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[1] Our objective was to measure the stable carbon isotope composition of leaf tissue and CO2 released by respiration (delta(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 Santarem forest site, delta(r) values showed a seasonal cycle varying from less than -29parts per thousand to approximately -26parts per thousand. The observed seasonal change in delta(r) was correlated with variation in the observed monthly precipitation. In contrast, there was no significant seasonal variation in delta(r) at the Manaus forest site (average delta(r) approximately -28parts per thousand), consistent with a narrower range of variation in monthly precipitation than occurred in Santarem. Despite substantial (9parts per thousand) vertical variation in leaf delta(13)C, the average delta(r) values observed for all forest sites were similar to the delta(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 delta(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 CO2 in pastures was enriched in C-13 by approximately 10parts per thousand compared to forest ecosystems. A significant temporal change occurred in delta(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 delta(r) values

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