<p>A long-term experiment on integrated nutrient management in a rice-wheat cropping system, initiated in 1984 at Bihar Agricultural College, was used to assess the effect of nutrient management practices on the kinetics of soil dehydrogenase activity. The objective was to determine how combinations of inorganic fertilizers and organic amendments influence substrate affinity and catalytic efficiency of dehydrogenase. Soil samples collected after the thirty-fourth crop cycle were analyzed for Michaelis constant (K_m) and maximum reaction velocity (V_max) of dehydrogenase. The control had the highest K_m, indicating the lowest enzyme–substrate affinity. In contrast, treatments with farmyard manure, wheat straw, or <i>Sesbania</i> manure reduced K_m by 63–73% as compared to the fully inorganic treatment, reflecting markedly higher affinity. In comparison to control, V_max increased by 19.47–23.47% with farmyard manure, 13.35–15.71% with wheat straw, and 10.52–19.47% with <i>Sesbania</i>. The highest catalytic efficiency (V_max/K_m) was observed in <i>Sesbania</i> treatments, particularly with reduced fertilizer dose. Overall, the study recommends integrating quality organic amendments, especially <i>Sesbania</i> green leaf manuring into rice–wheat systems to sustain soil enzymatic activity and long-term soil health.</p>