Remobilisation and utilization of seed P resources are crucial for achieving high yield potential in crops. The remobilization of stored maize seed phosphorus (P), and its allocation towards growing seedlings is critical for P-autotrophy during early ontogeny. We aimed to (i) evaluate the time frame of the origin of P utilized by maize seedlings including the heterotrophic, transitional and autotrophic phases, and (ii) compare P and carbon (C) dynamics in both seed and seedling compartments during the same phases. Using isotopic labelling (32P), we identified different P fluxes (P-heterotrophy, P-transition and P-autotrophy) and determined the proportion of P fluxes from heterotrophic seed P and external P uptake within seedling compartments during 23 days of early ontogeny. Phosphorus heterotrophic growth phase lasted from the first to the fourth day after sowing when seedlings were entirely made up of heterotrophic P originating from remobilized seed P pool. In our experimental conditions, the P-transitional phase, when growing seedlings were supported by both heterotrophic and autotrophic P, lasted from the 5th to the 15th day after sowing. Thereafter, seed P reserves were exhausted and seedlings depended entirely on external P uptake, indicating the beginning of Pautotrophic stage. Our results show that seed P reserve can sustain seedling early growth for up two weeks, but where external P availability are not limiting, seedlings switch to external resources within few days of germination. Further researches are needed to identify regulatory processes governing the transition from P and C heterotrophic to autotrophic growth in seedlings.