Effect Of Biochar Application On Carbon,nitrogen And Microbial Characteristics Within Aggregates Of Fluvo-aquic Soil

Soil aggregates are the material basis of soil fertility and the main site of soil microbial activities,which play a central role in the turnover of soil nutrients.As a kind of soil amendment,biochar can improve soil fertility and soil micro-ecological environment.Previous studies about the responses of soil physical,chemical and microbial properties to biochar application were mainly concentrated on the bulk soil level,but there is still a lack of systematic research on the soil aggregate level.In this study,a2-year field experiment was conducted in calcareous fluvo-aquic soil at the Modern Agricultural Research Base,Henan Province,China.The treatments comprised the application of no fertiliser(CK),chemical fertilisers(NPK),biochar(BC),and biochar plus chemical fertilisers(BC+NPK).This study applied multiple advanced techniques such as anion exchange chromatography,soil enzymology,molecular ecology,to investigate the stability of soil aggregates and the distributions of total organic carbon(TOC),total nitrogen(TN),microbial biomass carbon and nitrogen(MBC,MBN),enzyme activities,microbial community compositions and microbial residues within aggregates of fluvo-aquic soil after two year of biochar application.The main findings were as follows:(1)The distributions of organic carbon and nitrogen within aggregates of fluvo-aquic soil under biochar application.The combined application BC and chemical fertilisers significantly enhanced the stability of fluvo-aquic soil aggregates.Both the particle-size fraction and fertilisation had remarkably effects on TOC and TN contents.The 2–0.25 mm aggregate showed the highest TOC,TN contents,C/N ratio,and contribution rates of TOC and TN relative to other particle-size fractions.Compared with CK and NPK treatments,the BC and BC+NPK treatments notably increased the TOC content within each particle-size fraction,the TN content wihtin 2–0.25 mm aggregate,and the C/N ratio within 2–0.053 mm aggregates.(2)The changes of MBC,MBN contents and enzyme activities within aggregates of fluvo-aquic soil under biochar application.The MBC,MBN contents and enzyme activities were the highest within 2–0.25 mm aggregate,followed by within >2 mm aggregate.Compared with CK,the BC+NPK treatment significantly increased the MBC contents within >2 mm and 0.25–0.053 mm aggregates by 59.57% and 34.68%,and the MBN contents within >2 mm and 2–0.25 mm aggregates by42.24% and 19.28%,respectively.The addition of biochar significantly reduced the MBC/TOC ratio within 2–0.25 mm and 0.25–0.053 mm aggregates,which is beneficial to the sequestration of soil carbon.The application of chemical fertilisers significantly increased the activities of C cycle-related enzymes within 2–0.053 mm aggregates,as well as urease activity within >2 mm aggregate and acetyl-glucosaminidase activity within 0.25–0.053 mm aggregate.MBC had the highest explained proportion for the changes of enzyme activity within aggregate.(3)The changes of microbial community structure within aggregates of fluvo-aquic soil under biochar application.Both the particle-size fraction and the fertilization remarkably affected the composition of soil microbial community.The 2–0.25 mm aggregate showed the highest total amount of phospholipid fatty acid(PLFAs)relative to other fractions.The relative abundance of bacteria within >0.25 mm aggregates were higher than that in <0.25 mm aggregates,and the opposite was true for actinomycetes.Compared with CK,the NPK and BC+NPK treatments significantly increased the total amount of PLFAs within each aggregate,and BC+NPK treatment had the largest increase for that within 0.25–0.053 mm aggregate.BC+NPK treatment significantly increased the relative abundance of fungi within >2 mm aggregate,significantly reduced the relative abundance of bacteria within this fraction,and significantly increased the fungal/bacterial ratio and G+/G-ratio within >0.25 mm aggregates relative to CK.MBC had the highest explained proportion for the changes of microbial community structure within aggregate.(4)The changes of microbial residue within aggregates of fluvo-aquic soil under biochar application.The 2–0.25 mm aggregate had the highest amino sugars and microbial residues carbon contents,followed by >2 mm aggregate.Microbial residues within 0.25–0.053 mm aggregate was more sensitive to fertilisation,NPK treatment significantly increased muramic acid(Mur A)content within this fraction,BC significantly increased the glucosamine(Glu N)and fungal residues carbon(FRC)contents,and the above all increased significantly under BC+NPK treatment.Relative to CK,the BC and BC+NPK treatments significantly decreased the contribution of microbial residues to TOC within each particle-size aggregate and the contribution of amino sugar nitrogen to TN within 2–0.25 mm aggregate.Soil bacterial biomass and acetyl-glucosaminidase activity significantly positively affected amino sugar and microbial residue carbon.In summary,the addition of biochar to the soil had a longer effect.The combined application of biochar with chemical fertilisers significantly improved the aggregate stability and the C,N storage capacity within aggregates,and significantly increased the MBN content within >0.25 mm aggregate and the fungi/Bacteria ratio and G+/G-ratio,which provide scientific basis for improving soil microenvironment and soil quality.