Assessing blue carbon sink through the soil decomposition processes in tropical monsoon (Amazon) and Semiarid mangroves

Blue carbon (C) storage in mangroves soils is linked to the anaerobic organic matter (OM) decomposition under saline conditions. Therefore, studies concerning the decomposition in mangroves are relevant to evaluate factors that could affect C storage, a pivot strategy to climate change adaptation. Thus, OM decomposition was assessed using a standardized decomposition method (e.g., Tea Bag Index - TBI) in Brazilian mangroves in contrasting climate regions (monsoon Amazon and Semiarid). Our results indicated greater decomposition at monsoonal Amazon compared with a Semiarid region for green tea (similar to 25-days: Amazon = 75 %; Semiarid = 60 %; similar to 90-days: Amazon = 85 %; Semiarid = 70 %), and rooibos tea (similar to 25-days: Amazon = 28 %; Semiarid = 20 %; similar to 90-days: Amazon = 37 %; Semiarid = 30 %). The decomposition rate (k) and stabilisation factor (S) were consistent with observed results in wetland ecosystems, with higher values at the beginning of incubation (similar to 25 days), compared to similar to 90-days interval (kmean similar to 25-90 days: Amazon = 0.03-0.01; Semiarid = 0.02-0.01); (Smean similar to 25-90 days: Amazon = 0.10-0.13; Semiarid = 0.28-0.16), indicating a possible trend towards the stabilisation of OM decomposition. Climate differences aside, OM concentration, k, and S values in the Amazon and Semiarid regions can be linked to regional differences, such as tree species and size (net primary productivity), soil composition (physico-chemical conditions, clay assemblages, iron oxide content), river inputs, and tidal range. Therefore, regional differences in the decomposition process are likely to be a consistent indicator that the C cycle can vary substantially along tropical coastlines and is susceptible to global environmental changes.