A consistent observation of river waters in the Amazon Basin and elsewhere is that suspended fine particulate organic matter (FPOM) is compositionally distinct from coexisting dissolved organic matter (DOM). The present article presents experimental results that show that at least some of these compositional patterns are the outcome of selective partitioning of nitrogen-rich DOM components onto mineral surfaces. Nine laboratory experiments were conducted in which natural DOM from two rivers, one wetland, and two leachates from the Peruvian Amazon were mixed with natural suspended riverine minerals or organic-free kaolinite. Concentrations of organic carbon, organic nitrogen, and hydrolyzable amino acids were measured in both dissolved and particulate phases before and after mixing. In each of these trials, nitrogen was preferentially taken into the particulate fraction relative to the ''parent'' DOM, as were total hydrolyzable amino acids with respect to total organic carbon and nitrogen. Amino acid compositional patterns also indicated preferential sorption of basic amino acids, with positively charged nitrogen side chains, to the negatively charged aluminosilicate clay minerals. In short, sorption of natural DOM to minerals reproduced all contrasting organic nitrogen compositional patterns observed in the Amazon Basin. Although previously conjectured from FPOM-DOM compositional trends from river samples, this is the first direct evidence for preferential uptake of naturally occurring nitrogenous DOM by suspended riverine minerals. Last, nonprotein amino acids, which are commonly used as diagenetic indicators in sediments, preferentially remained dissolved, which suggests that sorptive fractionation may significantly complicate comparisons of FPOM and DOM diagenesis on the basis of interpretation of organic composition.
-The 13C : l*C of suspended particulate organic C (POC), dissolved organic C (DOC), and dissolved inorganic C (DIC) were measured during 1982-l 984 at 11 main-channel and 7 tributary stations over an 1,800 km reach of the Amazon River between Vargem Grande and Obidos, Brazil. The measured 613C range vs. marine carbonate (PDB) was -32 to -269~ for suspended POC, -30 to -289m for DOC, and -26 to -129~ for DIC. The 613C of the fine particulate organic C (FPOC) decreased downriver from Vargem Grande, with values lowest during the fallingwater portion of the runoff cycle; these trends were the result primarily of input of IC-depleted FPOC from tributaries draining the lowland regions of the Amazon basin and floodplain soils. The 613C of the FPOC at Obidos implies that at least 35% of the POC exported by the Amazon River is derived from the lowland portion of the Amazon basin. The 613C of DIC decreased downriver with the lowest values measured during falling water; these trends were due primarily to within-river respiration and tributary input. The 613C of the DIC suggests that -40% of the organic matter being respired in the river is C, plant material derived from floodplain grasses.Within its basin, the discharge of chemically diverse tributaries is blended by the Amazon River, resulting in main-channel concentrations of chemical species and sediment that are close to the world average for rivers (Stallard and Edmond 1983). In the main channel of the river, we found that -20% of the total C was particulate organic
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
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