The Effect Of Biochar Amendment On Soil Oragnic Carbon Sequestraton Of Deep Soil From Paddy Soil

Due to the large storage of SOC in deep soil,its dynamic change has an important impact on atmospheric carbon dioxide concentration and climate change.Therefore,in recent years,the dynamic of SOC in deep soil has received more and more attention.Understanding the stabilization mechanism of SOC in deep soil is great significance to accurately evaluate the potential of soil carbon sequestration.Biochar is a kind of special material with high carbon content and loose porous structure,which is produced by the pyrolysis of organic materials under the condition of oxygen limitation.Biochar application can have an important effect on soil function and health,especially on soil carbon sequestration.However,most studies have focused on topsoil,and there are few reports on the dynamic of soil organic carbon in deep soil.Soil carbon sequestration is related to biological,physical and chemical protection mechanisms.Comprehensive research of the stability of soil organic carbon in deep soil from paddy soil is great significance for the sustainable management of paddy soil organic carbon pool and the reduction of greenhouse emission.In this study,paddy soil in Yixing,Jiangsu Province were treated with corn straw biochar for 4 years.Two field treatments were setted:biochar treatment（BC）and non-biochar treatment（CK）,the biochar was pyrolyzed from corn straw at 450℃and the application rate was 15 t ha^-1.Soil samples of different depths were taken according to the soil layers（A,Ap and W）of paddy soil.The aggregates were separated by wet-sieving method,and the fractions with different densities were separated by density fraction method.The chemical compositions of organic carbon in different stable states were determined by acid hydrolysis method.The functional groups and molecular compositions of soil organic matter were determined by infrared spectroscopy and thermal pyrolysis method.Combined with laboratory incubation experiment,the effect of biochar application on soil microbial activity and carbon use efficiency in different depths were analyzed.The main results are as follows:1.The application of biochar only significantly changed the composition and distribution of surface soil aggregates,and there was no significant difference between the composition and distribution of soil aggregates in deep soil.Compared with the control,biochar application increased the mass ratio of macroaggregates and microaggregates by 23%and 10%,while reduced the mass ratio of silt and clay fractionin top soil;At the same time,the stability of soil aggregates also increased,and the average mass diameter of aggregates（MWD）increased by 17%.In deep soil,the mass ratio of macroaggregates decreased with the increase of soil depth,and the mass ratio of microaggregates and silt-clay fraction increased with the increase of soil depth.From the perspective of organic carbon content,biochar application also significantly changed the organic carbon content of aggregates of different particle sizes in surface soil.Compared with the control,the organic carbon content of macroaggregates,microaggregates,and silt-clay fraction under biochar application all increased with the increase of 27%,32%,and 11%respectively.In different soil depth,compared with the control,biochar application had a significant effect on the organic carbon content of microaggregates in A and Ap soil layers,with an increase of 32%and 13%,respectively.2.Biochar application only significantly changed the distribution of soil organic carbon in different density fractions in surface soil,but had no significant difference in deep soil.Compared with the control,biochar application did not change the mass distribution ratio of light fraction and mineral fraction in top soil,but significantly increased the contents of organic carbon of light fraction and mineral fraction by 40%and 14%,respectively.The mass distribution ratio of mineral fraction increased with the increase of depth,but the content of organic carbon of mineral fraction in deep soil had no difference between bifferent treatments.In terms of the proportion of organic carbon,biochar treatment significantly changed the proportion of organic carbon of light fraction and mineral fraction in A soil layer.And the proportion of organic carbon of mineral fraction increased with the increase of soil depth.3.Biochar application affected the distribution of organic carbon in different stable states.In the view of acid hydrolyzed organic carbon contents,compared with the control,biochar treatment significantly reduced the unstable organic carbon contents in A and Ap soil layers.The LPI-C decreased by 11%,14%,and LPII-C decreased by 8%and 17%in A and Ap soil layers.However,biochar treatment increased the recalcitrant organic carbon content in A and Ap soil layers by 30%and 14%,respectively.Similarly,the application of biochar also increased the recalcitrant index in A and Ap soil layers by 16%and 12%respectively.From the perspective of mineral-bound organic carbon,compared with no biochar treatment,biochar treatment significantly increased the iron-aluminum combined organic carbon content in the surface soil by 19%,but there was no significant difference in Ca combined organic carbon content in all soil depths.4.Biochar application significantly affected the molecular compositions of soil organic matter in different depths.Firstly,biochar application increased the peak area of some functional groups of organic matter at 3500-2 900 cm^-1 and 1630 cm^-1 in different soil depths,but decreased the peak area at 1034 cm^-1.In another,biochar application increased the percentage contents of phenols and phenolic esters in different soil depths,but decreased the percentage contents of amino acids,sugars,heterocyclic nitrogen compounds,dicarboxylic acids in different soil depths.5.Biochar application affected the microbial carbon use efficiency in different soil depths.Compared with the control,biochar and straw application only changed the organic carbon content of top soil,but had no significant difference in deep soil.After the straw was added,the contents of DOC and MBC in each soil layer increased rapidly,but top soil and deep soil had different responses to straw input.The increase of cumulative organic carbon mineralization in deep soil（160%～350%）was higher than that in surface soil（62%）,and the increase of microbial biomass in deep soil（36%～65%）was higher than that in surface soil（32%）.The increase of microbial metabolic quotient in deep soil（101%～214%）was higher than that in surface soil（23%）.However,in different soil depths under biochar application,the effects of straw input on the mineralization of organic carbon and microbial metabolism quotient were significantly lower than that of adding straw in the treatment without no biochar treatment.The cumulative mineralization of organic carbon in the A,Ap and W layers decreased by 10%,12%and 18%,and the microbial metabolism quotient decreased by 40%,29%and 28%in the A,Ap and W soil layers.6.The transformation of soil organic carbon in different soil depths was regulated by soil microbial and enzymes.Compared with the control,biochar application only increased the activity of carbon degrading enzymes in surface soil,but decreased the activity of nitrogen degrading enzyme in different soil layers.Long-term application of biochar changed the microbial activity and community structure compositions in different soil layers.After the input of exogenous straw carbon,the ratio of Gram⁺/Gram^-and Bacteria/Fungi showed a decreasing trend.In summary,the stability of soil organic carbon was highly dependened on the interaction between different mechanisms,and it was regulated by the molecular compositions of organic matter,the protection of aggregates and soil mineral and the microbial activity.The effects of biochar application on soil organic carbon protection mechanism in different soil depths was significantly different.Due to the strong physical protection of organic carbon in deep soil,the decomposition of organic carbon by microbial was inhibited.Although biochar application did not significantly changed the content of organic matter in deep soil,the results of FTIR and Py-GC-MS showed that the application of biochar could affected the molecular compositions of organic matter in deep soil.In terms of the effect of microbial carbon sequestration,biochar application affected soil microbial community compositions and reduced the decomposition rate of organic carbon and the microbial metabolic quotient,and improved the ability of microbial to assimilate carbon.Therefore,biochar application was beneficial to increase the carbon sequestration potential of in deep paddy soil.