Effects Of Straw Returning Methods On Soil Physicochemical Properties And Water-and Nitrogen-use Efficiency Under Brackish Water Drip Irrigation Cotton Fields

Objective:The use of brackish water to replace freshwater irrigation in arid regions can alleviate the shortage of freshwater resources,but it also brings a large amount of salt ions into the soil,causing soil salinization and inhibiting crop growth.Returning straw and biochar to the field can improve soil physical and chemical properties and increase soil nutrients.This study is aimed at the real problem of shortage of freshwater resources in arid regions,based on the long-term freshwater and brackish water drip irrigation positioning experiments,this study investigated the effects of cotton stalk and cotton stalk biochar on soil physicochemical properties,water and salt transport,enzyme activity,bacterial community structure,cotton growth,yield,and water-nitrogen use efficiency.It provides a theoretical basis for the rational utilization of brackish water resources in agriculture and the improvement of soil fertility in arid areas.Methods:This study was based on the field experiment of brackish water irrigation that has been carried out for 12 years（2009-2020）.The experiment is a two factors design,the irrigation water salinity（EC_W）is set as two treatments:0.35 d S·m^-1and 4.61 d S·m^-1,which represent fresh water（FW）and brackish water（BW）irrigation water quality,respectively.There are four treatments under the salinity of each irrigation water:no nitrogen application（N0）,N fertilizer application at 360 kg·hm^-2（N360）,nitrogen application plus cotton straw（N360ST）,and nitrogen application plus cotton straw biochar（N360BC）.The amount of nitrogen fertilizer in the nitrogen application area is kg·hm^-2.Cotton straw and cotton straw biochar are applied at one time before sowing.The amount of cotton straw is 6 t·hm^-2and biochar is 3.7 t·hm^-2（the dose of organic carbon addition of biochar was equivalent to straw returning to the field）.Results:（1）Brackish water irrigation significantly increased soil water content and salinity in each soil layer.The application of straw and biochar under freshwater treatment significantly reduced the water content and salinity of each soil layer,however,the effect of biochar treatment was more significant.Under the brackish water treatment,the application of straw significantly increased the water content of the 0-60cm soil layer,and the application of biochar significantly decreased the water content of the 0-60 cm soil layer.Both straw and biochar application significantly reduced the salinity content of the 40-100 cm soil layer.At the same time,straw and biochar can reduce the total salt residue by increasing the leaching of soil salt during the growth period,reducing 4.5%and 6.7%respectively.（2）Brackish water irrigation significantly increased soil bulk density,total carbon,total nitrogen and available phosphorus content,decreased soil porosity,pH and available potassium content,while affecting the content of each salt ions.Compared with FW treatment,BW treatment significantly increased the contents of soil Na⁺,Ca²⁺,and Cl^-,and significantly decreased the contents of soil K⁺,Mg²⁺,SO₄^2-,and HCO₃^-.The application of straw and biochar significantly increased soil nutrient content and pH,but significantly decreased soil bulk density and electrical conductivity.Biochar treatment significantly decreased K⁺and Mg²⁺contents by 34.4%and 60.7%,respectively,while straw treatment significantly decreased Na⁺,Ca²⁺,and Cl^-contents by 18.2%,22.7%and 12.3%,respectively.（3）Brackish water irrigation promoted the activities of enzymes related to nitrogen transformation and sucrase,but significantly inhibited themβ-Glucosidase and alkaline phosphatase activities.Application of straw significantly increased the activity of sucrase andβ-glucosidase by 42.5%and 25.4%,respectively,however,the application of biochar decreased its activity by 22.3%and 11.9%,respectively.The application of biochar under fresh water irrigation significantly increased the Shannon,Chao1,and ACE indexes of soil bacterial community,and the application of straw and biochar under brackish water irrigation significantly increased the Simpson index.The application of straw decreased the relative abundance of Proteobacteria and Verrucaria,but significantly increased the relative abundance of Thermobacteria.The application of biochar significantly increased the relative abundance of Proteus and Curvularia,but significantly decreased the relative abundance of Verrucaria.（4）Compared with fresh water irrigation,brackish water irrigation inhibited cotton growth and reduced plant height and biomass by 19.3%and 8.7%respectively.Brackish water irrigation also significantly inhibited the nutrient uptake of cotton,resulting in a significant decreased in yield,irrigation water and nitrogen use efficiency.The application of straw and biochar can promote cotton growth,significantly increase cotton plant height and biomass,promote nutrient uptake,and then increase seed cotton yield,irrigation water and nitrogen use efficiency.Conclusion:Brackish water irrigation increases soil salinity,leads to the deterioration of soil physical and chemical properties,reduces soil enzyme activity,significantly affects the structure and diversity of soil bacterial community,and inhibits the absorption of water and nutrients by cotton,so as to reduce the yield of seed cotton and water and nitrogen use efficiency.Straw and biochar application can improve soil porosity,increase soil nutrients,improve enzyme activity,and bacterial community diversity,promote cotton growth and nutrient uptake,increase yield,and increase irrigation water use efficiency and nitrogen use efficiency.