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

Humic materials make important contributions to soil organic carbon (SOC) and nitrogen (TN) accumulation. However, information on the correlation between humic substances and the accumulation of aggregate-protected C and N in response to biochar application to soils under tropical agroecosystems is limited. Therefore, a replicated 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 SOC and TN in a humid tropical agroecosystem. The treatments included no biochar/ control (CT), 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 BC-30 and BC-30+P treatments significantly increased the contents of humic materials (humic and fulvic acids) as compared to the CT. There was a significant increase in the mean weight diameter of soil aggregates by 3- and 4-fold in the BC-30 and BC-30+P treated soils, respectively. Similarly, the application of the BC-30 and BC-30+P treatments yielded significant increases in the soil structural coefficient by 3- and 4-fold, respectively, relative to the CT. A 10% decline in soil bulk density was observed with biochar applied at 30-Mg ha-1. Significant increases in SOC and TN accumulation were observed in the macroaggregates of the biochar-amended soils. An inverse relationship between the degree of polymerization and aggregate-protected C and N was observed, implying that low molecular weight aliphatic compounds favor SOC and TN accumulation with biochar amendments. The important soil property that contributed to SOC and TN accumulation in aggregates was FA, 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.