Organic Matter Content in Mangrove Soils from a Karstic Environment: Comparison between Thermogravimetric and Loss-on-Ignition Analytical Techniques

Mangroves represent one of the most important carbon sinks on the planet due to their ability to store a high organic matter (OM) concentration in their soils. Therefore, OM analysis is important for generating inventories that do not underestimate or overestimate carbon stocks and for reducing uncertainties. Accordingly, we propose the use of thermogravimetric analysis (TGA) against the classical ignition method to determine the OM content in mangrove soils from a karstic region of the Yucatan Peninsula, Mexico. Therefore, fifty-five soil profiles from El Playón in the Sian Ka’an Biosphere Reserve were analyzed and divided according to the condition of the mangrove: conserved, under restoration, or degraded. TGA analysis of mangrove soils, which are in direct contact with water bodies, can be used to accurately identify the thermal decomposition of hydrated calcium sulfates, which cannot be detected using the loss-on-ignition (LOI) method. Using TGA, it was determined that the water content in the internal structure of hydrated calcium sulfates was between 8 and 16% in the preserved mangrove soils, between 2 and 5% in the mangrove soils under restoration, and between 0.5 and 1% in the degraded mangrove soils. The OM content obtained using both techniques was similar; however, using TGA, the amount of water evaporated from calcium sulfates could be measured, and this was not possible with the LOI method. Therefore, the TGA technique can be used as a proxy analysis to determine the OM content in soils, including the amount of water from hydrated calcium sulfates that are found naturally.