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Combined Magnetic, Spectroscopic, and Analytical-Chemical Approach to Infer Genetic Information for a Vertisol

Inst. of Terrestrial Ecology, ETH Zurich, Grabenstrasse 3, CH-8925 Schlieren, Switzerland

G. Guggenberger and W. Zech

Inst. of Soil Science and Soil Geography, Univ. of Bayreuth, D-95440 Bayreuth, Germany

J. Luster

Swiss Federal Inst. for Forest, Snow and Landscape Research, WSL/ETH, CH-8903 Birmensdorf, Switzerland

*Corresponding author (gehring{at}sl.ethz.ch).

ABSTRACT

A Vertisol from a floodplain in southern Mali was investigated by a combination of magnetic and spectroscopic methods in addition to classical soil chemical and physical analyses. This combined approach allowed us to deduce the genesis of the soil. The Ustic Endoaquert profile extended to a water table at 1.2 m and was subdivided into four horizons, Ap (0–0.1 m), A2 (0.1–0.4 m), A3 (0.4–0.6 m), and Ag (0.6–1.2 m). Redoximorphic features were found at a depth between 0.6 and 0.8 m. Prevailing reducing conditions in the Ag horizon were documented by a maximum concentration of exchangeable Mn(II). High redox dynamics were indicated by minimum concentrations of total and extractable Fe in the A3 horizon as well as by minimum susceptibility and saturation isothermal remanent magnetization in the A3 and Ag horizons. The latter minimum was most likely caused by the reductive dissolution of lithogenic magnetite. Soil organic matter (SOM) was generally characterized by a high amount of aromatic compounds and a high degree of lignin decomposition with maxima in the Ag horizon as indicated by solution and solid-state ¹³C nuclear magnetic resonance as well as fluorescence spectroscopies. These characteristics of SOM suggested formation under highly oxidative conditions. The presence of such characteristics, in particular in the lower part of the Vertisol with prevailing reducing conditions, suggests an allochthonous origin of the SOM. Pedoturbation in the upper part of the profile was documented by a higher contribution of primary resources such as lignin, cellulose, and hemicellulose in the Ap and A2 horizons.

Contribution of the Swiss Federal Inst. for Forest, Snow and Landscape Research.

Received for publication December 11, 1995.