Cyclic nutrient addition drives the evolution of coal organic matter during biodegradation and aqueous immersion

Cyclic nutrient addition is a promising strategy for simulating in-situ hydrodynamic recharge in coal reservoirs to enhance coalbed methane production. However, the transformation mechanism of coal organic matter under the coupled effects of biodegradation and aqueous immersion with intermittent nutrient supply remains unclear. Taking high-volatile bituminous coals (C and B rank) as an example, this study applied a net-flux partitioning approach to quantitatively differentiate the contributions of abiotic and biotic processes to extractable organic matter, macromolecular structural parameters, and pore characteristics. Results show that abiotic processes primarily release pore-throat-entrapped organic matter through swelling, dissolution, and desorption, while causing aliphatic side-chain cleavage, oxygen-containing group removal, and micropore collapse. These actions serve as the prerequisite and sustained driving force for biogasification. Microorganisms rapidly utilize the released labile components to generate CH 4 and CO 2 , and compensate for abiotic losses by releasing bound organic matter and etching new micropores on the coal surface. The coal biogasification process comprises three phases: (1) launch phase, dominated by abiotic activation and microbial compensation; (2) transition phase, featuring peak microbial degradation, consumption of labile hydrocarbons, increased aromatic condensation, and formation of micro-mesopores; and (3) maturation phase, characterized by weakened microbial activity and coexistence of abiotic pore collapse with minor biotic pore generation. This study reveals a positive feedback “release–degradation–reconstruction” mechanism, providing theoretical guidance for optimizing in-situ cyclic nutrient addition strategies in coal biogasification.