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We investigated the forms and composition of dissolved and particulate organic matter in rivers of the Ji-Parana Basin, which is situated at the southern limit of the Amazon lowlands and has experienced extensive deforestation in the last three decades (similar to35 000 km(2)). Our objective was to investigate how extensive land-use changes, from forest to cattle pasture, have affected river biogeochemistry. We measured a series of chemical, biochemical, and isotopic tracers in three size classes of organic matter within five sites along Ji-Parana River and eight more sites in six tributaries. The results were compared with C-4 leaf and pasture soils end members in order to test for a pasture-derived signal in the riverine organic matter. The coarse size fraction was least degraded and derived primarily from fresh leaves in lowland forests. The fine fraction was mostly associated with a mineral soil phase but its ultimate source appeared to be leaves from forests; this fraction was the most enriched in nitrogen. The ultrafiltered dissolved organic matter (UDOM) appeared to have the same source as the coarse fraction, but it was the most extensively degraded of the three fractions. In contrast to Amazon white-water rivers, rivers of the Ji-Parana Basin had lower concentrations of suspended solids with a higher carbon and nitrogen content in the three size. fractions. However, principal component analyses showed a correlation between areas covered with pasture and the delta(13)C values of the three size fractions. The highest a delta(13)C values were observed in the ultrafiltered dissolved organic matter of the Rolim-de-Moura and Jaru rivers, which have the highest areas covered with pasture. The lower the order of the streams and the higher the pasture area, the greater is the possibility that the C-4-derived organic matter signal will be detected first in the faster-cycling fraction (UDOM). The large change in land use in the Ji-Parana Basin, replacement of primary forests by C-4 pastures for cattle feeding, that has taken place in the last 30-40 yr, has already changed the characteristics of the composition of the riverine organic matter

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