R_a and R_h are autotrophic and heterotrophic respiration, respectively. POM_O and POM_H are particulate organic matter (POM) decomposed by oxidative (EP_O) and hydrolytic enzymes (EP_H), respectively. MOM is mineral-associated organic matter, which is decomposed by a mixed enzyme group EM. Dissolved organic matter (DOM) interacts with the active layer of MOM (QOM) through sorption and desorption. Litter enters POM_O, POM_H, and DOM. Microbes consist of active (MB_A) and dormant microbes (MB_D). DOM can be assimilated by MB_A. Mineral N deposition and fertilization enter NH₄⁺ and NO₃^– that can be immobilized by microbes and taken up by plant roots. NH₄⁺ adsorption is also considered. N fixation, nitrification and denitrification are mediated by nitrogenase (nifH), ammonia oxidases (amoA, nxrA/B) and N-reductases (narG/napA, nirS/nirK, norB, nosZ), respectively. Mineral N loss pathways include leaching (NO₃^– and NO₂^–) and gas emission (NO, N₂O, and N₂) from soil to atmosphere.