Structure Of Humic Acids Oxidized By Hydrogen Peroxide And Its Regulation On Maize Roots

The biological activity of humic acids is closely related to its structure.Oxidation is an important method to change the structure of humic acids.In our research,weathered coal humic acids(HA)were used as materials to prepare a series of oxidized humic acids(coded OHA1~OHA9)by orthogonal design with four factors,including hydrogen peroxide concentration,reaction time,liquid/solid ratio and temperature.The structure of oxidized humic acids and its regulation on maize roots were studied combing ultrafiltration.The main results are as follows:1.Structural Characteristics of Humic Acids Oxidized by Hydrogen Peroxide.Compared with non-oxidized humic acids,hydrogen peroxide oxidation mostly reduced C,H and N content of humic acids,increased O content,C/N,C/H,O/C,E2/E3 and E4/E6.Alkyl carbon,O-alkyl carbon and aromatic carbon content of most oxidized humic acids decreased,carboxyl carbon content increased,hydrophobic index consistently decreased.Among the four preparation conditions,temperature had the biggest effect on the properties of humic acids,followed by hydrogen peroxide concentration and liquid/solid ratio,and time had the least effect.2.Structural Properties of Humic Acids Classified Using Ultrafiltration.1)Compared with HA,the yield of the molecular(10 k Da-50 k Da)and small(<10 k Da)molecular weight of OHA6(medium concentration-high liquid / solid ratio-short time-medium temperature oxidation)was higher,and that of large molecular weight(>50 k Da)was lower.The yield of medium and small molecular weight of OHA3(low concentration-high liquid / solid ratio-long time-high temperature oxidation)was lower than that of HA,and the yield of large molecular weight of OHA3 was higher than that of HA.2)The C content,N content,C/H ratio,aromatic carbon content,aromaticity and hydrophobicity index of humic acids increased with the increase of molecular weight,while,the O content,C/N ratio,O/C ratio,E2/E3,E4/E6 values and the degree of lipidation decreased with the increase of the molecular weight,and the carboxyl group content of the small-molecular-weight humic acids was the highest among the three different molecular weight humic acids.3)Compared with the corresponding fraction of HA,the content of H element of OHA3 and OHA6 decreased while the content of O element increased,the content of C element of large molecular weight fraction increased,while the content of C element of medium and small molecular weight fraction decreased;The C/H of the molecular weight of OHA3 fractions and the medium and small molecular weight fractions of OHA6 decreased,and the C/H of the other fractions increased.4)Compared with HA,all the three different molecular weight fractions of OHA6 have lower aromaticity and hydrophobicity index,higher degree of lipidation,and the effect is more obvious with the decrease of molecular weight;the aromaticity,lipidation degree and hydrophobicity index of different molecular fractions of OHA3 was basically similar to HA.3.Regulation of Different Structures of Oxidized Humic Acids on Maize Root Growth.1)Compared with HA,OHA6 significantly increased the root weight and activity of maize.On the contrary,OHA3 showed the opposite result.and the regulation effect of other oxidized humic acids(OHAn)on roots was between OHA6 and OHA3.2)Regardless of HA,OHA3 or OHA6,the regulation effects on the growth and activity of maize roots are small molecular weight > medium molecular weight > large molecular weight.Compared with the corresponding fraction of HA,the small molecular weight humic acids of OHA6 significantly increased root weight and root activity of maize;the small,medium and large molecular weight fraction of OHA3 mostly reduced the number of roots and root activity of maize roots.3)Regardless of HA,OHA3 or OHA6,when humic acids was applied unilaterally,the growth and activity of maize roots on the side of humic acids was significantly higher than that on the other side where humic acids was not applied,indicating that humic acids had “local” regulation on roots.The effect of unilateral application of humic acids on the other side of the root growth was better than that of the no humic acids addition treatments on both sides,indicating that humic acids has an “overall” effect on maize roots.4)The root dry weight,root activity,total TTC reducing amount,root volume and root surface area were positively correlated with O content,O/C ratio,carboxyl carbon content,O-aromatic carbon and E4/E6 of humic acids.The correlation between O/C ratio,O-aromatic carbon content,O content,carboxyl carbon content and maize roots growth is higher.The C content,N content,methoxy and N-alkyl carbon content,O-alkyl carbon content,alkyl carbon content,hydrophobicity index,and C/H ratio were negatively correlated with maize root growth.