| Biochar refers to highly aromatic and stable organic matter produced from pyrolysis under oxygen-free or oxygen-limited conditions.It is now considered as an important means to reduce greenhouse gas emissions and carbon sequestration.At the same time,the application of biochar in soil has a profound impact on soil organic carbon cycle.The stabilization mechanism of soil organic carbon can be roughly divided into physical isolation of soil aggregates,chemical adsorption of organic matter by minerals and chemical characteristics of organic matter itself.Systematic study on the turnover of organic carbon in different mechanisms after application of biochar is not only helpful to clarify the distribution of biochar in soil,but also important to mitigate global climate change and improve the efficient,coordinated and sustainable development of agriculture in China.In this study,paddy soils in southern China(C0)were treated with corn straw biochar(15 t ha-1)for 2 years(C15).The undisturbed soils were separated by wet-sieving method.Then 13C isotopes,microbial community structure and microbial residues were determined.The distribution of biochar in soil aggregates and the effect of biochar on microbial community structure and organic compounds from high stability microbial sources were studied.The dynamics of microorganisms in microenvironment and the mechanism of microbial sources organic matter accumulation under the improvement of biochar application were discussed;The undisturbed soils were separated by sequential density fractionation(SDF)method,13C isotope and minerals were analyzed by stable isotope tracer technology,X-ray diffraction(XRD)and infrared spectroscopy(IR).The binding of biochar on the surface of minerals with different densities and the effect of biochar on the binding of organic matter on the surface of different minerals were studied.Through the above two aspects of research,we hope to enhance our understanding between biochar and different protection mechanisms of organic matter,and become an important theoretical basis for the promotion of biochar as an important means of sustainable agricultural development.The results are as follows:1.By X-ray diffraction,the whole density fractions can be clearly divided into three regions according to the differences among minerals:light fraction(<1.85 g cm-3)dominated by plant residues and free biochar;clay mineral fraction(1.85-2.45 g cm-3)dominated by secondary minerals such as montmorillonite and chlorite;primary mineral fraction(>2.45 g cm-3)dominated by primary minerals such as mica and quartz.2.There are significant differences in the distribution of organic carbon in different density fractions.With the increase of density,the content of organic carbon in C15 and C0 treatments showed a decreasing trend.Infrared spectroscopy showed that the peak of O-H(2 923 cm-1)and C=C(1 610 cm-1)of aliphatic groups decreased with the increase of density,which indicated that the content and species of organic matter decreased gradually.The biochar application significantly increased the organic carbon content of light fraction(<1.65 g cm-3,1.65-1.85 g cm-3)by 65.54%,36.12%,respectively,but no significant effect on other fractions.3.Biochar application affected the turnover of native organic carbon.13C isotopes in C15 treatment were higher than those in C0 treatment,indicating that biochar could adhere to organic-mineral complexes in any density.However,with the increase of density,the relative proportion of biochar decreases gradually from 62.52%to 8.96%.Compared with C0 treatment,the application of biochar reduced the native organic carbon by 24.8%,which mainly occurred in the density range of 1.65-2.05 g cm-3.4.Compared with the C0,the application of biochar significantly promoted the formation of macroaggregates,while the proportion of silt and clay fraction decreased significantly.Biochar significantly increased the content of organic carbon in mac-and microaggregates by 50.14%and 41.89%.After excluding the contribution of biochar to organic carbon by 13C isotope analysis,it was found that biochar application resulted in a decrease by 30.61%in the native organic carbon in micro-aggregates and increase by 11.38%in macroaggregates.5.The contents of amino sugars in the same treated soil aggregates were as follows:macroaggregates>microaggregates=silt and clay fraction.The application of biochar significantly reduced the content of microbial residues.Compared with C0 treatment,the content of the total amino sugars decreased by 18.50%,and more specifically Glucosamine(GluN),Galactosamine(GalN)and Muramic acid(MurA)decreased by 15.01%,24.37%and 17.22%,respectively.6.The biochar application significantly changed the diversity of bacteria and fungi in the bulk soil.When C15 was compared with the C0 treatment,the number of OTU increased by 12.39%and the number of ITS increased by 27.60%in the bulk soil.Similarily for Chao 1 index,the increase for OUT and ITS was 13.89%and 15.47%,respectively,for the bulk soil.Finally,for Shannon index the increase for OUT was 2.84%and for ITS 12.32%,for the bulk soil.Although there was no significant difference in the bacterial ?-diversity among the aggregates treated in C15,the bacterial?-diversity was significantly higher than that of the corresponding size aggregates treated with C0.There was no significant difference in ?-diversity between the two fungi treatments.Biochar significantly changed the relative abundance of phyla in aggregates.Compared with C0 treatment,biochar application significantly increased the relative abundance of Proteobacteria and Mortierellomycota in macroaggregates by 14.37%and 33.28%,respectively.The relative abundance of Acidobacteria and Ascomycota in microaggregates decreased by 20.15%and 19.93%,respectively.The relative abundance of Basidiomycota in silt-clay fraction increased by 90.25%,while Ascomycta and Mortierellomycota decreased by 12.15%and 12.58%,respectively.In conclusion,two years after the biochar application,SOC was affected in different levels.From the point of view of chemically protected SOC,the content of total organic carbon increased significantly under the biochar application,but the native organic carbon protected by different minerals was decreased.From the point of view of physically protected SOC,biochar not only promoted the formation of macro-aggregates,but also increased the content of the total organic carbon in the macro-aggregates and significantly promoted the turnover of native organic carbon in the micro-aggregates,which may be related to the changes of the microbial communities observed.However,But the soil organic carbon turnover and its relationship with the microbial activity needs to be further studied. |
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