Stable carbon isotope variation in C₃ and C₄ plants along the Amazon River

Abstract

ALL plants assimilate ¹²C in preference to ¹³C. As a result of this isotope fractionation, the tissues of subaerial plants have lower ¹³C/¹²C ratios than that of atmospheric CO₂. By contrast, plant respiration and tissue decomposition are accompanied by little, if any, fractionation and hence release ¹³C-depleted biogenic CO₂ back into the atmosphere. If this biogenic CO₂ is reassimilated before it is thoroughly mixed into the atmosphere, a further depletion of ¹³C in the plant tissue will result. This recycling effect has been found most often in vertical variations of stable isotope composition in tropical forests where plant tissues near the forest floor are more depleted in ¹³C (refs 1–5). Here we show that the intensity of biogenic CO₂ recycling in flood plain forests of the Amazon systematically increases inland, in the western Amazon basin. We also show that a similar recycling mechanism affects the ¹³C composition of semiaquatic grasses owing to evasion of biogenic CO₂ from the Amazon river. But in this case the degree of recycling is more pronounced in the eastern basin because the flux of ¹³CO₂ out of the river is smaller there. Our data indicate that significant spatial carbon isotope gradients can exist across the same general ecosystem, both between different species and also within a single species. Recycling effects therefore need to be taken into account in studies that try to relate plant carbon composition to animal and human diet, and in those attempting to determine the carbon isotope composition of the ancient atmosphere from preserved plant tissues.