Microbiological Mechanisms of Humic Acid Formation in Soil
There are seven main hypotheses regarding the microbiological mechanisms of humic acid formation:
Waksman's Theory: This theory posits that during the transformation of plant material into humus, nitrogenous and non-nitrogenous organic compounds are decomposed by aerobic microorganisms, resulting in the formation of microbial biomass (soil protein) and lignin. These components then combine with alkaline substances to form humus.
Williams' Theory: According to this theory, humic acids are secretions of soil microorganisms.
Microbial Synthesis Hypothesis: This hypothesis suggests that microorganisms utilize plant material as a carbon and energy source to synthesize high-molecular-weight humic substances within their cells. After the microorganisms die, these substances are released into the soil and subsequently degraded extracellularly into humic acid and fulvic acid.
Cell Autolysis Hypothesis: This hypothesis proposes that humic substances are the products of cell autolysis following the death of plants and microorganisms.
Coal Chemistry Theory: This theory emphasizes two main conditions influencing the accumulation of plant residues in swamps and the formation of peat: (1) physical conditions that isolate plant residues from air, and (2) chemical conditions that maintain optimal microbial activity.
Kononova's Theory: This theory states that humus formation involves two stages—"decomposition" and "condensation"—both of which are mediated by soil microorganisms.
Anaerobic Fermentation Hypothesis: This hypothesis suggests that humic acid formation occurs in three stages—"hydrolysis," "acid production," and "synthesis"—with varying degrees of microbial involvement.
Regardless of the hypothesis, all theories unequivocally affirm the critical role of microorganisms in the formation of humic acids.