Study On The Effects Of Saline Water Irrigation On Soil Quality And Greenhouse Gases Emission In Cotton Field

Fresh water resources are scarce but underground salt-water resources are abundant in the low plain area of Hebei Province.The study on theory and technology of saline water irrigation for cotton is beneficial to alleviate the shortage of fresh water resources and promote the exploitation and utilization of salt water in this area.This study focuses on the tillage layer soil of cotton field irrigated with salt water.The local fresh water irrigation was taken as the control treatment（CK）,five salinity of irrigation water（S1,S2,S3,S4 and S5）were set up at 2,4,6,8 and 10 g·L^-1.The experiment was carried out in the dry farming water-saving agricultural experimental station from 2021 to 2022.This study analyzed the effect of cotton growth and yield under saline water irrigation.The effect of saline water irrigation on the soil environment was elucidated.The greenhouse（CO₂,N₂O and CH₄）effect of saline water irrigation on cotton field was clarified.The mechanism of soil greenhouse gas emission was revealed.The soil quality of cotton field was evaluated.The suitable salinity range of irrigation water was confirmed in this study.The main results were as follows:（1）Analyzing the effects of saline water irrigation on cotton growth and yield.The plant height,leaf area index and dry matter mass of cotton decreased with the increased irrigation water salinity at different growth stages.The differences in the yield components（single boll weight,harvest density,and number of bolls per plant）between S1,S2,and S3 and CK did not reach a significant level.As a result,there were no significant differences in cotton yield between S1,S2,S3 and CK.However,the cotton yield of S4 and S5 treatment was significantly reduced by 18.04%-20.81%and 18.51%-30.49%compared with CK（P<0.05）.（2）Elaborating the tillage player soil physical and chemical properties variations under saline water irrigation.With the increased irrigation water salinity,soil moisture,salinity and p H content increased gradually,but the soil moisture and salinity variations were also related to irrigation and rainfall.Soil organic matter content decreased with the increase of irrigation salinity,but there was no significant difference in soil organic matter content between saline irrigation treatment with salinity less than 4 g·L^-1 and CK.With the increase of salinity,soil ammonium nitrogen increased,while soil nitrate nitrogen decreased.This was related to the mutual conversion of soil ammonium nitrogen and nitrate nitrogen through the process of nitrification and denitrification.Saline water irrigation also changed the soil water-stable aggregates and porosity.The content of soil water-stable aggregates and porosity decreased with the increase of irrigation water salinity.（3）Elucidating the effects of saline water irrigation on soil biological properties.The abundance of soil carbon degradation genes sac C and van B decreased significantly with the increase of irrigation water salinity.The abundance of carbon fixation gene fac A in high salinity treatment(8 g·L^-1)was significantly lower than that in CK.The activities of cellulase,α-glucosidase and alkaline phosphatase decreased under saline water irrigation,which participate in the soil carbon cycle.The abundance of soil nitrogen degradation genes gln A,arc C and ure A decreased,while the abundance of DNRA gene nap B increased with the increase of salinity of irrigation water.Saline water irrigation decreased soil nitrification intensity and activities of urease,nitrate reductase and nitrite reductase,which were biological indicators of nitrogen cycle.（4）Clarifying the greenhouse effect of cotton soil under saline water irrigation.After saline water irrigation,CO₂ and N₂O emissions of each treatment appeared a peak,but emissions gradually decreased and tended to be stable with time.The CH₄ emission increased after saline water irrigation,and the CH₄emission of S5 changed from absorption to discharge.CO₂ emission rate was higher in the bud stage and the flower and boll stage,and the highest CO₂ cumulative emission value of different irrigation treatments appeared in the flower and boll stage.The N₂O emission rate was the highest at the bud stage and the lowest at the batting stage.The maximum N₂O cumulative emission rates appeared at the bud stage,while the lowest and appeared at the batting stage.The change of CH₄ emission rate in two years was different at different growth stages,which was related to the status of soil oxygen and moisture.The average rate of CO₂ and N₂O emission rates decreased with irrigation water salinity increased,while soil CH₄ emission increased.The decrease of soil carbon degradation genes and the inhibition of soil bacteria producing carbon degradation enzymes were the main reasons for the reduction of soil CO₂ emission,and the increase of mtt B abundance promoted the increase of soil CH₄ emission.The decrease of soil N₂O emission rate was related to the increase of nap B abundance and the decrease of soil nitrification intensity and activities of urease,nitrite reductase and nitrate reductase.In addition,the GHGI of saline water irrigation was lower than that of CK.With the increase of salinity of irrigation water,the global warming potential（GWP）decreased.It was found that soil moisture,porosity,R_0.25,organic matter and nitrate had a positive effect on GWP,while soil nitrate nitrogen and p H played a significant negative role for GWP.（5）Proposing the suitable salinity range of irrigation water for cotton field.The study employed principal component analysis to select a minimal dataset from a total of 16 soil indicators across the three aspects of soil physics,chemistry,and biology.This minimal dataset includes soil salinity,p H,urease activity,nitrate reductase activity,nitrification intensity and R_0.25.It was found that the soil in the CK and S1 belong to the high-productivity tillage layer soil,while the soil of S2,S3,and S4 belong to the medium-productivity tillage layer soil.The soil of S5 was classified as the low-productivity tillage layer soil.The suitable salinity range of irrigation water was proposed after combined with that information of yield,greenhouse gas emission and soil quality evaluation.Soil greenhouse effect of cotton field can be reduced and cotton yield and soil quality are not affected when the salinity of irrigation water is less than 2 g·L^-1.When irrigation water salinity is between 2 g·L^-1 and 6 g·L^-1,saline water irrigation has no significant impact on cotton yield,while it may lead to a decrease in greenhouse gas emissions and soil quality.The soil quality still belongs to the medium-productivity tillage layer soil under the circumstances.In addition,the greenhouse effect of cotton field,cotton yield and soil quality decreased significantly when irrigation water salinity≥8 g·L^-1.