Temporal carbon assessment through radiometric and repeat measure analysis in a mangrove

Mangrove ecosystems store large quantities of organic carbon for long periods of time. This study explores long-term 'blue carbon' stock change through the first comparative study of radiometric analysis and repeat measures over a multi-decadal period in a mangrove system. Examining a tall and stunted forest, radiometric analysis estimated a soil organic carbon accumulation rate of 4.3 ± 1 Mg C ha-1 y-1 in the tall gallery forest and 1.7 ± 0 Mg C ha-1 y-1 in a stunted mangrove encroachment zone. Repeat measures of only root carbon estimated 5.06 Mg C ha-1 y-1 in the tall forest and 6.63 Mg C ha-1 y-1 in the stunted forest - suggesting an underestimate by radiometric dating of 15% and 74% in the tall and stunted forest respectively. Past bulk carbon measures were not available, however it is likely this discrepancy would be higher if informed by repeat measures of bulk carbon. A higher carbon stock in the stunted forest was attributed to root mass increase, associated with landward mangrove encroachment. Extrapolated to the entire region of NSW, we estimate that mangrove encroachment has contributed at least ~1,853,000 Mg C sequestration over the 70 years for which this has been observed in NSW.