Rice cultivation in lowland swamp areas faces the challenge of waterlogging, which can affect nitrogen availability in the growing medium. The floating raft system is one alternative cultivation method that can be applied under these conditions; however, proper nutrient management is still necessary to improve nitrogen use efficiency. This study was conducted to analyze the effect of combined formulations of NPK fertilizer and biofertilizer (Bio N+) on nitrogen availability in rice cultivation using a floating raft system in lowland swamps. This study was designed using a one-factor randomized block design (RBD) consisting of six treatments: no treatment (F0), 100% NPK (F1), 75% NPK + Bio N+ (F2), 50% NPK + Bio N+ (F3), 25% NPK + Bio N+ (F4), and Bio N+ (F5). Each treatment was repeated four times, resulting in 24 experimental units. The results of the study showed that the application of NPK fertilizer, Bio N+, or a combination of the two affected nitrogen availability. At 45 DAP (days after planting), the best treatments for increasing available N content were the application of 75% NPK + Bio N+ and 25% NPK + Bio N+; for ammonium and organic C content, it was the application of 25% NPK + Bio N+; nitrate content and pH were highest with the 75% NPK + Bio N+ treatment; the 25% NPK + Bio N+ and Bio N+ treatments yielded the highest total nitrogen content in the growing medium, while the highest nitrogen content in plant tissues was observed in the 100% NPK treatment.Therefore, nitrogen availability can be attributed to the combined nitrogen supply from NPK fertilizer and the biological nitrogen fixation process carried out by Bio N+ microbes.