Sources of dissolved organic carbon in small volcanic mountainous tropical rivers, examples from Guadeloupe (French West Indies)

In the tropical zone, small watersheds are affected by intense meteorological events. These events play an important role in the erosion of soils and therefore on the sources of organic carbon in small tropical rivers. We studied the geochemistry of two soils on Basse-Terre Island (French West Indies, FWI): ferralitic soil and Andosol. The two studied soils are very similar in terms of soil organic matter (SOM) and soil solution parameters. The total organic carbon (TOC) and total nitrogen (TN) contents vary between 1.7 and 92 g kg(-1) and between 0.1 and 5.5 g kg(-1), respectively, with the highest concentrations observed in the topsoil. The C/N ratios are relatively constant throughout the soil profiles (ca. 12). The carbon isotopic composition of SOM varies between-273%. and-22.7 parts per thousand and presents an enrichment with increasing depth of soil profiles. Dissolved organic carbon (DOC) concentrations in soil solutions, varying from 3.2 to 913 mg L-1, are similar for the both extraction used in lab (with milliQwater and Ca(NO3)(2)) but are higher than those measured in soil solutions sampled from lysimeters (0.65-1.46 mg L-1). The isotopic compositions of DOC obtained by extractions and SOM are comparable, with delta C-13 values ranging from -28.6 parts per thousand to -25.8 parts per thousand. The DOC sampled from lysimeters is systematically depleted in C-13 compared to DOC obtained by extractions, with delta C-13 values of -33.8 parts per thousand to -30.6 parts per thousand. The enrichment of delta C-13 of SOM through the soil profiles is either consistent with the carbon isotopic fractionation of SOM by decomposing organisms, or the differential mineralization of both labile and stable carbon stocks in soils. DOC concentrations in stream waters vary between 0.46 and 5.75 mg L-1, and are generally lower during low water level than floods. The isotopic compositions of DOC in the rivers range from -38.9 parts per thousand to -27.2 parts per thousand, with delta C-13 values, which are more depleted in C-13 during low water level than flood events. The delta C-13(DOC) of water river samples and soil solutions obtained by extraction and collected with lysimeters demonstrates that the DOC in rivers derives essentially from both the lixiviation of the soil surface layers during floods and groundwater flow during low water levels. Lixiviation of soil surface layers can be boosted by significant increases of intensity and duration of meteorological events and can strongly favor the release of surface soil organic matter in rivers and the impoverishment in nutrients of soil surface layers. (C) 2016 Elsevier B.V. All rights reserved.