Soil carbon stocks in Gleysols in Brazil

Gleysols are typical mineral hydromorphic soils, characterized by a gley horizon near the surface due to prolonged saturation. These soils originate from a range of unconsolidated fluvial, marine, and lacustrine sediments of Pleistocene or Holocene age. Their texture varies from sandy to clayey, and they exhibit diverse mineralogical assemblages. Frequently, they also present high sodium (Na) saturation, which can lead to their classification as sodic, or solodic soil. Some contain significant amounts of sulfur, resulting in their classification as thionic. In Brazil, Gleysols comprise approximately 4% of the territory and are predominantly found in the Amazonian floodplains and mangrove along the Atlantic coast. We selected 20 soil profiles, comprising 88 layers, Measured soil carbon, bulk density, and layer thickness was used to estimate the soil carbon stocks. Gleysols exhibited a wide distribution of particle sizes, with sand fractions ranging from 6 to 840 g kg⁻¹, silt fractions from 10 to 810 g kg⁻¹, and clay fractions from 10 to 880 g kg⁻¹. Organic carbon values ranged from 0.02 to 138.50 g kg⁻¹, with a median of 1.09 g kg⁻¹. Bulk densities values ranged from 0.09 to 1.73, and this variation not correlated with sand and organic matter content, suggesting a complex soil structure. No significant linear correlation between clay content and organic l (R = 0.02) was found. We calculated the carbon stock to a depth of 0 to 30 cm. The modal carbon stock was 49 Mg C ha⁻¹. The largest soil carbon stock was found in Linhares, ES, with 1437 Mg C ha⁻¹, while the smallest was 18 Mg C ha⁻¹ in Benjamim Constant, AM. The median carbon stock was also 49 Mg C ha⁻¹. Carbon stocks in Gleysols in Brazil are highly variable, with low stocks observed in the Amazon's river floodplains. Similarly, mangrove Gleysols stocks are variable, with high carbon stocks in BA (Bahia) and low stocks in the Amazon. However, sampling and study locations are limited. Drained Gleysols may release high carbon concentrations and become highly acidic if they possess thionic characteristics. Enhanced research on these soil type and a better understanding of the drivers influencing carbon storage are crucial. The preservation of these soils and their associated vegetation (mangroves, riparian forests, igapó forests, and grasslands) is essential to maintain a large stocks of carbon