The stable carbon and nitrogen isotopic composition of vegetation in tropical forests of the Amazon Basin, Brazil

Here we present the within-site, seasonal, and interannual variations of the carbon (delta C-13) and nitrogen (delta N-15) isotope ratios of leaves, wood, bark and litter from four sites in the Amazon region, Brazil. Samples were collected in Manaus (3 degrees 06'07S; 60 degrees 01'30 W), Ji-Parana (10 degrees 53'07S; 61 degrees 57'06 W), and Santarem (2 degrees 26'35 S; 54 degrees 42'30 W) with mean annual precipitation of 2207, 2040 and 1909 mm respectively. The overall average for all leaf samples was -32.3 +/- 2.5 parts per thousand for delta C-13 and +/- 5.8 +/- 1.6 parts per thousand for delta N-15 (n=756). The leaf delta values at these sites were often but not always statistically distinct from each other. The delta C-13 values varied from -37.8 parts per thousand to -25.9 parts per thousand. Pronounced differences in delta C-13 values occurred with height associated with differences in forest structure. The delta C-13 of leaf dry matter showed seasonal variations associated with the length of the dry season, despite the fact that total annual precipitation was similar among the studied sites. Leaf delta N-15 values ranged from +/- 0.9 parts per thousand to a maximum value of +10.9 parts per thousand, and the Santarem sites showed more enriched values than Manaus and Ji-Parana sites. No seasonal variation was detected in the delta N-15 of leaves, but significant differences were observed among sites and with changes in canopy height. The isotope ratio data are consistent with our current understanding of the roles of light, water availability, and recycling of soil-respired CO2 influences on delta C-13 and consistent with our understanding that an open nitrogen cycle can lead to high delta N-15 values despite a significant number of legumes in the vegetation.