A comparison of δ13C ratios of surface soils in savannas and forests in Amazonia

Abstract

Aim: To determine the relationship between stable carbon isotope ratios in surface soils and the present vegetation in Amazonian savannas. Location: Areas in and around savannas in the Brazilian Amazonian States of Amazonas, Pará, Amapá, Roraima, Mato Grosso and Maranhão. Methods: δ13C ratios were measured from surface (0-5 cm depth) soils in fifty-two plots in savanna plots with different covers of trees, shrubs, and grasses, and in ten adjacent areas covered by forest. Results: Soil δ13C ratios varied widely (-24.9 to -15.2‰) among and within savannas, but were distinguishable from δ13C ratios under forest (-30.3 to -27.3‰). One site close to forest with 38% tree cover, 44% shrub cover and 45% grass cover was distinguishable from both forest and savanna on Landsat TM5 images, and had a δ13C ratio of -26.7‰. Tree density (TD), basal area (BA) and cover differed strongly between savanna and forest areas. However, most savanna areas had soil organic matter with δ13C ratios closer to those of forest trees than C4 grasses. Main conclusions: In Amazonia, soil δ13 C values >-25‰ can be unequivocally attributed to savannas irrespective of depth. However, there is no precise relationship between tree or grass cover in savannas and surface-soil δ13C values. This is partly because shrubs, as well as trees and grasses, contribute significantly to soil organic matter δ13C ratios, and partly because there is a stronger negative relationship between area of bare ground and cover of C4 grasses than between area of bare ground and cover of trees or shrubs. This means that the ratio of C3 to C4 plants tends to increase with a decrease in total cover. Areas with large amounts of open ground, may have a small proportion of grass cover relative to tree and shrub cover. Although we did not encounter any such places in this study, very harsh conditions could lead to desert-like formations with little grass cover and soil organic-matter δ13C ratios similar to forest areas.