Nutrient and water dynamics of Amazonian canga vegetation differ among physiognomies and from those of other neotropical ecosystems

Abstract

Ferriferous savannas, also known as cangas in Brazil, are nutrient-impoverished ecosystems adapted to seasonal droughts. These ecosystems support distinctive vegetation physiognomies and high plant diversity, although little is known about how nutrient and water availability shape these ecosystems. Our study was carried out in the cangas from Carajas, eastern Amazonia, Brazil. To investigate the N cycling and drought adaptations of different canga physiognomies and compare the findings with those from other ecosystems, we analyzed nutrient concentrations and isotope ratios (δ13C and δ15N) of plants, litter, and soils from 36 plots distributed in three physiognomies: typical scrubland (SB), Vellozia scrubland (VL), and woodland (WD). Foliar δ15N values in cangas were higher than those in savannas but lower than those in tropical forests, indicating more conservative N cycles in Amazonian cangas than in forests. The lower δ15N in savanna formations may be due to a higher importance of mycorrhizal species in savanna vegetation than in canga vegetation. Elevated δ13C values indicate higher water shortage in canga ecosystems than in forests. Foliar and litter nutrient concentrations vary among canga physiognomies, indicating differences in nutrient dynamics. Lower nutrient availability, higher C:N ratios, and lower δ15N values characterize VL, whereas WD is delineated by lower δ13C values and higher soil P. These results suggest lower water restriction and lower P limitation in WD, whereas VL shows more conserved N cycles due to lower nutrient availability. Differences in nutrient and water dynamics among physiognomies indicate different ecological processes; thus, the conservation of all physiognomies is required to ensure the maintenance of functional diversity in this unique ecosystem.