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MEND Modeling

Diagram of the C-N coupled Microbial-ENzyme Decomposition (MEND) model
Ra and Rh are autotrophic and heterotrophic respiration, respectively. POMO and POMH are particulate organic matter (POM) decomposed by oxidative (EPO) and hydrolytic enzymes (EPH), 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 POMO, POMH, and DOM. Microbes consist of active (MBA) and dormant microbes (MBD). DOM can be assimilated by MBA. 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.