Study On Accumulation Mechanism Of Soil Organic Matter Under Straw And Its Biochar Application

The black soil of Northeast China is a kind of high quality soil type with good character,high fertility and suitable for crop growth.However,the black soil in Northeast China is severely declining in fertility due to natural factors,fertilization,and intensive cultivation by traditional agricultural practices.As a result,the black soil in Northeast China has the problems of"becoming infertile,""thinning,"and"hardening".Soil organic matter（SOM）plays an important role in soil fertility and environmental quality.Exogenous carbon input and soil microbial necromass carbon play an important role in the accumulation and enhancement of SOM.Straw returning has become an important way to maintain soil fertility and improve the quantity and quality of SOM in Northeast China.Currently,biochar from straw has emerged as an emerging soil improvement technology with great potential to sequester carbon and reduce emissions.So far,there is limited research information on the synergistic effects of straw and biochar applied in the field of SOM enhancement and carbon sequestration,especially the comparative evaluation of straw-derived biochar and straw return to the field.In this study,the black soil of Northeast China was used as the research object,based on a field experiment of straw and biochar application for four consecutive years（2017–2021）,seven treatments were set up:no straw and biochar treatment（CK）,straw addition rate 0.25%treatment（CS1）,straw addition rate 0.53%treatment（CS2）,biochar addition rate 0.2%treatment（BC1）,biochar addition rate 0.37%treatment（BC2）,adding rate of 0.27%straw+0.1%biochar treatment（CB1）,adding rate 0.13%straw+0.19%biochar treatment（CB2）.The effects of corn straw and its biochar application on soil physicochemical properties,soil organic carbon（SOC）and its carbon management index（CMI）,aggregates stability and its organic carbon and total nitrogen content,humus composition and its structural characteristics,and microbial necromass carbon were studied.The molecular structure of humic acid（HA）was characterized by fourier transform infrared spectroscopy,thermogravimetry analysis and three-dimensional fluorescence.The main findings are as follows:1.The application of straw and biochar can improve soil physicochemical properties and increase soil fertilizer supply and protection capacity.Compared with CK,straw and biochar returning significantly increased soil p H（0.14–0.29 units）and cation exchange capacity（4.42%–24.29%）,and significantly reduced soil exchangable acid content（24.71%–52.94%）and soil bulk density（3.45%–8.76%）.The return of straw and biochar to the field was beneficial to the improvement of soil nutrients.Compared with CK,CS2,BC1,BC2 and CB1 treatments significantly increased the contents of soil hydrolysable nitrogen,available phosphorus,total nitrogen and total phosphorus.The content of hydrolysable nitrogen,total nitrogen and total phosphorus in BC2 treatment increased by 19.53%,34.31%and 14.45%,respectively,while the content of available phosphorus in CB1 treatment increased by 23.04%.2.The application of straw and biochar significantly increased SOC content by5.63%–23.21%,and increased with the increase of straw or biochar addition rate.The SOC content of CS2 and BC2 treatments was significantly increased by 23.21%and 11.61%compared with CK,respectively.The application of biochar alone was more beneficial to the increase of SOC content than that of straw alone.Straw and biochar application significantly increased soil easily oxidized organic carbon（EOC）,carbon pool index（CPI）and carbon management index（CMI）.Compared with straw and straw biochar,biochar application is more conducive to CPI increase,while straw return to the field is more conducive to improve CMI.Applying straw and biochar can not only improve soil carbon pool,but also improve SOM quality and SOM activity,which is conducive to soil fertility.Compared with applying biochar,applying straw was more beneficial to improving SOM quality.3.The application of straw and biochar significantly increased the amount of soil humic substances and changed the chemical composition of humic substances.Compared with straw,the application of biochar is more beneficial to the accumulation of humic substances.The results showed that straw returning significantly increased the aliphatic chain hydrocarbon carbon groups of HA,especially CS2 treatment had the strongest fluorescence intensity at peak A–C,HA had the most abundant hydroxyl,alkoxy,methoxy,amino and other electron donating substitutions,simplified the molecular structure of HA,indicating the regeneration of HA.Compared to straw and straw biochar,the application of straw increases the aliphatic nature of the HA molecular structure,renews the HA,"simplifies"and"rejuvenates"the HA,and contributes to the improvement of SOM quality.Application of straw biochar improves the aromaticity of HA,"complicates"HA,improves the stability of SOM and facilitates carbon sequestration.4.Soil microbial necromass carbon accounted for 39.78%–51.33%of the total SOC.Application of straw and biochar significantly increased the amount of soil microbial necromass carbon,and the increase rate increased with the increase of straw and biochar application amount.CS2 and BC2 treatments increased significantly by 21.65%and 20.19%compared with CK,respectively,but there was no significant difference between CS2 and BC2 treatments.In soil microbial necromass carbon,those from fungi were higher than those from bacteria,accounting for 66.45%–72.04%and 27.96%–33.55%,respectively.The application of straw and biochar significantly increased the microbial necromass carbon from fungi and bacteria,but the effect of straw application on microbial necromass carbon from bacteria was greater than that of straw biochar.Among the soil amino sugar fractions,the largest proportion of soil glucosamine（523.98–678.32 mg/kg）was 11.44–14.31 times greater than the proportion of muramic acid（44.44–52.10 mg/kg）,indicating that fungal microbial necromass play an important role in SOM important role in the stabilization mechanism.5.In the composition of soil aggregates,the dominant size was 2–0.25 mm large aggregates,accounting for 43.79%–49.08%,followed by 0.25–0.053 mm microaggregates.Compared to the CK,the application of straw and biochar significantly increased the proportion of>2 mm large aggregates by 19.56%–32.67%and significantly improved the stability of soil aggregates,with significantly higher mean weight diameter（MWD）,geometric mean diameter（GMD）and greater than 0.25 mm aggregates content(R_0.25).Compared with CK,the organic carbon content of each size aggregate treated with straw and biochar increased,and BC2 treatment increased the most.In conclusion,applying straw and biochar for four consecutive years improved the physicochemical properties of soil.Application of straw and biochar significantly increased the total SOC content and promoted the accumulation of soil humic substances carbon and microbial necromass carbon.Compared with the application of straw biochar,the application of straw improved the aliphatic property of HA molecular structure,HA was updated and"simplified"and"rejuvenated",and increased the activity of SOM,which was beneficial to the improvement of SOM quality.The application of straw biochar is more conducive to the accumulation of SOC and humic substances,increasing the aromaticity of HA,"complicating"HA,improving the stability of SOM and facilitating carbon sequestration.In the future,we should further study how the ratio of straw and biochar can better balance the stability of SOM and the activity of soil fertility,and clarify the synergic mechanism of carbon sequestration and fertilizer cultivation by straw and biochar application.