Landcover changes and delta C-13 composition of riverine particulate organic matter in the Piracicaba River Basin (southeast region of Brazil)

Assuming the paradigm that catchment vegetation is the main source of parti culate organic matter (POM) to rivers, the main objective of this study was to determine what the proportion of original C3 carbon from the forest had already been replaced by C4 carbon from sugar cane and pasture in the rive rs of the Piracicaba Basin. In order to achieve this objective, we first pr oduced a detailed landcover map using Landsat5-TM images, and then we measu red the carbon stable isotopic composition of the particulate riverine orga nic matter (delta(13)C-POM) in seven sites along the major rivers and in tw o sites along a small creek. Sugar cane and pasture (C4 plants) covered alm ost 60% of the basin area, while silviculture, mostly of other crops, citru s, and forest that are C3 plants, covered 35%. Isotopic studies conducted i n large pristine tropical rivers of South America and of Africa have shown that catchment vegetation is the main source of carbon in suspended POM. Ou r study demonstrates that relatively recent changes (70-80 yr ago) in landc over in the Piracicaba River Basin have already affected the composition of the riverine POM. Therefore, as in natural ecosystems, the vegetation (all ochthonous source) plays an important role in the composition of the riveri ne POM in agricultural systems such as the Piracicaba River Basin. This con trol can be supported by the good correlation between cumulative area of th e basin covered with C4 plants and the delta(13)C of the riverine POM. Howe ver, our study, differently from others, also shows that, during the low wa ter period, in situ processes, such as primary production, may be an import ant source of carbon to the riverine POM.