Inland variability of carbon-nitrogen concentrations and δ13C in Amazon floodplain (várzea) vegetation and sediment

Abstract

We analysed carbon, nitrogen and δ13C values in sediment sampled in the Amazon floodplains. The surface sediment samples were collected along inland transects divided according to their vegetation cover: (a) forest, where the dominant vegetation was inundation forests; (b) grass, where the dominant vegetation was grasses, mainly from Echnochloa polystachya; (c) mixed, where the vegetation changed from grasses near the river bank to forest inland. The average sediment carbon and nitrogen concentrations were significantly higher in forest samples (C = 0.99%, N = 0.12%) than in grass samples (C = 0.84%, N = 0.10%). The average sediment δ13C was significantly heavier in grass (-26.7%) than in forest samples (-28.6%), reflecting the isotopically heavier C4 grasses carbon. In the forest and mixed transects inland, increases in the carbon and nitrogen concentrations were observed. At approximately 50 to 60 m from the riverbank, the concentration of carbon and nitrogen became similar to the river particulate organic carbon (POC) concentration, which is the primary floodplain sediment source. We hypothesized that this trend is caused by the deposition of carbon- and nitrogen-depleted sand-size particles in the first few metres of the floodplain, whereas the more enriched silt-clay particles are preferentially deposited far inland. In contrast to inland trends, no significant downriver trend was observed in the carbon and nitrogen concentrations. However, a downriver δ13C increase was observed for sediment in grass-covered areas. Based on δ13C values in different várzea environments, we estimated the relative contribution of riverine POC and vegetation material present in the várzea sediments under different vegetation covers. For forest sediment samples we estimated that 60% of the carbon was derived from the riverine POC and the forests provided the remaining 40%. For open areas covered with grasses we estimated that only 10% was derived from these plants, and 90% was derived from riverine POC. © 2003 John Wiley & Sons, Ltd.