Increased humic materials explain aggregate-protected carbon and nitrogen accumulation in biochar-amended tropical soils

Humic materials make important contributions to soil organic carbon (C) and nitrogen (N) accumulation. However, information on the correlation between humic substances and the accumulation of soil aggregate-protected C and N in response to biochar application under tropical agroecosystems is limited. Therefore, a field trial was conducted to elucidate the effects of biochar on soil aggregate properties and humic materials and how these humic compounds affect aggregate-protected C and N in a humid tropical agroecosystem. The treatments included no-biochar control (CK), 15 Mg biochar ha-1 (BC-15), 30 Mg biochar ha-1 (BC-30), and 30 Mg biochar ha-1+ phosphate fertilizer (BC-30+P). The treatments BC-30 and BC-30+P significantly increased the contents of humic materials (humic and fulvic acids) as compared to CK. There was a significant increase in the mean-weight diameter of soil aggregates by 3- and 4-fold in BC-30 and BC-30+P, respectively. Similarly, BC-30 and BC-30+P led to significant increases in soil structural coefficient by 3- and 4-fold, respectively, relative to CK. Significant increases in organic C and N accumulation were observed in the macroaggregates of the biochar-amended soils. Inverse relationships between the degree of polymerization and aggregate-protected C and N were observed, implying that low-molecular-weight aliphatic compounds favor organic C and N accumulation with biochar amendments. The important soil property that contributed to organic C and N accumulation in aggregates was fulvic acid content, which could be used as an early and sensitive indicator to notice early changes in aggregate-protected C and N accumulation in soils of the humid tropics.