Soil C quantities of mangrove forests, their competing land uses, and their spatial distribution in the coast of Honda Bay, Philippines

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Mangrove forests provide many ecosystem goods and services and they contain large amount of carbon (C) especially in their soil. Yet, their global area is still declining owing to conversion to non-forest land uses. While studies have been conducted on mangrove soil C stocks, our knowledge on how C stocks of mangrove forests compare with those of non-forest land uses that replaced them is still limited. This knowledge is crucial in land use planning and decision-making in the coastal zone. Site-scale mapping and assessments of mangrove soil C stocks and the land uses that replaced them are also limited. The aim of this study was to quantify and compare the soil C stocks in mangrove forests and their competing non-forest land uses (represented by aquaculture pond, coconut plantation, salt pond and cleared mangrove), estimate soil C loss arising from conversion, and model the soil C stock distribution in the entire study site. On the average, the soil C stock of mangrove forests was 851.9 ± 87 MgC ha− 1 while that of their non-forest competing land uses was less than half at 365.1 ± 31 MgC ha− 1. Closed canopy mangrove was highest at 1040 ± 104 MgC ha− 1, followed by open canopy mangrove (640 ± 131 MgC ha− 1) while aquaculture, salt pond and cleared mangrove had comparable C stocks (454 ± 32, 401 ± 9, 413 ± 25 MgC ha− 1, respectively) and coconut plantation had the least (42 ± 0.7 MgC ha− 1). Overall, the reduction in soil C stock (soil C loss) due to land use conversion in mangrove ranged from 398 to 809 MgC ha− 1 (mean: 486.8 MgC ha− 1) or a decline of 57% in soil C stock, on the average. It was possible to model the site-scale spatial distribution of soil C stocks and predict their values with 85% overall certainty using the Ordinary Kriging approach. Results from this study could help inform current discussions on Blue Carbon and REDD + as well as policy and program development that advance research on soil C conservation and ecosystem services in coastal forested wetlands.