Sustainable Analysis of Maize Production under Previous Wheat Straw Returning in Arid Irrigated Areas

: Conservation tillage is widely recognized as an important way to improve soil quality, ensure food security and mitigate climate change. However, relatively little attention has been paid to the subject in terms of sustainable evaluation of environmental and economic beneﬁts of the combination of no tillage and straw returning for maize production in arid irrigated areas. In this study, grain yield (GY) and water use efﬁciency based on grain yield (WUE GY ), soil carbon emission characteristics and economic beneﬁts were investigated


Introduction
Water resources are the main limiting factor for agricultural production [1]. Declining water resources available for agriculture in the context of global climate change poses a challenge to ensuring food security [2]. In particular, soil moisture during the cropgrowing season is a key factor in ensuring the formation of crop yield and quality [3].
Thus, improving crop water use efficiency will help to ensure the sustainability of crop production. At the same time, the high consumption of fossil fuels contributes to global warming, but more fossil fuel inputs are often required in high-yield cropping systems [4,5]. As a result, the agricultural production sector is one of the major sources of greenhouse gas emissions, with agriculture and land use change accounting for about a quarter of total global greenhouse gas emissions [6]. In addition, a large amount of purchased resources are invested in conventional agricultural management [4], which in turn increases the cost of agricultural production and greenhouse gas emissions, making it possible to obtain poorer net income and ecological benefits [7]. Therefore, there is an urgent need to develop cropmanagement technologies that maintain and increase crop yield while reducing greenhouse gas emissions and improving crop water use efficiency to increase agricultural benefit.
Globally, conservation tillage practices such as reduced tillage or no tillage and straw returning play a key role in maintaining high soil moisture, increasing crop water use efficiency, and improving the ecological environment of farmland [8][9][10]. Straw returning has been regarded as an important agricultural water conservation technology for soil water storage and moisture retention, reducing ineffective evaporation and surface runoff [11,12]. At the same time, crop straws are a valuable renewable organic resource, which can effectively improve soil fertility and crop yield, thus obtaining higher crop productivity [13,14]. Published research showed long-term no tillage could improve crop water use efficiency by reducing deep soil root distribution and water uptake [9]. In addition, the combination of reduced tillage or no tillage and straw returning could further increase crop yield and water production benefit, and improve soil quality [11,15]. It has been shown that straw returning could increase water use efficiency by 5.5-36.4% over straw removal, with the combination of no tillage and straw returning having higher water use efficiency and economic benefit [11]. At the same time, conservation tillage has been widely adopted as an effective crop-management measure to reduce CO 2 emissions and ensure food security, with no tillage and straw covering combining the most prominent advantages [16,17]. Some studies found that no tillage plus straw mulching could increase crop yield and net income, reduce greenhouse gas emissions and carbon footprint, and ease the negative environmental impacts of crop production [18]. In addition, no tillage and maize residue covering integrated with suitable water and nitrogen supply could improve soil water storage capacity, reduce carbon emissions and increase soil carbon sequestration potential in arid irrigated areas [10]. Therefore, understanding how the combination of straw returning and reduced tillage or no tillage affects crop productivity and ecological environment of fields is important to improve crop yield and water use efficiency, reduce greenhouse gas emissions and increase economic benefit.
Northwestern China is a typical irrigated arid agricultural region [19], where wheat and maize are the main food crops [19]. Meanwhile, conservation tillage practices such as reduced tillage or no tillage and straw returning have positive effects on food security, improving soil fertility and water production benefit and reducing the environmental burden in this region [20,21]. However, the long-term continuous cropping pattern of maize in this region has seriously hampered maize production, resulting in massive damage to maize yield and soil quality [22]. Crop rotation is an agronomic practice that combines land use and land conservation, which can break the long-term continuous crop barriers, balance soil nutrients, improve the ecological environment of fields and ultimately achieve the purpose of increasing crop yield and net income [23,24]. At present, maize area accounts for more than 30% of the total food crop area in the region, and conservation tillage practices such as reduced tillage or no tillage and straw returning integrated in a wheat-maize rotation system are widely used [22]. It has been shown that long-term no tillage plus straw-management approaches could significantly reduce soil respiration rate and cumulative CO 2 emissions by increasing soil bulk density and reducing effective gas diffusivity in a wheat-maize rotation system, thereby reducing the net carbon flux of the wheat-maize rotation system, while improving the sustainability and carbon productivity of the wheat-maize rotation system [25]. In addition, several years of research showed that the combination of conservation tillage practices and straw returning was conducive to sustained improvement of maize yields and water use efficiency in arid irrigated areas, and it significantly increased maize yield in drought years [20]. Therefore, reduced tillage or no tillage combined with straw returning is a more suitable crop-management practice for long-term sustainable development of wheat-maize rotation systems in arid irrigated areas. However, few of these studies have focused on evaluating the environmental and economic benefits of maize production systems under wheat-maize rotation systems, especially in arid irrigated areas, with respect to sustainability [10]. Therefore, an integrated study of water production benefits, environmental costs, agricultural production costs and net income of maize production in wheat-maize rotation systems based on the integration of no tillage and straw returning is beneficial to providing a sufficient economic and environmental basis for the promotion of sustainable agricultural production technologies.
The objective of this study was to investigate the economic and environmental sustainability of maize production in arid irrigated areas. We hypothesized that previous wheat straw mulching with no tillage under a wheat-maize rotation system in arid irrigated areas could be a sustainable maize-management practice without increasing the environmental risks.

Test Area Description
The field experiment was conducted at Wuwei City, Gansu province, in northwest China (29 • 51 41 N, 105 • 59 53 E) from 2009 to 2012. The average annual temperature of the area is 7.2 • C, the accumulated temperature of ≥10 • C is 2985.4 • C and the total number of sunshine hours is 2945 h. It is suitable for wheat and maize growth. Precipitation in the region occurs mainly from July to September, and the average annual precipitation is about 150 mm, but the potential evaporation is over 2000 mm per year. The region is representative of arid irrigated agriculture. The soil at the Research Station is classified as a type of desert land filled with calcareous particles. At planting time in 2010, the soil contained 11.2 g kg −1 , 1.78 mg kg −1 , and 12.5 mg kg −1 of organic C, NH 4 + -N, and NO 3 -N, respectively, in the 0 to 30 cm soil layer; in 2012, these values were 12.0 g kg −1 , 1.88 mg kg −1 , and 12.8 mg kg −1 , respectively. The climatic conditions for the maizegrowing season (April-September) in 2010 and 2012 are shown in Figure 1. In a typical wheat-maize rotation system in northwest China, a consecutive four-year field experiment was conducted using various wheat straw returning-management practices. The main research components were as follows: (1) elucidating the effects of wheat straw returning on maize yield, water consumption characteristics of crops during the growing season, and water use efficiency; (2) understanding the response of carbon emission characteristics and economic efficiency of maize field with respect to different wheat straw returningmanagement practices; and (3) exploring wheat straw returning-management practices for sustainable production of maize in rotation of arid irrigated areas.

Experimental Design
In 2009, different wheat straw returning approaches were established. This study used data related to maize farmland in 2010 and 2012. There were four treatments in the experiment: (1) no tillage with 25 to 30 cm tall wheat straw mulching (NTSMP), (2) no tillage with 25 to 30 cm tall wheat straw standing (NTSSP), (3) conventional tillage with 25 to 30 cm tall wheat straw incorporation (CTSP), and (4) conventional tillage without wheat straw returning (CTP) (Figure 2). Each treatment was arranged according to a com-

Experimental Design
In 2009, different wheat straw returning approaches were established. This study used data related to maize farmland in 2010 and 2012. There were four treatments in the experiment: (1) no tillage with 25 to 30 cm tall wheat straw mulching (NTSMP), (2) no tillage with 25 to 30 cm tall wheat straw standing (NTSSP), (3) conventional tillage with 25 to 30 cm tall wheat straw incorporation (CTSP), and (4) conventional tillage without wheat straw returning (CTP) (Figure 2). Each treatment was arranged according to a completely randomized design and replicated three times. Maize (cultivar Wu-ke 2, a popular hybrid) was sown on 22 April 2010 and 20 April 2012, and harvested on 28 September 2010 and 2 October 2012, respectively. The planting density of maize was 82,500 plants ha −1 and the plot was 4.8 m 2 (10 by 4.8 m) with a 0.5 m wide by 0.3 m high ridge between two neighboring plots to eliminate the potential movement of irrigation water.

Experimental Design
In 2009, different wheat straw returning approaches were established. This study used data related to maize farmland in 2010 and 2012. There were four treatments in the experiment: (1) no tillage with 25 to 30 cm tall wheat straw mulching (NTSMP), (2) no tillage with 25 to 30 cm tall wheat straw standing (NTSSP), (3) conventional tillage with 25 to 30 cm tall wheat straw incorporation (CTSP), and (4) conventional tillage without wheat straw returning (CTP) (Figure 2). Each treatment was arranged according to a completely randomized design and replicated three times. Maize (cultivar Wu-ke 2, a popular hybrid) was sown on 22 April 2010 and 20 April 2012, and harvested on 28 September 2010 and 2 October 2012, respectively. The planting density of maize was 82,500 plants ha −1 and the plot was 4.8 m 2 (10 by 4.8 m) with a 0.5 m wide by 0.3 m high ridge between two neighboring plots to eliminate the potential movement of irrigation water. In 2009 and 2011, according to the experimental treatment, the corresponding straw returning approaches were adopted after wheat harvest. Maize was planted in 2010 and 2012, thus forming the wheat-maize rotation system, and the straw was removed from In 2009 and 2011, according to the experimental treatment, the corresponding straw returning approaches were adopted after wheat harvest. Maize was planted in 2010 and 2012, thus forming the wheat-maize rotation system, and the straw was removed from the maize field after the maize harvest. The CTSP and CTP treatments were tilled after wheat harvest at a depth of 30 cm; the following year, the base fertilizer was spread and harrowing and plastic mulching were carried out via machinery. The NTSMP and NTSSP treatments were no tillage practice after the tall wheat straw was harvested; the following year, the base fertilizer was spread, and rotary tillage, harrowing, and plastic mulching were carried out via machinery. Meanwhile, maize was sown in late April via a simple roller hole seeder. In addition, other field-management practices were the same as in the local high-yield maize field.
The irrigation and fertilization systems were the same as that of the local high-yield fields. The irrigation system was 120 mm of irrigation in late fall just before soil freezing, and 90, 75, 90, 75, and 75 mm of supplemental irrigation at the jointing, pre-heading, silking, flowering, and filling stages of maize. All treatments received 450 kg N ha −1 and 225 kg P 2 O 5 ha −1 . Meanwhile, phosphorus and nitrogen fertilizers were applied as diammonium phosphate and urea. All of the P was applied as base fertilizer. The N was applied three times: 30, 60, and 10% of the total top-dressing before sowing and at the jointing and grain-filling stages of maize, respectively.

Data Collection
Evapotranspiration (ET): The approximate evapotranspiration was calculated using the following field water balance equation [10,26].
where P and I are precipitation and irrigation, respectively, in each maize-growing stage (mm); ∆S is the difference value of soil water storage (mm) between the pre-growing and post-growing stages of maize; and i represents the various maize-growing stages. The upward and downward flows were measured previously at a nearby field, and these two items were found to be negligible in this semiarid area. Runoff was also negligible due to small rains, and irrigation was controlled via raised ridges between plots. Grain yield (GY): The grain yield was determined by using a small combine harvester at the physiological maturity stage of maize. A sampling square of 5 m was selected to investigate the ear number for maize in each plot. The grain yield per unit area was converted to the standard grain water content of 13%.
Water use efficiency based on grain yield (WUE GY ): where WUE GY (kg m −3 ), GY (kg ha −1 ), and ET (m 3 ha −1 ) are water use efficiency based on grain yield, grain yield, and evapotranspiration, respectively. Average soil CO 2 fluxes (AC f ): Soil CO 2 fluxes (C f , µmol CO 2 m −2 s −1 ) were measured using a CFX-2 system (Soil CO 2 Flux System, CFX-2, PP System, Hitchin, UK) connected with a proprietary respiration chamber. Before measuring, all crop residue and other refuse on the soil surface were removed, and a hole with a diameter the same as the respiration chamber size was made on the maize field to release the stored CO 2 efflux, at least 12 h before the measurement. The chamber, with a sharp edging point at the bottom, was placed on the soil surface and then pushed to a depth of 20 mm. Measurements were taken at three places randomly selected in each plot, five values were recorded for each place within 180 s, and the average value was used for each plot. The diurnal soil respiration was measured at 2 h intervals from 8:00 a.m. to 8:00 p.m. on the selected dates, the seasonal measurements started on 21 April 2011 and 22 April 2012, and the rest of the measurements were taken at 20-day intervals from April to September in each year. Average soil CO 2 fluxes (µmol CO 2 m −2 s −1 ) could be obtained by calculating the average of C f across the maize-growing season.
Soil carbon emission (CE): Soil carbon emission (CE) for the entire maize-growing season was based on C f . CE was calculated with the following equation [5,10].
where CE (kg C ha −1 ) is soil carbon emission, C f (µmol CO 2 m −2 s −1 ) is soil CO 2 fluxes, i and j are the current and last monitoring dates, respectively, t is days after maize emergence, 0.1584 is the conversion factor between mol CO 2 m −2 s −1 and g CO 2 m −2 h −1 , and 0.2727 is the conversion factor between g CO 2 m −2 h −1 and g C m −2 h −1 . Soil carbon emission efficiency (CEE): Soil carbon emission efficiency (CEE, kg kg −1 ) indicates how many kg of grain yield are produced for every 1 kg of carbon emission from soil [10]. The calculation of carbon emission efficiency (CEE, kg kg −1 ) quantifies the association between carbon emissions and grain yield, and it was described as follows.
where GY (kg ha −1 ) and CE (kg C ha −1 ) are grain yield and soil carbon emission, respectively. Water use efficiency based on soil carbon emission (WUE CE ): Water use efficiency based on soil carbon emission (kg C m −3 ) was determined using the following equation [26].
where CE (kg C ha −1 ) and ET (m 3 ha −1 ) are soil carbon emission and evapotranspiration, respectively. Economic benefit: In the two experimental years of this study, the inputs of agricultural materials such as human labor and farm machinery inputs (harrowing, mulching, fertilization, weeding, pest control, and harvesting), fertilizers, pesticides, seeds, mulch, drip irrigation tape, and irrigation volume were recorded in detail for the four wheat straw returning approaches. By combining the grain and straw yield of each plot, output, input, net income (NI), and input-output ratio were calculated for various wheat straw returning approaches. The price of grain, straw, agricultural materials, and labor costs were calculated according to the market price of the year. The benefit per cubic meter of water (BPW) was based on NI and ET. BPW was calculated with the following equation [27].
where BPW (¥ m −3 ), NI (¥ ha −1 ), and ET (m 3 ha −1 ) are benefit per cubic meter of water, net income, and evapotranspiration. Sustainable evaluation: the sustainability evaluation index (SEI) was based on GY, WUE GY , CE, CEE, WUE CE , NR, and BPW of various straw returning approaches. SEI was used to evaluate the straw returning approach with higher yield and efficiency, and a clean and friendly environment. A higher index indicates that the straw returning approach is more environmentally friendly and sustainable. To ensure that the evaluation component could be compared quantitatively, the variables were not dimensionalized. Three equations were used to determine the SEI, which was calculated as follows. Three equations were used to determine the SEI calculation equation. SEI was calculated with the following equation [4,5].
where ax ij is a standardized value (0 < ax ij ≤ 1) at i × j; ax ij is the corresponding actual value for the treatment i and variable j; x max and x min are the maximum and minimum value for each variable.
where bx ij is the coefficient of variation for each variable, the average of ax ij repetitions, and m is the maximum number for i or j.
where SEI is the sustainability evaluation index of various wheat straw returning approaches, and the higher the value of this index, the better the sustainability of the approach.

Statistical Analysis
All data on various parameters were analyzed via analysis of variance (ANOVA) for treatments using SPSS software 24.0 (IBM, Chicago, IL, USA). The mean values of various Wheat straw returning significantly increased grain yield (GY) of maize, but the difference between the test year, straw returning approaches, and their interaction were not significant ( Figure 3). In 2010 and 2012, compared to conventional tillage without wheat straw returning (CTP), wheat straw returning (NTSMP, NTSSP, and CTSP) treatments increased GY by 13.7-17.5%, 12.0-13.9%, and 4.4-11.9%, respectively. Meanwhile, no tillage with wheat straw mulching (NTSMP) was the most productive treatment and increased GY by 5.6-9.0% compared to conventional tillage with wheat straw returning (CTSP). Therefore, no tillage with 25 to 30 cm tall previous wheat straw mulching can effectively increase the grain yield of maize on the following occasion. treatments using SPSS software 24.0 (IBM, Chicago, IL, USA). The mean values of various treatments were tested for statistical significance at a 5% (p < 0.05) level of probability using Duncan's multiple range test [27].

Grain Yield of Maize
Wheat straw returning significantly increased grain yield (GY) of maize, but the difference between the test year, straw returning approaches, and their interaction were not significant ( Figure 3). In 2010 and 2012, compared to conventional tillage without wheat straw returning (CTP), wheat straw returning (NTSMP, NTSSP, and CTSP) treatments increased GY by 13.7-17.5%, 12.0-13.9%, and 4.4-11.9%, respectively. Meanwhile, no tillage with wheat straw mulching (NTSMP) was the most productive treatment and increased GY by 5.6-9.0% compared to conventional tillage with wheat straw returning (CTSP). Therefore, no tillage with 25 to 30 cm tall previous wheat straw mulching can effectively increase the grain yield of maize on the following occasion.

Evapotranspiration of Maize at Each Growth Stage
In 2012, NTSMP treatment had lower total evapotranspiration (ET) of maize during the whole growing period, reduced by 2.0%, 2.0%, and 2.5% compared to NTSSP, CTSP, and CTP treatments, respectively, but there was no significant difference between NTSSP, CTSP, and CTP treatments. While in 2010, NTSMP and NTSSP increased total ET by 3.2% and 3.7% over the CTSP treatments, but there was no significant difference between NTSMP and CTP treatments (Table 1). At the sowing to jointing stage of maize, ET accounted for 29.3-37.0% of the total ET, and the difference was significant between test years. In 2010, no tillage wheat with straw returning significantly increased maize ET, and NTSMP and NTSSP increased ET by 14.4% and 9.1% over the CTP treatment, and by 11.0% and 5.6% over the CTSP treatment, respectively, but there was no significant difference between the NTSMP and NTSSP treatments. In 2012, wheat straw returning significantly had lower ET of maize, NTSMP, NTSSP, and CTSP decreased by 9.9%, 5.1%, and 9.9% over the CTP treatment, and NTSMP decreased by 5.2% over the NTSSP treatment, respectively, while there was no significant difference between NTSMP and CTSP treatments. This indicates that no tillage with wheat straw returning facilitated the reduction of evapotranspiration from the sowing to jointing stage in maize as the test year was extended.
At jointing to the large bell mouth stage of maize, the NTSMP, NTSSP, and CTSP treatments had lower ET, reduced by 18.4-31.5%, 20.5-24.9%, and 15.5-18.0%, compared to CTP treatment, respectively, with the smallest ET in the NTSMP treatment. Similarly, at the large bell mouth to silking stage of maize, NTSMP had lower ET by 7.2-8.8%, 11.8-12.0%, and 11.4-14.2% than that of NTSSP, CTSP, and CTP treatments, respectively. In addition, at the silking to full-ripe stage of maize, NTSMP and NTSSP increased by 15.9-18.2% and 13.8-14.0% over the CTP treatment, respectively, with the NTSMP treatment having higher ET, increased by 4.0-14.5% over the CTSP treatment, but there was no significant difference between NTSMP and NTSSP treatments.
Overall, no tillage with wheat straw returning reduced evapotranspiration before the silking stage, increased evapotranspiration after the silking stage, and met the water demand for the formation of higher grain yield in the later stage of maize, with the NTSMP treatment having the most prominent effect.

Water Use Efficiency Based on Grain Yield of Maize
Wheat straw returning had the effect of significantly improving water use efficiency based on grain yield (WUE GY ) throughout the growing season of maize; the NTSMP treatment in particular was outstanding (Figure 4). In both trial years, NTSMP, NTSSP, and CTSP increased WUE GY by 15.4-16.7%, 11.3-12.6%, and 4.9-12.8% over the CTP treatment, respectively, and NTSMP increased by 2.3-11.2% over the CTSP treatment. In addition, in 2010, NTSMP increased WUE GY by 3.7% over the NTSSP treatment; in 2012, NTSMP and NTSSP were not significant with respect to each other, but NTSMP treatment had the highest WUE GY in both trial years. ment in particular was outstanding (Figure 4). In both trial years, NTSMP, NTSSP, and CTSP increased WUEGY by 15.4-16.7%, 11.3-12.6%, and 4.9-12.8% over the CTP treatment, respectively, and NTSMP increased by 2.3-11.2% over the CTSP treatment. In addition, in 2010, NTSMP increased WUEGY by 3.7% over the NTSSP treatment; in 2012, NTSMP and NTSSP were not significant with respect to each other, but NTSMP treatment had the highest WUEGY in both trial years.   Different wheat straw returning approaches had significant effects on soil carbon emissions during the maize-growing season ( Figure 5). Compared to 2010, the average soil CO 2 emission fluxes (AC f ) and carbon emission (CE) of wheat straw returning treatments were lower in 2012, reduced by 3.1% and 4.8%, especially the NTSMP treatment of 2012 decreased by 4.9% and 5.7% over the NTSMP treatment of 2010, respectively. Overall, NTSMP reduced AC f by 10.7-17.7%, 14.3-16.8%, and 17.7-18.9%, and CE decreased by 5.4-10.0%, 6.1-8.6%, and 11.1-11.2%, respectively. These values indicated that with the extension of the test year, the effect of wheat straw returning in reducing carbon emissions from maize field has gradually emerged, and the NTSMP treatment had the advantage of carbon reduction compared with other wheat straw returning approaches.

Soil Carbon Emission Efficiency and Water Use Efficiency Based on Soil Carbon Emission
The test years and wheat straw returning approaches had a significant effect on carbon emission efficiency (CEE) and water use efficiency based on carbon emissions (WUE CE ) of maize field ( Figure 6). Compared to 2010, CEE decreased by 3.5% and WUE CE increased by 12.9% in 2012. Wheat straw returning had a significant effect on CEE; NTSMP, NTSSP, and CTSP increased by 29.9-32.4%, 13.3-21.3%, and 10.2-14.7% over the CTP treatment, respectively, while NTSMP and NTSSP increased 15.4-16.1% and 2.9-5.8% over the CTSP treatment, respectively, with the NTSMP treatment having higher CEE by 9.1-12.8% over the NTSSP treatment. Meanwhile, NTSMP had lower WUE CE , reduced by 4.9-8.2%, 4.2-11.4%, and 8.8-12.8% over the NTSSP, CTSP, and CTP treatments, respectively.

Soil Carbon Emission Efficiency and Water Use Efficiency Based on Soil Carbon Emission
The test years and wheat straw returning approaches had a significant effect on carbon emission efficiency (CEE) and water use efficiency based on carbon emissions (WUECE) of maize field ( Figure 6). Compared to 2010, CEE decreased by 3.5% and WUECE increased by 12.9% in 2012. Wheat straw returning had a significant effect on CEE; NTSMP, NTSSP, and CTSP increased by 29.9-32.4%, 13.3-21.3%, and 10.2-14.7% over the CTP treatment, respectively, while NTSMP and NTSSP increased 15.4-16.1% and 2.9-5.8% over the CTSP treatment, respectively, with the NTSMP treatment having higher CEE by 9.1-12.8% over the NTSSP treatment. Meanwhile, NTSMP had lower WUECE, reduced by 4.9-8.2%, 4.2-11.4%, and 8.8-12.8% over the NTSSP, CTSP, and CTP treatments, respectively.

Input and Output Analysis
The test years and wheat straw returning approaches had a significant effect on grain output, straw output, and total output of maize production ( Table 2). Compared to 2010, grain output, straw output, and total output were significantly higher in 2012, with increases of 3.7%, 27.8%, and 7.5%, respectively. In both years, NTSMP and NTSSP increased grain output by 13.5-17.5% and 12.0-13.9% over the CTP treatment, respectively, with the

Input and Output Analysis
The test years and wheat straw returning approaches had a significant effect on grain output, straw output, and total output of maize production ( Table 2). Compared to 2010, grain output, straw output, and total output were significantly higher in 2012, with increases of 3.7%, 27.8%, and 7.5%, respectively. In both years, NTSMP and NTSSP increased grain output by 13.5-17.5% and 12.0-13.9% over the CTP treatment, respectively, with the NTSMP treatment standing out in output addition, increased by 5.8-5.9% over the CTSP treatment, but there was no significant difference between NTSMP and NTSSP treatments. In terms of straw output, compared to CTSP and CTP, the NTSM treatment increased by 7.7% and 7.0% in 2010, respectively, but there was no significant difference between NTSMP and NTSSP treatments, while NTSM decreased by 4.2%, 7.4%, and 18.1% over the NTSSP, CTSP, and CTP treatments in 2012, respectively. Meanwhile, NTSMP increased total output by 6.7-15.8% over the CTP treatment and by 5.8-5.9% over the CTSP treatment. In addition, compared to NTSMP in 2010, the NTSMP treatment increased grain output, straw output, and total output by 3.3%, 14.1%, and 4.7% in 2012, respectively. It can be seen that no tillage with 25 to 30 cm tall wheat straw covering is beneficial with respect to increasing grain and straw output, thus increasing total output of maize production. NTSMP and NTSSP had lower cost input of maize production, reduced by 5.6-5.7% and 4.9-5.0% over the CTP treatment, and reduced by 4.3-4.5% and 3.6-3.8% over the CTSP treatment, respectively ( Table 2). In particular, NTSMP and NTSSP reduced human labor and farm machinery by 10.4-10.7% and 9.3-9.5% over the CTP treatment and by 8.2-8.5% and 7.0-7.3% over the CTSP treatment, respectively. However, there was no significant difference in agricultural supplies and others between the treatments. Meanwhile, due to higher market prices in 2012, compared to 2010, total input increased by 6.1% in 2012, human labor and farm machinery and agricultural supplies increased by 8.0% and 5.9%, respectively, while the difference in other inputs was not significant. Therefore, at the same market prices, the NTSMP treatment can effectively reduce human labor and farm machinery input, and thus a certain degree of reduction in cost input of maize production.
The effect of wheat straw returning approaches on input-output ratio of maize was significant, while the test year did not have a significant effect on input-output ratio ( Table 2). In the two test years, no tillage with wheat straw returning had higher inputoutput ratio, with NTSMP and NTSSP increased by 13.1-22.8% and 11.7-19.0% over the CTP treatment, and by 10.6-10.9% and 7.4-9.3% over the CTSP treatment, respectively, among which the NTSMP treatment had obvious advantages.
Analysis of the above results shows that no tillage with 25 to 30 cm tall wheat straw mulching could increase total output and reduce total input by increasing grain and straw output and reducing human labor and farm machinery input, thus obtaining a higher inputoutput ratio, which was conducive to improving the economic benefits of maize production.

Net Income
The test years and wheat straw returning approaches had a significant effect on net income (NI) of maize production ( Table 2). Compared to 2010, NI increased by 7.8% in 2012. In both test years, no tillage with wheat straw returning had higher NI; NTSMP and NTSSP increased by 16.1-34.2% and 14.6-28.5% over the CTP treatment, and by 13.2-13.3% and 8.5-11.8% over the CTSP treatment, while the difference between NTSMP and NTSSP treatments was not significant, but the highest NI was observed with NTSMP treatment. Meanwhile, NTSMP improved NI by 4.2% in 2012 over the NTSMP treatment in 2010. Therefore, no tillage with 25 to 30 cm tall wheat straw mulching can gain higher net income, which helps farmers to increase their income, and the advantage become more and more prominent with the extension of the test year.

Benefit Per Cubic Meter of Water
No tillage with wheat straw returning had the effect of significantly increasing benefit per cubic meter of water (BPW) of maize production, with the most prominent advantage being with NTSMP treatment (Figure 7). In both trial years, NTSMP and NTSSP increased BPW by 19.1-31.8% and 15.2-25.2% over the CTP treatment, and by 9.8-15.6% and 4.6-11.8% over the CTSP treatment, respectively. In 2010, CTSP increased BPW by 20.0% over the CTP treatment, but the difference between CTSP and CTP treatment was not significant in 2012.

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income, which helps farmers to increase their income, and the advantage become more and more prominent with the extension of the test year.

Benefit Per Cubic Meter of Water
No tillage with wheat straw returning had the effect of significantly increasing benefit per cubic meter of water (BPW) of maize production, with the most prominent advantage being with NTSMP treatment (Figure 7). In both trial years, NTSMP and NTSSP increased BPW by 19.1-31.8% and 15.2-25.2% over the CTP treatment, and by 9.8-15.6% and 4.6-11.8% over the CTSP treatment, respectively. In 2010, CTSP increased BPW by 20.0% over the CTP treatment, but the difference between CTSP and CTP treatment was not significant in 2012.

Sustainable Evaluation of Maize Production in Arid Irrigated Areas
In this study, GY, WUEGY, CE, CEE, WUECE, NI, and BPW were used to assess the sustainability of various wheat straw returning approaches (Figure 8a). Among the four wheat straw returning approaches, the highest sustainable evaluation index (SEI) was found in the NTSMP treatment, where NTSMP, NTSSP, and CTSP increased by 21.1%,

Sustainable Evaluation of Maize Production in Arid Irrigated Areas
In this study, GY, WUE GY , CE, CEE, WUE CE , NI, and BPW were used to assess the sustainability of various wheat straw returning approaches (Figure 8a). Among the four wheat straw returning approaches, the highest sustainable evaluation index (SEI) was found in the NTSMP treatment, where NTSMP, NTSSP, and CTSP increased by 21.1%, 14.7%, and 9.1% over the CTP treatment, respectively; NTSMP and NTSSP increased by 11.0% and 5.2% over the CTSP treatment, respectively; NTSMP increased by 5.6% over the NTSSP treatment (Figure 8b). In particular, the NTSMP treatment had higher CEE and BPW, mainly because of higher GY and NR, and lower CE and ET (Figure 8c).

Crop Yield and Water Use Efficiency of Maize Affected by Various Wheat Straw Returning Approaches
Different wheat straw returning approaches had different effects on maize yield [17]. At present, under the conditions of straw returning, tillage practices commonly used in China's maize production are conventional tillage (plow tillage), no tillage, and reduced tillage (subsoil tillage and rotary tillage) [17]. In this study, we found that maize yield under no tillage conditions was significantly higher for the wheat straw returning treatments (NTSMP and NTSSP) than the CTP treatment, with the highest yield in the NTSMP treatment, which was similar to the results of previous studies (Figure 3). Previous studies showed that 33% straw mulching, 67% straw mulching, and 100% straw mulching significantly increased maize yield under no tillage over the conventional tillage [28]. However, another study found that crop yield of no tillage without straw returning was lower than plow tillage without straw returning, while no tillage with straw returning significantly increased crop yield compared to plow tillage without straw returning [29]. This suggests that no tillage can have a negative impact on crop yield to some extent under specific regions and climatic conditions [29]. Meanwhile, the combination of no tillage and straw returning could effectively increase soil organic carbon, effective potassium, effective ni-

Crop Yield and Water Use Efficiency of Maize Affected by Various Wheat Straw Returning Approaches
Different wheat straw returning approaches had different effects on maize yield [17]. At present, under the conditions of straw returning, tillage practices commonly used in China's maize production are conventional tillage (plow tillage), no tillage, and reduced tillage (subsoil tillage and rotary tillage) [17]. In this study, we found that maize yield under no tillage conditions was significantly higher for the wheat straw returning treatments (NTSMP and NTSSP) than the CTP treatment, with the highest yield in the NTSMP treatment, which was similar to the results of previous studies (Figure 3). Previous studies showed that 33% straw mulching, 67% straw mulching, and 100% straw mulching significantly increased maize yield under no tillage over the conventional tillage [28]. However, another study found that crop yield of no tillage without straw returning was lower than plow tillage without straw returning, while no tillage with straw returning significantly increased crop yield compared to plow tillage without straw returning [29]. This suggests that no tillage can have a negative impact on crop yield to some extent under specific regions and climatic conditions [29]. Meanwhile, the combination of no tillage and straw returning could effectively increase soil organic carbon, effective potassium, effective nitrogen, and water storage [30,31], thus improving soil quality and offsetting this negative impact and contributing to higher crop productivity. Meanwhile, it has been shown that no tillage, straw mulching, and crop rotation (three important techniques in conservation agriculture) used in combination could maintain the same or significantly increase crop yield with conventional tillage practices [32,33], which was consistent with the crop-management practices in this study, i.e., combination of no tillage and wheat straw mulching applied in a wheat-maize rotation system ( Figure 2). Therefore, the NTSMP treatment in a wheat-maize rotation system is an effective crop-management practice in arid irrigated areas that could improve maize yield. At the same time, this study found that the NTSMP treatment effectively regulated water consumption characteristics of maize during the growing season, which laid the foundation for grain yield formation in late maize reproduction, thus significantly improving water use efficiency based on grain yield (Table 1, Figure 4). It was also found that in semiarid areas, no tillage with straw mulching could retain and improve soil water and optimize crop water transfer compared to conventional tillage without straw returning, thus significantly improving crop yield and water use efficiency [34]. This was mainly because no tillage with straw mulching could effectively increase precipitation infiltration, reduce soil bulk density, and enhance soil water storage capacity, which was conducive to reducing field water consumption and increasing soil water content [35,36]. In addition, no tillage with straw mulching could also reduce ineffective evaporation of soil water, which contributed to the maintenance and regulation of field water and was conducive to improving crop water use efficiency [11]. Therefore, no tillage with wheat straw mulching can significantly improve maize yield and water use efficiency in a wheat-maize rotation system, thus ensuring food security in arid irrigated areas.

The Influence of Wheat Straw Returning Approaches on Soil Carbon Emission of Maize Field
Soil carbon sequestration is a clean and effective carbon emission mitigation strategy [37]. However, soil biodiversity had a strong influence on carbon sequestration and was influenced by factors such as temperature, rainfall, fossil fuels, and chemical fertilizers [38]. Rich substrates of renewable organic resources (crop straw) increased soil microbial and enzymatic activity related to carbon and nutrient cycling, and improved soil structure [39]. It has been shown that straw returning changed the community and function of soil bacteria and also increased the soil nutrient and soil organic carbon content in the subsoil [40], thus enhancing the carbon sequestration capacity of the crop field. However, numerous studies have shown that higher soil organic carbon content obtained through certain carbon sequestration measures might promote soil microbial respiration and lead to increased carbon emissions [25,41]. In terms of straw returning practices, previous studies have found that straw returning might also promote microbial decomposition of soil organic carbon, and accelerate soil carbon mineralization and acidification, thus significantly increasing carbon emissions from the field [42]. This process is mainly influenced by straw type, crop type, soil texture, temperature straw returning duration, and soil microbial species [38]. Nevertheless, straw returning had an important role in increasing crop yield and maintaining soil productivity [43,44]. Thus, there is an urgent need to optimize straw return methods to achieve a win-win situation in terms of increasing crop yield and reducing carbon emissions for crop production. At the same time, in our study, straw returning seems to reduce carbon emission from a field, as seen from the slightly reduced total soil carbon emission of CTSP compared to CTP, though this variation was not statistically significant in 2010 ( Figure 5). Other studies have shown that no tillage practice could significantly reduce carbon emissions from the field by increasing soil bulk density and reducing the effective oxygen diffusion coefficient due to reducing soil disturbance compared with conventional tillage, thus facilitating adaptation and mitigation of global climate change in the agricultural production sector [25,42]. In addition, a combination of conservation tillage practices such as straw returning and reduced tillage or no tillage in agricultural production could ensure food security, improve yield stability, and contribute to atmospheric CO 2 reduction through soil organic carbon sequestration [45]. It has been found that in arid irrigated areas, no tillage with straw mulching significantly reduced soil carbon emissions compared with conventional tillage, further improving carbon emission efficiency and having smaller carbon emissions per unit of water [26], which was similar to the results of this study. In this study, we found that the combination of no tillage and wheat straw mulching in wheat-maize rotation system had smaller average soil CO 2 fluxes, significantly reduced carbon emission from maize field, and lower water use efficiency based on carbon emission, which in turn improved carbon emission efficiency of maize fields in arid irrigated areas (Figures 5 and 6). Therefore, the combination of no tillage with straw mulching has the potential to mitigate climate change by reducing soil carbon emissions from crop production systems, providing an important theoretical basis for sustainable agriculture in arid irrigated areas.

The Sustainability of Maize Production by Wheat Straw Returning Approaches
In arid irrigated areas, the biggest challenge for maize production is to obtain maximum economic benefit with minimum risk input [46]. Previous studies have shown that no tillage with straw returning in maize production could achieve higher economic benefit and help increase farmers' income in the North China Plain [11]. Similarly, among the four treatments in this study, the NTSMP treatment had lower total input and higher total output, resulting in higher economic benefit ( Table 2). This result was largely attributed to the reduction of resource inputs such as fossil fuels, human labor, and farm machinery with no tillage practices [46], and the NTSMP treatment had the highest maize yield, which in turn resulted in higher grain and straw output. However, some studies found no significant effect of straw returning on economic benefit, which was contrary to the results of this study [47]. This possibility was due to the apparent differences in straw returning approaches and other crop-management practices as well as soil characteristics and climatic conditions [42,45]. Meanwhile, in this study, the NTSMP and NTSSP treatments had higher benefit per cubic meter of water ( Figure 7). This indicated that no tillage with wheat straw returning promoted transpiration of maize to a large extent and suppressed ineffective soil evaporation, thus increasing benefit per cubic meter of water [20]. Therefore, no tillage with wheat straw mulching can gain higher economic benefit for maize production under a wheat-maize rotation system with equal input of resources in arid irrigated areas.
From the sustainability analysis of the four wheat straw returning approaches, the NTSMP treatment had the highest sustainability evaluation index (Figure 8). This was mainly because no tillage with wheat straw mulching increased maize yield, and reduced water consumption and soil carbon emissions from maize field, and obtained higher net income, thus gaining higher carbon efficiency and benefit per cubic meter of water. Therefore, no tillage with wheat straw mulching is an effective crop-management practice with respect to achieving both economic and environmental win-win situations for maize production in arid irrigated areas. However, taking into account the great spatial heterogeneity of climate, soil characteristics, cropping systems, and agricultural management practices, it is not certain that the results obtained are fully applicable to other dry irrigated agricultural regions of the world. Future research is needed to evaluate the overall effectiveness of no tillage with wheat straw mulching at larger regional and even national scales to improve its application in sustainable maize production globally.

Conclusions
This study explored the effects of different wheat straw returning approaches on grain yield and water use efficiency based on grain yield, soil carbon emission characteristics, and economic benefits in maize production and sustainability analysis under a wheatmaize rotation system in arid irrigated areas. The NTSMP treatment could effectively regulate water consumption characteristics of a maize field during the entire growing season and meet the water demand of maize in the late growing season, thus significantly improving maize grain yield and water use efficiency based on grain yield. Meanwhile, the NTSMP treatment reduced average soil CO 2 fluxes and soil carbon emissions for the maize-growing season compared to the other three treatments, thus reducing water use efficiency based on soil carbon emissions. In addition, the NTSMP and NTSSP treatments significantly increased total output of maize fields and reduced total input compared to CTP treatment, which gained higher net income and input-output ratio. Among them, the NTSMP treatment had higher soil carbon emission efficiency and benefit per cubic meter of water, and the sustainability evaluation index was higher than the other treatments. In conclusion, no tillage with 25 to 30 cm tall wheat straw mulching is an effective and feasible way to achieve higher yields and improved economic benefits for maize and to reduce soil carbon emissions in arid irrigated areas.

Data Availability Statement:
The entire set of raw data presented in this study is available upon request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.