Land use change drives decadal-scale persistence of sediment organic carbon storage of restored mangrove

The restoration of blue carbon ecosystems is becoming increasingly recognised as a potentially important tool towards sustainable development because of their capacity to store carbon. However, the spatial and temporal variability of organic carbon storage is high, and the drivers regulating that variability remain poorly defined. This makes it difficult to plan habitat restoration with blue carbon objectives. Here, we quantified spatial and temporal variability of carbon storage in mangrove forests in northern Vietnam, comparing between old-growth forests, forests that have spontaneously naturally regenerated, and areas of intentional restoration. We found that sedimentation rates have increased over the past 25–50 years, but this did not translate to a general increase in carbon accumulation, suggesting a decrease in carbon burial efficiency. An exception was higher carbon burial in the restored mangrove area since the 1960s, followed by a decline since the 1990s. Microalgae were a consistent source (more than 50%) of the buried organic carbon, except in the post-1960 restored mangrove sediments - where the majority of the carbon originated from the mangroves and estuarine particulates. This suggests a shift in ecological structure towards a ‘closed loop’ of carbon recycling. Carbon burial spatial and temporal dynamics were well aligned to coastal land-use change, upstream dam construction and dry-wet climatic variability. These results suggest that both old-growth and restored mangrove forests are important carbon stores. Importantly, we show that mangrove re-establishment can have positive effects on carbon storage – supporting the application of habitat restoration for blue carbon-based sustainable development. However, this benefit can be transient if environmental conditions (because of natural or human factors) do not facilitate the development of ‘normal’ ecological functioning, emphasising the need for a holistic approach towards multiple conservation objectives.