Greenhouse Gas Emission From Soil Of Cotton Field Under Saline Water Irrigation

In recent years,the shortage of regional water resources has severely restricted the sustainable development of irrigated agriculture,while saline water irrigation is one of the effective ways to alleviate the shortage of irrigation water resources.Heilonggang area in Hebei Province has become the most serious groundwater"funnel"area in China due to severe groundwater over-exploitation.The water resources available for agricultural irrigation in this region are decreasing year by year.However,the area is rich in underground saline and brackish water resources.If agricultural irrigation is used properly,it can greatly reduce the impact of freshwater shortage on sustainable agricultural development.Farmland is one of the important sources of greenhouse gas emission,carrying out research on greenhouse gas emission from farmland is of great significance for achieving greenhouse gas emission reduction in agricultural units and mitigating climate change.This study takes the shortage of water resources in the Heilonggang area of Hebei as the background,and takes the safe use of saline/brackish water as the research goal to explore the impact of saline water?brackish water?irrigation on greenhouse gas emission from cotton fields.The results will help eliminate the uncertainty of greenhouse gas emission assessment due to changes in irrigation water quality in the region,and achieve accurate assessment of regional greenhouse gas emission,which is of great significance to reveal the environmental effects of saline water irrigation.In this experiment,six levels of saline water treatments?recorded as B1,B2,B3,B4,B5 and B6?were designed to systematically study the emission of greenhouse gas,soil physical and chemical properties,and yield in cotton fields irrigated by saline water.The main findings are as follows:?1?Effects of saline water irrigation on greenhouse gas emission from cotton fieldThe average soil CO₂ emission rate during cotton growth period in 2019 varied from152.39 to 332.50 mg·m^-2·h^-1,The maximum accumulated CO₂ flux appeared in B1(1273.31g·m^-2),while the minimum accumulated CO₂ flux in B6(584.48 g·m^-2).The average soil CH₄emission rate varied from-5.16 to-16.50?g m^-2 h^-1.The maximum accumulated CH₄ flux obtained in B1(-64.45 mg·m^-2),while the minimum accumulated CH₄ flux in B6(-21.44mg·m^-2).The average soil N₂O emission rate varied between 25.76-61.83?g·m^-2·h^-1.The maximum accumulated N₂O flux measured in B1(134.5 mg·m^-2),while the minimum accumulated N₂O flux in B6(73.18 mg·m^-2).The average CO₂,CH₄ and N₂O emission rates under freshwater irrigation?B1 treatment?were 332.47 mg·m^-2·h^-1?-16.50?g·m^-2·h^-1?61.83?g·m^-2·h^-1,respectively.Compared with freshwater irrigation,saline water irrigation can significantly reduce soil CO₂,N₂O emission rates and CH₄ absorption rate.With the increase of the salinity of irrigation water,the cumulative emission flux of soil CO₂ and N₂O was decreasing,the cumulative absorption flux of CH₄ under high salinity irrigation water treatment was significantly lower than that in freshwater irrigation treatment.In the 2019 season,the global warming potential?GWP?of cotton fields gradually decreased with the increase of salinity of irrigation water.The maximum GWP appeared in B1(13060.88 kg·hm^-2),and the minimum GWP in B6(6026.58kg·hm^-2).?2?Effects of saline water irrigation on soil physical and chemical properties of cotton fieldWith increasing salinity of irrigation water,soil moisture content,electrical conductivity,pH,and ammonium nitrogen content was increasing.Compared with B1,the saline water irrigation can increase soil moisture content by 0.5-13.0%,electrical conductivity by 24.7-275.4%,pH by 0.3-6.6%,and soil ammonium nitrogen content by 0.6%-10.7%,respectively.While soil organic matter and nitrate nitrogen content were decreasing.Compared with B1,the saline water irrigation can reduce soil organic matter content by 0.2-7.1%and soil nitrate nitrogen content by 1.4%-20.6%,respectively.?3?Effects of saline water irrigation on soil extracellular enzyme activity and microbial diversity in cotton fieldIrrigation water with different salinity significantly changed soil extracellular enzyme activity.With increasing salinity of irrigation water,soil xylosidase activity,?-glucosidase and phosphatase activities showed a decreasing trend,while the cellulose diglucosidase,alpha glucosidase,and acetylglucosidase activities were increasing.Salline water irrigation changed the ability of soil microorganisms to utilize carbon sources,and soil microorganism activity was significantly higher in freshwater irrigation than thant in high-mineralized irrigation water.?4?Effects of saline water irrigation on cotton yieldAs the mineralization of irrigation water increases,the yield of cotton increases first and then decreases.The cotton yield in B2 was the highest,which was 3672.02 kg·hm^-2.Compared with B1,the cotton yield in B3,B4,B5 and B6 were reduced by 3.6%,6.7%,27.9%,34.9%,respectively.Among them,the difference between B5,B6 treatment and B1 treatment reached a significant level.Considering cotton yield,greenhouse gas emission characteristics of cotton field and soil physical and chemical properties,it is suggested that the salinity of irrigation water in the test area should not exceed 6 g·L^-1.?5?Relationship between greenhouse gas emission and soil environmental factors under saline water irrigationThere is a significant negative correlation between soil CO₂,N₂O emissions and CH₄absorption and soil moisture,electrical conductivity,pH and ammonium nitrogen content,and a positive correlation with soil organic matter and nitrate nitrogen.The relationship between greenhouse gases,soil extracellular enzymes and microbial activity needs further study in the future.