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

Abstract

Here we present the within-site, seasonal, and interannual variations of the carbon (δ13C) and nitrogen (δ15N) isotope ratios of leaves, wood, bark and litter from four sites in the Amazon region, Brazil. Samples were collected in Manaus (3° 06′07′′ S; 60°01′30′′ W), Ji-Parana (10°53′07′′ S; 61°57′06′′ W), and Santarem (2°26′35′′ S; 54°42′30′′ W) with mean annual precipitation of 2207, 2040 and 1909 mm respectively. The overall average for all leaf samples was \(-32.3\pm2.5\permille\) for δ13C and \(+5.8\pm1.6\permille\) for δ15N (n=756). The leaf δ values at these sites were often but not always statistically distinct from each other. The δ13C values varied from \(-37.8\permille\) to \(-25.9\permille\). Pronounced differences in δ13C values occurred with height associated with differences in forest structure. The δ13C 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 δ15N values ranged from \(+0.9\permille\) to a maximum value of \(+10.9\permille\), and the Santarem sites showed more enriched values than Manaus and Ji-Parana sites. No seasonal variation was detected in the δ15N 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 δ13C and consistent with our understanding that an open nitrogen cycle can lead to high δ15N values despite a significant number of legumes in the vegetation.