The effects of biochars on humus composition in acidic purplish soil

Biochar characteristics are similar to feedstock lignin and cellulose contents. Two kinds of biochar were produced from cellulosic (maize straw) and ligneous (Eupatorium adenophorum) feedstocks, respectively, to evaluate how lignocellulosic feedstock biochar influence the humus composition in acidic purplish soil. Acidic purplish soil was treated with 5% maize straw biochar (MB) or 5% E. adenophorum biochar (EB) for 90 days, and the humic acid (HA), fulvic acid (FA), and humin (HM) contents, chemical elements, and HA color tonal coefficient were analyzed. The biochar specific surface areas were 2.32 m²·g^-1 (MB) and 0.72 m²·g^-1 (EB), and the total pore volumes were 42.71 mm³·g^-1(MB) and 12.59 mm³×g^-1 (EB). The carbon to hydrogen molar ratios (C/H) were 1.91 (MB) and 1.46 (EB), and the oxygen and sulfur to carbon molar ratios(O+S)/C were 0.09 (MB) and 0.16 (EB), indicating that the MB had a stronger adsorption capacity, a higher organic matter, condensation degree, and a lower oxidation degree. Biochar application significantly increased the soil HA, FA, and HM contents (P < 0.05) compared to no application (CK). After 90 days, biochar amendment increased the HA content by 65.59%-102.82%, increased the FA content by 85.87%-118.54%, and increased the HM content by 137.25%-161.23%. The MB increased the humus composition contents more than EB, and both treatments reduced the soil HA/soil organic carbon (SOC) values by 13.53%-27.06% and the FA/SOC values by 6.81%-18.03% (EB treatment P < 0.05). Both treatments also increased the HM/SOC values by 4.58%-11.40% (MB treatment P < 0.05). Biochar amendment increased the HA color tonal coefficient (ΔlgK) degree by 2.40%-5.60%, reduced the C/H of HA by 3.51%-11.81%, and increased the (O+S)/C by 1.51%-8.74%. The biochar application increased the content of each humic component, reduced the C/H condensation degree, and improved the HA oxidation degree; the effect was more prominent when MB was applied. MB application significantly increased the proportion and stability of HM/SOC, but reduced the stability of HAC/H decreased, (O+S)/C increased. EB biochar significantly reduced the proportion of HA/SOC and FA/SOC, and had no effect on the proportion of HM/SOC, and increased the proportion of labile organic carbon.