Tropical Histosols of the lower Orinoco Delta, features and preliminary quantification of their carbon storage

Abstract

Histosols are potentially important in the global carbon cycle, since they show significant carbon accumulation. Variations in Histosol features may influence both vegetation types and carbon storage amounts and rates. In this paper, we compare Histosols of the Lower Orinoco River Delta by examining relationships among the vegetation communities they maintain and differences among organic and mineral layers. We preliminarily assess their potential organic carbon accumulation. Soil samples were collected from 227 sites and analyzed for soil texture, organic matter, pH, salinity, organic carbon, exchangeable cations, nutrients (P, N, K) and sulphate acid potential. Histosols accounted for more than 80%, whilst Entisols represented 11.8% of the studied samples. The interdistributary peats may be ombrogenous, since they receive inorganic elements mainly by atmospheric deposition, whereas coastal and some riverine peats may be topogenous. Like other ombrogenous tropical peats, topsoils had higher levels of P, N, Ca and Mg than bottom soils. Total nitrogen suggested some degree of N deficiency due to extreme acidity. Total phosphorus was probably associated with humic and fulvic acids. When compared to other ombrogenous tropical peat soils, our total phosphorus values were higher, which is consistent with the younger age of the lower Orinoco Delta peat. The area showed potential sulphate acid soils. Estimations of organic carbon stored yielded 45.0–97.0 kg m − 2 , similar to the range reported for other peatland soils of the world of similar age and accumulation rate. The study area contributed approximately 0.049 Gt to the global carbon storage in soils. Extrapolating from these results, the carbon storage of the study in the lower Orinoco Delta (roughly 7000 km 2) is approximately 0.067 Gt in the first metre of soil. This is not a negligible contribution when compared with the 19.9 Gt of carbon in the first metre of soil of all tropical peatland soils (roughly 444,000 km 2). When combined with the 150,000 km 2 estimated for the Amazonia, the importance of neotropical peatlands as sinks or sources of carbon appears greater than previously thought.