Soil redistribution in abandoned raised fields in French Guiana assessed by radionuclides

Abstract

AbstractPre‐Columbian agricultural raised fields are widespread in Neotropical wetlands. Many seasonally flooded coastal savannahs of French Guiana are dotted with thousands of small, round mounds, which current research indicates are vestiges of raised fields. It is surprising that these elevated structures persist under the erosive force of heavy tropical rainfall. One hypothesis for this stability is that “engineer” organisms such as plants, earthworms, termites and ants colonize abandoned raised fields and maintain the raised structures against erosion. This “erosion/deposition balance” hypothesis assumes that the landscape is subject to non‐negligible rates of erosion, but that actions of engineer organisms reduce the erodibility of mounds, compensate for erosion by transporting materials to mounds, or both. The hypothesis also predicts greater bioturbation on mounds than in other parts of the landscape. To test these assumptions and predictions, we estimated soil erosion and bioturbation rates in abandoned raised fields in a seasonally flooded savannah in French Guiana by using two radionuclides (137Cs and unsupported 210Pb) as tracers for soil redistribution. Analysis of the vertical and horizontal distribution of radionuclides at the study site showed that soil erosion in the mounds either occurs at a rather low rate or that organisms counteract erosion, or both. We also showed that mounds were heavily influenced by bioturbation. These results validate the fundamental assumptions of the erosion/deposition balance hypothesis.