Characterization of dissolved organic carbon in cleared forest soils converted to maize cultivation

The impact of cultivation on the nature of dissolved organic carbon (DOC) has been studied in order to understand differences in denitrification rates observed in situ. The quantitative aspect of DOC was studied by means of porous-cup vacuum samplers and water extraction. Qualitative characterization was achieved using delta(13)C analysis and tangential ultrafiltration with nominal cut-offs at 10000 and 100000 Da. During the first stage, cultivation intensified mineralization of the initial organic matter. This led to an increase of 2- to 5-fold in the quantity of DOC, with an important spatial variability. This phenomenon generated mainly medium molecules (MM) and large molecules (LM). These molecules were supposed to be fulvic and humic acids, and large colloids or humic metal complexes. During the second stage, once the original organic matter is stabilized, DOC concentrations decreased with time of cultivation. Both soil organic carbon and DOC were enriched in carbon originating from maize with time of cultivation. DOC was mainly composed of small molecules (SM) and MM. These molecules might be organic or amino acids, polysaccharides, or fulvic acids. From a methodological point of view, it is noteworthy that DOC characteristics determined following extraction by means of porous cups differed from those for water extraction. (C) 1997 Elsevier Science B.V.