Year-round plastic film mulch to increase wheat yield and economic returns while reducing environmental risk in dryland of the Loess Plateau

Field Crops Research 225 (2018) 1–8

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Year-round plastic film mulch to increase wheat yield and economic returns while reducing environmental risk in dryland of the Loess Plateau

T

⁎

Gang Hea,b, Zhaohui Wanga,b, , Hanbing Caoa,b, Jian Daia,b, Qiang Lia,b, Cheng Xuea,b a

State Key Laboratory of Crop Stress Biology in Arid Areas, Northwest A&F University, Yangling, 712100, Shaanxi, China Key Laboratory of Plant Nutrition and Agri-environment in Northwest China, Ministry of Agriculture/College of Natural Resources and Environment, Northwest A&F University, Yangling, 712100, Shaanxi, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Life cycle assessment Greenhouse gas Nitrate-N leaching Soil water Soil temperature

In the coming decades, humanity will face great challenges in ensuring food and environmental security while reducing poverty through increasing economic profits. Plastic film mulch is an effective management practice for ensuring food security by improving crop productivity per unit area. However, its effects on environmental and economic benefits have not been well evaluated. Here, a location-fixed field experiment was performed to determine the effect of year-round plastic film mulch (YPM, mulching soil surface with plastic film during the growing season and fallow season of winter wheat) on wheat yield, environment (nitrate-N leaching and greenhouse gas (GHG) emissions), and economic returns. Compared with farmer practice (the local traditional practice), adoption of YPM increased the mean yield by 11%, which is attributed to the fact that YPM increased soil water storage at wheat sowing and soil temperature during wheat growing season by a mean of 7% and 0.6 °C, respectively. The increased grains induced a 12% increase in net economic returns. The YPM decreased the soil nitrate-N leaching by 51%, which was explained by decreasing soil nitrate-N residue caused by the increased yield. The YPM also reduced GHG emissions intensity by an average of 12%. As a result, YPM was a better choice for increasing yield and economic returns while reducing environmental risk. In the future, we should develop better mulching systems to further improve food, environmental benefits, and economic returns in dryland farming production.

1. Introduction

increased by 29% and 5% in western China and in Pakistan, respectively (Rehman et al., 2009; Xie et al., 2005). However, the plastic film mulch practices as these studies were usually only applied during growing season of wheat. Just because of failed to manage fallow season, this may not fully tap the potential of plastic film mulch to increase yield. In order to further improve yield, a year-round plastic film mulch (YPM), covering topsoil using clear plastic film through whole year (including growing season and summer fallow of winter wheat), was designed and conducted in this study. For a long time, increasing yield has received greater attentions due to high demand for food, particularly for dryland regions with low yield (Ren et al., 2016). A series of environmental issues with the increased yield have been overlooked. Soil nitrate-N leaching and greenhouse gas (GHG) emissions are the typical environmental damage in wheat production systems (Cuello et al., 2015; Ju et al., 2009). In northern China, a survey of nitrate-N concentrations in groundwater confirmed that almost half of groundwater samples exceeded the WHO drinking water standards (Ju et al., 2006). Nitrate-N leaching should be responsible for this. In the Loess Plateau, the soil surface of farmland was usually bare

Dryland accounts for about 41% of the planet’s land area, and dryland agriculture supply more than 38% of the world’s food production (Reynolds et al., 2007; Stewart et al., 2006). The Loess Plateau, located in northwestern China, is a typical dryland agriculture region. Here, due to the lack of surface water and groundwater, insufficient annual precipitation (200–600 mm) is the only source of water (Zhang et al., 2011), the absence of soil water is therefore the most fundamental factor limiting crop productivity. In this region, winter wheat (Triticum aestivum L.)-summer fallow is the most common cropping system. More than 50% of precipitation occurs during summer fallow (Deng et al., 2006), the mismatch between the most important rainy season and growing season of winter wheat aggravates the water stress of wheat growth. Plastic film mulch is an effective practice for improving soil water content, alleviating water stress, and increasing wheat yield, thus it has been widely applied in dryland wheat production (Li et al., 2004; Sharma et al., 2011). Due to adoption of plastic film mulch, wheat yield ⁎

Corresponding author at: State Key Laboratory of Crop Stress Biology in Arid Areas, Northwest A&F University, Yangling, 712100, Shaanxi, China. E-mail address: [email protected] (Z. Wang).

https://doi.org/10.1016/j.fcr.2018.05.019 Received 15 February 2018; Received in revised form 27 May 2018; Accepted 27 May 2018 0378-4290/ © 2018 Elsevier B.V. All rights reserved.

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and available potassium (K) of 130 mg kg−1.

during the summer fallow of winter wheat, and a heavy rainfall over a short time usually induced nitrate-N leaching (Yang et al., 2015). In this study, we hypothesis that covering soil surface during summer fallow using plastic film has the capacity to decrease nitrate-N leaching and attempts to verify it by a location-fixed field experiment. In dryland, adoption of plastic film mulch to alleviate the limitation of soil water for crop growth is a helpless choice, since it requires extra inputs, including material of plastic film and machinery. The production and application of plastic film and the diesel consumption by machinery would induce GHG emissions (Wang et al., 2017). Although plastic film mulch has the ability to increase yield, it is still unclear whether it can reduce GHG emissions intensity (GHG emissions per unit yield). Apart from food and environment benefits, economic returns play a predominant role in evaluating the feasibility of new technologies. Farmers often carefully consider inputs and outputs before using a new technology, because they have little capacity of undertaking the economic risk (Zhang et al., 2015a). In pursuit of economic returns, new technologies are mostly applied for high value-added agricultural products, such as strawberries (Steinmetz et al., 2016), whereas it is usually omitted for low value-added products like wheat, particularly for YPM system with costs increase. However, it is still unknown whether YPM application can increase the net economic returns in dryland wheat production systems. Overall, the yield, environmental impacts, and economic returns associated with YPM system in different precipitation levels is unclear. The objectives of this study were to: (1) detect YPM effect on wheat yield depending on soil water and soil temperature, (2) quantify environmental risk (soil nitrate-N leaching and GHG emissions intensity) and economic returns associated with YPM application, and (3) estimate the feasibility of adopting YPM in dryland wheat production in terms of yield, environmental risk, and economic returns.

2.2. Experiment establishment The field experiment designed and tested two treatments: farmer practice (the local traditional practice) and year-round plastic film mulch (YPM). For farmer practice, winter wheat was planted using traditional flat way without covering soil surface during wheat growing season; after the harvest of wheat, removing wheat straw from the field and ploughing the soil to a depth of 40 cm were performed. For YPM, alternating ridges and furrows were shaped on soil surface before wheat sowing, and the ridges were covered using a clear plastic film and the furrows were bare for seeding. The wide of ridge and furrow was 40 cm and 20 cm, respectively. The innovations of YPM reflected the management of soil surface during summer fallow. After the harvest of wheat, the plastic films were still remained on the ridges, and the shredded straws were returned to the furrows to cover topsoil. Until the end of summer fallow (i.e., before the beginning of subsequent wheat growing season), removing the plastic film from the ridges, ploughing the soil to a depth of 40 cm, and incorporating straw segments into soil were performed. This meant that soil surface in YPM has been covered throughout whole year. The N fertilizer rates were 162 kg N ha–1 in all years for farmer practice, and 138 and 150 kg N ha–1 in 2008–2010 and 2010–2015 for YPM, respectively. In 2008–2012, three quarters of N fertilizer was used before wheat sowing, and one quarters of N fertilizer was used at the frozen soil melted (February 20 in 2009, March 8 in 2010, March 5 in 2011, and March 18 in 2012). In 2012–2015, all N fertilizers were used before wheat sowing. All N inputs of YPM and farmer practice in 2008–2015 were shown in Table S1. In all years for farmer practice and YPM, the P fertilizer rates were 105 kg P2O5 ha–1, and applied before wheat sowing. Because soil available K supply is sufficient, no K fertilizer was applied in this study. In all years, the plots (22 m × 6 m) were arranged in a randomized block design with four replicates. The seeding rate of winter wheat was 150 kg ha–1. Herbicides, fungicides, and insecticides were applied to control weeds, diseases, and pests each year.

2. Materials and methods 2.1. Site description A location-fixed field experiment was established in 2008 at Changwu (35.20 °N, 107.75 °E, and 1200 m above sea level), located in the central Loess Plateau. As groundwater (50–80 m depth) is not used for crop production, precipitation is the only source of water. The precipitation over the seven consecutive experimental years and their long-term average were shown in Table 1. The annual mean air temperature is 9.1 °C. The soil at this experimental site is a silt loam texture, with a pH of 8.18, organic carbon of 0.853%, total N of 0.077%, nitrateN of 13.1 mg kg−1, available Olsen-phosphorus (P) of 4.50 mg kg−1,

2.3. Sampling and measurement At the maturity period of winter wheat, the fresh grains for each plot were harvested using a combine harvester and weighed to obtain a fresh weight. ∼1 kg fresh grains per plot was collected to calculate its water content, and thus obtain dry weight. Soil samples from 0 to 300 cm profile were collected before sowing and after harvest of winter wheat each year to determine soil water

Table 1 Monthly precipitation (mm) at the experimental site in seven experimental years (2008–2015). Month

2008-2009

2009-2010

2010-2011

2011-2012

2012-2013

2013-2014

2014-2015

Long-term average

July August September October November December January February March April May June Annual precipitation Rainfall in fallow season Precipitation in growing season

185a 57 108 19 11 0 0 21 21 11 64 15 513 270 243

135 97 47 19 9 0 0 34 19 45 39 31 475 280 195

207 211 77 27 0 0 0 14 18 8 70 34 666 458 208

127 111 227 52 54 4 0 13 17 38 50 29 722 453 269

100 146 75 10 0 0 0 3 9 25 69 10 447 285 161

259 57 106 32 23 0 0 21 31 97 35 46 706 332 374

31 139 181 18 9 0 8 7 34 65 42 104 638 351 287

110 107 98 51 20 5 6 9 23 39 54 58 580 316 264

a b

Precipitation was obtained from a standard rain gauge installed at the experimental site. Long-term average represents the mean precipitation over 58 years from 1957–2015. 2

b

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Fig. 1. The distribution of soil temperature during growing seasons of winter wheat from 2009 to 2011 in farmer practice and year-round plastic film mulch (YPM).

wheat; I (mm) is the amount of irrigation; C (mm) is the upward flow into root zone; D (mm) is the downward drainage out of root zone; R (mm) is the surface runoff; ΔSWS (mm) is the difference in soil water storage (i.e., soil water storage at sowing of winter wheat minus soil water storage at harvest of winter wheat in 0–300 cm soil depth in one wheat growing season). For this experiment, I was not applied; C was ignored because the groundwater table more than 50 m; D was ignored because the depth of soil water monitoring was much deeper than the effective root zone; R was not observed due to flat experimental field. ET (mm) of winter wheat growing season was therefore determined according to the following formula:

content and soil nitrate-N concentration; the detailed process can refer to this report (He et al., 2016b). The concentration of soil organic carbon (SOC) was measured by a CN analyzer (Vario Max CN, Elementar, Hanau, Germany) and soil bulk density was measured by cutting-ring method to calculate SOC stock. Soil temperature at 5-cm depth over whole growing season of winter wheat was recorded using an automatic geothermal data logger (StowAway TidbiT v2, once each hour). Soil temperature in 2009–2015 has been recorded and only in 2009–2011 was shown in Fig. 1, since it had a similar pattern. 2.4. Data calculation and analyses

ET = P + △SWS 2.4.1. Soil water storage (SWS) SWS (mm) in 0–300 cm soil layer was determined by following formula:

2.4.3. Soil nitrate-N (SN) SN (kg N ha−1) was calculated by following formula:

300

SWS =

∑ wi × ρi × hi × 10/100,

300

i = 20

SN =

where i is the number of soil depth (20, 40, 60, …, 300); wi (%) is soil water content; ρi (g cm−3) is soil bulk density; hi (cm) is the thickness of soil layer.

∑ Ni × ρi × hi/10, i = 20

where, Ni is soil nitrate-N concentration (mg kg–1). Soil nitrate-N in 0–300 cm profile at wheat harvest was defined as soil nitrate-N residue. The difference in nitrate-N of consecutive topsoil was used to estimate soil nitrate-N leaching in summer fallow. When soil nitrate-N at the end of summer fallow was significantly lower than that at the beginning of summer fallow, nitrate-N leaching was calculated by the soil nitrate-N at the end of summer fallow minus that at the beginning of summer fallow (Dai et al., 2016).

2.4.2. Evapotranspiration (ET) ET (mm) of winter wheat growing season for each plot was calculated using soil water balance equation (Li et al., 2013):

ET = (P + I + C)−(D + R−△SWS ), where, P (mm) is the precipitation during growing season of winter 3

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water storage at wheat sowing increased by 7% in YPM (Table 2). In details, soil water storage under YPM increased by 7%, 7%, 13%, and 14% in 2010–2011, 2012–2013, 2013–2014, and 2014–2015, respectively. Mean daily soil temperature during wheat growing season under YPM increased by 0.6 °C in 2009–2010 and 0.5 °C in 2010–2011, compared with farmer practice (Fig. 1). Increasing soil temperature by YPM occurred at each growth stage of wheat. Mean soil temperature over the two years under YPM increased by 0.5–0.7 °C from sowing to over-wintering stage, 1.0–1.4 °C from reviving to jointing stage, and 0.2–0.5 °C from heading to grain-filling stage.

2.4.4. SOC stock (SOCs)

SOCs = SOC × ρi × hi / 10, where, SOC is the SOC concentration (g kg−1). 2.4.5. GHG emissions The methodology of Life Cycle Analysis was applied to estimate GHG emissions. The system boundary included the burden of all material inputs and agricultural processes in wheat production, which involved three subsystems: raw materials, farming inputs, and arable farming (Fig. S1). Estimating the GHG emissions in wheat production involved: (1) production, transportation, and applications of fertilizers, pesticides, seeds, and plastic film; (2) production and transportation of diesel for agricultural machinery used; (3) manpower input. The GHG emissions were calculated by following formula:

3.1.2. ET and wheat yield In comparison with farmer practice, the yearly mean ET increased by 5% under YPM (Table 2). The increased ET by YPM occurred in most cases. ET increased by 10%, 5%, 10%, 8%, and 9% for YPM in 2008–2009, 2009–2010, 2010–2011, 2013–2014, and 2014–2015, respectively. The yearly mean wheat yield in YPM increased by 11% (Table 2). Wheat yield had a variation in different years with varied annual precipitation. It was increased by 41% in 2008–2009, 15% in 2009–2010, 39% in 2010–2011, and 15% in 2014–2015, and not affected in 2011–2012 and 2013–2014, while reduced by 19% in 2012–2013.

n

GHG emissions =

∑ AIi × EFi + EN2 O + δSOCs i=1

where AIi is the amount of agricultural inputs during wheat production, including fertilizers, pesticides, seeds, diesel, manpower, and plastic film (Table S3); EFi is the GHG emissions coefficient of an individual agricultural input in life cycle (Table S4); EN2O is the N2O emission from N fertilizer application in the field (Table S5); δSOCs is the CO2 emission from the SOC stock in the field (Table S2); n is the number of agriculture inputs. The specific GHG emissions of agricultural inputs are shown in Table S5.

3.2. Environmental risk under YPM 3.2.1. Soil nitrate-N residue and nitrate-N leaching Compared with farmer practice, total N input in YPM increased by an average of 6% (Table S1), while the yearly mean soil nitrate-N residue in YPM decreased by 52% (Fig. 2A). The decreased soil nitrate-N residue by YPM occurred in most cases. Soil nitrate-N residue under YPM decreased by 51%, 48%, 59%, and 69% in 2009–2010, 2010–2011, 2011–2012, and 2012–2013, respectively. It is clear that the reduction in soil nitrate-N residue can be attributed to the increased wheat yield (Fig. 3A). Notably, the decreased soil nitrate-N residue is the key driving force for the reduction in soil nitrate-N leaching (Fig. 3B). As a result, the yearly mean soil nitrate-N leaching in YPM also decreased by 51% in comparison with farmer practice. Soil nitrateN leaching under YPM decreased by 51% in the summer of 2010, and 48% in the summer of 2011 (Fig. 2B).

2.4.6. Economic returns A survey was performed in the local villages (30–40 households) and markets to estimate the economic returns of YPM system in wheat production. The inventory was shown in Table S6. Net economic returns are calculated by output minus input. 2.5. Statistical analyses The MIXED procedure of SAS program was performed to determine the significance. When analysis of variance was significant, the Duncan’s multiple range test (P < 0.05) were used to compare the differences between means. Correlation analysis was conducted to determine the relationships and obtain a best-fit equations.

3.2.2. GHG emissions intensity Compared with farmer practice, the yearly mean GHG emissions intensity under YPM reduced 12% (Fig. 2C). GHG emissions intensity in YPM has a variation in different years. It was reduced by 32%, 16%, 27%, and 13% in 2008–2009, 2009–2010, 2010–2011, and 2014–2015, respectively, whereas not affected in 2011–2012 and 2013–2014, even increased by 23% in 2012–2013.

3. Results 3.1. Wheat yield affected by year-round plastic film mulch (YPM) 3.1.1. Soil water and soil temperature Compared with farmer practice, the yearly means show that soil

Table 2 Soil water storage in 0–300 cm soil layer at sowing, ET, and grain yield of winter wheat affected by year-round plastic film mulch (YPM) from 2008 to 2015. Treatments

2008–2009

2009–2010

2010–2011

2011–2012

2012–2013

2013–2014

2014–2015

Average

Soil water storage (mm) Farmer practice 506a YPM 514a Average 510E

509a 523a 516E

644b 687a 666C

745a 771a 758A

571b 613a 592D

569b 642a 605D

661b 753a 707B

601b 643a

ET (mm) Farmer practice YPM Average

344b 378a 361D

294b 308a 301E

381b 418a 399C

554a 525a 540A

306a 310a 308E

471b 509a 490B

406a 442b 424C

394b 413a

Grain yield (kg ha−1) Farmer practice YPM Average

3301b 4741a 4053C

3168b 3646a 3407D

4150b 5751a 4951B

7512a 7278a 7395A

4026a 3243b 3634CD

7623a 8141a 7882 A

3925a 4518b 4221C

4824b 5331a

Data are given as the mean of four replicates. Different lowercase letters in the same column indicate significant differences between treatments at P < 0.05. Different upper case letters in the same row indicate significant differences among years at P < 0.05. 4

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Fig. 2. Soil nitrate-N residue at winter wheat harvest (A), soil nitrate-N leaching during summer fallow (B), greenhouse gas (GHG) emissions intensity (C), and net economic benefit (D) affected by year-round plastic film mulch (YPM). Error bars indicate standard error. “*” indicate significant differences between farmer practice and YPM at P < 0.05.

the most fundamental strategy due to the limited available land for agricultural expansion (Foley et al., 2011). Our results showed that adoption of YPM is an efficient approach for improving yield per unit area. For YPM, mulching on the ridge soil using clear plastic film declined the exchange of soil water and air water, and harvested more rainfall into furrow soil, which helped to decline water loss by evaporation and increase water storage by infiltration into the soil (Gao et al., 2014; Lentz and Bjorneberg, 2003). YPM application is therefore beneficial for increasing soil water content, alleviating soil water stress, and improving wheat yield (Ding et al., 2018; He et al., 2016a). A field experiment, located in our experimental site nearby, also showed that adoption of conventional plastic film mulch (mulching soil surface only during growing season of winter wheat) resulted in a 18% yield increase in 2008–2009 (Chen et al., 2015). The yield increase was lower than that in our study in this year. The greater yield benefits in YPM

3.3. Economic returns affected by YPM In comparison with farmer practice, the yearly mean net economic returns over the seven years increased by 12% under YPM (Fig. 2D). The results also varied with years. Net economic returns under YPM increased by 63% in 2008–2009 and 56% in 2010–2011, and not affected in 2009–2010, 2011–2012, 2013–2014, and 2014–2015, whereas decreased by 40% in 2012–2013. 4. Discussion 4.1. Effect of year-round plastic film mulch (YPM) on wheat productivity 4.1.1. YPM increases wheat yield by improving soil environment In order to ensuring food security, improving yield per unit area is

Fig. 3. Relastionships between wheat yield and soil nitrate-N residue at wheat harvest (A), and soil nitrate-N residue at wheat harvest and soil nitrate-N leaching during summer fallow (B). Gray areas indicate a confidence interval at 95%. “**” indicates significance at the 0.01 level. 5

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that wheat can tolerate, thus restricted yield formation (He et al., 2017). To addressing this problem, covering the surface of film with a thin layer of soil should be carefully considered. In the future, more strategies for optimizing YPM should receive more attention to steadily increase wheat yield.

indicated the significance of covering soil surface during summer fallow. Even so, it is a pity that we can not provide a direct evidence to confirm the yield benefit of YPM, because there is no a conventional plastic film mulch as a control in this study. Increasing soil temperature was also a key factor for improving crop productivity (Hoegy et al., 2013), whereas its contribution was often neglected due to the limit of manual measure methods (Anikwe et al., 2007; Mo et al., 2018). In this study, soil temperature was recorded automatically over whole growing season of winter wheat. The results clearly showed that YPM practice can increase soil temperature in each stage of winter wheat, particularly in the vegetative period (Fig. 1). During the seedling establishment stage, increasing soil temperature helped to increase emergence rates and develop strong seedlings (Li et al., 1999), which is the most basis guarantee for forming more spikes per unit area. During the reviving stage, increasing soil temperature not only contributed to avoid freeze injury caused by the coldness in the late spring, but also helped to break the dormancy earlier that can amplify the growth period, particularly from jointing to flowering stage (Li et al., 2016). As a result, improving soil water and soil temperature by YPM helped to alleviate or even avoid drought and cold stress, and provided better seedling establishment and longer effective growth stages; all of these contributed to increase wheat yield.

4.2. Adoption of YPM to reduce environmental risk 4.2.1. YPM reduces soil nitrate-N leaching Until recently, most agricultural paradigms still focused on increasing yield due to high demand for food, often to the detriment of the natural environment (DeFries et al., 2004; Foley et al., 2011). Approximately 100 Tg N in 2010 lost to environment through leakages and emissions for global agriculture production (Oenema et al., 2009; Zhang et al., 2015b). Excessive N fertilizer application was considered to be the most contributor for this, thus optimal N management was the most common strategy for addressing this challenge (Cui et al., 2014, 2013). Our results clearly showed that adoption of YPM was beneficial for reducing environmental risk. Soil nitrate-N leaching had a strong positive relationship with soil nitrate-N residue (Fig. 3B), which gradually decreased with increasing wheat yield (Fig. 3A). Adoption of YPM had the ability to increase wheat yield, hence it contributed to reduce soil nitrate-N leaching. In addition, covering soil surface using plastic film under YPM strengthened the nitrate-N retention (Ruidisch et al., 2013), which contributed to reduce nitrate-N leaching. Further, the YPM practice reduced the infiltration rate of rainfall because of straw retention in the furrow; this was also important for declining nitrate-N leaching. All of these helped to reduce nitrate-N leaching, although the total N input in YPM practice is higher than that in farmer practice (Table S1).

4.1.2. YPM does not always increase wheat yield Wheat yield under YPM had a greater variation with the varied precipitation. In 2011–2012 and 2013–2014 with more than 700 mm annual precipitation, soil water was no longer the most crucial factor restricting the growth of wheat, thus YPM system lost the capacity to improve yield by increasing soil water. Our report on different precipitation areas also confirmed that plastic film mulch cannot effectively increase wheat yield in the regions with sufficient precipitation (He et al., 2018). A similar result was also reported in dryland maize production by Wang et al. (2016). Notably, YPM even reduced wheat yield in 2012–2013. For this year, the annual precipitation of 447 mm is higher than the average ET, which seems to mean that it has the ability to maintain the normal growth of winter wheat. The higher coefficient of determination (R2) in Fig. 4B imply that the precipitation during vegetative period was also very crucial for yield formation. However, the precipitation during the vegetative period of winter wheat is only 47 mm, which is the lowest in history. The mismatch between the distribution of precipitation and water demand of winter wheat exacerbated water stress and limited the development of wheat canopy, which allowed soil below plastic film to receive more solar radiation during reproductive period to heat topsoil. As a result, the sharp increased soil temperature exceeded the threshold

4.2.2. YPM reduces GHG emissions intensity Agriculture is the main sector of GHG emissions, and contributes to the atmosphere GHG emissions about 5.0–5.8 G t CO2 eq yr−1, which is 11–14% of the total global GHG emissions (Mohammadi et al., 2013; Wollenberg et al., 2016). Fortunately, our results showed that adoption of YPM had a potential opportunity for reducing GHG emissions intensity. The reason for this was the increased yield rather than the reduction in GHG emissions per unit area; a similar result was also reported by Chen et al. (2017). Although there is no difference in GHG emissions between YPM and farmer practice, their production process is quite different (Fig. S2B). Compared with farmer practice, YPM has a higher GHG emissions in the production and transportation of agriculture input because of additional input from plastic film (Cambria and Pierangeli, 2011), and a lower GHG emissions in wheat production. The

Fig. 4. Relastionships between soil water storage at sowing and wheat yield (A), and soil water storage at sowing plus precipitation during vegetative period of winter wheat and wheat yield (B). Gray areas indicate a confidence interval at 95%. “**” indicates significance at the 0.01 level. 6

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References

larger SOC stock in YPM was the most contributor for the lower GHG emissions (Table S2), as better soil water and soil temperature situation in YPM increased N2O emission (Kim et al., 2017). In this study, CO2 emission (especially from the SOC stock) was the most important component of GHG emissions compared with N2O emission (Fig. S2A, Table S5). As a result, the larger SOC stock in YPM plays a crucial role in offsetting the increase in GHG emissions caused by the production, transportation, and application of plastic film. In the future, the changes in SOC stock should be given more attention for better evaluating GHG emissions of plastic film mulch systems in dryland crop production.

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4.3. Adoption of YPM to increase economic returns Economic returns are the most concerned by farmers, which determine whether new technologies can be widely used (Zhang et al., 2017). Considering all the costs of agricultural inputs in YPM system, we have performed a comprehensive assessment of economic profits. The results clearly showed that YPM system significantly increased net economic returns. In this study, the most important output value was wheat grain, thus increasing grain yield by YPM was the fundamental cause for increasing net economic returns. Notably, extra cost from plastic film in YPM system decreased the net economic benefits. If the extra cost could not be offset by the increased economic income, this will result in a decrease in net economic returns (Steinmetz et al., 2016). Indeed, our finding also confirmed that adoption of YPM had the potential to reduce net economic returns owing to undesirable yield benefits, although this situation only occurred in extreme weather. To date, most of plastic film at the end of the useful life-span was considered as a waste and disposed of via incineration or landfill. If plastic film waste can be recycled, it will bring a small amount of income, which will be effective for increasing net economic returns. In the future, the net economic returns would be greater if environmental profits obtained by the decrease of environmental costs were included in the cost-benefit analysis (Xia et al., 2017), which is similar to the subsidies of the “Carbon Farming Initiative” in Australia (Lam et al., 2013). We believe that the strategies with further optimized YPM and effective policies will provide a strong guarantee for eliminating the economic barriers of farmers.

5. Conclusion Adoption of YPM offers a better opportunity for increasing yield and net economic returns while reducing soil nitrate-N leaching and GHG emissions intensity. Meanwhile, we also believe that YPM technology provides a new notion on how to tap the potential of crops fallow seasons, especially for farming in dryland. In the future, we hope not only to be concerned with the yield profits of plastic film mulch, but also to pay more attention to its environmental and economic benefits.

Acknowledgements This work was supported by the Chinese National Basic Research Program (973, Program: 2015CB150404), China Agricultural Research System (CARS-3-1-31), the Special Fund for Agro-scientific Research in the Public Interest under Grant (201303104), the Agricultural Scientific Research Talent and Team Program, and the National Natural Science Foundation of China (NSFC) (41401330).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.fcr.2018.05.019. 7

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