Effect Of Subsurface Water Retention Technology On Cotton Root Growth And Water-and Nitrogen-use Efficiency Under Drip-irrigation Cotton Field

【Objective】Excessive irrigation and nitrogen fertilization leads to deep percolation and mineral nitrogen leaching loss,which is one of the main pathways of water and nitrogen loss from drip irrigation field.Subsurface water retention technology（SWRT）can effectively reduce the deep percolation and nitrogen leaching,and improve the water-and nitrogen-use efficiency.This study,which included three years of fix-position experiments,was to investigate（1）the effects of SWRT on soil water and nitrogen migration transformation,distribution,water deep percolation losses and N leaching loss;（2）root growth of cotton influenced by SWRT;（3）the different water and nitrogen management on cotton N uptake,yield,WUE and ANR in SWRT.The results of this study provide a theoretical basis for improving the utilization of water nitrogen in drip irrigation and the rational application of SWRT.【Method】Field experiments were carried out in 2015-2017 years,including different irrigation amounts and different depth of SWRT,and different N application rates and different depth of SWRT.（1）The 3×3 factorial experiment field experiment about the effects drip-irrigated,irrigation amount and SWRT membrane depth were determined using samples collected from 2015 to 2016.The three irrigation amounts were 100%ET_C,80%of ET_C,and 60%of ET_C（referred to as ET1.0,ET0.8 and ET0.6,respectively）.The three membrane treatments were control（no membrane）,40-cm-deep membrane,and 60-cm-deep membrane（referred to as CK,D40,D60）.（2）The experiment of N application rate and SWRT,which was conducted in 2017,used a complete randomized block design with two N application rates(0,360 kg N·hm^-2respectively referred to as N0,N360)and three SWRT treatments（control,40 cm-deep membrane,and 60cm-deep membrane;respectively referred to as CK,D40,D60）.【Result】（1）The treatments of 40 cm-or 60 cm-deep membrane significant increased soil water content（0-60 cm）and net soil water storage compared with control.Water deep percolation losses below 100-cm depth were by 5.7 times higher in the ET0.8 treatment and 16.3 times higher in the ET1.0 treatment compared with the ET0.6 treatment.The amount of water deep percolation varied from 14-62 mm,accounting for 3%to 15%of irrigation amount under the ET1.0 plots.D40,D60treatment significant reduced water deep percolation losses compared with control.In the ET0.6plots,there was no significant difference in water deep percolation losses among three SWRT treatments.In the ET0.8,ET1.0 plots,water deep percolation losses were by 64%,38%less in the40 cm-deep membrane treatment and 72%,76%less in the 60 cm-deep membrane treatment compared with the control.The treatments of 40 cm-or 60 cm-deep membrane both significantly reduced evapotranspiration in the ET0.6 plots.However,in the ET1.0 plots,the 60 cm-deep membrane treatment significantly increased evapotranspiration compared with the control.（2）The treatments of 40 cm-,60 cm-deep membrane significant increased soil inorganic nitrogen contents in the 0-40 cm,0-60 cm depth compared with the control,respectively;however,the 60-100 cm soil layer of NO₃^--N contents in D40,D60 treatments were significant less than the control,respectively.Irrigation and nitrogen application significantly increased the amount of N Leaching.In the ET1.0 polts,the amount of N leaching varied from 4.0 to 28.6 kg N·hm^-2.The amount of N leaching varied from 0.3 to 28.6 kg N·hm^-2,accounting for 2.2%to 7.7%of the urea-N applied under different SWRT and N application rate treatments.In the ET0.6,ET0.8 plots,N Leaching were by 64%,38%less in the 40 cm-deep membrane treatment and 72%,76%less in the 60 cm-deep membrane treatment compared with the control,but the differences were not significant between the treatments of 40 cm-and 60 cm-deep membrane.In the ET1.0 plots,N Leaching were by 45%less in the 40 cm-deep membrane treatment and 78%less in the 60cm-deep membrane treatment compared with the control.The amount of ammonia volatilization varied from 8 to 26 kg N·hm^-2,accounting for 6.3%to 7.3%of the urea-N applied under different SWRT and N application rate treatments.D40 significantly increased the amount of ammonia volatilization in the N360 plots.There were no significant differences in the amount of ammonia volatilization between CK and D60 under the N360 plots.（3）D40 and D60 both significantly increased total root biomass,average root length density,root surface area,and root volume,in the ET0.6 plots and ET0.8 plots.However,in the ET1.0plots,the total root biomass and average root length density significantly decreased in the order D60>CK>D40.Nitrogen application significantly increased cotton total root biomass,root activity and significantly reduced average root length density,root surface area,root volume,and root average diameter.In the unfertilized plots,cotton total root biomass and average root length density were significantly higher in both the D40 and D60 treatments than in the CK treatment.In the fertilized plots（N360）,cotton total root biomass,average root length density,root surface area,and root volume were all higher in the D60 treatment than in either the D40 or the CK treatments;there was no significant difference in cotton total root biomass,average root length density,root surface area,and root volume between the D40 and CK treatments.D40 and D60 both significantly increased root mass,root length density and root activity in the 20-40 cm layer.In the40-60 cm layer,root mass,root length density and root activity significantly decreased in the order60>CK>D40（P<0.05）.However,the 60-100 cm soil layer of root mass,root length density and root activity in D40,D60 treatments were significant less than the control,respectively.In the fertilized plots（N360）,root biomass of 0-10 cm layer was less in the D40 treatment than in either the CK or the D60 treatments;there was no significant difference in root biomass of 0-10 cm layer between the D60 and CK treatments.（4）Irrigation and nitrogen application significantly increased cotton biomass,N uptake,cotton yield.However,there were no significant differences in root/shoot ratio and cotton yield between ET0.8 and ET1.0 treatments.Average WUE decreased in the order of ET0.8>ET1.0>ET0.6.IWUE significantly increased as irrigation amount decreased.In the ET0.6 and ET0.8 plots or N0 plots,the D40 both D60 treatments significantly increased cotton biomass,N uptake,cotton yield,water use efficiency;There were no significant differences between the D40 and D60treatments.In the ET1.0 or application of N fertilizer(360 kg N·hm^-2)plots,the D60 treatment significantly increased cotton biomass,N uptake,cotton yield,water-and N-use efficiency;there were no significant differences between the D40 and CK treatments.【Conclusion】Excessive irrigation leads to deep percolation and mineral nitrogen leaching loss,and no obvious increase in cotton yield,resulted in the decrease of WUE;Nitrogen application significantly increased N Leaching and ammonia volatilization.The 40 cm-and 60 cm-deep membrane treatments significantly increased water content and NH₄⁺-N and NO₃^--N contents of the soil layer above the SWRT membrane（20-40 cm,20-60 cm）,and significantly reduced the amount of the deep percolation and N leaching.In high irrigation amount level（ET1.0）or application of N fertilizer（N360）,the amount of the deep percolation and N leaching reduced significantly as SWRT membrane depth increased;the 40 cm-deep membrane treatment significantly increased ammonia volatilization,have negative effect on root growth,and have no obvious effects on nutrient uptake,seed yield,the water-and nitrogen-use efficiency of cotton,but the 60 cm-deep membrane treatments promote root growth of the 20-60 cm layer,significantly increased N uptake,seed yield,the water-and nitrogen-use efficiency of cotton.