| Drought has become a worldwide problem seriously affecting crop yield and quality,which poses a major challenge to the stability and improvement of agricultural production.The Northwestern China has been facing drought for a long time,and optimizing irrigation strategy and adjusting crop structure are effective ways to improve water use efficiency and agricultural productivity.Compared with traditional grain crops,forage crops have stronger water adaptability,but the lack of water resources still limits forage production.Alfalfa(Medicago sativa L.)is one of the main high-quality forages,and it is of great significance to study the water management and water use characteristics of alfalfa grassland for alfalfa cultivation.Regulated deficit irrigation,aiming at the maximum water productivity,is an important way to balance water input and yield output in arid areas,while application of regulated deficit irrigation in forage still needs further study.A three-year trial(2017-2019)was conducted with two irrigation systems(flood irrigation,FI and subsurface drip irrigation,SDI)and seven irrigation treatments.Irrigation treatments included a full irrigation treatment for the whole growth period(Ifu)and six regulated deficit irrigation(RDI)treatments:slight water deficit for the whole growth period(Isl),moderate water deficit for the whole growth period(Imo),moderate water deficit at the branching and budding stage(Ibb),moderate water deficit at the regrowth stage(Ire),moderate water deficit at the branching stage(Ibr)and moderate water deficit at the budding stage(Ibu).Alfalfa yield and quality,plant height,leaf area index,radiation interception,soil water and evaporation were measured regularly.The effects of regulated deficit irrigation on production performance and growth dynamics of alfalfa under different irrigation systems were analyzed,and the changes of water utilization and movement under different irrigation treatments were simulated.The main results are as follows:1)The yield and quality of alfalfa were significantly affected by the regulated deficit irrigation under two irrigation systems.Irrigation treatment had no significant effect on the dry matter yield of alfalfa in the planting year,but had a significant effect on the yield in the next years.Compared with slight deficit during the whole growth period,moderate deficit irrigation at single growth stage did not lead to a significant decrease in yield.With regulated deficit irrigation treatment,there was a significant quadratic relationship between dry matter yield and plant height,and a significant linear relationship between dry matter yield and leaf area index.Regulated deficit irrigation treatment significantly affected the crude protein content and relative feeding value of alfalfa,which was regulated by alfalfa growth.With regulated deficit irrigation,there was a significant linear relationship between quality indexes and stem leaf ratio,but the R~2(coefficient of determination)was low.There was a quadratic relationship between dry matter yield and crude protein content,neutral detergent fiber,acid detergent fiber and relative feeding value.With the increase of dry matter yield,the quality decreased first and then increased.The irrigation water use efficiency under subsurface drip irrigation was significantly higher than that under flood irrigation.2)The regulated deficit irrigation affected the growth of alfalfa shoot mainly by reducing the radiation use efficiency.Under the whole growth irrigation treatments,the stem leaf ratio,plant height and leaf area index decreased with the increase of the deficit degree,while the difference between treatments was small under the growth stage deficit treatments.The K value(reflecting the upper limit of plant height and leaf area index)of plant height and leaf area index model(logistic)was reduced by deficit irrigation under the whole growth treatments,however,the accumulated temperature for the rapid growth stage was barely affected;the effect of growth stage deficit treatment on K value was small in most cuts.The radiation interception fraction of alfalfa between treatments were smaller at most cuts,especially in the middle and late stages of each cut.Under whole growth deficit irrigation treatments,the cumulative radiation interception decreased with the increase of deficit degree at most cuts,and the difference between treatments was small under growth stage deficit treatments.Under whole growth deficit irrigation treatments,the radiation use efficiency decreased significantly with the increase of deficit degree,while under growth stage deficit treatments,the difference between treatments was small and lower than that under Ifu treatment.The principal component analysis showed that at most cuts,there was a strong positive correlation between radiation use efficiency and dry matter accumulation,and the correlation was greater than that between accumulated radiation interception and dry matter accumulation.3)Irrigation method and regulated deficit irrigation affected the water use of alfalfa grassland.At the experimental site,the evapotranspiration of alfalfa in the planting year could reach 650 mm,and that of 2-and 3-year-old alfalfa could be more than 800 mm.Under the whole growth deficit irrigation,the evapotranspiration and transpiration decreased with the increase of the deficit degree,but the evaporation increased;under the growth stage deficit irrigation,the difference between the treatments was small.At the site,capillary rise accounted for a relatively small proportion of water input(9%-26%),and evaporation accounted for a small proportion of water output(13%-26%).The transpiration and evaporation under subsurface drip irrigation were less than that under flood irrigation.The model simulation also showed that the soil evaporation measured by micro lysimeter at the middle and later stages of each cut was higher than the simulation value.4)Regulated deficit irrigation affected the vertical movement of soil water and root water uptake under flood irrigation.Irrigation had a great influence on soil water content at0-60 cm depth under flood irrigation.The water content at 10 and/or 30 cm depth under different treatments was similar,and the soil water content below 30 cm had a small fluctuation and tended to decrease.Under whole growth deficit irrigation,the soil water content below 30 cm decreased with the increase of deficit degree;the difference between treatments was small under growth stage deficit irrigation.There were great differences in the vertical movement direction of water in different years and irrigation treatments.The result of Hydrus model also showed that the root water uptake of alfalfa in the planting year was lower,and the root water uptake of the first cut was higher than those of the second and third cuts.5)Regulated deficit irrigation affected the vertical and horizontal movement of soil water under surface drip irrigation.Under subsurface drip irrigation,there was a point seepage water source on each side of boundary at the depth of 20 cm,which forms a semi elliptical or nearly fan-shaped humid area after irrigation.The simulation results of Hydrus-2D model showed that larger irrigation amount could lead to change from fan-shaped horizontal distribution to vertical distribution in a shorter time.Under full irrigation,the vertical distribution of soil water was restored after 3-4 d of irrigation,while under moderate deficit irrigation,the vertical distribution was restored on the 7th-8th day.Under the growth stage deficit irrigation,the duration of fan-shaped water distribution under Ibb treatment was the longest at the branching stage in most cases,and the duration under Ibr treatment was also longer.The duration under Ibb and Ibu treatments was longer at the budding stage.In conclusion,the results showed that regulated deficit irrigation could regulate the yield and quality of alfalfa,and higher production performance could be obtained under single growth stage deficit irrigation,especially at the budding stage.Subsurface drip irrigation had more advantages than flood irrigation in improving water use efficiency.Due to the closed canopy structure,radiation use efficiency was more sensitive to water stress than cumulative radiation interception,and had a greater impact on dry matter accumulation.In the study site,the evapotranspiration of 2-and 3-year-old alfalfa grassland could be more than 800 mm,and the transpiration accounted for more than 70%.With the increase of deficit degree,evapotranspiration and transpiration decreased significantly,but evaporation did not change significantly.There were differences in soil water distribution and movement patterns between different irrigation systems,and regulated deficit irrigation also had a significant impact on them.Irrigation mainly affected soil water content at 40cm depth.In the inland arid area,the subsurface drip irrigation would be performed well in alfalfa production and moderate deficit irrigation at the budding stage of alfalfa was also recommended. |
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