Chemical composition of precipitation and throughfall in a riparian fragment in southwestern Amazonia (Rondonia, Brazil)

Abstract: Nutrient cycling in riparian forests play an important role in terrestrial and aquatic ecosystems coupling. Recent deforestation in Amazonia has promoted various perturbations, not only in the landscape, but also on nutrient dynamics. Due to this new reality, it is essential to increase the knowledge about the nutrient status/cycling within these altered systems. In this study carbon deposition via throughfall was analyzed for 7 months (Nov/05–May/06) in a riparian fragment of the Urupá River - Rondônia, Brazil. A total of 12 individual events were sampled on a bi-weekly basis, to determine rain and throughfall chemistry in a 2-ha plot of riparian forest. Throughfall samples were collected in 11 samplers distributed randomly among three transects situated on the riparian forest between the Urupá River and the adjacent pasture area (where the precipitation sampler was installed). Throughfall (TF) input from the wet-to-dry season was 1,5 times greater for DOC (dissolved organic carbon) than those from the dry-to-wet season, with an average concentration of 647,7 µM, while for precipitation (P), in which average DOC concentrations were 132,8 µM, this ratio was nearly 2,5 times. The pH was largely driven by this high concentration of organics showing acidic values (along the entire studied period), with an average of 6,226 (TF) and 5,926 (P) during the wet-to-dry season and 5,763 (TF) and 5,503 (P) during dry-to-wet. The average E.C. (electrical conductivity) in throughfall (19,99 µS.cm^-1) was much higher (2,5 times) than those in rainwater (7,79 µS.cm^-1) during the whole sampling period. Since E.C. reflects the ionic content, this shows the enrichment of throughfall after passing through the forest canopy.