[1] Our objective was to measure the stable carbon isotope composition of leaf tissue and CO 2 released by respiration (d r ), and to use this information as an estimate of changes in ecosystem isotopic discrimination that occur in response to seasonal and interannual changes in environmental conditions, and land-use change (forest-pasture conversion). We made measurements in primary forest and pastures in the Amazon Basin of Brazil. At the Santarém forest site, d r values showed a seasonal cycle varying from less than À29% to approximately À26%. The observed seasonal change in d r was correlated with variation in the observed monthly precipitation. In contrast, there was no significant seasonal variation in d r at the Manaus forest site (average d r approximately À28%), consistent with a narrower range of variation in monthly precipitation than occurred in Santarém. Despite substantial (9%) vertical variation in leaf d 13 C, the average d r values observed for all forest sites were similar to the d 13 C values of the most exposed sun foliage of the dominant tree species. This suggested that the major portion of recently respired carbon dioxide in these forests was metabolized carbohydrate fixed by the sun leaves at the top of the forest canopy. There was no significant seasonal variation observed in the d 13 C values of leaf organic matter for the forest sites. We sampled in pastures dominated by the C 4 grass, Brachiaria spp., which is planted after forest vegetation has been cleared. The carbon isotope ratio of respired CO 2 in pastures was enriched in 13 C by approximately 10% compared to forest ecosystems. A significant temporal change occurred in d r after the Manaus pasture was burned. Burning removed much of the encroaching C 3 shrub vegetation and so allowed an increased dominance of the C 4 pasture grass, which resulted in higher d r values.

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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

Several studies in tropical watersheds have evaluated the impact of urbanization and agricultural practices on water quality. In Brazil, savannas (known regionally as Cerrados) represent 23% of the country's surface, representing an important share to the national primary growth product, especially due to intense agriculture. The purpose of this study is to present a comprehensive evaluation, on a yearly basis, of carbon, nitrogen and major ion fluxes in streams crossing areas under different land use (natural vegetation, sugar cane and eucalyptus) in a savanna region of SE Brazil. Eucalyptus and sugar cane alter the transport of the investigated elements in small watersheds. The highest concentration of all parameters (abiotic parameters, ions, dissolved organic carbon -DOC -and dissolved inorganic carbon -DIC) were found in Sugar Cane Watersheds (SCW). The observed concentrations of major cations in Eucalyptus Watersheds (EW) (Mg, Ca, K, Na), as well as DIN and DOC, were found frequently to be intermediate values between those of Savanna Watersheds (SW) and SCW, suggesting a moderate impact of eucalyptus plantations on the streamwater. Same trends were found in relation to ion and nutrient fluxes, where the higher values corresponded to SCW. It is suggested that sugar cane plantations might be playing an important role in altering the chemistry of water bodies.

Abstract. We investigated the forms and composition of dissolved and particulate organic matter in rivers of the Ji-Paraná Basin, which is situated at the southern limit of the Amazon lowlands and has experienced extensive deforestation in the last three decades (ϳ35 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-Paraná 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 JiParaná 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 ␦ 13 C values of the three size fractions. The highest ␦ 13 C values were observed in the ultrafiltered dissolved organic matter of the Rolim-de-Moura and Jarú 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-Paraná 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.

Riverine nitrogen distribution is increasingly controlled by anthropogenic activities in their watersheds, regardless of spatial scale, climate, and geographical zone. Consequently, modelling efforts to predict the export of nitrogen from rivers worldwide have used attributes such as population density, land use, urbanization and sanitation. These models have greatly enhanced our understanding of the sources and fate of nitrogen added to terrestrial systems and transported to rivers and streams, especially for developed countries of the North temperate zone. However, much of the world's population lives in developing countries of the tropics, where the effects of human activities on riverine N exports are still poorly understood. In an effort to close this gap, we compare riverine nitrogen data from 32 Brazilian rivers draining two contrasting regions in this tropical country in terms of economic development -the State of São Paulo and the Amazon. Our data include nitrogen in different dissolved forms, such as Dissolved Inorganic Nitrogen (DIN) and Dissolved Organic Nitrogen (DON). The results show that nitrogen concentrations decreased as river runoff increased in both study areas, and that concentrations were significantly higher in rivers draining the most economically developed region. The relationships between nitrogen concentrations and fluxes with demographic parameters such as population density were also determined and compared to those in temperate systems. In contrast to temperate watersheds, we found that nitrogen fluxes increased only after population densities were higher than 10 individuals per km 2 .Keywords: nitrogen, river, pollution, Piracicaba basin, Amazon basin, Brazil. Nitrogênio dissolvido em rios de regiões distintas do Brasil ResumoA carga do nitrogênio em rios é cada vez mais controlada por atividades humanas independentemente da escala espacial, do clima, ou da região geográfica. Consequentemente, esforços de modelagem que predizem exportação de nitrogênio usam atributos que refletem atividades humanas, tais quais, densidade populacional, uso do solo, urbanização e saneamento ambiental. Estes modelos têm aumentado significativamente nosso entendimento sobre as fontes e destino do nitrogênio adicionado ao sistema terrestre e transportado aos sistemas aquáticos, especialmente em países desenvolvidos localizados no Hemisfério Norte. Entretanto, a maioria da população mundial vive em países em desenvolvimento dos trópicos, onde os efeitos das atividades humanas sobre a carga de nitrogênio nos rios são pobremente entendidos. Em um esforço para aumentar nossa compreensão sobre este fenômeno, comparamos a distribuição de nitrogênio dissolvido em 32 rios brasileiros, drenando duas regiões contrastantes em termos de desenvolvimento econômico -rios do Estado de São Paulo e da região Amazônica. Nossos dados incluem nitrogênio em duas formas dissolvidas: inorgânica (NID) e orgânica (NOD

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