Blue carbon gain by plant invasion in saltmarsh overcompensated carbon loss by land reclamation

Saltmarsh is a major contributor to global blue carbon (C) sink. However, plant invasion and anthropic activities have greatly altered its distribution and its blue C function in the last decades. We used remote sensing and data synthesis to investigate how the four decades of plant invasion and land reclamation affect the spatiotemporal variation of blue C in Yancheng saltmarshes, the largest coastal ecosystem in eastern China. We revealed that the invasion of exotic cordgrass, Spartina alterniflora , greatly increased the regional blue C storage, mainly by the seaward expansion of cordgrass in mudflats and its high ecosystem C stock. The iconic Suaeda salsa “Red Beach” marshes shrank by 90%, mainly due to reed occupation by the native reed, Phragmites australis . Phragmites australis marsh area was greatly occupied by reclamation, but well offset by its intrusion into Suaeda salsa . The blue C gain by cordgrass invasion overcompensated C loss due to land reclamation and the loss of “Red Beach”. Our findings highlight that coastal blue C is greatly regulated by vegetation coverage, and the seaward cordgrass invasion can benefit the saltmarsh C sequestration. Such a similar process may apply in other regions of the world, with potentially large implications for blue C sinks. Graphical Abstract