Effect Of Biochar Application On The Mechanism Of Soil Aggregates Carbon Sequestration Under Freeze-thaw Conditions

Inner Mongolia Autonomous Region is located in the northern frontier of China,spanning a wide range of longitude and latitude.The winter in the autonomous region is long and cold,and the cold season in most areas lasts for five months to half a year.Freezing-thawing cycle is one of the important factors affecting the formation and stability of soil aggregates in the region.Freezing-thawing alternation caused soil aggregate fragmentation and increased CO₂ emissions.Biochar application can change the composition of soil aggregates and the distribution characteristics of soil organic carbon,which has an important impact on soil carbon sequestration However,the response mechanism of CO₂ emission to biochar application under freezing-thawing conditions is still unclear.In this study,the effects of biochar application on soil CO₂ emission,soli aggregate composition and stability,and organic carbon distribution in soil aggregates under different freezing-thawing conditions were studied by laboratory simulation experiments,and the proportion of biochar source carbon in soil CO₂ emission and its distribution in soil aggregates were determined byδ¹³C tracer technology.The main results of the study were as follows:（1）Low temperature freezing increased the emission rate of CO₂ in soil,which was in the order of high amplitude freezing and thawing（HF）>low amplitude freezing and thawing（LF）>constant temperature control（CK）;At the same time,the CO₂ emission rate increased first and then decreased with the increase of freeze-thaw cycles.The application of biochar could reduce the CO₂ emissions under freeze-thaw conditions.The cumulative CO₂ emission of HF treatment was 16.7%higher than that of LF treatment.While the cumulative CO₂emission of BHF treatment with biochar was only 8.6%higher than that of BLF treatment.The carbon derived from biochar accounted for about 34.3%of the soil CO₂ emissions under freeze-thaw conditions after biochar application.（2）Low temperature freezing and thawing reduced the content of 0.25～2 mm water-stable aggregates,and the reduction of the size of water-stable aggregates increased with the increase of freezing and thawing intensity,with a maximum reduction of about 34.5%.The content of water-stable aggregates in 0.053～0.25 mm and<0.053 mm fractions showed an increasing trend.Biochar application increased the content of macroaggregate,and the breakage of macroaggregate were reduced by 4.11%～7.74%under freeze-thaw conditions after biochar application.（3）Compared with constant temperature treatment,the mean weight diameter（MWD）,geometric mean diameter（GMD）and water-stable aggregate(WR_0.25)of soil under freeze-thaw conditions decreased by 18.5%～25.5%,20.9%～26.3%and 23.9%～34.1%,respectively.Biochar application increased the MWD,GMD and WR_0.25 of soil aggregates,and the stability of soil aggregates was enhanced.The three stability indexes of the treatment with biochar were 6.98%～16.54%higher than those of the treatment without biochar under freeze-thaw action.（4）The content of organic carbon in 0.25～2 mm water-stable aggregates decreased while the content of organic carbon in 0.053～0.25 mm and<0.053 mm water-stable aggregates increased under freeze-thaw conditions.Biochar application significantly increased the soil organic carbon content of all particle sizes,and the increase effect of 0.25～2 mm particle size was significant.At the same time,the distribution of organic carbon in soil water-stable aggregates was changed by biochar application under freeze-thaw conditions.Compared with the treatment without biochar application,the application of biochar under freeze-thaw condtions increased the distribution of organic carbon in 0.25～2mm water-stable aggregates.（5）Biochar was mainly accumulated in 0.25～2 mm aggregates,followed by 0.053～0.25 mm and<0.053 mm aggregates,and the proportion of biochar in 0.25～2 mm aggregates was about 50%.The distribution of biochar from 0.25～2 mm soil into smaller soil fractions increased significantly due to freeze-thaw.In conclusion,freezing and thawing caused the decrease of soil stability,the fragmentation of aggregate structure,and the increase of soil organic carbon content,which led to the increase of soil CO₂ emission.Biochar is mainly enriched in macro-aggregates,which can increase the distribution of organic carbon in macroaggregates,not only improve the stability of soil aggregates,but also play a certain role in protecting the original soil organic carbon.Therefore,biochar application can play a certain role in carbon sequestration and emission reduction under freeze-thaw conditions.