Active chemical fractions with growth promotion activity of humic substances obtained from different animal manure...

Humic acid (HA) and fulvic acid (FA) are the most abundant components of the organic matter in the compost. However, the key chemical structures for the bioactivity of HA/FA and how these structures being affected by composting conditions are not fully understood. The changes in chemical compositions between HA and FA were primarily driven by differences in the contents of carboxyl C, aromatic C, O- alkyl C and C/N ratio. FA was generally more active in promoting plant biomass accumulation and uptake of nitrogen, phosphorous and potassium due to the higher carboxyl C. Raw materials with a C/N ratio of 35 and pH 8 were more conducive for the syntheisis of HS and demonstrated the most significant plant growth stimulation acvitity. On average, the plant dry weight treated by the C/N 35_HA, C/N 35_FA, pH 8_HA and pH 8_FA increased by 79.41 %, 107.69 %, 76.47 % and 120.70 %, respectively. Linear regression analysis showed that plant fresh weight increased with higher alkyl index, carboxyl C content and oxygen content, C/N ratio and O/C ratio, while it was negatively correlated with E₄/E₆ ratio. This study provided theoretical support for the production of high-value-added compost for sustainable reuse of animal manures.