FAN Ting-lu,LI Shang-zhong,ZHAO Gang,WANG Shu-ying,ZHANG Jian-jun,WANG Lei,DANG Yi,CHENG Wan-li

1 Key Laboratory for Efficient Utilization of Water Resources in Dryland Areas in Gansu Province,Lanzhou 730070,P.R.China

2 Dryland Agriculture Institute,Gansu Academy of Agricultural Sciences,Lanzhou 730070,P.R.China

Abstract Climate change has a significant impact on agriculture. However,the impact investigation is currently limited to the analysis of meteorological data,and there is a dearth of long-term monitoring of crop phenology and soil moisture associated with climate change. In this study,temperature and precipitation (1957-2020) were recorded,crop growth(1981-2019) data were collected,and field experiments were conducted at central and eastern Gansu and southern Ningxia,China. The mean temperature increased by 0.36°C,and precipitation decreased by 11.17 mm per decade.The average evapotranspiration (ET) of winter wheat in 39 years from 1981 to 2019 was 362.1 mm,demonstrating a 22.1-mm decrease every 10 years. However,the ET of spring maize was 405.5 mm over 35 years (1985-2019),which did not show a downward trend. Every 10 years,growth periods were shortened by 5.19 and 6.47 d,sowing dates were delayed by 3.56 and 1.68 d,and maturity dates advanced by 1.76 and 5.51 d,respectively,for wheat and maize. A film fully-mulched ridge-furrow (FMRF) system with a rain-harvesting efficiency of 65.7-92.7% promotes deep rainwater infiltration into the soil. This leads to double the soil moisture in-furrow,increasing the water satisfaction rate by 110-160%. A 15-year grain yield of maize increased by 19.87% with the FMRF compared with that of half-mulched flat planting. Grain yield and water use efficiency of maize increased by 20.6 and 17.4% when the density grew from 4.5×104 to 6.75×104 plants ha-1 and improved by 12.0 and 12.7% when the density increased from 6.75×104 to 9.0×104 plants ha-1,respectively. Moreover,responses of maize yield to density and the corresponding density of the maximum yield varied highly in different rainfall areas. The density parameter suitable for water planting was 174 maize plants ha-1 with 10 mm rainfall. Therefore,management strategies should focus on adjusting crop planting structure,FMRF water harvesting system,and water-suitable planting to mitigate the adverse effects of climate change and enhance sustainable production of maize in the drylands.

Keywords: climate change,dryland maize and wheat,plastic mulch,water-suitable planting

1.Introduction

Agricultural production is highly dependent on climatic conditions and is extremely sensitive to climate change(Lin and Yang 2003;Wanget al.2003). Under global warming conditions,increasing instability in agricultural production,changes in crop planting structure and variety distribution,and increasing extreme weather events pose a severe threat to food security (IPCC 2014;Guo 2015). The drylands in northern China are among the regions experiencing significant temperature increases(Denget al.2010). The change in crop phenology,which effectively reflects the response process of the crops to climate change,has emerged as a prime focus in research on the impact of global climate change on agriculture.

The relationship between crop phenology and climate change has received increasing attention in recent years(Denget al.2010). The climate in northern China has demonstrated the characteristics of warming and drying with increasing temperature and decreasing precipitation over the last 50 years (Lin and Qian 2003;Maet al.2003). The sowing date of spring crops has shifted earlier,while that of overwintering crops is delayed (Denget al.2008). Climate change has caused crop areas to expand to high altitude and high latitude areas,which significantly affects the crop yield in rainfed areas (Denget al.2010). In the United States,the sowing date of maize advanced by 10 d,the growth period extended by 12 d,and the accumulated temperature increased by 14% (Sacks and Kucharik 2011). The emergence rate of maize increased by 18%,the growth rate from emergence to maturity increased by 17%,and the growth period of maize shortened by 14 d when the average temperature increased by 1°C (Wanget al.2011). A majority of these studies use statistical methods,such as correlation analysis and linear regression,to predict the changes in the potential growth periods that may be caused by climate change (Muet al.2014;Suet al.2021). However,the long-term field crop observation data are insufficient in these studies.

Dryland agriculture (crops and pastures) accounts for more than 70% of the total and is located primarily between 30°N and 50°N in China (Li and Xiao 1992). The maize cultivated in the drylands of northern China covers 13 million ha,accounts for 35% of the sown area,and produces approximately 45% of the the country’s total maize production. Maize is a C4heat-loving crop with high yield potential. The highest yield record of 37.77 Mg ha-1was set in the United States in 2021,while a yield of 24.95 Mg ha-1was recorded in Xinjiang,China,in 2019.However,maize production is significantly affected by climate change. The maize yield and water use efficiency(WUE) are relatively low in northern China because of water shortages and increasingly frequent droughts (Renet al.2016). The limited precipitation and short growing seasons present major challenges for sustainable crop production. Crops may sometimes fail due to the erratic nature of precipitation,resulting in a substantial loss of the producers’ farm income (Fanet al.2019). Dryland crop production depends on soil water storage during the planting season and precipitation during the growing season. Water from precipitation should therefore be appropriately captured and stored in the soil for efficient use by crops to sustain dryland crop yields and meet food demands for the growing population (Ungeret al.2006;Nielsenet al.2010). The adoption of advanced water management systems and water-suitable planting techniques is an appropriate strategy for determining the response of maize growth to climate change and enhancing grain production and supply in drylands (Wright and Nageswara 1994;Stewart and Lal 2018).

The Chinese government has implemented several scientific and technological research projects on dryland agriculture over the last 40 years to solve the problem of low yield and WUE caused by drought since the 1970s(Xinet al.2001;Fanet al.2019;Li F Met al.2020;Zhanget al.2021). Great progress has been made in the breeding of drought-resistant and water-saving varieties,effective use of precipitation,rainwater harvesting and irrigation,plastic mulching,and crop compound planting(Liet al.2012,2022;Wanget al.2016;Gaoet al.2019;Li Met al.2020;Chuet al.2022). In particular,the innovative technology of the fully-mulched ridgefurrow rainwater harvesting and maize planted in furrow areas,as shown in Fig.1 (FMRF),has been widely adopted in the Loess Plateau for nearly 20 years (Ganet al.2013;Fanet al.2019). The adoption of drought resistance varieties,critical period deficit irrigation (Fanet al.2005),straw mulching,and fertilizer management(Liuet al.2010;Zhenget al.2020) have greatly improved crop yield and WUE. Increased seeding rate may improve water-use efficiency by controlling weed growth and enhancing crop water uptake because of increased plant density (Tompkinset al.1991).Currently,adaptation strategies using drought-resistant and water-suitable planting technology are proposed for increasing resilience to combat climate change-related effects (Ungeret al.2006;Nielsenet al.2010;Zhang 2019;Li Met al.2020;Upendraet al.2021;Liet al.2022). However,the research on this technology is still insufficient. It is,therefore,urgent to determine technical parameters as per crop demands through a large number of observations and experimental verification. Based on the long-term observation data of crop growth period and climate factors of the Zhenyuan Experimental Station(ZES),as well as a multi-year field experiment,this study revealed the response of spring maize and winter wheat to climate change. It additionally put forward the technical strategy of drought-resistant and watersuitable planting of maize,providing technical support for sustainable grain production.

2.Materials and methods

2.1.Collection of meteorological,crop evapotranspiration,and growth data

The annual precipitation and temperature data were obtained from the weather station at ZES from 1957 to 2020,and the climate change trend over the years was analyzed. The crop evapotranspiration (ET) during the growing seasons of dryland winter wheat from 1981 to 2019 and of dryland maize from 1985 to 2019 were calculated annually using the water balance equation(Crstina and Salvatore 2013):

where P is the total growing season precipitation (mm)(precipitation received from planting to harvesting),I is the total irrigation quota (mm) that was negligible due to the complete rainfed area,W1 and W2 are the soil water storage (mm) at sowing and harvest (mm). Under the FMRF system,W1 and W2 were averaged in the ridges and furrows;C,D,and R are the upward flow into the root zone,the downward drainage out of the root zone,and the runoff (mm),respectively,which were neglected in this study because of the over 100 m groundwater table,high water-holding capacity,and less drainage. The water-use efficiency (WUE,kg ha-1mm-1) of mulched maize was calculated as the ratio of maize yield (kg ha-1) to ET (mm)in a crop-growing season.

The growth data of wheat were recorded yearly from the winter wheat regional tests (1981-2019) and the spring maize field experiment (1985-2019). The ET was determined,and potential evapotransporation (PET)was recorded from 1980 to 2020 in the 4-year winter wheat rotated annually with a 2-year maize experiment,Pingliang,Gansu. Crop water satisfaction rates (ET/PET) of maize and wheat were calculated. The change parameters of the growth data of winter wheat and spring maize related to years were then analyzed.

2.2.Harvested water and crop water satisfaction from the FMRF system

Ridge-furrow planting has been effectively used for insite water harvesting (Liuet al.2009;Ganet al.2013). In this study,the FMRF system was used to cover the soil surface completely,and maize was planted in furrows,as presented in Fig.1. Runoff rainwater from the FMRF was collected from April to June by installing plastic buckets of 30 cm in height with 20 cm in diameter between the ridge and furrow connection. A runoff harvesting area of 1.65 m2(a half area summing 70 cm ridges and 40 cm furrows along with 600 cm strips) was set up. The rainharvesting efficiency (α,%) at the ridge-furrow surface was calculated based on the water volume (kg) stored in the plastic bucket divided by the rainfall amount (kg) in the harvesting area. In the FMRF system,the potential water amount (W,mm) accumulated at the crop root zone was expressed as follows (Fanet al.2019):

where GP (mm) is the growing season rainfall (rainfall received from planting to harvest),N1 and N2 (cm) are the ridge width and furrow width (Fig.1),respectively,andkis the ridge to furrow rate.

In the FMRF system,the actual crop water satisfaction rate (WSR) of harvested water to potential crop evapotranspiration (ETm) was calculated using the following formula (Richardet al.1998):

whereWSR0is referred to as WSR under flat planting techniques with no plastic mulching and WSR toWSR0rate＞1.

2.3.Suitable planting density of maize in areas with different rainfall

Fig. 1 Configuration view and field operation by tractors of film fully-mulched ridge-furrow planting system (FMRF).

Fig. 2 Changes of annual air temperature and precipitation (1957-2020) in Zhenyuan County,Gansu Province,China.

Fig. 3 Evapotranspiration (ET) trends of winter wheat (1981-2019) and spring maize (1985-2019) in the dryland of Zhenyuan County,Gansu Province,China.

Fig. 4 Water satisfaction rate (ET/PET) changes during the years of 1980 to 2020 in the 4-year winter wheat rotated annually with 2-year maize experiment,Pingliang,Gansu Province,China.

Fig. 5 Growth period and date changes of winter wheat (1981-2019) and spring maize (1985-2019) in the dryland of Zhenyuan County,Gansu Province,China.

Fig. 6 Grain yield of dryland maize covered by plastic film mulching from 2007 to 2021 in Zhenyuan County,Gansu Province,China. FMRF,film fully-mulched ridge-furrow planting and maize was planted in furrows;HMFP,film half-mulched flat planting and two rows of maize were seeded on the plastic film strips. Means for grain yield followed by a and b letters between FMRF and HMFP are significant at P=0.032＜0.05.

Fig. 7 Relationship of spring maize yield to density in different sites or years with rainfall amount of 300 to 650 mm in Gansu Province,China.

Fig. 8 Maize planting density determination by annual rainfall amount in dryland areas,Gansu Province,China.

Under the FMRF planting system,a 4-year field experiment was done with three maize planting densities of 4.5×104plant ha-1(low density) and 6.75×104plant ha-1(medium density),and 9.0×104plant ha-1(high density) at Pengyang County of Ningxia Hui Autonomous Region from 2012 to 2015. Field experiments with maize densities ranging from 3.0×104to 10.5×104plant ha-1were also conducted at eight sites with annual precipitation ranging from 300 to 650 mm in middle and eastern Gansu to illustrate the relationship between maize yield and planting densities. The unitary quadratic regression relationship between grain yield (Y,kg ha-1)and density (X,plant ha-1) was established in areas with different precipitation. Suitable planting density (Xmax)corresponding to the maximum yield (Ymax) of maize was then calculated under different annual precipitation (P,mm) levels. The linear increasing relationship betweenXmaxand P was then established,where the slope represents the suitable density parameter determined by the precipitation level. Field experiments with different maize varieties and densities were also carried out to determine the grain yield and ET changes.

2.4.Statistical analysis

The experimental data were calculated using Microsoft Excel 2003. The change in the trends of the annual mean temperature and precipitation was analyzed,in addition to the analysis of crop growth data in years and maize yield in density. The different maize densities were compared using Duncan’s multiple range tests. AP-value＜0.05 was considered significant.

3.Results

3.1.Annual air temperature and precipitation trends

The temperature and precipitation data of ZES in Gansu from 1957 to 2020 showed a decrease in precipitation by 11.17 mm and an increase in temperature by 0.36°C every 10 years since the 1950s,the 20-year mean temperature increased from 8.85°C in 1957-1976 to 9.59°C in 1977-1996 and 10.50°C in 1997-2016,and the corresponding precipitation decreased respectively from 550.2 mm to 487.1 and 463.2 mm (Fig.2). This indicates a noticeable trend of warming and drying. Compared to that of the 20 years from 1957 to 1976,the precipitation decreased by 63.1 mm from 1976 to 1996 and 87.0 mm from 1997 to 2016. The trend of temperature increase was especially evident in the 1980s. From 2011 to 2020,the annual average temperature reached 10.41°C,which is 0.96 and 0.31°C higher than that during 1981-1990 and 1991-2000,respectively,with an increase of 9.2 and 3.0%. Over the last 60 years,the precipitation during the summer fallow period decreased by 7 mm from July to September every decade and decreased by 2.1-2.6 mm in April when spring maize was sown and in October when winter wheat was sown. At the same time,extreme drought events increased,such as in 1992,1995,2000,2002,2005,2009,2015,and 2021,which resulted in more than a 30% loss of grains and failure of wheat and maize harvests in central and eastern Gansu.

3.2.Changes in crop evapotranspiration (ET)

The ETs in the dryland crops of wheat and maize present different trends in the process of warming and drying(Fig.3). The average ET of winter wheat in 39 years from 1981 to 2019 was 362.1 mm,demonstrating a 22.1-mm decrease every 10 years (Fig.3). The average ET of winter wheat was 302.3 mm from 2001 to 2010 and 353.9 mm from 2011 to 2019,which was 13.5-26.1% lower than the average ET of 409.1 mm from 1981 to 1991. This is equivalent to a reduction of 5-10 mm per year. However,the ET of spring maize was 405.5 mm over 35 years(1985-2019),which did not show a downward trend like that of winter wheat. The inter-annual coefficient of variation of the ET in maize was 18.8%,nearly 3 percentage points lower than that of wheat.

In addition,the rainfall from July to September accounts for 60-70% of the annual total in northern China. Its distribution was highly consistent with the water demand of the growing season of spring maize from April to September. On the other hand,the growing season of winter wheat is in the dry season with scarce rainfall from October to next June. The crop water satisfaction rate(ET/PET) of maize increased by 3.4 percentage points every 10 years,while that of winter wheat decreased by 5.0 percentage points (Fig.4). The water satisfaction rate and ET of winter wheat,therefore,decreased because of the warming and drying climate. However,the high coincidence of rainfall to the demand for maize water and the increase in the water satisfaction rate was beneficial to maize production and acreage expansion.

3.3.Response of dryland crops to climate change

Over the past 30 years,wheat (1981-2019) and maize(1985-2019) demonstrated a shortening growth period,a delay in the sowing period,and an advanced harvest period (Fig.5). The growth period of winter wheat was found to be shortened by 5.2 d,the sowing date delayed by 3.6 d,and the maturity date advanced by 1.8 d every 10 years. The growth period of spring maize in the drylands is affected by the combined effect of temperature and precipitation,leading to the advance of each growth stage of maize,similar to that of winter wheat. The growth period of maize was shortened by 6.5 d,the sowing date delayed by 1.7 d,and the maturity date advanced by 5.5 d.Thus,the warm and dry climate significantly affected the growth and development of dryland winter wheat and spring maize,and the crops,in return,adapted to the climate change by adjusting their growth periods.

3.4.Rain-harvesting and grain yield of maize in the FMRF system

Unlike the half-mulched flat planting (HMFP),the runoff from the FMRF system was collected by installing plastic buckets from April to June. The results indicated that the rain-harvesting efficiency (α) of the ridge-furrow system in different rainfall intensities varied greatly,which could attribute to the yield of dryland maize and gains in WUE.The FMRF system allowed more runoff to accumulate in the furrow,thereby letting the collected rainwater infiltrate deep into the soil. In the 1.65 m2water catchment area of the furrow-ridge system,the amount of collected rainwater ranged from 8.11 to 13.32 kg when a rainfall of 5-10 mm occurred (equal to 80.7-92.7% inαvalue),from 1.41 to 6.41 kg in a less than 5 mm rain (equal to 65.7-82.7%inαvalue),and from 9.43 to 18.95 kg in a 10-15 mm rain (equal to 48.9-78.2 % inαvalue) (Table 1). It might be deduced that with 70 cm film-ridge rainfall harvesting and 40 cm film-furrow planting zone (k (ridge to furrowratio)=1.75),the potential water (W) amount collected at the root zone of maize would be 2.14,11.56,and 25.75 mm if the averageαvalue of the FMRF system was 0.65 in 1 mm,0.75 in 5 mm,and 0.90 in 10 mm rainfall,respectively. This implies that the soil water amount in the root zone may double in amount (Table 2). The WSR increased with the increase inαand k. For example,whenαwas 0.65 and 0.9,k increased from 1:1 to 1.75:1,and the WSR increased from 65 to 113.8% and 90 to 150%,respectively. An increase in soil moisture and WSR in the FMRF system may bring a significant increase in grain yield and WUE. In the 15-year field experiment,the mean yield of maize in the FMRF system was 12.76 Mg ha-1(P=0.032),19.81% higher than 10.65 Mg ha-1in the FMHP system (Fig.6). From 2007 to 2010,another field experiment showed that the average WUE of maize planted using the FMRF system was 44.9 kg ha-1mm-1,33.6% higher than that of 33.6 kg ha-1mm-1in FMHP. In particular,the FMRF-planted maize,with ET of 223 mm and growing season rainfall of 239 mm,demonstrated a grain yield and WUE as high as 12.17 Mg ha-1and 54.6 kg ha-1mm-1,respectively,especially during the severe drought in 2009.

Table 1 Harvesting water and rain-harvesting efficiency in the film fully-mulched ridge-furrow system (FMRF) in Zhenyuan County,Gansu Province,China

Table 2 Expected root zone water (W) of maize in planting furrow and crop water satisfaction rate (WSR) increase in the film fully-mulched ridge furrow system (FMRF)

3.5.Response of grain yield and ET to maize density

More than 1 530 maize fields were investigated in the dryland areas of northwestern China,where the average density of maize was 5.25×104plants ha-1. From 2012 to 2015,the 4-year experiment in southern Ningxia demonstrated that the grain yield increased by 20.62%from 8.34 to 10.06 Mg ha-1when the density went from 4.5×104(low) to 6.75×104(medium) plants ha-1,and it increased by 12.03% from 10.06 to 11.27 Mg ha-1when the density increased from 6.75×104to 9.0×104(high) plants ha-1. Similar to grain yield,the WUE value increased by 17.35% from 19.88 to 23.33 kg ha-1mm-1with 2.25×104plants ha-1,a plant density increased from the low to medium level,and it increased by 12.73%from 23.33 to 26.30 kg ha-1mm-1with the same amount maize plants increasing from the medium to high density.Like the grain yield and WUE,ET was also significantly influenced by plant density (P＜0.001). ET increased by 10.6 mm from the low to medium plant density but kept constant from the medium to high density. Grain yield,ET,and WUE increased simultaneously when the density changed from low to medium;grain yield and WUE increased without an increase in ET when the density raised from medium to high. In dryland conditions,the impact of plant density treatments on grain yield and WUE of spring maize was more pronounced than their effect on ET. In the same experiment from 2016 to 2019 in the drylands of eastern Gansu,with the increase of the maize density from 4.5×104,6.0×104,7.5×104,and 9.0×104to 10.5×104plants ha-1,the mean ET values of three maize hybrids of Yuyu 22,Xianyu 335,and Longdan 9 were found to be 438.4,450.4,465.2,457.4,and 450.7 mm,respectively. There was no significant difference in the ET among the planting densities (P=0.335＞0.05). However,the grain yield increased from 11.3 to 14.5 Mg ha-1(P=0.001＜0.05),and the WUE increased from 25.8 to 32.0 kg ha-1mm-1(P=0.02＜0.05).

Maize density experiments conducted in five counties in Gansu with an annual rainfall of 300-650 mm showed that the quadratic parabola of grain yield is related to maize density (Fig.7),where the response of maize yield to density varied highly in different rainfall areas or years. The maize density in the drylands was strongly dependent on the amount of rainfall. The suitable density range for achieving increased maize yield in areas with annual precipitation of 300-450 mm was lower than that in areas with annual precipitation of 450-650 mm. Additionally,the maximum planting density (Xmax) corresponding to the maximum yield(Ymax) varied significantly among areas with different rainfall or across years. For example,in the areas with annual precipitation of 300,420,480,and 540 mm,the maximum planting densities were 4.97×104,5.84×104,6.54×104,and 9.50×104plants ha-1,respectively. With an annual rainfall range of 300-500 mm,the suitable planting density (Xmax) of maize increased with the annual rainfall (P) ofXmax=0.0174P-1.0147 (R2=0.886,P＜0.001) (Fig.8). The slope of the equation was defined as the density parameter for water-suitable planting for maize,i.e.,1 mm rainfall can sustain 174 maize plants per ha (approximately 12 plants of maize per 666.7 m2are planted with 1 mm rainfall). The increase in grain yield did not lead to an increase in ET when the rainfall was more than 500 mm (red line and dots in Fig.8).The planting density,therefore,did not improve with the increase in rainfall but remained within the range of 8.3×104to 9.0×104plants ha-1.

4.Discussion

4.1.Climate change affects dryland crops production

Global climate change,associated with increased temperatures,reduced precipitation,and erratic wind patterns (Solomon 2007),poses a great threat to world food security and sustainable development. This is evident in the global drylands that are affected by increasing temperatures and water shortages. Dryland agriculture accounts for more than 70% of total farmland in northern and northwestern China and produces 58%of the country’s grain (Fanet al.2019). A large number of studies have shown that in the past 50 years,the mean temperature in the eastern and western parts of Northwest China has increased by 0.98 and 0.91°C(Denget al.2010),respectively,and the precipitation has decreased by 20-40 mm,which is consistent with the trend observed in this study. The observation data of ZES showed that the mean temperature increased by 0.36°C and precipitation decreased by 11.2 mm every decade. Winter wheat and spring maize in Gansu were affected by the reduction in precipitation and the increase in temperature. Their growth periods were shortened by 5-6 d,demonstrating a trend of late sowing and early harvesting. This coincided with the growth period of winter wheat shortened by 6-9 d (Denget al.2007;Liet al.2009) and that of spring maize shortened by approximately 6 d (Wanget al.2004). Therefore,climate warming is more beneficial to the growth and yield of winter wheat and maize,which can be expanded northward and westward to high latitudes and altitudes(Wuet al.2021). With climate warming,spring maize should be timely sown in advance to compensate for the heat lack in the late growth period and avoid the harm of frost in the morning and evening and high-temperature stress and drought in mid-summer. As temperatures become warmer during autumn and winter,winter wheat planting should be delayed to prevent overgrowth before winter. As precipitation decreases and drought intensifies,new varieties with drought resistance,water saving,and heat resistance should be introduced,while varieties should be diversified (Wang 2010).

In this study,the water satisfaction rate increased for spring maize but decreased for winter wheat,while the ET decreased for winter wheat and did not change for spring maize. This was conducive to reducing the area of wheat and expanding the area of maize. It also facilitated the establishment of a planting structure for water-suitable crops adapted to rainfall distribution and climate change.By optimizing the crop planting structure and developing climate resilience in dryland agriculture,the precipitation utilization rate of the planting system may improve. The strategy of planting structure adjustment by expanding spring maize and reducing wheat was implemented in the drylands of Gansu for climate change adaptation.The planting proportion of overwinter crops (e.g.,winter wheat) to spring crops (e.g.,maize and potato) changed from 58.7:41.3 in 1995 to 36.6:63.4 in 2020 in Gansu,and the grain yield increased from 2.19 to 4.02 Mg ha-1and the WUE from 7.1 to 11.8 kg ha-1mm-1.

4.2.The FMRF system increases water use efficiency

Climate warming and drying severely affected the crop climate yield in dryland areas and contributed to the improvement of the climate yield of spring maize and winter (Denget al.2010). However,increased water stress to crops and a drastic decrease in water availability for agricultural use are predicted. Water and its effective use thus present one of the foremost challenges to crop production in rain-fed agriculture. A set of agricultural technology systems suitable for dryland farming were established to adapt to climate change (Denget al.2010;Renet al.2017). Deep plowing,generally carried out in the summer,has been widely used in dryland farming as an effective method for storing and keeping soil water.However,the summer fallow efficiency (percentage of rainfall stored in the soil during the fallow period) is only 30% under bare fallow because of high temperatures and the consequent high evaporation (Li and Xiao 1992;Petersonet al.1996). In dry farmlands,50-60% of the rainfall is lost through soil evaporation,while only 26-34%is attributable to crop transpiration (Fanet al.2019).Results from our lysimeter indicated that soil evaporation accounted for 57.5 and 32.3% of ET in dryland winter wheat and spring maize. Thus,reducing evaporation loss and increasing water availability are key to droughtresistant and water-suitable planting. Ridge-furrow planting is the most effective technique to improve soil moisture and crop water use efficiency (Liet al.2002;Li and Gong 2002). In this study,with the FMRF system,the rain-harvesting efficiency changed from 48.9 to 92.7%with different rainfall intensities,particularly exceeding 80%in light rainfall. The soil water storage in the root zone may increase two folds compared with rainfall amount.In the growth period of dryland maize in northwestern China,rainfall ≤10 mm and ≤5 mm makes up 40-50%and 20% of the annual rainfall. Such low rainfall has poor effectiveness and a low utilization rate for crop production.The soil moisture content was 2.0-3.5 percent points(equivalent to 25-45 mm of rainfall) higher in the 0-100 cm soil layer under the FMRF system compared with that of the HMFP. This may primarily be attributed to two reasons.First,the ridge-furrow system could accumulate rainfall to enhance water infiltration and retention. Second,the film fully mulched on ridge-furrow could prevent the soil water exchange between the soil and air,thereby decreasing the evaporation of soil water (Renet al.2017). However,most soil moisture may evaporate directly from the bare flat surface in the HMFP (Liet al.2016). The FMRF system increased the maize yield by 19.81% compared with that of the FMRF system. This increase may be attributed to soil water availability and the increase in planting density.The maize yield and ET increased with an increase in the planting density from 4.5×104to 9.0×104plant ha-1,which could be attributed to the leave area of maize and the increase in transpiration in high-density conditions(Liuet al.2010,2021). However,the ET value increased slightly only. There was a small change in the ET in the FMRF system when the maize density was above 6.75×104plant ha-1while the WUE and grain yield continuously increased. These results were consistent with the findings where no difference in the ET was observed among maize densities (Zhanget al.2014). Increasing maize density intensifies soil water extraction from the deep layers and has little influence on soil water consumption in dryland maize production. Some studies showed that ET in maize production was persistent under a certain grain yield level and specific area. This is because ET is calculated by the evaporation from the leaves and stems,and the transpiration ratio (transpiration to evapotranspiration,T/ET) could be changed by agronomic measures (Wanget al.2013). The FMRF system improved the T/ET ratio by decreasing the evaporation to a minimum within a definite amount of water loss and thus increased grain yield and WUE without an increase in water consumption,compared with no mulch (Liuet al.2014a).

4.3.Dryland yield of maize is correlated with rainfall and density

With the application of the FMRF system,the northern boundary of crop cultivation was extended by 100-150 km.The expansion of maize planting became a reality in the semi-arid area of 300-400 mm,the unstable yield area of 400-500 mm became a stable increase and high-yield area,and the planting area of dryland maize increased by over 50% (Houet al.2009;Denget al.2010). The acreage of maize planted in the dryland areas in Gansu increased from 17.7 million ha in 1991 to 66.7 million ha in 2020,producing half of the province’s grain with 1/4 of the grain acreage. In the past halfcentury,an important reason for the increased maize yield was the improvement of density tolerance,drought tolerance,and disease resistance (Wanget al.2015;Assefaet al.2016). The increasing tolerance of maize hybrids to high density has made planting density the most important agronomic measure for increasing yield in the past 60 years (Sangoiet al.2002). Duvick (2005)evaluated the annual yield gains of 36 maize hybrids from 1934 to 1991 at 7.9,5.4,and 3.0 plants m-2densities as 110,88,and 39 kg ha-1,respectively. The average yield of maize in the US increased by about three folds due to increased planting densities (Assefaet al.2016). The corresponding density for the high grain yield of maize ranged from 8.55×104to 10.95×104plants ha-1(Minget al.2017). Over 12 years,from 2005 to 2016,the planting density of spring maize in northern China increased by 1.5×104plants ha-1(Minget al.2017).

This study indicated that maize yield increased with increasing planting density and then declined in different rainfall areas or years,following quadratic functions supported by findings from similar climate conditions in the Shanxi Province (Zhanget al.2014). The maximum density for achieving the highest yield varied greatly with the rainfall amount,suggesting that a suitable density for dryland maize production should be determined by rainfall. The density parameter of water-suitable planting for maize (1 mm of rainfall can plant 174 maize ha-1) was obtained in this study to develop and evaluate optimum water management strategies and to ensure the most efficient use of water resources. Maize production in dryland should follow the principle of determining density by precipitation and grain yield by density. This analysis presents certain guidelines for the sustainable increase in yield and WUE in dryland maize production.

5.Conclusion

This study found that the climate in eastern Gansu exhibited warming and drying characteristics with increasing temperature and decreasing precipitation,which shortened the growth periods of winter wheat and spring maize by 5-6 days. Moreover,such climatic changes decreased the ET and water satisfaction rate of winter wheat and the water satisfaction rate of spring maize but kept spring maize’s ET constant. This climate change trend is conducive to dryland maize yield increase and acreage expansion. However,the warm and dry climate intensified soil water loss from evaporation and the water stress of winter wheat. The film fully-mulched ridge-furrow (FMRF) water harvesting is emerging as a major technology for dryland maize to cope with warm and dry climates and has been widely adopted in northern China. The FMRF system increased the effective use of light rainfall and doubled the soil water availability. Increasing plant density according to rainfall is an important measure to improve yield and water use efficiency in dryland maize production. Given that 1 mm of rainfall can sustain 174 maize plants ha-1,under the current conditions,an increase in the density by 1.5×104plants ha-1will increase the yield by 10-20%.The management strategy of dryland maize for watersuitable planting is an environmentally friendly and yieldincreasing technology in response to climate change.

Acknowledgements

We gratefully acknowledge the funding support from the National Key Research and Development Program of China (2012BAD0903 and 2018YFD0100200) and the China Agriculture Research System (CARS-02-77).

Declaration of competing interest

The authors declare that they have no conflict of interest.