Qualitative and quantitative monitoring of changes in soil organic carbon (SOC) is very much essential to keep the soil viable and productive for long-term sustainability. In the present study, long-term enforcement of farmyard manure (FYM) and inorganic nutrient management practices on changes in physio-chemical, microbiological properties and dehydrogenase activity of flooded rice soil was assessed. The treatments consisted of unfertilized (control), inorganic N fertilizer (180 kg ha-1), inorganic fertilizer (NPK @ 180-90-60 kg ha- 1 ), FYM @ 10 Mg ha-1 ) and inorganic fertilizer NPK @ 180-90-60 kg ha-1 and FYM @ 5 Mg ha-1 in rice-rice cropping system at Andhra Pradesh Rice Research Institute, Maruteru, West Godavari District, Andhra Pradesh. The results revealed that among the different treatments, NPK fertilizer + FYM recorded significantly higher Microbial biomass carbon and dehydrogenase activity followed by FYM, inorganic fertilizer NPK and control. A significant four-fold increased activity of dehydrogenase was recorded at the panicle initiation stage of crop growth. The dehydrogenase activity was significantly correlated with MBC (0.883**).