Effects Of Plastic Film Mulch On Winter Wheat Growth And Water Use And Its Mechanisms In The Loess Plateau

Wheat is one of the main food crops in the Loess Plateau of China.With the improvement of soil fertility,water has become the main factor limiting wheat yield in this area.Plastic film mulch has been proven to be the most effective measure to improve crop yield in comparison to the other mulch measures in the Loess Plateau.However,plastic film mulch significantly changes soil moisture and thermal conditions as compared with the bare ground practice,and thus crop water use has drastically altered.It is of great scientific significance to further study soil water use dynamics during wheat growing season and to fully understand the mechanism of water use by wheat with plastic film mulch,to achieve a desired yield and water use efficiency.In this study,a four-year field experiment was conducted on the Loess Plateau to investigate the interactive effects of pre-sowing soil water,season,and soil cover（plastic film mulch and bar ground）on soil hydrothermal environment,winter wheat growth,water use,yield and water use efficiency.The main results and conclusions obtained were as follows:（1）Plastic film mulch increased the average soil temperature at 0.1–0.5 m soil layer in the wheat season by about 0.3–0.5°C compared with bare ground practice.The interaction between plastic film mulch and environmental factors（pre-sowing soil water and rainfall during growth period）had a significant impact on soil temperature.Plastic film mulch increased soil temperature at low air temperature（early growth stage of winter wheat）and decreased soil temperature at high air temperature（later growth stage of winter wheat）.From sowing to jointing stage,plastic film mulch increased soil temperature by about 0.5–0.7°C.From jointing to anthesis stage,the soil temperature of 0.1-0.5 m soil layer was similar between the plastic film mulch and bare ground.From anthesis to maturity stage,plastic film mulch reduced soil temperature by about 0.2°C at 0.1 m soil depths,while the soil temperature at 0.2-0.5m soil layer was similar.Plastic film mulch significantly increased soil water storage in 0–0.2 m soil layer during wheat growth period compared with conventional treatment in dry and moderate season,but they were similar in wet season.And plastic film mulch decreased soil water storage in 0.4–2.0 m soil layer at the maturity stage of wheat in dry season.（2）The interaction between plastic film mulch and environmental factors significantly affected wheat population at jointing stage.Compared with bare ground practice,plastic film mulch significantly increased the spike number at anthesis and maturity stages（excluding freezing season）,and the positive effect of plastic film mulch on wheat population was more obvious in dry season.Plastic film mulch increased winter wheat biomass at jointing,anthesis and maturity stages by about 17%,6%and 10%,respectively.Although plastic film mulch had no significant effect on the senescence characteristics of wheat flag leaves after anthesis,plastic film mulch increased the wheat dry matter accumulation after anthesis by about 22%.Water-soluble carbohydrates content at anthesis and maturity,and water-soluble carbohydrates translocation ranged from 129 kg ha^-1 to3903 kg ha^-1,81 kg ha^-1 to 1101 kg ha^-1and 78 kg ha^-1 to 3034 kg ha^-1,respectively.Plastic film mulch significantly increased water-soluble carbohydrates content at anthesis and water-soluble carbohydrates translocation by about 14%and 16%,respectively.However,plastic film mulch had no significant effect on the water-soluble carbohydrates content at maturity and water-soluble carbohydrates translocation ratio.In addition,plastic film mulch increased root length density and root surface density in 0.4–2.0 m soil layer at wheat anthesis stage.（3）The ranges of pre-anthesis,post-anthesis and total evapotranspiration were 23–324mm,54–160 mm and 155–444 mm,respectively.The pre-anthesis evapotranspiration under plastic film mulch was similar to that under bare ground.However,their interaction between plastic film mulch and environmental factors significantly affected the post-anthesis and total evapotranspiration（excluding freezing season）.Plastic film mulch increased the post-anthesis and total evapotranspiration in dry season or initial soil water level,and reduced the post-anthesis and total evapotranspiration in wet season or initial soil water level.Combination of isotope technique with MixSIAR model found that winter wheat mainly used soil water in 0–0.2 m depth at jointing and anthesis stages,and then secondly used water from 0.4–1.2 m soil depth,and the utilization rate of soil water in the remaining soil layers was similar.At the jointing stage,compared with conventional practice,plastic film mulch reduced the utilization rate of soil water in 0–0.2 m soil layer by about 14%,increased the utilization rate of soil water in 0.2–0.4 m and 0.4–1.2 m soil layers by 14%and 9%,and increased the utilization rate of 1.2–2.0 m and 2.0–3.0 m soil layers by 3–5%.At the anthesis stage,plastic film mulch reduced the utilization rate of soil water in 0–0.2 m soil layer by 18%,compared with bare ground,and increased the utilization rate of soil water in 0.2–0.4 m,0.4–1.2 m,1.2–2.0 m and 2.0–3.0 m soil layers by 31%,14%,20%and 30%,respectively.Plastic film mulch had no significant effect on the depth of water use,but significantly increased the water use from 1.2–3 m soil layer,and this effect was more significant with dry pre-sowing soil water level.（4）The yield of winter wheat ranged from 1646 kg ha^-1–7955 kg ha^-1.Plastic film mulch increased winter wheat yield by 10–120%（excluding freezing season）,with an average increase of 47%,and the effect of increase yield was most obvious in the dry season or pre-sowing soil water level.Plastic film mulch significantly increased the wheat grains number by about 7%,but had no significant effect on grain weight and harvest index.The SEM model showed that increasing the pre-anthesis dry matter accumulation would increase the pre-anthesis and post-anthesis evapotranspiration,which was beneficial to the formation of grain number.At the same time,it would increase the water-soluble carbohydrates translocation and grain weight,which was conducive to the improvement of yield and harvest index.Increasing post-anthesis dry matter accumulation was beneficial to increase yield.Water use efficiency of wheat ranged from 8 kg ha^-1 mm^-1 to 20 kg ha^-1mm^-1.Plastic film mulch significantly increased the water use efficiency by about 13–100%（excluding freezing season）,with an average increase of 43%,and the effect of increase water use efficiency was most obvious in the dry season.In summary,under different water environments in the Loess Plateau,plastic film mulch can improve soil temperature during the winter wheat growth period,increase soil moisture,enhance deep root distribution and utilize deep soil water,promote wheat growth,increase biomass accumulation and water-soluble carbohydrates translocation,and improve crop yield components,and thus increase wheat yield and water use efficiency,especially in dry growth period or initial soil water level.