Human population density and blue carbon stocks in mangroves soils

Mangrove soils provide many important ecosystem services such as carbon sequestration, yet they are vulnerable to the negative impacts brought on by anthropogenic activities. Research in recent decades has shown a progressive loss of blue carbon in mangrove forests as they are converted to aquaculture, agriculture, and urban development. We seek to study the relationship between human population density and soil carbon stocks in urban mangrove forests to quantify their role in the global carbon budget. To this end, we conducted a global analysis, collecting mangrove soil carbon data from previous studies and calculating population density for each study location utilizing a recent database from the European Commission. Results indicate population density has a negative association with mangrove soil carbon stocks. When human population density reaches 300 people km-2, which is defined as 'urban domains' in the European Commission database, mangrove soil carbon is estimated to be lower than isolated mangrove forests by 37%. Nonetheless, after accounting for climatic factors in the model, we see the negative relationship between population density and soil carbon is reduced and is even non-significant in mixed effects models. This suggests population density is not a good measure for the direct effects of humans on mangrove ecosystems and further implies mangrove ecosystems in close proximity to very high population density can still possess valuable carbon stocks. Our work provides a better understanding of how soil carbon stocks in existing mangrove forests correlate with different levels of population density, underscores the importance of protecting existing mangroves and especially those in areas with high human population density, and calls for further studies on the association between human activities and mangrove forest carbon stocks.