Understanding N-P interactions is crucial for co-limitation in ecosystems but remains unclear due to oversimplified frameworks. Here, a large-scale N and P addition experiment in early soils revealed four key processes driving nutrient coupling, emphasizing carbon’s central role. N addition significantly increases P availability. P availability drives biological N fixation. Both nutrients stimulate carbon fixation, with N having a stronger effect. N addition also intensifies ammonia nitrification, further reducing pH; P has no significant effect on nitrification. Based on these findings and prior research, we establish two models: one depicting P-driven N availability, the other N-driven P availability. These models highlight carbon-related processes as central to N-P coupling, enhancing nutrient availability in early soils. The results reveal a synergistic pathway where nutrient enrichment boosts co-availability rather than intensifying limitation. This framework clarifies the driving mechanisms of N-P bioavailability interactions that support positive ecosystem succession during soil development.