CHARACTERIZATION OF DISSOLVED ORGANIC-CARBON IN CLEARED FOREST SOILS CONVERTED TO MAIZE CULTIVATION

The impact of cultivation on the nature of dissolved organic carbon (D OC) has been studied in order to understand differences in denitrifica tion rates observed in situ. The quantitative aspect of DOC was studie d by means of porous-cup vacuum samplers and water extraction. Qualita tive characterization was achieved using delta(13)C analysis and tange ntial ultrafiltration with nominal cut-offs at 10000 and 100000 Da. Du ring the first stage, cultivation intensified mineralization of the in itial organic matter. This led to an increase of 2- to 5-fold in the q uantity of DOC, with an important spatial variability. This phenomenon generated mainly medium molecules (MM) and large molecules (LM). Thes e molecules were supposed to be fulvic and humic acids, and large coll oids or humic metal complexes. During the second stage, once the origi nal organic matter is stabilized, DOC concentrations decreased with ti me of cultivation. Both soil organic carbon and DOC were enriched in c arbon originating from maize with time of cultivation. DOC was mainly composed of small molecules (SM) and MM. These molecules might be orga nic or amino acids, polysaccharides, or fulvic acids. From a methodolo gical point of view, it is noteworthy that DOC characteristics determi ned following extraction by means of porous cups differed from those f or water extraction. (C) 1997 Elsevier Science B.V.