Assessing Carbon Stability in Mangroves Soil by Comparing Two Fractionation Methods

Session: 37th Annual Undergraduate Research Exhibition Sponsored by Sigma Gamma Epsilon (Posters)

Presenting Author:

Andrea Vázquez Marrero

Authors:

Vázquez Marrero, Andrea¹, Aguilar Enriquez, Beatriz Alejandra², Smoak, Joseph M.³, Romero, Isabel C.⁴, Breithaupt, Josh L.⁵, Rosenheim, Brad E.⁶

(1) Environmental Sciences, University of Puerto Rico, Río Piedras, Gurabo, PR, USA, (2) University of Florida, College of Marine Science, Saint Petersburg, FL, USA, (3) University of South Florida, Saint Petersburg, FL, USA, (4) University of South Carolina, Columbia, SC, USA, (5) Florida State University, Tallahassee, FL, USA, (6) University of South Florida, College of Marine Science, St Petersburg, FL, USA,

Abstract:

Mangroves are vital ecosystems that help shield coastlines, provide habitat for diverse species, and act as significant carbon sinks. A large portion of that carbon is locked away in the Soil Organic Matter (SOM), which consists of two general components: Particulate Organic Matter (POM) and Mineral-Associated Organic Matter (MAOM). The POM is primarily composed of fragmented structural material from both plants and microbes, whereas the MAOM is mineral-bound organic molecules in aggregates that tend to be more stable and resist microbial decomposition.  

This study evaluates whether size fractionation can approximate density-based MAOM measurements in Florida mangroves. Despite its importance, few studies have assessed MAOM in mangrove soils, considering the heterogeneity of these environments. A significant challenge is that quantifying MAOM typically relies on density separation techniques, and it remains unclear whether simpler, more accessible size-based fractionation methods can serve as effective proxies in organic-rich, low-mineral settings. By using sites from a study comparing restored and natural mangroves, organic matter composition was compared between density and size fractionation methods across three sedimentary units: peat, organic-rich mud, and sand. MAOM was defined as material smaller than 63 µm for size fractionation and as the heavy fraction material (>1.83 g/cm^-3) for density separation. For both MAOM characterizations, we determined the percentage of organic carbon using an elemental analyzer to calculate the MAOC of the mass obtained by both methods, hypothesizing that there would be no difference in the %OC between the two approaches.  

Results exhibit that peat, the most carbon-rich sedimentary unit, contained the highest mass by size fractionation, in the fine size fraction (<63 µm), but also showed the highest percentage of MAOC in the heavier fraction. This highlights peat’s key role in long-term carbon storage and suggests that the choice of method can influence how that storage is interpreted. Sand-dominated soils retained more mass in the heavy fraction from density separation, indicating more mineral association, while organic-rich muds exhibited high variability across both methods. Although both fractionation methods aligned broadly, variation in peat layers revealed that they are not fully interchangeable. For mineral-rich units like sand, size fractionation underestimates MAOM compared to density separation. Therefore, we recommend density-based methods for quantifying MAOM in mangrove soils, especially where mineral content is high.