Lvet al.2013).Consistent with this, we found positive relationships between WUEyand WUEbm. Hsiao (1993) reported that WUEbmis generally closely related to the photosynthetic WUE of single leaves (short-term gas exchange on a photosynthesis basis), but that WUEyis not closely related to WUEbm. In contrast, Qiuet al.(2008) showed that WUEbmis positively related to WUEy, with increases in WUEbmhaving positive effects on WUEy, thereby enhancing ultimate yields (Qiuet al.2008). The results of the present study are consistent with those of Qiuet al.(2008) as we found that WUEywas positively associated with WUEbm. A positive relationship between WUEyand WUEbmcould be used to provide valuable information to guide agricultural production and farming practices. By monitoring WUEbmthroughout the growing season, and adjusting it by reasonable irrigation,farmers may simultaneously achieve relatively high yield and optimal WUEy. Our results showed that in all the four years we investigated, AE decreased with increasing fertilizer application. Moreover, there was an upward trend in AE with increasing irrigation. These results are consistent with those of Monet al.(2016), who found that AE was negative at low irrigation levels and high N rates.

Xueet al.(2006) reported that in the Texas High Plain,irrigation increases ET, and that biomass and grain yield also increases as ET increased. However, although our results showed that ET increased as irrigation increased in the NCP,biomass and grain yield did not show similar increases.Although the yield of wheat with two irrigations was not significantly different from that with three irrigations, the ET of two-irrigation wheat was lower than that of three-irrigation wheat. Similar results were obtained by Sunet al.(2011)who indicated that ET tended to increase with an increasing level of irrigation. Increases in irrigation frequency and amount of water might increase soil evaporation. The major factor affecting soil evaporation fluctuation was surface soilmoisture (Gaoet al.2005). Compared with three irrigations,the two-irrigation treatment was associated with less surface soil moisture and lower vapor flux, which decreases soil evaporation. Furthermore, larger amounts of irrigated water may cause unfavorable delayed senescence in winter wheat and thus raise the ET.

Table 7 The output-input ratio of different irrigation and nitrogen treatments

An important goal of agricultural practices is to achieve continuous and efficient development of farmland, which takes into consideration both production and environmental bene fits. We found that among the different treatments, the highest net income (11 457 CNY ha–1) was obtained using the W2N1 treatment. The corresponding output-input ratio for this treatment (2.5) was also the highest among those treatments examined.

There remains considerable potential for further savings in the NCP with regards to water and fertilizer use. In the late 1970s and early 1980s, there were as many as 8–10 irrigation events during the winter wheat growing season,which maintained a moist soil surface during the entire growing period. Subsequent research on water-saving irrigation systems for winter wheat, has indicated that the number of irrigation events per growing season could be reduced to five (Zhang 2011). In the 21st century,Zhanget al.(2003, 2008) devised irrigation regimes for high-yielding and water-saving winter wheat cultivation that entailed one irrigation in a wet year, two irrigations in a normal year, and three irrigations in a drought year.In the present study, two irrigation treatments sufficed for wheat production in the drought-prone NCP. This is still potentially higher than that found in previous studies. We found that the difference in grain yield between N0 and N1 application was larger under W2 irrigation than that under W1 irrigation. It has previously been shown that a water deficit can restrict the effectiveness of N fertilizer (Guet al.2015). Under two irrigations, 100 kg N ha–1can satisfy nutritional requirements for the period of crop growth.Compared with current practices, 60 mm of irrigated water and 200 kg N ha–1could be saved without yield reductions in a drought year. In a normal or wet year, the water-saving potential could be improved even further, when rainfall coincides with the key wheat growing period. In addition,the irrigation-saving effect of W2N1 may have important implications for relieving groundwater over-exploitation and improving the ecological environment. In the NCP,the area planted with winter wheat covers over 5 million ha, and on the basis of our experimental results, there is tremendous potential to save 3 billion m3of irrigation water in this region. Further, the low fertilizer input of the W2N1 treatment may reduce the risk of environmental degradation produced by excess nitrogen including nitrate pollution of groundwater, ammonia volatilization, and other forms of air pollution.

5. Conclusion

In the present study, we found that different irrigation and N fertilization treatments significantly affected grain yield,water use efficiency, and fertilizer agronomic efficiency.This four-year experiment showed that in drought years in the North China Plain: (1) a high yield of 7 500–9 200 kg ha–1can be achieved with two irrigation events (one at the jointing stage and one at the heading stage) coupled with 100 kg N ha–1; (2) two rounds of irrigation per growth season can produce a higher WUEycompared with the local conventional three rounds of irrigation; and (3) the W2N1 treatment achieved the maximum economic returns and has important implications for the sustainable development of agriculture and the ecological environment.

Acknowledgements

This study was supported by the National Key Research and Development Program of China (2016YFD0300808), the National Key Technologies R&D Program of China during the 12th Five-Year Plan period (2013BAD05B02), the National Natural Science Foundation of China (31571612 and 31100191), the Science and Technology Service Network Initiative of Chinese Academy of Sciences (KFJ-STSZDTP-001), and the Hebei Key Research and Development Program, China (15226407D and 17227006D).

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