This paper presents carbon isotope data on soil organic matter (SOM) collected along an ecosystem transect that includes a wooded savannah (cerrado), a tropical semideciduous forest (cerradão), a forest tran sition type and a tropical forest. The study area is located in the Rondonia state, southwestern Brazilian Amazon region. 14C data of total soil organic matter and charcoal indicate that the organic matter in these soils is at least Holocene in age. The forest and forest transition sites are characterized by δ13C soil depth profiles gener ated typically by C3 plants, indicating no major changes in plant communities have occurred in this region during the time period represented by the isotope data. In contrast, the cerrado and cerradão have experienced significant vegetation changes during the Holocene. The d13C data (-30‰ to -27‰) obtained in the deepest part of the profile at the cerradão site show the expansion of the C3 forest vegetation into this region during early Holocene. A vegetation change consisting of increased C4 plant influence is reflected in the 13C-enriched 13C record shows a clear expansion of C3 vegetation, particularly at the cerradão site. The regression/expansion of the forest and savannah vegetation documented at the cerradão and cerrado sites is probably related to changes from a humid to a drier climate and a return to more humid conditions and is in agreement with palaeoclimatic information reported for Brazil and the Bolivian Altiplano. This study suggests that large areas in the Amazon basin have been affected by vegetation changes during the Holocene and that soil organic matter in the transition areas between savannah and forest ecotones contains a valuable palaeorecord of vegetation changes in the Ama zon region.

[1] During September and October 1999, dynamic chamber measurements were carried out to determine nitric oxide (NO) fluxes from a primary forest soil and an old pasture in the Brazilian Amazon basin as part of the project ''European Studies of Trace Gases and Atmospheric Chemistry as a Contribution to the Large-Scale Biosphere-Atmosphere Experiment in Amazonia'' (LBA-EUSTACH). In addition, soil samples were collected from these two sites, and laboratory experiments were conducted to determine the NO production and consumption rate constants as functions of soil temperature and soil moisture. These laboratory results were converted into NO fluxes using a simple algorithm, which required additional information on the gas diffusion in soil, the soil bulk density, and the field conditions (soil temperature and soil moisture). Over the entire measurement period, the calculated and measured NO fluxes agreed well both for the forest (6.9 ± 2.9 and 5.0 ± 4.6 ng m À2 s À1 , respectively) and for the pasture (0.67 ± 0.09 and 0.65 ± 0.37 ng m À2 s À1 , respectively). Forest to pasture conversion decreased NO production and gas diffusion and resulted in smaller NO fluxes from pasture than forest soil.

ABSTRACT. This study, which was carried out in the southern Brazilian Amazon region (Rond8nia state and Humaita, Southern Amazon state), presents and discusses the significance of carbon isotope data measured in soil profiles collected across natural boundaries of forest to savanna vegetation. The main objective of this study was to evaluate the expansionregression dynamics of these vegetation units in relation to climate changes during the Holocene. 14C data from charcoal, soil organic matter (SOM) and its component humin fraction indicate that the organic matter in the studied soils is essentially Holocene in origin. 13C data indicate that C3 type plants were the dominant vegetation at all study areas in the early Holocene, and during the entire Holocene, in the forest sites of Central Rond8nia state and in the forest site 50 km from the city of Humaita.13C data also indicate that C4 plants have influenced significantly the vegetation at the transitional forest and the Cerrado (wooded savanna) sites of Southern Rond8nia state and the forest ecosystem located 20 km from the Humaita city.These typical C4 type isotopic signatures probably reflect a drier climate during the mid-Holocene. The 13C records representing probably the last 3000 yr show an expansion of the forest, due to a climatic improvement, in areas previously occupied by savanna vegetation. These results and other published data for the Amazon region indicate that the areas representing today's forest-savanna boundaries have been determined by significant vegetation changes during the Holocene. The boundary between forest and savanna vegetation seems to be quite sensitive to climatic change and should be the focus of more extensive research to correlate climate and past vegetation dynamics in the Amazon region.

RESUMOEste trabalho discute os efeitos das mudanças do uso do solo na biogequímica dos rios da bacia de drenagem do rio Ji-Paraná (Rondônia). Nesta região, a distribuição espacial do desmatamento e das propriedades do solo resultam em sinais diferentes, possibilitando a divisão dos sistemas fluviais em três grupos: rios com águas pobres em íons e baixo impacto; rios com conteúdo iônico intermediário e impacto médio e rios com elevados conteúdo iônico e impacto antropogênico. As características biogeoquímicas dos rios têm relação significativa com a área de pasto, melhor parâmetro para prever a condutividade elétrica (r 2 = 0,87) e as concentrações de sódio (r 2 = 0,75), cloreto (r 2 = 0,69), potássio (r 2 = 0,63), fosfato (r 2 = 0.78), nitrogênio inorgânico (r 2 = 0.52), carbono inorgânico (r 2 = 0.81) e carbono orgânico (rain 2 = 0.51) dissolvidos. Cálcio e magnésio tiveram sua variância explicada pelas características do solo e pastagem. Nossos resultados indicam que as mudanças observadas na micro-escala constituem "sinais biogeoquímicos" gerados pelo processamento do material nas margens dos rios. A medida em que os rios evoluem para ordens superiores, os sinais persistentes nos canais fluviais estão mais associdados às características da bacia de drenagem (solos e uso da terra). Apesar dos efeitos das mudanças observadas no uso do solo não serem ainda detectáveis na macro-escala (bacia amazônica), a disrupção da estrutura e funcionamento dos ecossistemas é detectável nas micro e meso escalas, com alterações significativas na ciclagem de nutrientes nos ecossistemas fluviais. PALAVRAS-CHAVEAmazônia, rios e igarapés, biogeoquímica, mudanças no uso da terra.Effects of land use changes in the biogeochemistry of fluvial systems of the Ji-Paraná river basin, Rondônia. ABSTRACT In this article we present the results of the effects of land use change on the river biogeochemistry of the Ji-Paraná basin (Rondônia). In this region, the spatial distribution of deforestation and soil properties result in different biogeochemical signals, allowing the division of the fluvial systems into three groups: rivers with low ionic concentration and low impact; rivers with intermediate ionic content and medium impact; and rivers with high ionic content and anthropogenic impact. River biogeochemical characteristics present KEY WORDSAmazonia, Rivers and streams, biogeochemistry, land-use change

Concentrations of cations (Na ? , Ca 2? , Mg 2? , K ? , NH 4 ? ), anions (HCO 3 -, Cl -, NO 3 -, SO 4 2-, PO 4 3-) and suspended sediments in the Madeira River water were determined near the city of Porto Velho (RO), in order to assess variation in water chemistry from 2004 to 2007. Calcium and bicarbonate were the dominant cation and anion, respectively. Significant seasonal differences were found, with highest concentrations occurring during the dry season, as expected from the drainage of Andean carbonate-rich substratum. Interannual variations were also observed, but became significant only when annual average discharge was 25% less than normal. Under this atypical discharge condition, bicarbonate was replaced by sulfate, and higher suspended sediment concentrations and loads were also observed. Compared to previously published studies, it appears that no significant changes in water chemistry have occurred during the last 20-30 years, although differences in approaches and sampling designs among this and previous studies may not allow detection of modest changes. The calculated suspended sediment load reported here is close to the values presented elsewhere, reinforcing the relative importance of this river as a sediment supplier for the Amazon Basin. Seasonality has a significant control on the chemistry of Madeira River waters, and severe decrease in discharge due to anthropogenic changes, such as construction of reservoirs or the occurrence of drier years-a plausible consequence of global climate change-may lead to modification in the chemical composition as well in the sediment deliver to the Amazon River.