Effects Of Biochar On The Physicochemical Properties And Annual Greenhouse Gas Emissions Of Saline Soils And Freeze-thaw Effect

As an important grain and oil production base in China,the river-loop irrigation area in Inner Mongolia has more serious soil salinisation,a cold and arid climate and soil freezing and thawing for up to about 180 days per year.In recent years,with global warming,the snowpack and soil freeze-thaw environment in the middle and high latitudes will change,in which water and heat transfer,water phase change and salt accumulation will have different degrees of impact on soil moisture,density,temperature and nutrients,etc.Coupled with the rising concentration of greenhouse gases in the atmosphere,which is now considered to be one of the main causes of global warming,the 10 year Inner Mongolia The average warming since 1961 has reached 0.36°C,which is significantly higher than the global（0.12°C）and national（0.23°C）warming over the same period,making it one of the most significant areas of warming in China.Therefore,it is particularly important to study the nature of saline soils in the river-loop irrigation area,dissect the mechanism of soil greenhouse gas emissions,and propose appropriate measures to reduce emissions and increase production.In this study,three biochar additive treatments（biochar dosage 15（D15）,30（D30）t/hm²and a control（CK）without biochar application）were set up before the freeze-thaw period in 2019,based on the existing results at home and abroad,Soil water-thermal-salt-fertility,carbon and nitrogen conditions were monitored during the two-year freeze-thaw period as well as the annual CO₂,CH₄and N₂O emission fluxes from different treatments of soil using static chamber-gas chromatography.The main conclusions are as follows:（1）The application of biochar could increase the temperature,p H,conductivity,nutrient content and thaw index of the tillage layer during the freeze-thaw period to a certain extent,and reduce the freezing index.Compared with the control,the increase in conductivity of 0-20 cm soil layer during the thawing period was 10.6%,while the increase in potassium,alkaline nitrogen and phosphorus contents during the freeze-thawing period ranged from 3.1%to 38.1%,1.3%to 44.6%and 5.4%to 80.4%,respectively,with the application of 30 t/hm²biochar being superior.（2）Compared with the control,biochar treatment increased soil organic carbon,whole carbon content and carbon pool index in the 0-40 cm soil layer during the freeze-thaw period,with the largest increases of 12.5%to 33.6%,12.4%to 23.0%and15.0%to 59.0%,respectively,and the magnitude of soil carbon index content in each soil layer showed that 0-20 cm was greater than that in the 20-40 cm soil layer.The application of biochar reduced the content of soil inorganic N fraction during the first thaw period,while the ability of biochar to increase soil NO₃^--N and NH₄⁺-N content gradually increased with the increase of biochar application time,which was more significant with the application of 30 t/hm²biochar,and the maximum increase was up to 37.6%during the thaw period.（3）Under the freeze-thaw conditions,the application of biochar reduced soil water loss,and the 0-40 cm soil layer had a clear vertical distribution pattern and better water storage capacity than the 40-100 cm soil layer.During the freeze-thaw period,the soil salinity changes were similar in all treatments,with the soil total salt content remaining at a high level in the 0-40 cm soil layer and at a low level in the 40-100 cm soil layer.was reduced,providing a good growing environment for spring crop seedling growth.（4）A comprehensive analysis of the changes in annual greenhouse gas emission fluxes of soils showed that,compared with the control,there were differences in the inhibition of CO₂cumulative emissions of salinized soils by biochar treatment in each period,among which the inhibition effect was significant for soils in the crop growing period,with a reduction of 9.9%～14.4%.The soils in the test periods were all dominated by CH₄uptake,and the application of biochar could promote CH₄uptake in salinized soils,but its ability to reduce the accumulated soil CH₄emissions weakened as the application time of biochar increased.The dynamic changes of annual N₂O emission fluxes were basically the same among different treatments,and the peak of emission occurred in the soil melting period and the crop growing period,especially the crop growing period accounted for nearly half of the N₂O emission in the whole experiment period.In summary,the application of biochar effectively improved the physical and chemical properties of saline soils during the freeze-thaw period.After two rounds of freeze-thaw,biochar could significantly inhibit N₂O emissions and promote CH₄uptake without significantly reducing soil CO₂emissions.Therefore,the application of biochar to saline soils in the river-loop irrigation area is of great significance for saline land management,agricultural economic development and ecological environmental protection,and can provide a good environment for crop growth and reproduction in the coming year,and the application effect is better at a biochar dosage of 30 t/hm².