Urbanization reshapes coastal blue carbon, but its effects on how much carbon is stored, who supplies it, and how long it persists remain poorly integrated. We investigated natural and restored mangroves and adjacent tidal flats along an urbanization gradient, and quantified soil organic carbon stocks, burial from <sup>210</sup>Pb profiles, source composition using isotope end-member mixing, and stability from turnover metrics. Our results showed a coordinated triad response to urbanization. Carbon stocks and burial declined, sources shifted away from mangrove detritus toward planktonic and algal inputs, and turnover accelerated, lowering stability. Responses were habitat dependent and nonlinear. Natural mangroves in low urbanization settings maintained the highest sequestration with mangrove-dominated inputs and slower turnover. Restored stands and tidal flats showed steeper stock losses, stronger source substitution, and faster cycling under higher urban pressure. We introduced a triad framework that treats stocks, sources, and stability as coupled state variables along the urbanization gradient and identified two reproducible system states: carbon anchors in low urbanization natural mangroves and instability fronts in restored stands and tidal flats. Shifts in sources and stability precede stock losses, providing clear early warnings of urban impact. A simple diagnostic that combines connectivity and accretion with source composition and a composite stability index guides anchor protection and stability-first restoration. These results link urban growth to blue-carbon performance and define actionable thresholds for sustaining coastal carbon.